Patent Publication Number: US-6987930-B2

Title: Lens barrel incorporating the advancing/retracting mechanism

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an advancing/retracting mechanism incorporated in a photographing (imaging) lens (lens barrel). 
   2. Description of the Related Art 
   An advancing/retracting mechanism for advancing or retracting a linearly guided movable member (linearly movable member) by a rotation of a cam ring is often used in a lens barrel and the like. In such an advancing/retracting mechanism which is incorporated in a photographing lens barrel, the maximum amount of movement of the linearly guided movable member along the photographing optical axis has been required to be great due to a growing trend in recent years to increase a variable-power ratio of a zoom lens. A simple way to increase the maximum amount of movement of the linearly guided movable member along the photographing optical axis is to increase the cam ring and a linear guide member in size, specifically the lengths of these elements in the optical axis direction. However, this increases the size of the photographing lens, which is not desirable, especially for compact cameras, which are required to be designed as small as possible to satisfy the current trend towards further miniaturization. The linear guide member which guides the movable member linearly without rotating the movable member is generally coupled to the cam ring to be immovable relative to the cam ring along the rotational axis thereof while allowing the cam ring to rotate relative to the movable member. Due to this structure, the size of the linear guide member (especially the length thereof) is limited by the cam ring. Accordingly, if the maximum amount of movement of the movable member is relatively great compared to the size of the cam ring, the movable member is difficult to be guided linearly without rotating with reliability over the entire range of movement of the movable member. 
   SUMMARY OF THE INVENTION 
   The present invention provides an advancing/retracting mechanism incorporated in a photographing lens (lens barrel), wherein the mechanism can move the movable member by a great amount of movement though the mechanism is simple and small. 
   According to an aspect of the present invention, a guide mechanism for a lens barrel is provided, the mechanism including a support frame supporting an imaging component; and a linear guide configured to guide the support frame along an axis. The linear guide can includes a ring portion defining an opening through which the support frame can pass, and further includes at least one linear guide key extending along the axis from the ring portion and positioned substantially radially inwardly of the opening. The support frame can have at least one linear guide groove located at the outer peripheral surface thereof and configured to slidably engage with a respective the at least one linear guide key, and each of the opposite ends of the at least one linear guide groove are open such that the support frame is movable to extend from either of the sides of the ring portion. 
   The at least one linear guide key can extend along the axis on one side of the ring portion only. 
   It is desirable for the at least one linear guide key can extend generally perpendicularly from the ring portion. 
   It is desirable for the linear guide to have a plurality of linear guide keys provided at different circumferential positions of the ring portion, and wherein the support frame has a corresponding plurality of linear guide grooves. 
   It is desirable for the linear guide key to be substantially planar and be provided substantially perpendicular to a radius of the ring portion. 
   The ring portion can have at least one guide projection projecting radially outwardly therefrom and configured to engage a linear guide ring. 
   The guide mechanism can further include a linear guide ring having at least one guide portion, formed on an inner peripheral surface thereof, engageable with the guide projection and configured to guide the linear guide along the axis without rotating. 
   The guide mechanism can further include a cam ring rotatable about the axis, the cam ring having at least one cam groove located on an inner peripheral surface thereof. The support frame can include at least one cam follower which projects from an outer peripheral surface thereof engageable with the cam groove. 
   It is desirable for the cam ring to have a plurality of cam grooves located at different positions on the inner peripheral surface thereof in at least the axis direction to respectively trace a plurality of reference cam diagrams having generally the same shape and size, respectively. It is desirable for a rearmost cam groove of the plurality of cam grooves in the axis direction is located such that a rear portion of the rearmost cam groove is missing and forms at least one rear end opening of the rearmost cam groove at the rear end of the cam ring. The support frame can have a plurality of cam followers which are located at different positions on the outer peripheral surface thereof in at least the axis direction, the plurality of cam followers respectively engageable in the plurality of cam grooves. A rearmost cam follower of the plurality of cam followers can be movable out of the rear end opening to be disengaged from the rearmost cam groove when the support frame is positioned at a rear movement limit thereof. The ring portion can include at least one cam follower passage recess, which is formed on an inner peripheral surface thereof, and allows the plurality of cam followers to pass through the cam follower passage recess in the axis direction, when the plurality of cam followers are disengaged from the rearmost cam groove. 
   The ring portion can be supported by a circumferential portion of the cam ring such that the cam ring is rotatable relative to the ring portion and immovable relative to the ring portion in the axis direction. 
   In another embodiment, a drive mechanism of a lens barrel is provided, including a cam ring rotatable about a rotational axis, including at least one cam groove located on an inner peripheral surface of the cam ring; a movable frame including at least one cam follower which projects from an outer peripheral surface of the movable frame and is configured to engage in the at least one cam groove; and a linear guide configured to guide the movable frame linearly in an optical axis direction without rotating the movable frame, the movable frame moving linearly in the optical axis direction by a rotation of the cam ring. The movable frame can include at least one linear guide groove located on an outer peripheral surface of the movable frame to extend generally parallel to the optical axis, each of opposite ends of the linear guide groove being open. The linear guide can include a ring portion which includes a central opening through which the movable frame can pass; and at least one linear guide key which projects from the ring portion and is positioned radially inside the central opening and is further slidably engageable in the at least one linear guide groove. At least part of the movable frame is positioned in front of the ring portion of the linear guide when the movable frame is positioned at a front movement limit thereof, and the at least part of the movable frame is configured to pass through the central opening of the ring portion to be positioned behind the ring portion of the linear guide when the movable frame moves to a rear movement limit thereof from the front movement limit. 
   The cam ring can include a circumferential groove located proximate a rear end of the cam ring in the optical axis direction, such that the ring portion is engageable in the circumferential groove so as not to move in the optical axis direction relative to the cam ring while allowing the cam ring to rotate relative to the ring portion. It is desirable for the at least one linear guide key to extend forward along an inner peripheral surface of the cam ring. The entire the movable frame can be positioned in front of the ring portion of the linear guide when the movable frame is positioned at the front movement limit thereof. It is desirable for a portion of the movable frame to project rearward through the central opening of the ring portion and is positioned behind the ring portion of the linear guide when the movable frame is positioned at the rear movement limit thereof. 
   The at least one cam groove can include a plurality of cam grooves located at different positions on the inner peripheral surface of the cam ring in at least the optical axis direction, the plurality of cam grooves configured to trace a respective plurality of reference cam diagrams having generally the same shape and size, respectively. A rearmost cam groove of the plurality of cam grooves in the optical axis direction can be configured such that a rear portion of the rearmost cam groove is missing and forms at least one rear end opening of the rearmost cam groove at the rear end of the cam ring. The at least one cam follower can include a plurality of cam followers located at different positions on the outer peripheral surface of the movable frame in at least the optical axis direction, the plurality of cam followers engageable in a respective the plurality of cam grooves. It is desirable for a rearmost cam follower of the plurality of cam followers to exits the rear end opening and to be disengaged from the rearmost cam groove when the movable frame is positioned at the rear movement limit thereof. The ring portion of the linear guide can include at least one cam follower passage recess located on an inner peripheral surface of the ring portion and allows the plurality of cam followers to pass the ring portion through the cam follower passage recess in the optical axis direction, when the plurality of cam followers are disengaged from the plurality of cam grooves through the least one rear end opening. 
   A rear end of the at least one linear guide key can project rearward from the at least one linear guide groove when the movable frame is positioned at the front movement limit thereof, and a front end of the at least one linear guide key can project forward from the at least one linear guide groove when the movable frame is positioned at the rear movement limit thereof. 
   The at least one linear guide groove can be a plurality of linear guide grooves located at different circumferential positions, and the at least one linear guide key can be a plurality of linear guide keys located at different circumferential positions. 
   The drive mechanism can further include a stationary barrel, the linear guide and the cam ring being movable relative to the stationary barrel in the optical axis direction. 
   The lens barrel can include a plurality of movable lens groups movable relative to each other in the optical axis direction, the movable frame supporting at least one of the plurality of movable lens groups. 
   In another embodiment, a drive mechanism of a lens barrel is provided, including a cam ring including a plurality of cam grooves located on an inner peripheral surface of the cam ring at different positions thereon in at least an optical axis direction, and a circumferential groove formed in the vicinity of a rear end of the cam ring, wherein the plurality of cam grooves are configured to respectively trace a plurality of reference cam diagrams having generally the same shape and size, and wherein a rearmost cam groove of plurality of cam grooves in the optical axis direction is configured such that a rear portion of the rearmost cam groove is missing and forms at least one rear end opening of the rearmost cam groove at the rear end of the cam ring; a movable frame including a plurality of cam followers located at different positions in at least the optical axis direction and are respectively engageable in the plurality of cam grooves, and at least one linear guide groove located on an outer peripheral surface of the movable frame to extend generally parallel to the optical axis, each of opposite ends of the linear guide groove being open; and a linear guide including a ring portion, at least one linear guide key and at least one cam follower passage recess, wherein the ring portion is engageable in the circumferential groove so as not to move in the optical axis direction relative to the cam ring and be rotatable relative to the cam ring, and the ring portion including a central opening through which the movable frame can pass, wherein the at least one linear guide key projects forward from the ring portion along an inner peripheral surface of the cam ring and is positioned radially inside the central opening and is further slidably engageable in the at least one linear guide groove, and wherein the at least one cam follower passage recess is located on an inner peripheral surface of the ring portion, the at least one cam follower passage recess generally aligned with the rear end opening of the rearmost cam groove in the optical axis direction to allow the plurality of cam followers to pass the ring portion through the at least one cam follower passage recess in the optical axis direction when the cam ring and the linear guide are positioned relative to each other such that a rearmost cam follower of the plurality of cam followers reaches the rear end opening of the rearmost cam groove. The entire movable frame can be positioned in front of the ring portion when the movable frame is positioned at a front movement limit thereof, and at least part of the movable frame can pass through the central opening of the ring portion such that the rearmost cam follower is disengaged from the rearmost cam groove through the rear end opening and the at least one cam follower passage recess when the movable frame moves to a rear movement limit thereof from the front movement limit. 
   It is desirable for a rear end of the at least one linear guide key to project rearward from the at least one linear guide groove when the movable frame is positioned at the front movement limit thereof, and a front end of the at least one linear guide key to project forward from the at least one linear guide groove when the movable frame is positioned at the rear movement limit thereof. 
   The at least one linear guide groove can include a plurality of linear guide grooves located at different circumferential positions, and the at least one linear guide key can include a plurality of linear guide keys located at different circumferential positions. 
   The drive mechanism can further include a stationary barrel, the linear guide and the cam ring being movable relative to the stationary barrel in the optical axis direction. 
   The lens barrel can include a plurality of movable lens groups movable relative to each other in the optical axis direction, the movable frame supporting at least one of the plurality of movable lens groups. 
   It is desirable for the distance between the front movement limit and the rear movement limit of the movable frame to be a greater than an axial length of the at least one linear guide key. 
   The present disclosure relates to subject matter contained in Japanese Patent Application Nos. 2002-247338 (filed on Aug. 27, 2002) and 2003-25447 (filed on Feb. 3, 2003) which are expressly incorporated herein by reference in their entireties. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described below in detail with reference to the accompanying drawings in which: 
       FIG. 1  is an exploded perspective view of an embodiment of a zoom lens according to the present invention; 
       FIG. 2  is an exploded perspective view of a structure supporting a first lens group of the zoom lens; 
       FIG. 3  is an exploded perspective view of a structure supporting a second lens group of the zoom lens; 
       FIG. 4  is an exploded perspective view of a lens barrel advancing-retracting structure of the zoom lens for advancing and retracting a third external barrel from a stationary barrel; 
       FIG. 5  is a perspective view, partly exploded, of the zoom lens, showing a fixing procedure of a viewfinder unit to the zoom lens and a fixing procedure of a gear train to the zoom lens; 
       FIG. 6  is a perspective view of a zoom lens assembly made from the elements shown in  FIG. 5 ; 
       FIG. 7  is a side elevational view of the zoom lens assembly shown in  FIG. 6 ; 
       FIG. 8  is a perspective view of the zoom lens assembly shown in  FIG. 6 , viewed obliquely from behind; 
       FIG. 9  is an axial cross sectional view of an embodiment of a digital camera incorporating the zoom lens assembly shown in  FIGS. 6 through 8 , wherein an upper half above a photographing optical axis and a lower half below the photographing optical axis show a state of the zoom lens at telephoto extremity and a state of the zoom lens at wide-angle extremity, respectively; 
       FIG. 10  is an axial cross sectional view of the digital camera shown in  FIG. 9  in the retracted state of the zoom lens; 
       FIG. 11  is a developed view of the stationary barrel shown in  FIG. 1 ; 
       FIG. 12  is a developed view of a helicoid ring shown in  FIG. 4 ; 
       FIG. 13  is a developed view of the helicoid ring shown in  FIG. 1 , showing a structure of the inner peripheral surface thereof by broken lines; 
       FIG. 14  is a developed view of the third external barrel shown in  FIG. 1 ; 
       FIG. 15  is a developed view of a first linear guide ring shown in  FIG. 1 ; 
       FIG. 16  is a developed view of a cam ring shown in  FIG. 1 ; 
       FIG. 17  is a developed view of the cam ring shown in  FIG. 1 , showing a structure of the inner peripheral surface thereof by broken lines; 
       FIG. 18  is a developed view of a second linear guide ring shown in  FIG. 1 ; 
       FIG. 19  is a developed view of a second lens group moving frame shown in  FIG. 1 ; 
       FIG. 20  is a developed view of a second external barrel shown in  FIG. 1 ; 
       FIG. 21  is a developed view of a first external barrel shown in  FIG. 1 ; 
       FIG. 22  is a conceptual diagram of elements of the zoom lens, showing the relationship among these elements in relation to the operations thereof; 
       FIG. 23  is a developed view of the helicoid ring, the third external barrel and the stationary barrel, showing the positional relationship thereamong in the retracted state of the zoom lens; 
       FIG. 24  is a developed view of the helicoid ring, the third external barrel and the stationary barrel, showing the positional relationship thereamong at the wide-angle extremity the zoom lens; 
       FIG. 25  is a developed view of the helicoid ring, the third external barrel and the stationary barrel, showing the positional relationship among thereamong at the telephoto extremity the zoom lens; 
       FIG. 26  is a developed view of the helicoid ring, the third external barrel and the stationary barrel, showing a positional relationship thereof; 
       FIG. 27  is a developed view of the stationary barrel, showing the positions of a set of rotational sliding projections of the helicoid ring with respect to the stationary barrel in the retracted state of the zoom lens; 
       FIG. 28  is a view similar to that of  FIG. 27 , showing the positions of the set of rotational sliding projections of the helicoid ring with respect to the stationary barrel at the wide-angle extremity of the zoom lens; 
       FIG. 29  is a view similar to that of  FIG. 27 , showing the positions of the set of rotational sliding projections of the helicoid ring with respect to the stationary barrel at the telephoto extremity of the zoom lens; 
       FIG. 30  is a view similar to that of  FIG. 27 , showing the positions of the set of rotational sliding projections of the helicoid ring with respect to the stationary barrel; 
       FIG. 31  is a cross sectional view taken along M 2 -M 2  line shown in  FIG. 27 ; 
       FIG. 32  is a cross sectional view taken along M 1 -M 1  line shown in  FIG. 23 ; 
       FIG. 33  is an enlarged cross sectional view of a portion of the upper half of the zoom lens shown in FIG.  9 ; 
       FIG. 34  is an enlarged cross sectional view of a portion of the lower half of the zoom lens shown in  FIG. 9 ; 
       FIG. 35  is an enlarged cross sectional view of a portion of the upper half of the zoom lens shown in  FIG. 10 ; 
       FIG. 36  is an enlarged cross sectional view of a portion of the lower half of the zoom lens shown in  FIG. 10 ; 
       FIG. 37  is an enlarged perspective view of a portion of the connecting portion between the third external barrel and the helicoid ring; 
       FIG. 38  is a view similar to that of  FIG. 37 , showing a state where a stop member has been removed; 
       FIG. 39  is a view similar to that of  FIG. 38 , showing a state where the third external barrel and the helicoid ring have been disengaged from each other in the optical axis direction from the state shown in  FIG. 38 ; 
       FIG. 40  is a perspective view of a portion of the stationary barrel, the stop member and a set screw therefor, showing a state where the stop member and the set screw have been removed from the stationary barrel; 
       FIG. 41  is a perspective view similar to that shown in  FIG. 40 , showing a state where the stop member is properly fixed the stationary barrel via the set screw; 
       FIG. 42  is an enlarged developed view of a portion of helicoid ring in relation to a corresponding portion of the stationary barrel; 
       FIG. 43  is a view similar to that of  FIG. 42 , showing the positional relationship between the specific rotational sliding projection of the helicoid ring and the circumferential groove of the stationary barrel; 
       FIG. 44  is a developed view of the third external barrel and the first linear guide ring in relation to a set of roller followers fixed to the cam ring, showing the positional relationship between the helicoid ring and the stationary barrel in the retracted state of the zoom lens; 
       FIG. 45  is a view similar to that of  FIG. 44 , showing the positional relationship between the helicoid ring and the stationary barrel at the wide-angle extremity of the zoom lens; 
       FIG. 46  is a view similar to that of  FIG. 44 , showing the positional relationship between the helicoid ring and the stationary barrel at the telephoto extremity of the zoom lens; 
       FIG. 47  is a view similar to that of  FIG. 44 , showing the positional relationship between the helicoid ring and the stationary barrel; 
       FIG. 48  is a developed view of the helicoid ring and the first linear guide ring, showing the positional relationship therebetween in the retracted state of the zoom lens; 
       FIG. 49  is a view similar to that of  FIG. 48 , showing the positional relationship between the helicoid ring and the first linear guide ring at the wide-angle extremity of the zoom lens; 
       FIG. 50  is a view similar to that of  FIG. 48 , showing the positional relationship between the helicoid ring and the first linear guide ring at the telephoto extremity of the zoom lens; 
       FIG. 51  is a view similar to that of  FIG. 48 , showing the positional relationship between the helicoid ring and the first linear guide ring; 
       FIG. 52  is a developed view of the cam ring, the first external barrel, the second external barrel and the second linear guide ring, showing the positional relationship thereamong in the retracted state of the zoom lens; 
       FIG. 53  is a view similar to that of  FIG. 52 , showing the positional relationship among the cam ring, the first external barrel, the second external barrel and the second linear guide ring at the wide-angle extremity of the zoom lens; 
       FIG. 54  is a view similar to that of  FIG. 52 , showing the positional relationship among the cam ring, the first external barrel, the second external barrel and the second linear guide ring at the telephoto extremity of the zoom lens; 
       FIG. 55  is a view similar to that of  FIG. 52 , showing the positional relationship among the cam ring, the first external barrel, the second external barrel and the second linear guide ring; 
       FIG. 56  is an exploded perspective view of elements of the zoom lens, showing a state where the third external barrel has been removed from the first linear guide ring; 
       FIG. 57  is an exploded perspective view of elements of the zoom lens, showing a state where the second external barrel and a follower-biasing ring spring have been removed from the block of the zoom lens shown in  FIG. 56 ; 
       FIG. 58  is an exploded perspective view of elements of the zoom lens, showing a state where the first external barrel has been removed from the block of the zoom lens shown in  FIG. 57 ; 
       FIG. 59  is an exploded perspective view of elements of the zoom lens, showing a state where the second linear guide ring has been removed from the block of the zoom lens shown in  FIG. 58  while the set of roller followers have been removed from the cam ring included in the block; 
       FIG. 60  is a developed view of the helicoid ring, the third external barrel, the first linear guide ring and the follower-biasing ring spring in relation to the set of roller followers fixed to the cam ring, showing the positional relationship thereamong in the retracted state of the zoom lens; 
       FIG. 61  is a view similar to that of  FIG. 60 , showing the positional relationship among the helicoid ring, the third external barrel and the first linear guide ring at the wide-angle extremity of the zoom lens; 
       FIG. 62  is a view similar to that of  FIG. 60 , showing the positional relationship among the helicoid ring, the third external barrel and the first linear guide ring at the telephoto extremity of the zoom lens; 
       FIG. 63  is a view similar to that of  FIG. 60 , showing the positional relationship among the helicoid ring, the third external barrel and the first linear guide ring; 
       FIG. 64  is an enlarged developed view of portions of the third external barrel and the helicoid ring in relation to the set of roller followers fixed to the cam ring, viewed from radially inside the third external barrel and the helicoid ring; 
       FIG. 65  is a view similar to that of  FIG. 64 , showing a state where the helicoid ring is rotated in a lens barrel advancing direction thereof; 
       FIG. 66  is an enlarged developed view of portions of the third external barrel and the helicoid ring shown in  FIG. 64 ; 
       FIG. 67  is an enlarged developed view of portions of a front rind and a rear ring of a comparative example which are to be compared with the third external barrel and the helicoid ring shown in  FIGS. 64 through 66 ; 
       FIG. 68  is a view similar to that of  FIG. 67 , showing a state where the rear ring has slightly rotated with respect to the front ring from the state shown in  FIG. 67 ; 
       FIG. 69  is a magnified view of a part of the drawing shown in  FIG. 60  (FIG.  44 ); 
       FIG. 70  is a magnified view of a part of the drawing shown in  FIG. 61  (FIG.  45 ); 
       FIG. 71  is a magnified view of a part of the drawing shown in  FIG. 62  (FIG.  46 ); 
       FIG. 72  is a magnified view of a part of the drawing shown in  FIG. 63  (FIG.  47 ); 
       FIG. 73  is an axial cross sectional view of an upper half of elements of a linear guide structure of the zoom lens shown in  FIGS. 5 and 10 , showing the linear guide structure at the wide-angle extremity of the zoom lens; 
       FIG. 74  is a view similar to that of  FIG. 73 , showing the linear guide structure at the wide-angle extremity of the zoom lens; 
       FIG. 75  is a view similar to that of  FIG. 74 , showing the linear guide structure in the retracted state of the zoom lens; 
       FIG. 76  is a perspective view of a subassembly of the zoom lens shown in  FIGS. 5 through 10  which includes the first external barrel, the external barrel, the second linear guide ring, the cam ring and other elements, showing the positional relationship between the first external barrel and the second linear guide ring that are positioned radially inside and outside the cam ring, respectively; 
       FIG. 77  is a perspective view of a subassembly of the zoom lens shown in  FIGS. 5 through 10  which includes all the elements shown in FIG.  77  and the first linear guide ring, showing a state where the first external barrel has been extended forward to its assembling/disassembling position; 
       FIG. 78  is a perspective view of the subassembly shown in  FIG. 77 , viewed obliquely from behind the subassembly; 
       FIG. 79  is a developed view of the cam ring, the second lens group moving frame and the second linear guide ring, showing the positional relationship thereamong in the retracted state of the zoom lens; 
       FIG. 80  is a view similar to that of  FIG. 79 , showing the positional relationship among the cam ring, the second lens group moving frame and the second linear guide ring at the wide-angle extremity of the zoom lens; 
       FIG. 81  is a view similar to that of  FIG. 79 , showing the positional relationship among the cam ring, the second lens group moving frame and the second linear guide ring at the telephoto extremity of the zoom lens; 
       FIG. 82  is a view similar to that of  FIG. 79 , showing a positional relationship among the cam ring, the second lens group moving frame and the second linear guide ring; 
       FIG. 83  is developed view of the cam ring, showing a state where a set of front cam followers of the second lens group moving frame pass through the points of intersection between a set of front inner cam grooves and a set of rear inner cam grooves of the cam ring; 
       FIG. 84  is a perspective view of a portion of the zoom lens shown in  FIGS. 5 through 10  which includes the second lens group moving frame, the second linear guide ring, a shutter unit and other elements, viewed obliquely from the front thereof; 
       FIG. 85  is a perspective view of the portion of the zoom lens in  FIG. 84 , viewed obliquely from behind; 
       FIG. 86  is a view similar to that of  FIG. 84 , showing the positional relationship between the second lens group moving frame and the second linear guide ring when the second lens group moving frame is positioned at its front limit for the axial movement thereof with respect to the second linear guide ring; 
       FIG. 87  is a perspective view of the portion of the zoom lens in  FIG. 86 , viewed obliquely from behind; 
       FIG. 88  is a front elevational view of the second linear guide ring; 
       FIG. 89  is a rear elevational view of the second lens group moving frame, the second linear guide ring and other elements in an assembled state thereof; 
       FIG. 90  is a developed view of the first external barrel and the cam ring in relation to a set of cam followers of the first external barrel, showing the positional relationship between the first external barrel and the cam ring in the retracted state of the zoom lens; 
       FIG. 91  is a view similar to that of  FIG. 90 , showing a state where each cam follower of the first external barrel is positioned at the insertion end of the inclined lead section of the associated outer cam groove of a set of outer cam grooves of the cam ring by a rotation of the cam ring in a lens barrel advancing direction thereof; 
       FIG. 92  is a view similar to that of  FIG. 90 , showing the positional relationship between the first external barrel and the cam ring at the wide-angle extremity of the zoom lens; 
       FIG. 93  is a view similar to that of  FIG. 90 , showing the positional relationship between the first external barrel and the cam ring at the telephoto extremity of the zoom lens; 
       FIG. 94  is a view similar to that of  FIG. 90 , showing a positional relationship between the first external barrel and the cam ring; 
       FIG. 95  is a magnified view of a part of the drawing shown in  FIG. 90 ; 
       FIG. 96  is a magnified view of a part of the drawing shown in  FIG. 91 ; 
       FIG. 97  is view similar to those of  FIGS. 95 and 96 , showing a state where each cam follower of the first external barrel are positioned in the inclined lead section of the associated outer cam groove of the cam ring; 
       FIG. 98  is a magnified view of a part of the drawing shown in  FIG. 92 ; 
       FIG. 99  is a magnified view of a part of the drawing shown in  FIG. 93 ; 
       FIG. 100  is a magnified view of a part of the drawing shown in  FIG. 94 ; 
       FIG. 101  is a view similar to that of  FIG. 95 , showing another embodiment of the structure of the set of outer cam grooves of the cam ring, showing the positional relationship between the first external barrel and the cam ring in the retracted state of the zoom lens; 
       FIG. 102  is an exploded perspective view of a structure of the zoom lens for supporting a second lens frame which holds the second lens group, for retracting the second lens frame to a radially retracted position thereof, and for adjusting the position of the second lens frame; 
       FIG. 103  is a perspective view of the structure for the second lens frame shown in  FIG. 102  in an assembled state and a position-control cam bar of a CCD holder, viewed obliquely from the front; 
       FIG. 104  is a perspective view of the structure for the second lens frame and the position-control cam bar shown in  FIG. 103 , viewed obliquely from behind; 
       FIG. 105  is a view similar to that of  FIG. 104 , showing a state where the position-control cam bar is in the process of entering the cam-bar insertable hole of a rear second lens frame support plate fixed to the second lens group moving frame; 
       FIG. 106  is a front elevational view of the second lens group moving frame; 
       FIG. 107  is a perspective view of the second lens group moving frame; 
       FIG. 108  is a perspective view of the second lens group moving frame and the shutter unit fixed thereto, viewed obliquely from front; 
       FIG. 109  is a perspective view of the second lens group moving frame and the shutter unit shown in  FIG. 108 , viewed obliquely from behind; 
       FIG. 110  is a front elevational view of the second lens group moving frame and the shutter unit shown in  FIG. 108 ; 
       FIG. 111  is a rear elevational view of the second lens group moving frame and the shutter unit shown in  FIG. 108 ; 
       FIG. 112  is a view similar to that of  FIG. 111 , showing a state where the second lens frame has retracted to the radially retracted position; 
       FIG. 113  is a cross sectional view taken along M 3 —M 3  line shown in  FIG. 110 ; 
       FIG. 114  is a front elevational view of the structure for the second lens frame shown in  FIGS. 105 and 108  through  112 , showing a state where the second lens frame is held at a photographing position thereof as shown in  FIG. 110 ; 
       FIG. 115  is a front elevational view of a portion of the structure for the second lens frame shown in  FIG. 114 ; 
       FIG. 116  is a view similar to that of  FIG. 115  in a different state; 
       FIG. 117  is a front elevational view of a portion of the structure for the second lens frame shown in  FIGS. 105 and 108  through  116 ; 
       FIG. 118  is a front elevational view of a portion of the structure for the second lens frame shown in  FIGS. 105 and 108  through  116 , showing the positional relationship between the second lens frame and the position-control cam bar of the CCD holder when the second lens frame is held in a photographing position thereof as shown in  FIGS. 109 and 111 ; 
       FIG. 119  is a view similar to that of  FIG. 118 , showing a positional relationship between the second lens frame and the position-control cam bar of the CCD holder; 
       FIG. 120  is a view similar to that of  118 , showing the positional relationship between the second lens frame and the position-control cam bar of the CCD holder when the second lens frame is held in the radially retracted position as shown in  FIG. 112 ; 
       FIG. 121  is a perspective view of an AF lens frame and the CCD holder shown in  FIGS. 1 and 4 , showing a state where the AF lens frame is fully retracted to contact with and the CCD holder, viewed obliquely from lower front of the CCD holder; 
       FIG. 122  is a front elevational view of the CCD holder, the AF lens frame and the second lens group moving frame; 
       FIG. 123  is a perspective view of the CCD holder, the AF lens frame, the second lens group moving frame, the second lens frame and other elements; 
       FIG. 124  is a view similar to that of  FIG. 123 , showing a state where the second lens frame has fully moved rearward and fully rotated to the radially retracted position; 
       FIG. 125  is an axial cross sectional view of a portion of the upper half of the zoom lens shown in  FIG. 9 , showing a structure wiring a flexible PWB for exposure control in the zoom lens; 
       FIG. 126  is a perspective view of the second lens frame, the flexible PWB and other elements, showing a manner of supporting the flexible PWB by the second lens frame; 
       FIG. 127  is a perspective view of the second lens frame and the AF lens frame, showing a state where the second lens frame has retracted closely to the AF lens frame; 
       FIG. 128  is a side elevational view of the second lens frame and the AF lens frame, showing a state immediately before the second lens frame comes into contact with the AF lens frame; 
       FIG. 129  is a view similar to that of  FIG. 128 , showing a state where the second lens frame is in contact with the AF lens frame; 
       FIG. 130  is a front elevational view of the second lens frame and the AF lens frame, showing a positional relationship therebetween; 
       FIG. 131  is a perspective view of the first external barrel that surrounds the second lens group moving frame, and the first lens frame for the first lens group that is held by the first external barrel; 
       FIG. 132  is a front elevational view of the first external barrel and the first lens frame; 
       FIG. 133  is a perspective view of the first lens frame, the second lens group moving frame, the AF lens frame and the shutter unit, viewed obliquely from front, showing the positional relationship thereamong at a ready-to-photograph state of the zoom lens; 
       FIG. 134  is a perspective view of the first lens frame, the second lens group moving frame, the AF lens frame and the shutter unit which are shown in  FIG. 133 , viewed obliquely from rear thereof; 
       FIG. 135  is a view similar to that of  FIG. 133 , showing the positional relationship among the first lens frame, the second lens group moving frame, the AF lens frame and the shutter unit, showing the positional relationship thereamong in the retracted state of the zoom lens; 
       FIG. 136  is a perspective view of the first lens frame, the second lens group moving frame, the AF lens frame and the shutter unit which are shown in  FIG. 135 , viewed obliquely from rear thereof; 
       FIG. 137  is a rear elevational view of the first lens frame, the second lens group moving frame, the AF lens frame and the shutter unit which are shown in  FIG. 135 ; 
       FIG. 138  is a perspective view, of the first lens frame, the first external barrel, the second lens group moving frame, the AF lens frame and the shutter unit in the retracted state of the zoom lens, showing the positional relationship thereamong in the retracted state of the zoom lens; 
       FIG. 139  is a front elevational view of the first lens frame, the first external barrel, the second lens group moving frame, the AF lens frame and the shutter unit which are shown in  FIG. 138 ; 
       FIG. 140  is an exploded perspective view of the shutter unit of the zoom lens; 
       FIG. 141  is a longitudinal cross sectional view of a portion of the zoom lens in the vicinity of the first lens group in the upper half of the zoom lens shown in  FIG. 9 , in which the zoom lens is in a ready-to-photograph state; 
       FIG. 142  is a view similar to that of FIG.  141  and shows the same portion in the upper half of the zoom lens shown in  FIG. 10 , in which the zoom lens is in the retracted state; 
       FIG. 143  is an exploded perspective view of the viewfinder unit shown in  FIGS. 5 through 8 ; 
       FIG. 144  is a developed view, similar to that of  FIG. 23 , of the helicoid ring and the third external barrel in relation to a zoom gear and a viewfinder drive gear, showing the positional relationship thereamong in the retracted state of the zoom lens; 
       FIG. 145  is a developed view, similar to that of  FIG. 24 , of the helicoid ring and the stationary barrel in relation to the zoom gear and the viewfinder drive gear, showing the positional relationship thereamong at the wide-angle extremity the zoom lens; 
       FIG. 146  is a perspective view of a power transmission system of the zoom lens for imparting rotation of a zoom motor from the helicoid ring to movable lenses of a viewfinder optical system incorporated in the viewfinder unit; 
       FIG. 147  is a front elevational view of the power transmission system shown in  FIG. 148 ; 
       FIG. 148  is a side elevational view of the power transmission system shown in  FIG. 148 ; 
       FIG. 149  is an enlarged developed view of the helicoid ring and the viewfinder drive gear, showing a positional relationship therebetween in the middle of rotation of the helicoid ring in the lens barrel advancing direction from the retracted position shown in  FIG. 144  to the wide-angle extremity shown in FIG.  145 . 
       FIG. 150  is a view similar to that of  FIG. 149 , showing a state subsequent to the state shown in  FIG. 149 ; 
       FIG. 151  is a view similar to that of  FIG. 149 , showing a state subsequent to the state shown in  FIG. 150 ; 
       FIG. 152  is a view similar to that of  FIG. 149 , showing a state subsequent to the state shown in FIG.  151 ; 
       FIG. 153  is a front elevational view of the helicoid ring and the viewfinder drive gear which are shown in  FIG. 150 ; 
       FIG. 154  is a front elevational view of the helicoid ring and the viewfinder drive gear which are shown in  FIG. 151 ; 
       FIG. 155  is a front elevational view of the helicoid ring and the viewfinder drive gear which are shown in  FIG. 152 ; 
       FIG. 156  is a developed view of a cam-incorporated gear of the viewfinder unit; and 
       FIG. 157  is a developed view, similar to that of  FIG. 156 , of a comparative example of a cam-incorporated gear incorporating an idle running section which is to be compared with the cam-incorporated gear shown in FIG.  156 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   In some of the drawings, lines of different thicknesses and/or different types of lines are used as the outlines of different elements for the purpose of illustration. Additionally, in some cross sectional drawings, several elements are shown on a common plane, though positioned in different circumferential positions, for the purpose of illustration. 
   In  FIG. 22 , the symbols “(S)”, “(L)”, “(R)” and “(RL)” which are each appended as a suffix to the reference numeral of some elements of a present embodiment of a zoom lens (zoom lens barrel)  71  (see  FIGS. 5 through 10 ) indicate that the element is stationary, the element is solely movable linearly along a lens barrel axis Z 0  (see  FIGS. 9 and 10 ) without rotating about the lens barrel axis Z 0 , the element is rotatable about the lens barrel axis Z 0  without moving along the lens barrel axis Z 0 , and the element is solely movable along the lens barrel axis Z 0  while rotating about the lens barrel axis Z 0 , respectively. Additionally, in  FIG. 22 , the symbol “(R, RL)” which is appended as a suffix to the reference numeral of some elements of the zoom lens  71  indicates that the element rotates about the lens barrel axis Z 0  without moving along the lens barrel axis Z 0  during a zooming operation and that the element moves along the lens barrel axis Z 0  while rotating about the lens barrel axis Z 0  during the time the zoom lens  71  advances from or retracts into a camera body  72  upon power being turned ON or OFF, while the symbol “(S, L)” which is appended as a suffix to the reference numeral of some elements of the zoom lens  71  indicates that the element is stationary when the zoom lens  71  in a zooming range in which a zooming operation is possible and that the element moves linearly along the lens barrel axis Z 0  without rotating about the lens barrel axis Z 0  during the time the zoom lens  71  advances from or retracts into the camera body  72  upon power being turned ON or OFF. 
   As shown in  FIGS. 9 and 10 , the present embodiment of the zoom lens  71  incorporated in a digital camera  70  is provided with a photographing optical system consisting of a first lens group LG 1 , a shutter S, an adjustable diaphragm A, a second lens group LG 2 , a third lens group LG 3 , a low-pass filter (optical filter) LG 4 , and a CCD image sensor (solid-state image pick-up device)  60 . “Z 1 ” shown in  FIGS. 9 and 10  designates the optical axis of the photographing optical system. The photographing optical axis Z 1  is parallel to a common rotational axis (the lens barrel axis Z 0 ) of external barrels which form an outward appearance of the zoom lens  71 . Moreover, the photographing optical axis Z 1  is positioned below the lens barrel axis Z 0 . The first lens group LG 1  and the second lens group LG 2  are driven along the photographing optical axis Z 1  in a predetermined moving manner to perform a zooming operation, while the third lens group L 3  is driven along the photographing optical axis Z 1  to perform a focusing operation. In the following descriptions, the term “optical axis direction” means a direction parallel to the photographing optical axis Z 1  unless there is a different explanatory note on the expression. 
   As shown in  FIGS. 9 and 10 , the camera  70  is provided in the camera body  72  thereof with a stationary barrel  22  fixed to the camera body  72 , and a CCD holder  21  fixed to a rear portion of the stationary barrel  22 . The CCD image sensor  60  is mounted to the CCD holder  21  to be held thereby via a CCD base plate  62 . The low-pass filter LG 4  is held by the CCD holder  21  to be positioned in front of the CCD  60  via a filter holder portion  21   b  and an annular sealing member  61 . The filter holder portion  21   b  is a portion formed integrally with the CCD holder  21 . The camera  70  is provided behind the CCD holder  21  with an LCD panel  20  which indicates a live image so that the user can see how the image about to be taken looks before photographing, captured images so that the user can review pictures which he or she has already taken, and also various photographing information. 
   The zoom lens  71  is provided in the stationary barrel  22  with an AF lens frame (a third lens frame which supports and holds the third lens group LG 3 )  51  which is guided linearly in the optical axis direction without rotating about the photographing optical axis Z 1 . Specifically, the zoom lens  71  is provided with a pair of AF guide shafts  52  and  53  which extend parallel to the photographing optical axis Z 1  to guide the AF lens frame  51  in the optical axis direction without rotating the AF lens frame  51  about the photographing optical axis Z 1 . Front and rear ends of each guide shaft of the pair of AF guide shafts  52  and  53  are fixed to the stationary barrel  22  and the CCD holder  21 , respectively. The AF lens frame  51  is provided on radially opposite sides thereof with a pair of guide holes  51   a  and  51   b  in which the pair of AF guide shafts  52  and  53  are respectively fitted so that the AF lens frame  51  is slidable on the pair of AF guide shafts  52  and  53 . In this particular embodiment, the amount of clearance between the AF guide shaft  53  and the guide hole  51   b  is greater than that between the AF guide shaft  52  and the guide hole  51   a . Namely, the AF guide shaft  52  serves as a main guide shaft for achieving a great positioning accuracy, while the AF guide shaft  53  serves as an auxiliary guide shaft. The camera  70  is provided with an AF motor  160  (see  FIG. 1 ) having a rotary drive shaft which is threaded to serve as a feed screw shaft, and this rotary drive shaft is screwed through a screw hole formed on an AF nut  54  (see FIG.  1 ). The AF nut  54  is provided with a rotation-preventing protrusion  54   a . The AF lens frame  51  is provided with a guide groove  51   m  (see FIG.  127 ), extending in a direction parallel to the optical axis Z 1 , in which the rotation-preventing protrusion  54   a  is slidably fitted. Furthermore, the AF lens frame  51  is provided with a stopper protrusion  51   n  (see  FIG. 127 ) which is positioned behind the AF nut  54 . The AF lens frame  51  is biased forward in the optical axis direction by an extension coil spring  55  serving as a biasing member, and the forward movement limit of the AF lens frame  51  is determined via engagement between the stopper protrusion  51   n  and the AF nut  54 . The AF lens frame  51  can be moved rearward against the biasing force of the extension coil spring  55  when a rearward force is applied by the AF nut  54 . Due to this structure, rotating the rotary drive shaft of AF motor  160  forward and rearward causes the AF lens frame  51  to move forward and rearward in the optical axis direction. In addition, the AF lens frame  51  can be moved rearward against the biasing force of the extension coil spring  55  when a rearward force is directly applied to the AF lens frame  51 . 
   As shown in  FIGS. 5 and 6 , the camera  70  is provided above the stationary barrel  22  with a zoom motor  150  and a reduction gear train box  74  which are mounted on the stationary barrel  22 . The reduction gear train box  74  contains a reduction gear train for transferring rotation of the zoom motor  150  to a zoom gear  28  (see FIG.  4 ). The zoom gear  28  is rotatably fitted on a zoom gear shaft  29  extending parallel to the photographing optical axis Z 1 . Front and rear ends of the zoom gear shaft  29  are fixed to the stationary barrel  22  and the CCD holder  21 , respectively. Rotations of the zoom motor  150  and the AF motor  160  are controlled by a control circuit  140  (see  FIG. 22 ) via a flexible PWB (printed wiring board)  75  which is partly positioned on an outer peripheral surface of the stationary barrel  22 . The control circuit  140  comprehensively controls the overall operation of the camera  70 . 
   As shown in  FIG. 4 , the stationary barrel  22  is provided on an inner peripheral surface thereof with a female helicoid  22   a , a set of three linear guide grooves  22   b , a set of three inclined grooves  22   c , and a set of three rotational sliding grooves  22   d . Threads of the female helicoid  22   a  extend in a direction inclined with respect to both the optical axis direction and a circumferential direction of the stationary barrel  22 . The set of three linear guide grooves  22   b  extend parallel to the photographing optical axis Z 1 . The set of three inclined grooves  22   c  extend parallel to the female helicoid  22   a . The set of three rotational sliding grooves  22   d  are formed in the vicinity of a front end of the inner peripheral surface of the stationary barrel  22  to extend along a circumference of the stationary barrel  22  to communicate the front ends of the set of three inclined grooves  22   c , respectively. The female helicoid  22   a  is not formed on that specific front area (non-helicoid area  22   z ) of the inner peripheral surface of the stationary barrel  22  which is positioned immediately behind the set of three rotational sliding grooves  22   d  (see  FIGS. 11 ,  23  through  26 ). 
   The zoom lens  71  is provided in the stationary barrel  22  with a helicoid ring  18 . The helicoid ring  18  is provided on an outer peripheral surface thereof with a male helicoid  18   a  and a set of three rotational sliding projections  18   b . The male helicoid  18   a  is engaged with the female helicoid  22   a , and the set of three rotational sliding projections  18   b  are engaged in the set of three inclined grooves  22   c  or the set of three rotational sliding grooves  22   d , respectively (see FIGS.  4  and  12 ). The helicoid ring  18  is provided on threads of the male helicoid  18   a  with an annular gear  18   c  which is in mesh with the zoom gear  28 . Therefore, when a rotation of the zoom gear  28  is transferred to the annular gear  18   c , the helicoid ring  18  moves forward or rearward in the optical axis direction while rotating about the lens barrel axis Z 0  within a predetermined range in which the male helicoid  18   a  remains in mesh with the female helicoid  22   a . A forward movement of the helicoid ring  18  beyond a predetermined point with respect to the stationary barrel  22  causes the male helicoid  18   a  to be disengaged from the female helicoid  22   a  so that the helicoid ring  18  rotates about the lens barrel axis Z 0  without moving in the optical axis direction relative to the stationary barrel  22  by engagement of the set of three rotational sliding projections  18   b  with the set of three rotational sliding grooves  22   d.    
   The set of three inclined grooves  22   c  are formed on the stationary barrel  22  to prevent the set of three rotational sliding projections  18   b  and the stationary barrel  22  from interfering with each other when the female helicoid  22   a  and the male helicoid  18   a  are engaged with each other. To this end, each inclined groove  22   c  is formed on an inner peripheral surface of the stationary barrel  22  to be positioned radially outwards (upwards as viewed in  FIG. 31 ) from the bottom of the female helicoid  22   a  as shown in  FIG. 31. A  circumferential space between two adjacent threads of the female helicoid  22   a  between which one of the three inclined grooves  22   c  is positioned is greater than that between another two adjacent threads of the female helicoid  22   a  between which none of the three inclined grooves  22   c  is positioned. The male helicoid  18   a  includes three wide threads  18   a -W and twelve narrow threads. The three wide threads  18   a -W are positioned behind the three rotational sliding projections  18   b  in the optical axis direction, respectively (see FIG.  12 ). The circumferential width of each of the three wide threads  18   a -W is greater than that of each of the twelve narrow threads so that each of the three wide threads  18   a -W can be positioned in the associated two adjacent threads of the female helicoid  22   a  between which one of the three inclined grooves  22   c  is positioned (see FIGS.  11  and  12 ). 
   The stationary barrel  22  is provided with a stop-member insertion hole  22   e  which radially penetrates through the stationary barrel  22 . A stop member  26  having a stop projection  26   b  is fixed to the stationary barrel  22  by a set screw  67  so that the stop projection  26   b  can be inserted into and removed from the stop-member insertion hole  22   e  (see FIGS.  40  and  41 ). 
   As will be appreciated from  FIGS. 9 and 10 , the zoom lens  71  of the camera  70  is of a telescoping type having three external telescoping barrels: a first external barrel  12 , a second external barrel  13  and a third external barrel  15  which are concentrically arranged about the lens barrel axis Z 0 . The helicoid ring  18  is provided, on an inner peripheral surface thereof at three different circumferential positions on the helicoid ring  18 , with three rotation transfer recesses  18   d  (see FIGS.  4  and  13 ) front ends of which are open at the front end of the helicoid ring  18 , while the third external barrel  15  is provided, at corresponding three different circumferential positions on the third external barrel  15 , with three pairs of rotation transfer projections  15   a  (see  FIGS. 4 and 14 ) which project rearward from the rear end of the third external barrel  15  to be inserted into the three rotation transfer recesses  18   d  from the front thereof, respectively. The three pairs of rotation transfer projections  15   a  and the three rotation transfer recesses  18   d  are movable relative to each other in a direction of the lens barrel axis Z 0 , and are not rotatable relative to each other about the lens barrel axis Z 0 . Namely, the helicoid ring  18  and the third external barrel  15  rotate in one piece. Strictly speaking, the three pairs of rotation transfer projections  15   a  and the three rotation transfer recesses  18   d  are slightly rotatable relative to each other about the lens barrel axis Z 0  by the amount of clearance between the three pairs of rotation transfer projections  15   a  and the three rotation transfer recesses  18   d , respectively. This structure will be discussed in detail later. 
   The helicoid ring  18  is provided, on front faces of the three rotational sliding projections  18   b  at three different circumferential positions on the helicoid ring  18 , with a set of three engaging recesses  18   e  which are formed on an inner peripheral surface of the helicoid ring  18  to be open at the front end of the helicoid ring  18 . The third external barrel  15  is provided, at corresponding three different circumferential positions on the third external barrel  15 , with a set of three engaging projections  15   b  which project rearward from the rear end of the third external barrel  15 , and also project radially outwards, to be engaged in the set of three engaging recesses  18   e  from the front thereof, respectively. The set of three engaging projections  15   b , which are respectively engaged in the set of three engaging recesses  18   e , are also engaged in the set of three rotational sliding grooves  22   d  at a time, respectively, when the set of three rotational sliding projections  18   b  are engaged in the set of three rotational sliding grooves  22   d  (see FIG.  33 ). 
   The zoom lens  71  is provided between the third external barrel  15  and the helicoid ring  18  with three compression coil springs  25  which bias the third external barrel  15  and the helicoid ring  18  in opposite directions away from each other in the optical axis direction. The rear ends of the three compression coil springs  25  are respectively inserted into three spring support holes (non-through hole)  18   f  which are formed on the front end of the helicoid ring  18 , while the front ends of the three compression coil springs  25  are respectively in pressing contact with three engaging recesses  15   c  formed at the rear end of the third external barrel  15 . Therefore, the set of three engaging projections  15   b  of the third external barrel  15  are respectively pressed against front guide surfaces  22   d -A (see  FIGS. 28 through 30 ) of the rotational sliding grooves  22   d  by the spring force of the three compression coil springs  25 . At the same time, the set of three rotational sliding projections  18   b  of the helicoid ring  18  are respectively pressed against rear guide surfaces  22   d -B (see  FIGS. 28 through 30 ) of the rotational sliding grooves  22   d  by the spring force of the three compression coil springs  25 . 
   The third external barrel  15  is provided on an inner peripheral surface thereof with a plurality of relative rotation guide projections  15   d  which are formed at different circumferential positions on the third external barrel  15 , a circumferential groove  15   e  which extends in a circumferential direction about the lens barrel axis Z 0 , and a set of three rotation transfer grooves  15   f  which extend parallel to the lens barrel axis Z 0  (see FIGS.  4  and  14 ). The plurality of relative rotation guide projections  15   d  are elongated in a circumferential direction of the third external barrel to lie in a plane orthogonal to the lens barrel axis Z 0 . As can be seen in  FIG. 14 , each rotation transfer groove  15   f  intersects the circumferential groove  15   e  at right angles. The circumferential positions of the three rotation transfer grooves  15   f  are formed to correspond to those of the three pairs of rotation transfer projections  15   a , respectively. The rear end of each rotation transfer groove  15   f  is open at the rear end of the third external barrel  15 . The helicoid ring  18  is provided on an inner peripheral surface thereof with a circumferential groove  18   g  which extends in a circumferential direction about the lens barrel axis Z 0  (see FIGS.  4  and  13 ). The zoom lens  71  is provided inside the third external barrel  15  and the helicoid ring  18  with a first linear guide ring  14 . The first linear guide ring  14  is provided on an outer peripheral surface thereof with a set of three linear guide projections  14   a , a first plurality of relative rotation guide projections  14   b , a second plurality of relative rotation guide projections  14   c , and a circumferential groove  14   d  in this order from rear to front of the first linear guide ring  14  in the optical axis direction (see FIGS.  4  and  15 ). The set of three linear guide projections  14   a  project radially outwards in the vicinity of the rear end of the first linear guide ring  14 . The first plurality of relative rotation guide projections  14   b  project radially outwards at different circumferential positions on the first linear guide ring  14 , and are each elongated in a circumferential direction of the first linear guide ring  14  to lie in a plane orthogonal to the lens barrel axis Z 0 . Likewise, the second plurality of relative rotation guide projections  14   c  project at different circumferential positions on the first linear guide ring  14 , and are each elongated in a circumferential direction of the first linear guide ring  14  to lie in a plane orthogonal to the lens barrel axis Z 0 . The circumferential groove  14   d  is an annular groove with its center on the lens barrel axis Z 0 . The first linear guide ring  14  is guided in the optical axis direction with respect to the stationary barrel  22  by engagement of the set of three linear guide projections  14   a  with the set of three linear guide grooves  22   b , respectively. The third external barrel  15  is coupled to the first linear guide ring  14  to be rotatable about the lens barrel axis Z 0  relative to the first linear guide ring  14  by both the engagement of the second plurality of relative rotation guide projections  14   c  with the circumferential groove  15   e  and the engagement of the plurality of relative rotation guide projections  15   d  with the circumferential groove  14   d . The second plurality of relative rotation guide projections  14   c  and the circumferential groove  15   e  are engaged with each other to be slightly movable relative to each other in the optical axis direction. Likewise, the plurality of relative rotation guide projections  15   d  and the circumferential groove  14   d  are engaged with each other to be slightly movable relative to each other in the optical axis direction. The helicoid ring  18  is coupled to the first linear guide ring  14  to be rotatable about the lens barrel axis Z 0  relative to the first linear guide ring  14  by engagement of the first plurality of relative rotation guide projections  14   b  with the circumferential groove  18   g . The first plurality of relative rotation guide projections  14   b  and the circumferential groove  18   g  are engaged with each other to be slightly movable relative to each other in the optical axis direction. 
   The first linear guide ring  14  is provided with a set of three through-slots  14   e  which radially penetrate the first linear guide ring  14 . As shown in  FIG. 15 , each through-slot  14   e  includes a front circumferential slot portion  14   e - 1 , a rear circumferential slot portion  14   e - 2 , and an inclined lead slot portion  14   e - 3  which connects the front circumferential slot portion  14   e - 1  with the rear circumferential slot portion  14   e - 2 . The front circumferential slot portion  14   e - 1  and the rear circumferential slot portion  14   e - 2  extend parallel to each other in a circumferential direction of the first linear guide ring  14 . The zoom lens  71  is provided with a cam ring  11  a front portion of which is positioned inside the first external barrel  12 . A set of three roller followers  32  fixed to an outer peripheral surface of the cam ring  11  at different circumferential positions thereon are engaged in the set of three through-slots  14   e , respectively (see FIG.  3 ). Each roller follower  32  is fixed to the cam ring  11  by set screw  32   a . The set of three roller followers  32  are further engaged in the set of three rotation transfer grooves  15   f  through the set of three through-slots  14   e , respectively. The zoom lens  71  is provided between the first linear guide ring  14  and the third external barrel  15  with a follower-biasing ring spring  17 . A set of three follower pressing protrusions  17   a  protrude rearward from the follower-biasing ring spring  17  to be engaged in front portions of the set of three rotation transfer grooves  15   f , respectively (see FIG.  14 ). The set of three follower pressing protrusions  17   a  press the set of three roller followers  32  rearward to remove backlash between the set of three roller followers  32  and the set of three through-slots  14   e  when the set of three roller followers  32  are engaged in the front circumferential slot portions  14   e - 1  of the set of three through-slots  14   e , respectively. 
   Advancing operations of movable elements of the zoom lens  71  from the stationary barrel  22  to the cam ring  11  will be discussed hereinafter with reference to the above described structure of the digital camera  70 . Rotating the zoom gear  28  in a lens barrel advancing direction by the zoom motor  150  causes the helicoid ring  18  to move forward while rotating about the lens barrel axis Z 0  due to engagement of the female helicoid  22   a  with the male helicoid  18   a . This rotation of the helicoid ring  18  causes the third external barrel  15  to move forward together with the helicoid ring  18  while rotating about the lens barrel axis Z 0  together with the helicoid ring  18 , and further causes the first linear guide ring  14  to move forward together with the helicoid ring  18  and the third external barrel  15  because each of the helicoid ring  18  and the third external barrel  15  is coupled to the first linear guide ring  14  to make respective relative rotations between the third external barrel  15  and the first linear guide ring  14  and between the helicoid ring  18  and the first linear guide ring  14  possible and to be movable together along a direction of a common rotational axis (i.e., the lens barrel axis Z 0 ) due to the engagement of the first plurality of relative rotation guide projections  14   b  with the circumferential groove  18   g , the engagement of the second plurality of relative rotation guide projections  14   c  with the circumferential groove  15   e  and the engagement of the plurality of relative rotation guide projections  15   d  with the circumferential groove  14   d . Rotation of the third external barrel  15  is transferred to the cam ring  11  via the set of three rotation transfer grooves  15   f  and the set of three roller followers  32 , which are engaged in the set of three rotation transfer grooves  15   f , respectively. Since the set of three roller followers  32  are also engaged in the set of three through-slots  14   e , respectively, the cam ring  11  moves forward while rotating about the lens barrel axis Z 0  relative to the first linear guide ring  14  in accordance with contours of the lead slot portions  14   e - 3  of the set of three through-slots  14   e . Since the first linear guide ring  14  itself moves forward together with the third lens barrel  15  and the helicoid ring  18  as described above, the cam ring  11  moves forward in the optical axis direction by an amount of movement corresponding to the sum of the amount of the forward movement of the first linear guide ring  14  and the amount of the forward movement of the cam ring  11  by engagement of the set of three roller followers  32  with the lead slot portions  14   e - 3  of the set of three through-slots  14   e , respectively. 
   The above described rotating-advancing operations of the cam ring  11 , the third external barrel  15  and the helicoid ring  18  are performed while the set of three rotational sliding projections  18   b  are moving in the set of three inclined grooves  22   c , respectively, only when the male helicoid  18   a  and the female helicoid  22   a  are engaged with each other. When the helicoid ring  18  moves forward by a predetermined amount of movement, the male helicoid  18   a  and the female helicoid  22   a  are disengaged from each other so that the set of three rotational sliding projections  18   b  move from the set of three inclined grooves  22   c  to the set of three rotational sliding grooves  22   d , respectively. Since the helicoid ring  18  does not move in the optical axis direction relative to the stationary barrel  22  even if rotating upon the disengagement of the male helicoid  18   a  from the female helicoid  22   a , the helicoid ring  18  and the third external barrel  15  rotate at respective axial fixed positions thereof without moving in the optical axis direction due to the engagement of the set of three rotational sliding projections  18   b  with the set of three rotational sliding grooves  22   d . Furthermore, at substantially the same time when the set of three rotational sliding projections  18   b  slide into the set of three rotational sliding grooves  22   d  from the set of three inclined grooves  22   c , respectively, the set of three roller followers  32  enter the front circumferential slot portions  14   e - 1  of the set of three through-slots  14   e , respectively. In this state, since the first linear guide ring  14  stops while the set of three roller followers  32  have respectively moved into the front circumferential slot portions  14   e - 1 , the cam ring  11  is not given any force to make the cam ring  11  move forward. Consequently, the cam ring  11  only rotates at an axial fixed position in accordance with rotation of the third external barrel  15 . 
   Rotating the zoom gear  28  in a lens barrel retracting direction thereof by the zoom motor  150  causes the aforementioned movable elements of the zoom lens  71  from the stationary barrel  22  to the cam ring  11  to operate in the reverse manner to the above described advancing operations. In this reverse operation, the above described movable elements of the zoom lens  71  retract to their respective retracted positions shown in  FIG. 10  by rotation of the helicoid ring  18  until the set of three roller followers  32  enter the rear circumferential slot portions  14   e - 2  of the set of three through-slots  14   e , respectively. 
   The first linear guide ring  14  is provided on an inner peripheral surface thereof with a set of three pairs of first linear guide grooves  14   f  which are formed at different circumferential positions to extend parallel to the photographing optical axis Z 1 , and a set of six second linear guide grooves  14   g  which are formed at different circumferential positions to extend parallel to the photographing optical axis Z 1 . Each pair of first linear guide grooves  14   f  are positioned on the opposite sides of the associated linear guide groove  14   g  (every other linear guide groove  14   g ) in a circumferential direction of the first linear guide ring  14 . The zoom lens  71  is provided inside the first linear guide ring  14  with a second linear guide ring  10 . The second linear guide ring  10  is provided on an outer edge thereof with a set of three bifurcated projections  10   a  which project radially outwards from a ring portion  10   b  of the second linear guide ring  10 . Each bifurcated projection  10   a  is provided at a radially outer end thereof with a pair of radial projections which are respectively engaged in the associated pair of first linear guide grooves  14   f  (see FIGS.  3  and  18 ). On the other hand, a set of six radial projections  13   a  which are formed on an outer peripheral surface of the second external barrel  13  at a rear end thereof to project radially outwards (see  FIG. 3 ) are engaged in the set of six second linear guide grooves  14   g , respectively to be slidable therealong. Therefore, each of the second external barrel  13  and the second linear guide ring  10  is guided in the optical axis direction via the first linear guide ring  14 . 
   The zoom lens  71  is provided inside the cam ring  11  with a second lens group moving frame  8  which indirectly supports and holds the second lens group LG 2  (see FIG.  3 ). The first external barrel  12  indirectly supports the first lens group LG 1 , and is positioned inside the second external barrel  13  (see FIG.  2 ). The second linear guide ring  10  serves as a linear guide member for guiding the second lens group moving frame  8  linearly without rotating the same, while the second external barrel  13  serves as a linear guide member for guiding the first external barrel  12  linearly without rotating the same. 
   The second linear guide ring  10  is provided on the ring portion  10   b  with a set of three linear guide keys  10   c  (specifically two narrow linear guide keys  10   c  and a wide linear guide key  10   c -W) which project forward in parallel to one another (see  FIGS. 3 and 18 ) from the ring portion  10   b . The second lens group moving frame  8  is provided with a corresponding set of three guide grooves  8   a  (specifically two narrow guide grooves  8   a  and a wide guide groove  8   a -W) in which the set of three linear guide keys  10   c  are engaged, respectively. As shown in  FIGS. 9 and 10 , a discontinuous outer edge of the ring portion  10   b  is engaged in a discontinuous circumferential groove lie formed on an inner peripheral surface of the cam ring  11  at the rear end thereof to be rotatable about the lens barrel axis Z 0  relative to the cam ring  11  and to be immovable relative to the cam ring  11  in the optical axis direction. The set of three linear guide keys  10   c  project forward from the ring portion  10   b  to be positioned inside the cam ring  11 . Opposite edges of each linear guide key  10   c  in a circumferential direction of the second linear guide ring  10  serve as parallel guide edges which are respectively engaged with circumferentially-opposed guide surfaces in the associated guide groove  8   a  of the second lens group moving frame  8 , which is positioned in the cam ring  11  to be supported thereby, to guide the second lens group moving frame  8  linearly in the optical axis direction without rotating the same about the lens barrel axis Z 0 . 
   The wide linear guide key  10   c -W has a circumferential width greater than those of the other two linear guide keys  10   c  to also serve as a support member for supporting a flexible PWB (printed wiring board)  77  (see  FIGS. 84 through 87 ) used for exposure control. The wide linear guide key  10   c -W is provided thereon with a radial through hole  10   d  through which the flexible PWB  77  passes (see FIG.  18 ). A portion of the ring portion  10   b  from which the wide linear guide key  10   c -W projects forward is partly cut out so that the rear end of the radial through hole  10   d  extends through the rear end of the ring portion  10   b . As shown in  FIGS. 9 and 125 , the flexible PWB  77  for exposure control passes through the radial through hole  10   d  to extend forward along an outer surface of the wide linear guide key  10   c -W from the rear of the ring portion lob, and subsequently bends radially inwards in the vicinity of the front end of the wide linear guide key  10   c -W to extend rearward along an inner surface of the wide linear guide key  10   c -W. The wide guide groove  8   a -W has a circumferential width greater than those of the other two guide grooves  8   a  so that the wide linear guide key  10   c -W can be engaged in the wide guide groove  8   a -W to be slidable therealong. As can be clearly seen in  FIG. 19 , the second lens group moving frame  8  is provided in the wide guide groove  8   a -W with a radial recess  8   a -Wa in which the flexible PWB  77  can lie and two separate bottom walls  8   a -Wb positioned on opposite sides of the radial recess  8   a -Wa to support the wide linear guide key  10   c -W thereon. Whereas, each of the other two guide grooves  8   a  is formed as a simple bottomed groove that is formed on an outer peripheral surface of the second lens group moving frame  8 . The second lens group moving frame  8  and the second linear guide ring  10  can be coupled to each other only when the wide linear guide key  10   c -W and the wide guide groove  8   a -W are aligned in the direction of the lens barrel axis Z 0 . 
   The cam ring  11  is provided on an inner peripheral surface thereof with a plurality of inner cam grooves  11   a  for moving the second lens group LG 2 . As shown in  FIG. 17 , the plurality of inner cam grooves  11   a  are composed of a set of three front inner cam grooves  11   a - 1  formed at different circumferential positions, and a set of three rear inner cam grooves  11   a - 2  formed at different circumferential positions behind the set of three front inner cam grooves  11   a - 1 . Each rear inner cam groove  11   a - 2  is formed on the cam ring  11  as a discontinuous cam groove (see FIG.  17 ), the detail thereof will be discussed later. 
   The second lens group moving frame  8  is provided on an outer peripheral surface thereof with a plurality of cam followers  8   b . As shown in  FIG. 19 , the plurality of cam followers  8   b  include a set of three front cam followers  8   b - 1  which are formed at different circumferential positions to be respectively engaged in the set of three front inner cam grooves  11   a - 1 , and a set of three rear cam followers  8   b - 2  which are formed at different circumferential positions behind the set of three front cam followers  8   b - 1  to be respectively engaged in the set of three rear inner cam grooves  11   a - 2 . 
   A rotation of the cam ring  11  causes the second lens group moving frame  8  to move in the optical axis direction in a predetermined moving manner in accordance with contours of the plurality of inner cam grooves  11   a  since the second lens group moving frame  8  is guided linearly in the optical axis direction without rotating via the second linear guide ring  10 . 
   The zoom lens  71  is provided inside the second lens group moving frame  8  with a second lens frame (radially-retractable lens frame)  6  which supports and holds the second lens group LG 2 . The second lens frame  6  is pivoted on a pivot shaft  33  front and rear ends of which are supported by front and rear second lens frame support plates (a pair of second lens frame support plates)  36  and  37 , respectively (see  FIGS. 3 and 102  through  105 ). The pair of second lens frame support plates  36  and  37  are fixed to the second lens group moving frame  8  by a set screw  66 . The pivot shaft  33  is a predetermined distance away from the photographing optical axis Z 1 , and extends parallel to the photographing optical axis Z 1 . The second lens frame  6  is swingable about the pivot shaft  33  between a photographing position shown in  FIG. 9  where the optical axis of the second lens group LG 2  coincides with the photographing optical axis Z 1  and a radially retracted position (retracted away from the optical axis)shown in  FIG. 10  where the optical axis of the second lens group LG 2  is eccentric from the photographing optical axis Z 1 . A rotation limit shaft  35  which determines the photographing position of the second lens frame  6  is mounted to the second lens group moving frame  8 . The second lens frame  6  is biased to rotate in a direction to come into contact with the rotation limit shaft  35  by a front torsion coil spring  39 . A compression coil spring  38  is fitted on the pivot shaft  33  to remove backlash of the second lens frame  6  in the optical axis direction. 
   The second lens frame  6  moves together with the second lens group moving frame  8  in the optical axis direction. The CCD holder  21  is provided on a front surface thereof with a position-control cam bar  21   a  which projects forward from the CCD holder  21  to be engageable with the second lens frame  6  (see FIG.  4 ). If the second lens group moving frame  8  moves rearward in a retracting direction to approach the CCD holder  21 , a retracting cam surface  21   c  (see  FIG. 103 ) formed on a front end surface of the position-control cam bar  21   a  comes into contact with a specific portion of the second lens frame  6  to rotate the second lens frame  6  to the radially retracted position. 
   The second external barrel  13  is provided, on an inner peripheral surface thereof, with a set of three linear guide grooves  13   b  which are formed at different circumferential positions to extend parallel to one another in the optical axis direction. The first external barrel  12  is provided on an outer peripheral surface at the rear end thereof with a set of three engaging protrusions  12   a  which are slidably engaged in the set of three linear guide grooves  13   b , respectively (see  FIGS. 2 ,  20  and  21 ). Accordingly, the first external barrel  12  is guided linearly in the optical axis direction without rotating about the lens barrel axis Z 0  via the first linear guide ring  14  and the second external barrel  13 . The second external barrel  13  is further provided on an inner peripheral surface thereof in the vicinity of the rear end of the second external barrel  13  with a discontinuous inner flange  13   c  which extends along a circumference of the second external barrel  13 . The cam ring  11  is provided on an outer peripheral surface thereof a discontinuous circumferential groove  11   c  in which the discontinuous inner flange  13   c  is slidably engaged so that the cam ring  11  is rotatable about the lens barrel axis Z 0  relative to the second external barrel  13  and so that the second external barrel  13  is immovable in the optical axis direction relative to the cam ring  11 . On the other hand, the first external barrel  12  is provided on an inner peripheral surface thereof with a set of three cam followers  31  which projects radially inwards, while the cam ring  11  is provided on an outer peripheral surface thereof with a set of three outer cam grooves  11   b  (cam grooves for moving the first lens group LG 1 ) in which the set of three cam followers  31  are slidably engaged, respectively. 
   The zoom lens  71  is provided inside the first external barrel  12  with a first lens frame  1  which is supported by the first external barrel  12  via a first lens group adjustment ring  2 . The first lens group LG 1  is supported by the first lens frame  1  to be fixed thereto. The first lens frame  1  is provided on an outer peripheral surface thereof with a male screw thread  1   a , and the first lens group adjustment ring  2  is provided on an inner peripheral surface thereof with a female screw thread  2   a  which is engaged with the male screw thread  1   a . The axial position of the first lens frame  1  relative to the first lens group adjustment ring  2  can be adjusted via the male screw thread  1   a  and the female screw thread  2   a . A combination of the first lens frame  1  and the first lens group adjustment ring  2  is positioned inside the first external barrel  12  to be supported thereby and to be movable in the optical axis direction relative to the first external barrel  12 . The zoom lens  71  is provided in front of the first external barrel  12  with a fixing ring  3  which is fixed to the first external barrel  12  by two set screws  64  to prevent the first lens group adjustment ring  2  from moving forward and coming off the first external barrel  12 . 
   The zoom lens  71  is provided between the first and second lens groups LG 1  and LG 2  with a shutter unit  76  including the shutter S and the adjustable diaphragm A (see  FIGS. 1 ,  9  and  10 ). The shutter unit  76  is positioned in the second lens group moving frame  8  to be supported thereby. The air-distance between the shutter S and the second lens group LG 2  is fixed. Likewise, the air-distance between the diaphragm A and the second lens group LG 2  is fixed. The zoom lens  71  is provided in front of the shutter unit  76  with a shutter actuator  131  for driving the shutter S, and is provided behind the shutter unit  76  with a diaphragm actuator  132  for driving the diaphragm A (see FIGS.  140 ). The flexible PWB  77  extends from the shutter unit  76  to establish electrical connection between the control circuit  140  and each of the shutter actuator  131  and the diaphragm actuator  132 . Note that, in  FIG. 9 , the flexible PWB  77  is shown in a cross sectional view of a lower half portion of the zoom lens  71  below the photographing optical axis Z 1  (the zoom lens  71  set at wide-angle extremity) for the purpose of making the relative locations between the flexible PWB  77  and peripheral elements clearly understandable though the flexible PWB  77  is actually disposed only in the space above the photographing optical axis Z 1  in the zoom lens  71 . 
   The zoom lens  71  is provided at the front end of the first external barrel  12  with a lens barrier mechanism which automatically closes a front end aperture of the zoom lens  71  when the zoom lens  71  is retracted into the camera body  72  to protect the frontmost lens element of the photographing optical system of the zoom lens  71 , i.e. the first lens group LG 1 , from getting stains and scratches thereon when the digital camera  70  is not in use. As shown in  FIGS. 1 ,  9  and  10 , the lens barrier mechanism is provided with a pair of barrier blades  104  and  105 . The pair of barrier blades  104  and  105  are rotatable about two pivots projecting rearward therefrom to be positioned on radially opposite sides of the photographing optical axis Z 1 , respectively. The lens barrier mechanism is further provided with a pair of barrier blade biasing springs  106 , a barrier blade drive ring  103 , a drive ring biasing spring  107  and a barrier blade holding plate  102 . The pair of barrier blades  104  and  105  are biased to rotate in opposite directions to be closed by the pair of barrier blade biasing springs  106 , respectively. The barrier blade drive ring  103  is rotatable about the lens barrel axis Z 0 , and is engaged with the pair of barrier blades  104  and  105  to open the pair of barrier blades  104  and  105  when driven to rotate in a predetermined rotational direction. The barrier blade drive ring  103  is biased to rotate in a barrier opening direction to open the pair of barrier blades  104  and  105  by the drive ring biasing spring  107 . The barrier blade holding plate  102  is positioned between the barrier blade drive ring  103  and the pair of barrier blades  104  and  105 . The spring force of the drive ring biasing spring  107  is greater than the spring force of the pair of barrier blade biasing springs  106  so that the barrier blade drive ring  103  is held by the spring force of the drive ring biasing spring  107  in a specific rotational position thereof to open the pair of barrier blades  104  and  105  against the biasing force of the pair of barrier blade biasing springs  106  in the state shown in  FIG. 9  where the zoom lens  71  has been extended forward to a point in a zooming range (zooming operation performable range) where a zooming operation can be carried out. In the course of the retracting movement of the zoom lens  71  to the retracted position shown in  FIG. 10  from a position in the zooming range, the barrier blade drive ring  103  is forcefully rotated in a barrier closing direction opposite to the aforementioned barrier opening direction by a barrier drive ring pressing surface  11   d  (see  FIGS. 3 and 16 ) formed on the cam ring  11 . This rotation of the barrier blade drive ring  103  causes the barrier blade drive ring  103  to be disengaged from the pair of barrier blades  104  and  105  so that the pair of barrier blades  104  and  105  are closed by the spring force of the pair of barrier blade biasing springs  106 . The zoom lens  71  is provided immediately in front of the lens barrier mechanism with a substantially round lens barrier cover (decorative plate)  101  which covers the front of the lens barrier mechanism. 
   A lens barrel advancing operation and a lens barrel retracting operation of the zoom lens  71  having the above described structure will be discussed hereinafter. 
   The stage at which the cam ring  11  is driven to advance from the retracted position shown in  FIG. 10  to the position shown in  FIG. 9  where the cam ring  11  rotates at the axial fixed position without moving in the optical axis direction has been discussed above, and will be briefly discussed hereinafter. 
   In the state shown in  FIG. 10  in which the zoom lens  71  is in the retracted state, the zoom lens  71  is fully accommodated in the camera body  72  so that the front face of the zoom lens  71  is substantially flush with the front face of the camera body  72 . Rotating the zoom gear  28  in the lens barrel advancing direction by the zoom motor  150  causes a combination of the helicoid ring  18  and the third external barrel  15  to move forward while rotating about the lens barrel axis Z 0  due to engagement of the female helicoid  22   a  with the male helicoid  18   a , and further causes the first linear guide ring  14  to move forward together with the helicoid ring  18  and the third external barrel  15 . At this time, the cam ring  11  which rotates by rotation of the third external barrel  15  moves forward in the optical axis direction by an amount of movement corresponding to the sum of the amount of the forward movement of the first linear guide ring  14  and the amount of the forward movement of the cam ring  11  by a leading structure between the cam ring  11  and the first linear guide ring  14 , i.e., by engagement of the set of three roller followers  32  with the lead slot portions  14   e - 3  of the set of three through-slots  14   e , respectively. Once the combination of the helicoid ring  18  and the third external barrel  15  advances to a predetermined point, the male helicoid  18   a  is disengaged from the female helicoid  22   a  while the set of three roller followers  32  are disengaged from the lead slot portions  14   e - 3  to enter the front circumferential slot portions  14   e - 1 , respectively. Consequently, each of the helicoid ring  18  and the third external barrel  15  rotates about the lens barrel axis Z 0  without moving in the optical axis direction. 
   A rotation of the cam ring  11  causes the second lens group moving frame  8 , which is positioned inside the cam ring  11 , to move in the optical axis direction with respect to the cam ring  11  in a predetermined moving manner due to the engagement of the set of three front cam followers  8   b - 1  with the set of three front inner cam grooves  11   a - 1  and the engagement of the set of three rear cam followers  8   b - 2  with the set of three rear inner cam grooves  11   a - 2 , respectively. In the state shown in  FIG. 10  in which the zoom lens  71  is in the retracted state, the second lens frame  6 , which is positioned inside the second lens group moving frame  8 , has rotated about the pivot shaft  33  to be held in the radially retracted position above the photographing optical axis Z 1  by the position-control cam bar  21   a  so that the optical axis of the second lens group LG 2  moves from the photographing optical axis Z 1  to a retracted optical axis Z 2  positioned above the photographing optical axis Z 1 . In the course of movement of the second lens group moving frame  8  from the retracted position to a position in the zooming range as shown in  FIG. 9 , the second lens frame  6  is disengaged from the position-control cam bar  21   a  to rotate about the pivot shaft  33  from the radially retracted position to the photographing position shown in  FIG. 9  where the optical axis of the second lens group LG 2  coincides with the photographing optical axis Z 1  by the sprig force of the front torsion coil spring  39 . Thereafter, the second lens frame  6  remains to be held in the photographing position until when the zoom lens  71  is retracted into the camera body  72 . 
   In addition, a rotation of the cam ring  11  causes the first external barrel  12 , which is positioned around the cam ring  11  and guided linearly in the optical axis direction without rotating about the lens barrel axis Z 0 , to move in the optical axis direction relative to the cam ring  11  in a predetermined moving manner due to engagement of the set of three cam followers  31  with the set of three outer cam grooves  11   b , respectively. 
   Therefore, an axial position of the first lens group LG 1  relative to a picture plane (a light-sensitive surface of the CCD image sensor  60 ) when the first lens group LG 1  is moved forward from the retracted position is determined by the sum of the amount of forward movement of the cam ring  11  relative to the stationary barrel  22  and the amount of movement of the first external barrel  12  relative to the cam ring  11 , while an axial position of the second lens group LG 2  relative to the picture plane when the second lens group LG 2  is moved forward from the retracted position is determined by the sum of the amount of forward movement of the cam ring  11  relative to the stationary barrel  22  and the amount of movement of the second lens group moving frame  8  relative to the cam ring  11 . A zooming operation is carried out by moving the first and second lens groups LG 1  and LG 2  on the photographing optical axis Z 1  while changing the space therebetween. When the zoom lens  71  is driven to advance from the retracted position shown in  FIG. 10 , the zoom lens  71  firstly goes into a state shown below the photographing lens axis Z 1  in  FIG. 9  in which the zoom lens  71  is set at wide-angle extremity. Subsequently, the zoom lens  71  goes into the state shown above the photographing lens axis Z 1  in  FIG. 9  in which the zoom lens  71  is set at telephoto extremity by a further rotation of the zoom motor  150  in a lens barrel advancing direction thereof. As can be seen from  FIG. 9 , the space between the first and second lens groups LG 1  and LG 2  when the zoom lens  71  is set at the wide-angle extremity is greater than that when the zoom lens  71  is set at the telephoto extremity. When the zoom lens  71  is set at the telephoto extremity as shown above the photographing lens axis Z 1  in  FIG. 9 , the first and second lens groups LG 1  and LG 2  have moved to approach each other to have some space therebetween which is smaller than the space in the zoom lens  71  set at the wide-angle extremity. This variation of the space between the first and second lens groups LG 1  and LG 2  for zooming operation is achieved by contours of the plurality of inner cam grooves  11   a  ( 11   a - 1  and  11   a - 2 ) and the set of three outer cam grooves  11   b . In the zooming range between the wide-angle extremity and the telephoto extremity, the cam ring  11 , the third external barrel  15  and the helicoid ring  18  rotate at their respective axial fixed positions, i.e., without moving in the optical axis direction. 
   When the first through third lens groups LG 1 , LG 2  and LG 3  are in the zooming range, a focusing operation is carried out by moving the third lens group L 3  along the photographing optical axis Z 1  by rotation of the AF motor  160  in accordance with an object distance. 
   Driving the zoom motor  150  in a lens barrel retracting direction causes the zoom lens  71  to operate in the reverse manner to the above described advancing operation to fully retract the zoom lens  71  into the camera body  72  as shown in FIG.  10 . In the course of this retracting movement of the zoom lens  71 , the second lens frame  6  rotates about the pivot shaft  33  to the radially retracted position by the position-control cam bar  21   a  while moving rearward together with the second lens group moving frame  8 . When the zoom lens  71  is fully retracted into the camera body  72 , the second lens group LG 2  is retracted into the space radially outside the space in which the third lens group LG 3 , the low-pass filter LG 4  and the CCD image sensor  60  are retracted as shown in  FIG. 10 , i.e., the second lens group LG 2  is radially retracted into an axial range substantially identical to an axial range in the optical axis direction in which the third lens group LG 3 , the low-pass filter LG 4  and the CCD image sensor  60  are positioned. This structure of the camera  70  for retracting the second lens group LG 2  in this manner reduces the length of the zoom lens  71  when the zoom lens  71  is fully retracted, thus making it possible to reduce the thickness of the camera body  72  in the optical axis direction, i.e., in the horizontal direction as viewed in FIG.  10 . 
   As described above, the helicoid ring  18 , the third external barrel  15  and the cam ring  11  move forward while rotating at the stage at which the zoom lens  71  changes from the retracted state shown in  FIG. 10  to a ready-to-photograph state shown in  FIG. 9  (in which the first through third lens groups LG 1 , LG 2  and LG 3  remain within the zooming range), whereas the helicoid ring  18 , the third external barrel  15  and the cam ring  11  rotate at the respective axial fixed positions thereof without moving in the optical axis direction when the zoom lens  71  is in the ready-to-photograph state. The third external barrel  15  and the helicoid ring  18  are engaged with each other to be rotatable together about the lens barrel axis Z 0  by making the three pairs of rotation transfer projections  15   a  inserted into the three rotation transfer recesses  18   d , respectively. In this state where the three pairs of rotation transfer projections  15   a  are respectively engaged in the three rotation transfer recesses  18   d , the set of three engaging projections  15   b  are respectively engaged in the set of three engaging recesses  18   e , which are formed on inner peripheral surfaces of the helicoid ring  18  in three rotational sliding projections  18   b , respectively (see FIGS.  37  and  38 ). In a state where the relative rotational angle about the lens barrel axis Z 0  between the third external barrel  15  and the helicoid ring  18  is such that the three pairs of rotation transfer projections  15   a  are respectively engaged in the three rotation transfer recesses  18   d  and that the set of three engaging projections  15   b  are respectively engaged in the set of three engaging recesses  18   e , the front ends of the three compression coil springs  25 , the rear ends of which are respectively inserted in the three spring support holes  18   f  on the front end of the helicoid ring  18 , are respectively in pressing contact with the three engaging recesses  15   c  that are formed at the rear end of the third external barrel  15 . 
   Each of the helicoid ring  18  and the third external barrel  15  is coupled to the first linear guide ring  14  to make respective relative rotations between the third external barrel  15  and the first linear guide ring  14  and between the helicoid ring  18  and the first linear guide ring  14  possible due to the engagement of the first plurality of relative rotation guide projections  14   b  with the circumferential groove  18   g , the engagement of the second plurality of relative rotation guide projections  14   c  with the circumferential groove  15   e  and the engagement of the plurality of relative rotation guide projections  15   d  with the circumferential groove  14   d . As can be seen in  FIGS. 33 through 36 , the second plurality of relative rotation guide projections  14   c  and the circumferential groove  15   e  are engaged with each other to be slightly movable relative to each other in the optical axis direction, the plurality of relative rotation guide projections  15   d  and the circumferential groove  14   d  are engaged with each other to be slightly movable relative to each other in the optical axis direction, and the first plurality of relative rotation guide projections  14   b  and the circumferential groove  18   g  are engaged with each other to be slightly movable relative to each other in the optical axis direction. Accordingly, the helicoid ring  18  and the third external barrel  15  are slightly movable relative to each other in the optical axis direction even though prevented from being separated totally from each other in the optical axis direction via the first linear guide ring  14 . The amount of play (clearance) between the helicoid ring  18  and the first linear guide ring  14  in the optical axis direction is greater than that between the third external barrel  15  and the first linear guide ring  14 . 
   When the third external barrel  15  and the helicoid ring  18  are engaged with each other to be rotatable relative to the first linear guide ring  14 , the spaces between the three spring support holes  18   f  and the three engaging recesses  15   c  in the optical axis direction are smaller than the free lengths of the three compression coil springs  25  so that the three compression coil springs  25  are compressed and held between opposed end surfaces of the third external barrel  15  and the helicoid ring  18 . The three compression coil springs  25  compressed between the opposed end surfaces of the third external barrel  15  and the helicoid ring  18  bias the third external barrel  15  and the helicoid ring  18  in opposite directions away from each other by the resilience of the three compression coil springs  25 , i.e., bias the third external barrel  15  and the helicoid ring  18  forward and rearward in the optical axis direction by the resilience of the three compression coil springs  25 , respectively. 
   As shown in  FIGS. 27 through 31 , the stationary barrel  22  is provided in each of the three inclined grooves  22   c  with two opposed inclined surfaces  22   c -A and  22   c -B which are apart from each other in a circumferential direction of the stationary barrel. The helicoid ring  18  is provided, on opposite side edges of each of the three rotational sliding projections  18   b  in a circumferential direction of the helicoid ring  18 , with two circumferential end surfaces  18   b -A and  18   b -B which face the two opposed inclined surfaces  22   c -A and  22   c -B in the associated inclined grooves  22   c , respectively. Each of the two opposed inclined surfaces  22   c -A and  22   c -B in each inclined groove  22   c  extend parallel to threads of the female helicoid  22   a . The two circumferential end surfaces  18   b -A and  18   b -B of each of the three rotational sliding projections  18   b  are parallel to the two opposed inclined surfaces  22   c -A and  22   c -B in the associated inclined groove  22   c , respectively. The two circumferential end surfaces  18   b -A and  18   b -B of each rotational sliding projection  18   b  are shaped so as not to interfere with the two opposed inclined surfaces  22   c -A and  22   c -B in the associated inclined groove  22   c , respectively. More specifically, when the male helicoid  18   a  are engaged with the female helicoid  22   a , the two opposed inclined surfaces  22   c -A and  22   c -B in each inclined groove  22   c  do not hold the associated rotational sliding projection  18   b  therebetween as shown in FIG.  31 . In other words, the two opposed inclined surfaces  22   c -A and  22   c -B in each inclined groove  22   c  are not engaged with the two circumferential end surfaces  18   b -A and  18   b -B of the associated rotational sliding projection  18   b , respectively, when the male helicoid  18   a  are engaged with the female helicoid  22   a.    
   One of the three rotational sliding projections  18   b  is provided on the circumferential end surface  18   b -A thereof with an engaging surface  18   b -E (see  FIGS. 37 ,  38 ,  39 ,  42  and  43 ) with which the stop projection  26   b  of the stop member  26  can be engaged. The engaging surface  18   b -E is parallel to the lens barrel axis Z 0 . 
   As described above, the stationary barrel  22  is provided in each of the set of three rotational sliding grooves  22   d  with two opposed surfaces: the front guide surface  22   d -A and the rear guide surface  22   d -B which are apart from each other in the optical axis direction to extend parallel to each other. Each of the three rotational sliding projections  18   b  is provided with a front sliding surface  18   b -C and a rear sliding surface  18   b -D which extend parallel to each other to be slidable on the front guide surface  22   d -A and the rear guide surfaces  22   d -B, respectively. As shown in  FIGS. 37 through 39 , the set of three engaging recesses  18   e  are respectively formed on front sliding surfaces  18   b -C of the three rotational sliding projections  18   b  of the helicoid ring  18  to be open at the front end of the helicoid ring  18 . 
   In the state shown in  FIGS. 23 and 27  in which the zoom lens  71  is in the retracted state, the two circumferential end surfaces  18   b -A and  18   b -B of each rotational sliding projection  18   b  are not in contact with the two opposed inclined surfaces  22   c -A and  22   c -B in each inclined groove  22   c  though the set of three rotational sliding projections  18   b  are positioned in the set of three inclined grooves  22   c , respectively, as shown in FIG.  31 . In the retracted state of the zoom lens  71 , the male helicoid  18   a  is engaged with the female helicoid  22   a  while the set of three rotational sliding projections  18   b  are engaged in the set of three inclined grooves  22   c , respectively. Therefore, if the helicoid ring  18  is rotated in a lens barrel advancing direction (in an upward direction as viewed in  FIG. 23 ) by rotation of the zoom gear  28  that is in mesh with the annular gear  18   c  of the helicoid ring  18 , the helicoid ring  18  moves forward in the optical axis direction (in a leftward direction as viewed in  FIG. 23 ) while rotating about the lens barrel axis Z 0  due to engagement of the male helicoid  18   a  with the female helicoid  22   a . During this rotating-advancing operation of the helicoid ring  18 , the set of three rotational sliding projections  18   b  do not interfere with the stationary barrel  22  since the set of three rotational sliding projections  18   b  move in the set of three set of three inclined grooves  22   c  therealong, respectively. 
   When the set of three rotational sliding projections  18   b  are respectively positioned in the set of three set of three inclined grooves  22   c , positions of the set of three engaging projections  15   b  in the optical axis direction are not limited by the set of three inclined grooves  22   c , respectively, and also a position of the front sliding surface  18   b -C and a position of the rear sliding surface  18   b -D of each rotational sliding projection  18   b  in the optical axis direction are not limited by the associated inclined groove  22   c . As shown in  FIGS. 35 and 36 , the third external barrel  15  and the helicoid ring  18 , which are biased in opposite directions away from each other by the spring force of the three compression coil springs  25 , are slightly apart from each other in the optical axis direction by a distance corresponding to the amount of clearance between the relative rotation guide projections  14   b ,  14   c  and  15   d  and the circumferential grooves  18   g ,  15   e  and  14   d , respectively, i.e., by a distance corresponding to the sum of the amount of play (clearance) between the helicoid ring  18  and the first linear guide ring  14  in the optical axis direction and the amount of play (clearance) between the third external barrel  15  and the first linear guide ring  14  in the optical axis direction. In this state, the spring force of the three compression coil springs  25  by which the third external barrel  15  and the helicoid ring  18  are biased in opposite directions away from each other is small because the three compression coil springs  25  are not compressed largely, so that the space between the third external barrel  15  and the helicoid ring  18  is loosely maintained. The existence of this loosely maintained space does not become a substantial problem because any pictures are not taken during the translation of the zoom lens  71  from the retracted state to the ready-to-photograph state, i.e., when the set of three rotational sliding projections  18   b  are engaged in the set of three inclined grooves  22   c . In retractable telescoping type zoom lenses including the preset embodiment of the zoom lens  71 , it is generally the case that the total time in which the zoom lens is in the retracted position (including the time when the power is OFF) is greater than the service hours (operating time). Accordingly, it is desirable to apply no heavy load to biasing members such as three compression coil springs  25  to prevent the biasing members from deteriorating with time unless the zoom lens is in the ready-to-photograph state. In addition, if the spring force of the three compression coil springs  25  is small, only a little load is applied to the associated moving parts of the zoom lens  71  during the translation of the zoom lens  71  from the retracted state to the ready-to-photograph state. This lessens the loads applied to the zoom motor  150 . 
   A forward movement of the helicoid ring  18  in the optical axis direction causes the first linear guide ring  14  to move together with the helicoid ring  18  in the optical axis direction due to engagement of the engagement of the first plurality of relative rotation guide projections  14   b  with the circumferential groove  18   g . At the same time, a rotation of the helicoid ring  18  is transferred to the cam ring  11  via the third external barrel  15  to move the cam ring  11  forward in the optical axis direction while rotating the cam ring  11  about the lens barrel axis Z 0  relative to the first linear guide ring  14  by engagement of the set of three roller followers  32  with the lead slot portions  14   e - 3  of the set of three through-slots  14   e , respectively. This rotation of the cam ring  11  causes the first lens group LG 1  and the second lens group LG 2  to move along the photographing optical axis Z 1  in a predetermined moving manner in accordance with contours of the set of three outer cam grooves  11   b  for moving the first lens group LG 1  and the plurality of inner cam grooves  11   a  ( 11   a - 1  and  11   a - 2 ) for moving the second lens group LG 2 . 
   Upon moving beyond the front ends of the set of three inclined grooves  22   c , the set of three rotational sliding projections  18   b  enter the set of three rotational sliding grooves  22   d , respectively. The ranges of formation of the male helicoid  18   a  and the female helicoid  22   a  on the helicoid ring  18  and the stationary barrel  22 , respectively, are determined so that the male helicoid  18   a  and the female helicoid  22   a  are disengaged from each other at the time when the set of three rotational sliding projections  18   b  enter the set of three rotational sliding grooves  22   d , respectively. More specifically, the stationary barrel  22  is provided, on an inner peripheral surface thereof immediately behind the set of three rotational sliding grooves  22   d , with the aforementioned non-helicoid area  22   z , on which no threads of the female helicoid  22   a  are formed, and the width of the non-helicoid area  22   z  in the optical axis direction is greater than the width of that area on the outer peripheral surface of the helicoid ring  18  on which the male helicoid  18  is formed in the optical axis direction. On the other hand, the space between the male helicoid  18   a  and the set of three rotational sliding projections  18   b  in the optical axis direction is determined so that the male helicoid  18   a  and the set of three rotational sliding projections  18   b  are positioned within the non-helicoid area  22   z  in the optical axis direction when the set of three rotational sliding projections  18   b  are positioned in the set of three rotational sliding grooves  22   d , respectively. Therefore, at the time when the set of three rotational sliding projections  18   b  respectively enter the set of three rotational sliding grooves  22   d , the male helicoid  18   a  and the female helicoid  22   a  are disengaged from each other, so that the helicoid ring  18  does not move in the optical axis direction even if rotating about the lens barrel axis Z 0  relative to the stationary barrel  22 . Thereafter, the helicoid ring  18  rotates about the lens barrel axis Z 0  without moving in the optical axis direction in accordance with rotation of the zoom gear  28  in the lens barrel advancing direction. As shown in  FIG. 24 , the zoom gear  28  remains engaged with the annular gear  18   c  even after the helicoid ring  18  has moved to the fixed axis position thereof, at which the helicoid ring  18  rotates about the lens barrel axis Z 0  without moving in the optical axis direction due to the engagement of the set of three rotational sliding projections  18   b  with the set of three rotational sliding grooves  22   d . This makes it possible to continue to transfer rotation of the zoom gear  28  to the helicoid ring  18 . 
   The state of the zoom lens  71  shown in  FIGS. 24 and 28  in which the helicoid ring  18  can rotate at the axial fixed position while the set of three rotational sliding projections  18   b  have slightly moved in the set of three rotational sliding grooves  22   d  corresponds to a state in which the zoom lens  71  is set at the wide-angle extremity. As shown in  FIG. 28  in which the zoom lens  71  is set at the wide-angle extremity, each rotational sliding projection  18   b  is positioned in the associated rotational sliding groove  22   d  with the front sliding surface  18   b -C and the rear sliding surface  18   b -D of the rotational sliding projection  18   b  facing the front guide surface  22   d -A and the rear guide surface  22   d -B in the associated rotational sliding groove  22   d , so that the helicoid ring  18  is prevented from moving in the optical axis direction relative to the stationary barrel  22 . 
   When the set of three rotational sliding projections  18   b  move into the set of three rotational sliding grooves  22   d , respectively, as shown in  FIG. 33 , the set of three engaging projections  15   b  of the third external barrel  15  move into the set of three rotational sliding grooves  22   d  at the same time, respectively, so that the set of three engaging projections  15   b  are respectively pressed against the front guide surfaces  22   d -A in the set of three rotational sliding grooves  22   d  by the spring force of the three compression coil springs  25  and so that the set of three rotational sliding projections  18   b  of the helicoid ring  18  are respectively pressed against the rear guide surfaces  22   d -B in the set of three rotational sliding grooves  22   d  by the spring force of the three compression coil springs  25 . The space between the front guide surfaces  22   d -A and the rear guide surfaces  22   d -B in the optical axis direction is determined to make the set of three rotational sliding projections  18   b  and the set of three engaging projections  15   b  positioned closer to each other in the optical axis direction than those when the set of three rotational sliding projections  18   b  and the set of three engaging projections  15   b  are respectively positioned in the set of three inclined grooves  22   c . At this time when the set of three rotational sliding projections  18   b  and the set of three engaging projections  15   b  are made to be positioned closer to each other in the optical axis direction, the three compression coil springs  25  are largely compressed to thereby apply a stronger spring force to the set of three engaging projections  15   b  and the set of three rotational sliding projections  18   b  than the spring force which is applied thereto by the three compression coil springs  25  when the zoom lens  71  is in the retracted state. Thereafter, while the set of three rotational sliding projections  18   b  and the set of three engaging projections  15   b  are positioned in the set of three rotational sliding grooves  22   d , the set of three engaging projections  15   b  and the set of three rotational sliding projections  18   b  are pressed against each other by the spring force of the three compression coil springs  25 . This stabilizes axial positions of the third external barrel  15  and the helicoid ring  18  relative to the stationary barrel  22  in the optical axis direction. Namely, the third external barrel  15  and the helicoid ring  18  are supported by the stationary barrel  22  with no play between the third external barrel  15  and the helicoid ring  18  in the optical axis direction. 
   Rotating the third external barrel  15  and the helicoid ring  18  in the lens barrel advancing direction from their respective wide-angle extremities (from the positions shown in  FIGS. 24 and 28 ) causes the set of three engaging projections  15   b  and the set of three rotational sliding projections  18   b  (the rear sliding surface  18   b -D thereof) to firstly move toward the terminal ends of the set of three rotational sliding grooves  22   d  (upwards as viewed in  FIG. 28 ) while being guided by the front guide surfaces  22   d -A and the rear guide surfaces  22   d -B and subsequently reach telephoto extremities of the third external barrel  15  and the helicoid ring  18  (the positions shown in FIGS.  25  and  29 ). Since the set of three engaging projections  15   b  and the set of three rotational sliding projections  18   b  remain engaged in the set of three rotational sliding grooves  22   d , respectively, the helicoid ring  18  and the third external barrel  15  are prevented from moving in the optical axis direction relative to the stationary barrel  22  and accordingly rotate about the lens barrel axis Z 0  without moving in the optical axis direction relative to the stationary barrel  22 . In this state, the helicoid ring  18  is guided to be rotatable about the lens barrel axis Z 0  mainly by the rear sliding surfaces  18   b -D of the set of three rotational sliding projections  18   b  and the rear guide surfaces  22   d -B of the stationary barrel  22  because the helicoid ring  18  is biased rearward in the optical axis direction by the three compression coil springs  25 , i.e., in a direction to make the rear sliding surfaces  18   b -D come into pressing contact with the rear guide surfaces  22   d -B, respectively (see FIG.  32 ). 
   When the helicoid ring  18  rotates at the axial fixed position, the cam ring  11  also rotates at the axial fixed position without moving in the optical axis direction relative to the first linear guide ring  14  because the set of three roller followers  32  are engaged in the front circumferential slot portions  14   e - 1  of the set of three through-slots  14   e , respectively. Accordingly, the first and second lens groups LG 1  and LG 2  move in the optical axis direction relative to each other in a predetermined moving manner to perform a zooming operation in accordance with contours of respective zooming sections of the plurality of inner cam grooves  11   a  ( 11   a - 1  and  11   a - 2 ) and the set of three outer cam grooves  11   b.    
   Further rotating the external barrel  15  and the helicoid ring  18  in the lens barrel advancing direction to move the external barrel  15  and the helicoid ring  18  in the optical axis direction beyond their respective telephoto extremities causes the set of three rotational sliding projections  18   b  to reach the terminal ends (assembly/disassembly sections) of the set of three rotational sliding grooves  22   d  as shown in  FIGS. 26 and 30 . In this state shown in  FIGS. 26 and 30 , movable elements of the zoom lens  71  such as the first through third external barrels  12 ,  13  and  15  can be removed from the stationary barrel  22  from the front thereof. However, if the stop member  26  is provided fixed to the stationary barrel  22  as shown in  FIG. 41 , such movable elements cannot be removed from the stationary barrel  22  unless the stop member  26  is removed from the stationary barrel  22  because the engaging surface  18   b -E, which is provided on specific one of the three rotational sliding projections  18   b , comes into contact with the stop projection  26   b  of the stop member  26  to prevent the set of three rotational sliding projections  18   b  from reaching the terminal ends (assembly/disassembly sections) of the set of three rotational sliding grooves  22   d , respectively. 
   Rotating the third external barrel  15  and the helicoid ring  18  in a lens barrel retracting direction (downwards as viewed in  FIG. 25 ) from their respective telephoto extremities causes the set of three rotational sliding projections  18   b  and the set of three engaging projections  15   b  to move toward the set of three inclined grooves  22   c  in the set of three rotational sliding grooves  22   d , respectively. During this movement, the third external barrel  15  and the helicoid barrel  18  rotate together about the lens barrel axis Z 0  with no play between the third external barrel  15  and the helicoid ring  18  in the optical axis direction because the set of three engaging projections  15   b  are respectively pressed against the front guide surfaces  22   d -A in the set of three rotational sliding grooves  22   d  by the spring force of the three compression coil springs  25  while the set of three rotational sliding projections  18   b  of the helicoid ring  18  are respectively pressed against the rear guide surfaces  22   d -B in the set of three rotational sliding grooves  22   d  by the spring force of the three compression coil springs  25 . 
   Further rotating the external barrel  15  and the helicoid ring  18  in the lens barrel retracting direction beyond their respective wide-angle extremities (the positions shown in  FIGS. 24 and 28 ) causes the circumferential end surfaces  18   b -B of the set of three rotational sliding projections  18   b  to come into contact with the inclined surfaces  22   c -B in the set of three inclined grooves  22   c , respectively. Thereupon, the movement of the helicoid ring  18  in the lens barrel retracting direction generates a component force in a direction to make the circumferential end surfaces  18   b -B of the set of three rotational sliding projections  18   b  move rearward in the optical axis direction along the inclined surfaces  22   c -B in the set of three inclined grooves  22   c  while sliding thereon, respectively, because the two circumferential end surfaces  18   b -A and  18   b -B of each of the three rotational sliding projections  18   b  are parallel to the two opposed inclined surfaces  22   c -A and  22   c -B in the associated inclined groove  22   c  as shown in  FIG. 31 , respectively. Therefore, the helicoid ring  18  starts moving rearward in the optical axis direction while rotating about the lens barrel axis Z 0  in the reverse manner to when the helicoid ring  18  moves forward while rotating. A slight rearward movement of the helicoid ring  18  in the optical axis direction by the engagement of the set of three rotational sliding projections  18   b  with the set of three inclined grooves  22   c , respectively, causes the male helicoid  18   a  to be engaged with the female helicoid  22   a  again. Thereafter, further rotating the helicoid ring  18  in the lens barrel retracting direction causes the helicoid barrel  18  to keep moving rearward in the optical axis direction by the engagement of the set of three rotational sliding projections  18   b  with the set of three inclined grooves  22   c , respectively, until the helicoid ring  18  reaches a retracted position thereof shown in  FIGS. 23 and 27 , i.e., until the zoom lens  71  is fully retracted. The third external barrel  15  moves rearward in the optical axis direction while rotating about the lens barrel axis Z 0  due to the structures of the helicoid ring  18  and the first linear guide ring  14 . During this rearward movement of the third external barrel  15 , the set of three engaging projections  15   b  moves together with the set of three rotational sliding projections  18   b  in the set of three inclined grooves  22   c , respectively. When the helicoid ring  18  and the third external barrel  15  move rearward in the optical axis direction, the first linear guide ring  14  also moves rearward in the optical axis direction, which causes the cam ring  11 , which is supported by the first linear guide ring  14 , to move rearward in the optical axis direction. In addition, at the time when the helicoid ring  18  starts moving rearward while rotating after rotating at the axial fixed position, the set of three roller followers  32  are disengaged from the front circumferential slot portions  14   e - 1  to be engaged in the lead slot portions  14   e - 3 , respectively, while the cam ring  11  moves rearward in the optical axis direction while rotating about the lens barrel axis Z 0  with respect to the first linear guide ring  14 . 
   Upon the set of three rotational sliding projections  18   b  entering the set of three inclined grooves  22   c  from the set of three rotational sliding grooves  22   d , respectively, the third external barrel  15  and the helicoid ring  18  change the relationship therebetween from the relationship in the ready-to-photograph state shown in  FIGS. 33 and 34 , in which the relative axial positions of the third external barrel  15  and the helicoid ring  18  in the optical axis direction are finely determined, back to the relationship shown in  FIGS. 35 and 36 , in which the axial positions of the third external barrel  15  and the helicoid ring  18  are coarsely determined due to the engagement of the third external barrel  15  with the first linear guide ring  14  with a clearance therebetween in the optical axis direction and the engagement of the helicoid barrel  18  with the first linear guide ring  14  with a clearance therebetween in the optical axis direction since either positions of the set of three engaging projections  15   b  in the optical axis direction or positions of the set of three rotational sliding projections  18   b  in the optical axis direction are not limited by the set of three rotational sliding grooves  22   d , respectively. In the state shown in  FIGS. 35 and 36  in which the set of three rotational sliding projections  18   b  are engaged in the set of three inclined grooves  22   c , the respective positions of the third external barrel  15  and the helicoid ring  18  in the optical axis direction do not need to be determined finely since the zoom lens  71  is no longer in the ready-to-photograph state. 
   As can be understood from the above descriptions, in the present embodiment of the zoom lens  71 , a simple mechanism having the male and female helicoids  18   a  and  22   a  (that have male threads and female threads which are formed on radially-opposed outer and inner peripheral surfaces of the helicoid ring  18  and the stationary barrel  22 , respectively), the set of three rotational sliding projections  18   b , the set of three inclined grooves  22   c  and the set of three rotational sliding grooves  22   d  can make the helicoid ring  18  perform a rotating-advancing/rotating-retracting operation in which the helicoid ring  18  rotates while moving forward or rearward in the optical axis direction, and a fixed-position rotating operation in which the helicoid ring  18  rotates at a predetermined axial fixed position without moving in the optical axis direction relative to the stationary barrel  22 . A simple fit between two ring members such as the helicoid ring  18  and the stationary barrel  22  with a highly reliable precision in driving one of the two ring members relative to the other can generally be achieved with a fitting structure using helicoids (male and female helicoid threads). Moreover, the set of three rotational sliding projections  18   b  and the set of three rotational sliding grooves  22   d , which are adopted to make the helicoid ring  18  rotatable at the axial fixed position which cannot be achieved by helicoids, also constitute a simple projection-depression structure similar to the above fitting structure using helicoids. Furthermore, the set of three rotational sliding projections  18   b  and the set of three rotational sliding grooves  22   d  are formed on the outer and inner peripheral surfaces of the helicoid ring  18  and the stationary barrel  22  on which the male helicoid  18   a  and the female helicoid  22   a  are also formed. This does not require any additional space for the installation of the set of three rotational sliding projections  18   b  and the set of three rotational sliding grooves  22   d  in the zoom lens  71 . Accordingly, the aforementioned rotating-advancing/rotating-retracting operation and the fixed-position rotating operation that are performed by rotation of the helicoid ring  18  are achieved with a simple, compact and low-cost structure. 
   The zoom gear  28  has a sufficient length in the optical axis direction to remain engaged with the annular gear  18   c  of the helicoid ring  18  regardless of variations of the position thereof in the optical axis direction. Therefore, the zoom gear  28 , that is provided as a single gear, can transfer rotation thereof to the helicoid ring  18  at all times in each of the rotating-advancing/rotating-retracting operation and the fixed-position rotating operation of the helicoid ring  18 . Accordingly, a simple and compact rotation transfer mechanism for transferring rotation to the helicoid ring  18  that presents intricate movements is achieved in the present embodiment of the zoom lens, and the helicoid ring  18  and components associated therewith which are positioned inside the helicoid ring  18  can be driven with a high degree of precision. 
   As shown in  FIGS. 31 and 32 , the tooth depth of each rotational sliding projection  18   b  of the female helicoid  18   a  is greater than that of each thread of the female helicoid  18   a , and accordingly the set of three inclined grooves  22   c  and the set of three rotational sliding grooves  22   d  are formed to have greater tooth depths than the threads of the female helicoid  22   a . On the other hand, the zoom gear  28  is supported by the stationary barrel  22  so that the gear teeth formed around the zoom gear  28  project radially inwards from an inner peripheral surface of the stationary barrel  22  (from a tooth flank of the female helicoid  22   a ) to be engaged with the annular gear  18   c , which is formed on an outer peripheral surface of each thread of the male helicoid  18   a . Therefore, the set of three rotational sliding projections  18   b  and gear teeth of the zoom gear  28  are positioned in the same annular range (radial range) about the lens barrel axis Z 0  as viewed from the front of the zoom lens  71 . However, the zoom gear  28  does not overlap the moving paths of set of three rotational sliding projections  18   b  because the zoom gear  28  is positioned between two of the set of three inclined grooves  22   c  in a circumferential direction of the stationary barrel  22  and because the zoom gear  28  is installed on the stationary barrel  22  at a position different from the position of the set of three rotational sliding grooves  22   d  in the optical axis direction. Accordingly, the set of three rotational sliding projections  18   b  do not interfere with the zoom gear  28  even though engaged in either the set of three inclined grooves  22   c  or the set of three rotational sliding grooves  22   d.    
   It is possible that the set of three rotational sliding projections  18   b  and the zoom gear  28  be prevented from interfering with each other by reducing the amount of projection of the gear teeth of the zoom gear  28  from an inner peripheral surface of the stationary barrel  22  (from a tooth flank of the female helicoid  22   a ) so that the tooth depth of the zoom gear  28  becomes smaller than that of the male helicoid  18   a . However, in this case, the amount of engagement of the teeth of the zoom gear  28  with the teeth of the male helicoid  18   a  will be small, which makes it difficult to achieve a stable rotation of the helicoid ring  18  when it rotates at the axial fixed position. Alternatively, if the tooth depth of the male helicoid  18   a  is increased without changing the amount of projection of each rotational sliding projection  18   b , both the diameter of the stationary barrel  22  and the radial distance between the zoom gear  28  and the lens barrel axis Z 0  increase accordingly. This increases the diameter of the zoom lens  71 . Accordingly, if either the tooth depth of the male helicoid  18   a  or the amount of projection of the set of three rotational sliding projections  18   b  in radial directions of the helicoid ring  18  is changed to prevent the set of three rotational sliding projections  18   b  and the zoom gear  28  from interfering with each other, the helicoid ring  18  may not be driven with stability; moreover, a sufficient downsizing of the zoom barrel  71  may not be done. In contrast, according to the configurations of the zoom gear  28  and the set of three rotational sliding projections  18   b  shown in  FIGS. 27 through 30 , the set of three rotational sliding projections  18   b  and the zoom gear  28  can be prevented from interfering with each other without such problems. 
   In the present embodiment of the zoom lens  71 , a rotatable portion of the zoom lens  71  which rotates at an axial fixed position at one time and also rotates while moving forward or rearward in the optical axis direction at another time is divided into two parts: the third external barrel  15 , and the helicoid ring  18  that are slightly movable relative to each other in the optical axis direction. In addition, the third external barrel  15  and the helicoid ring  18  are biased in opposite directions away from each other in the optical axis direction by the resilience of the three compression coil springs  25  to press the set of three engaging projections  15   b  of the third external barrel  15  against the front guide surfaces  22   d -A in the set of three rotational sliding grooves  22   d , respectively, and to press the set of three rotational sliding projections  18   b  of the helicoid ring  18  against the rear guide surfaces  22   d -B in the set of three rotational sliding grooves  22   d , respectively, to eliminate backlash between the third external barrel  15  and the stationary barrel  22  and backlash between the helicoid ring  18  and the stationary barrel  22 . As described above, the set of three rotational sliding grooves  22   d  and the set of three rotational sliding projections  18   b  are elements of a drive mechanism for rotating the helicoid ring  18  at the axial fixed position or rotating the helicoid ring  18  while moving the same in the optical axis direction, and are also used as elements for removing the aforementioned backlashes. This reduces the number of elements of the zoom lens  71 . 
   The zoom lens  71  does not have to secure an additional space in the vicinity of the stationary barrel  22  in which the three compression coil springs  25  adopted for removing backlash are accommodated because the three compression coil springs  25  are compressed and held between opposed end surfaces of the third external barrel  15  and the helicoid ring  18  that rotate in one piece about the lens barrel axis Z 0 . In addition, the set of three engaging projections  15   b  are respectively received in the set of three engaging recesses  18   e . This achieves a space-saving connected portion between the third external barrel  15  and the helicoid ring  18 . 
   As described above, the three compression coil springs  25  are largely compressed to apply a strong spring force to the set of three engaging projections  15   b  and the set of three rotational sliding projections  18   b  only when the zoom lens  71  is in the ready-to-photograph state. Namely, the three compression coil springs  25  are not largely compressed to apply a strong spring force to the set of three engaging projections  15   b  and the set of three rotational sliding projections  18   b  when the zoom lens  71  is not in the ready-to-photograph state, e.g., the retracted state. This reduces load on the associated moving parts of the zoom lens  71  during the translation of the zoom lens  71  from the retracted state to the ready-to-photograph state, especially at the beginning of driving the zoom lens in the lens barrel advancing operation, and also increases durability of the three compression coil springs  25 . 
   The helicoid ring  18  and the third external barrel  15  are disengaged from each other firstly in the disassembling operation of the zoom lens  71 . A zoom lens assembling mechanism which makes it easy for the zoom lens  71  to be assembled and disassembled, mainly elements of the zoom lens assembling mechanism which are associated with the helicoid ring  18  and the third external barrel  15 , will be discussed hereinafter. 
   As described above, the stationary barrel  22  is provided with the stop-member insertion hole  22   e  that radially penetrates the stationary barrel  22 , from an outer peripheral surface of the stationary barrel  22  to a bottom surface of specific one of the three rotational sliding grooves  22   d . The stationary barrel  22  is provided on a surface thereof in the vicinity of the stop-member insertion hole  22   e  with a screw hole  22   f  and a stop member positioning protrusion  22   g . The stop member  26 , which is fixed to the stationary barrel  22  as shown in  FIG. 41 , is provided with an arm portion  26   a  which extends along an outer peripheral surface of the stationary barrel  22 , and the aforementioned stop projection  26   b  which projects radially inwards from the arm portion  26   a . The stop member  26  is provided at one end thereof with an insertion hole  26   c  into which the set screw  67  is inserted, and is further provided at the other end thereof with a hook portion  26   d . The stop member  26  is fixed to the stationary barrel  22  by screwing the set screw  67  into the screw hole  22   f  through the insertion hole  26   c  with the hook portion  26   d  being engaged with the stop member positioning protrusion  22   g  as shown in FIG.  41 . In a state where the stop member  26  is fixed to the stationary barrel  22  in this manner, the stop projection  26   b  is positioned in the stop-member insertion hole  22   e  so that the tip of the stop projection  26   b  projects inside a specific rotational sliding groove  22   d  among the set of three rotational sliding grooves  22   d . This state is shown in FIG.  37 . Note that the stationary barrel  22  is not shown in FIG.  37 . 
   The stationary barrel  22  is provided, at the front end thereof on the front walls of the three rotational sliding grooves  22   d , with three insertion/removable holes  22   h  through which the front of the stationary barrel  22  communicate with the three rotational sliding grooves  22   d  in the optical axis direction, respectively. Each of the three insertion/removable holes  22   h  has a sufficient width allowing the associated one of the three engaging projections  15   b  to be inserted into the insertion/removable hole  22   h  in the optical axis direction.  FIG. 42  shows one of the three insertion/removable holes  22   h  and peripheral parts when the zoom lens  71  is set at the telephoto extremity as shown in  FIGS. 25 and 29 . As can be clearly seen in  FIG. 42 , in the case where the zoom lens  71  is set at the telephoto extremity, the set of three engaging projections  15   b  cannot be removed, toward the front of the zoom lens  71 , from the three rotational sliding grooves  22   d  through the three insertion/removable holes  22   h  because the three engaging projections  15   b  and the three insertion/removable holes  22   h  are not aligned in the optical axis direction (horizontal direction as viewed in FIG.  42 ), respectively. This positional relationship is true for the remaining two insertion/removable holes  22   h  though only one of the three insertion/removable holes  22   h  is shown in FIG.  42 . On the other hand, when the zoom lens  71  is set at the wide-angle extremity as shown in  FIGS. 24 and 28 , the three engaging projections  15   b  are respectively positioned further from the three insertion/removable holes  22   h  than the three engaging projections  15   b  shown in  FIGS. 25 and 29  in which the zoom lens  71  is set at the telephoto extremity. This means that the set of three engaging projections  15   b  cannot be removed from the three rotational sliding grooves  22   d  through the three insertion/removable holes  22   h , respectively, when the zoom lens  71  is in the ready-to-photograph state, i.e., when the zoom lens  71  is set at a focal length between the wide-angle extremity and the telephoto extremity. 
   In order to align the three engaging projections  15   b  and the three insertion/removable holes  22   h  in the optical axis direction, respectively, from the state shown in  FIG. 42  in which the zoom lens  71  is set at the telephoto extremity, the third external barrel  15  needs to be further rotated together with the helicoid ring  18  counterclockwise as viewed from the front of the zoom lens  71  relative to the stationary barrel  22  (upwards as viewed in  FIG. 42 ) by a rotational angle (disassembling rotational angle ) Rt 1  (see FIG.  42 ). However, in a state where the stop projection  26   b  is inserted into the stop-member insertion hole  22   e  as shown in  FIG. 41 , if the third external barrel  15  is rotated together with the helicoid ring  18  counterclockwise as viewed from the front of the zoom lens  71  relative to the stationary barrel  22  by a rotational angle (allowable rotational angle) Rt 2  (see FIG.  42 ), which is smaller than the disassembling rotational angle Rt 1 , from the state shown in  FIG. 42  in which the zoom lens  71  is set at the telephoto extremity, the engaging surface  18   b -E that is formed on one of the three rotational sliding projections  18   b  comes into contact with the stop projection  26   b  of the stop member  26  to prevent the third external barrel  15  and the helicoid ring  18  from further rotating (see FIG.  37 ). Since the allowable rotational angle Rt 2  is smaller than the disassembling rotational angle Rt 1 , the three engaging projections  15   b  and the three insertion/removable holes  22   h  cannot be aligned in the optical axis direction, respectively, which makes it impossible to remove the set of three engaging projections  15   b  from the three rotational sliding grooves  22   d  through the three insertion/removable holes  22   h , respectively. Namely, although terminal end portions of the set of three rotational sliding grooves  22   d , which respectively communicate with the front of the stationary barrel  22  through the three insertion/removable holes  22   h , serve as assembly/disassembly sections, the third external barrel  15  cannot be rotated together with the helicoid ring  18  to a point where the set of three engaging projections  15   b  are positioned in the terminal end portions of the set of three rotational sliding grooves  22   d , respectively, as long as the stop member  26  remains fixed to the stationary barrel  22  with the stop projection  26   b  in the stop-member insertion hole  22   e.    
   In the disassembling operation of the zoom lens  71 , the stop member  26  needs to be removed from the stationary barrel  22  in the first place. If the stop member  26  is removed, the stop projection  26   b  comes out of the stop-member insertion hole  22   e . Once the stop projection  26   b  comes out of the stop-member insertion hole  22   e , the third external barrel  15  and the helicoid ring  18  can be rotated together by the disassembling rotational angle Rt 1 . Rotating the third external barrel  15  and the helicoid ring  18  together by the disassembling rotational angle Rt 1  in a state where the zoom lens  71  is set at the telephoto extremity causes the third external barrel  15  and the helicoid ring  18  to be positioned to their respective specific rotational positions relative to the stationary barrel  22  (hereinafter referred to as assembling/disassembling angular positions) as shown in  FIGS. 26 ,  63 .  FIGS. 26 and 30  show a state of the zoom lens  71  where the third external barrel  15  and the helicoid ring  18  have been rotated together by the disassembling rotational angle Rt 1  to be positioned in the respective assembling/disassembling angular positions from a state where the zoom lens  71  is set at the telephoto extremity. This state of the zoom lens  71 , in which the third external barrel  15  and the helicoid ring  18  are positioned in the respective assembling/disassembling angular positions, is hereinafter referred to as an assemblable/disassemblable state.  FIG. 43  shows a portion of the stationary barrel  22  on which one of the three insertion/removable holes  22   h  is formed and portions of peripheral elements in the able-to-be assembled/disassembled state. As can be clearly understood from  FIG. 43 , if the third external barrel  15  and the helicoid ring  18  have rotated by the disassembling rotational angle Rt 1  as shown in  FIG. 43 , the three insertion/removable holes  22   h  and the three engaging recesses  18   e  that are formed on the set of three rotational sliding projections  18   b  are aligned in the optical axis direction so that the set of three engaging projections  15   b  accommodated in the set of three engaging recesses  18   e  can be removed therefrom through the three insertion/removable holes  22   h  from the front of the zoom lens  71 , respectively. Namely, the third external barrel  15  can be removed from the stationary barrel  22  from the front thereof. Removing the set of three engaging projections  15   b  from the set of three engaging recesses  18   e , respectively, causes the set of three engaging projections  15   b  of the third external barrel  15  and the set of three rotational sliding projections  18   b  of the helicoid ring  18  to be free from the spring force of the three compression coil springs  25 , which are adopted to bias the set of three engaging projections  15   b  and the set of three rotational sliding projections  18   b  in opposite directions away from each other in the optical axis direction. At the same time, a function of the three rotational sliding projections  18   b  for removing backlash between the third external barrel  15  and the stationary barrel  22  and backlash between the helicoid ring  18  and the stationary barrel  22  is cancelled. The three engaging projections  15   b  and the three insertion/removable holes  22   h  are aligned in the optical axis direction when the set of three engaging projections  15   b  are in contact with the terminal ends (upward ends as viewed in  FIG. 28 ) of the set of three rotational sliding grooves  22   d , respectively. Accordingly, the three engaging projections  15   b  and the three insertion/removable holes  22   h  are automatically aligned in the optical axis direction if the third external barrel  15  and the helicoid ring  18  are fully rotated together counterclockwise as viewed from the front of the zoom lens  71  relative to the stationary barrel  22 , i.e., if the third external barrel  15  and the helicoid ring  18  are rotated together to the respective assembling/disassembling angular positions. 
   Although the third external barrel  15  can be removed from the stationary barrel  22  when rotated to the assembling/disassembling angular position as shown in  FIGS. 26 and 30 , the third external barrel  15  is still engaged with the first linear guide ring  14  by the engagement of the plurality of relative rotation guide projections  15   d  with the circumferential groove  14   d  and the engagement of the second plurality of relative rotation guide projections  14   c  with the circumferential groove  15   e . As can be seen in  FIGS. 14 and 15 , the second plurality of relative rotation guide projections  14   c  are formed on the first linear guide ring  14  at irregular intervals in a circumferential direction thereof, and some of the second plurality of relative rotation guide projections  14   c  have different circumferential widths than another ones. Likewise, the plurality of relative rotation guide projections  15   d  are formed on the third external barrel  15  at irregular intervals in a circumferential direction thereof, and some of the relative rotation guide projections  15   d  have different circumferential widths than another ones. The third external barrel  15  is provided at a rear end thereof with a plurality of insertion/removable holes  15   g  through which the second plurality of relative rotation guide projections  14   c  can be removed from the circumferential groove  15   e  in the optical axis direction, respectively, only when the first linear guide ring  14  is positioned in a specific rotational position relative to the third external barrel  15 . Likewise, the first linear guide ring  14  is provided at the front end thereof with a plurality of insertion/removable holes  14   h  through which the plurality of relative rotation guide projections  15   d  can be removed from the circumferential groove  14   d  in the optical axis direction, respectively, only when the third external barrel  15  is positioned in a specific rotational position relative to the first linear guide ring  14 . 
     FIGS. 44 through 47  are developed views of the third external barrel  15  and the first linear guide ring  14 , showing the relationship of coupling therebetween in different states. Specifically,  FIG. 44  shows a state of coupling between the third external barrel  15  and the first linear guide ring  14  when the zoom lens  71  is in the retracted state (which corresponds to the state shown in each of FIGS.  23  and  27 ),  FIG. 45  shows the same when the zoom lens  71  is set at the wide-angle extremity (which corresponds to the state shown in each of FIGS.  24  and  28 ),  FIG. 46  shows the same when the zoom lens  71  is set at the telephoto extremity (which corresponds to the state shown in each of FIGS.  25  and  29 ), and  FIG. 47  shows the same when the zoom lens  71  is in the assemblable/disassemblable state (which corresponds to the state shown in each of FIGS.  26  and  30 ). As can be seen from  FIGS. 44 through 47 , all of the second plurality of relative rotation guide projections  14   c  and the plurality of relative rotation guide projections  15   d  cannot be inserted into or removed from the circumferential groove  15   e  and the circumferential groove  14   d  in the optical axis direction through the plurality of insertion/removable holes  15   g  and the plurality of insertion/removable holes  14   h  at the same time, respectively, when the zoom lens  71  is in between the wide-angle extremity and the telephoto extremity, or even in between the wide-angle extremity and the retracted position, because some of the second plurality of relative rotation guide projections  14   c  and some of the plurality of relative rotation guide projections  15   d  are engaged in the circumferential groove  15   e  and the circumferential groove  14   d , respectively. Only when the third external barrel  15  and the helicoid ring  18  are rotated together to the respective assembling/disassembling angular positions as shown in  FIGS. 26 and 63  with the stop member having been removed, the second plurality of relative rotation guide projections  14   c  reach respective specific positions in the circumferential groove  15   e  at which the second plurality of relative rotation guide projections  14   c  and the plurality of insertion/removable holes  15   g  are aligned in the optical axis direction and at the same time the plurality of relative rotation guide projections  15   d  reach respective specific positions in the circumferential groove  14   d  at which the plurality of relative rotation guide projections  15   d  and the plurality of insertion/removable holes  14   h  are aligned in the optical axis direction. This makes it possible to remove the third external barrel  15  from the first linear guide ring  14  from the front thereof as shown in  FIGS. 47 and 56 . Note that the stationary barrel  22  is not shown in FIG.  56 . If the third external barrel  15  is removed, the three compression coil springs  25 , which are to be held between the third external barrel  15  and the helicoid ring  18 , are exposed to the outside of the zoom lens  71 , and can be removed accordingly (see FIGS.  39  and  56 ). 
   Therefore, if the third external barrel  15  and the helicoid ring  18  are rotated together to the respective assembling/disassembling angular positions as shown in  FIGS. 26 and 63  after the stop member has been removed, the third external barrel  15  can be removed from both the stationary barrel  22  and the first linear guide ring  14  at the same time. In other words, the stop member  26  serves as a rotation limiting device for limiting the range of rotation of each of the third external barrel  15  and the helicoid ring  18  about the lens barrel axis Z 0  relative to the stationary barrel  22  therein so that the third external barrel  15  and the helicoid ring  18  cannot be rotated together to the respective assembling/disassembling angular positions in a normal operating state of the zoom lens  71 . As can be understood from the above descriptions, a guiding structure consisting of the set of three rotational sliding projections  18   b , the set of three rotational sliding grooves  22   d  and the set of three inclined grooves  22   c  is simple and compact; moreover, if only the stop member  26  is added to the guiding structure, the range of rotation of each of the third external barrel  15  and the helicoid ring  18  about the lens barrel axis Z 0  relative to the stationary barrel  22  can be securely limited so that the third external barrel  15  and the helicoid ring  18  cannot be rotated together to the respective assembling/disassembling angular positions in a normal operating state of the zoom lens  71 . 
   Removing the third external barrel  15  from the zoom lens  71  makes it possible to further disassemble the zoom lens  71  in a manner which will be discussed hereinafter. As shown in  FIGS. 9 and 10 , the third external barrel  15  is provided at the front end thereof with a frontmost inner flange  15   h  which projects radially inwards to close the front ends of the set of six second linear guide grooves  14   g . The second external barrel  13 , the set of six radial projections  13   a  of which are respectively engaged in the set of six second linear guide grooves  14   g , cannot be removed from the front of the zoom lens  71  in a state where the third external barrel  15  and the first linear guide ring  14  are coupled to each other because the frontmost inner flange  15   h  prevents the set of six radial projections  13   a  from being removed from the set of six second linear guide grooves  14   g , respectively. Hence, the second external barrel  13  can be removed from the first linear guide ring  14  once the third external barrel  15  is removed. However, the second external barrel  13  cannot be removed from the cam ring  11  in the optical axis direction if the discontinuous inner flange  13   c  remains engaged in the discontinuous circumferential groove  11   c  of the cam ring  11 . As shown in  FIG. 20 , the discontinuous inner flange  13   c  is formed as a discontinuous groove which is disconnected at irregular intervals in a circumferential direction of the second external barrel  13 . On the other hand, as shown in  FIG. 16 , the cam ring  11  is provided on outer peripheral surface thereof with a set of three external protuberances  11   g  which project radially outwards, while the discontinuous circumferential groove  11   c  is formed discontinuously on only respective outer surfaces of the set of three external protuberances  11   g . The discontinuous circumferential groove  11   c  is provided on each of the three external protuberances  11   g  with an insertion/removable hole  11   r  which is open at the front end of the external protuberance  11   g . The insertion/removable holes  11   r  are arranged at irregular intervals in a circumferential direction of the cam ring  11 . 
     FIGS. 52 through 55  are developed views of the cam ring  11 , the first external barrel  12  and the second external barrel  13 , showing the relationship of coupling of each of the first external barrel  12  and the external barrel  13  to the cam ring  11  in different states. Specifically,  FIG. 52  shows a state of coupling of the first external barrel  12  and the external barrel  13  to the cam ring  11  when the zoom lens  71  is in the retracted state (which corresponds to the state shown in each of FIGS.  23  and  27 ),  FIG. 53  shows the same when the zoom lens  71  is set at the wide-angle extremity (which corresponds to the state shown in each of FIGS.  24  and  28 ),  FIG. 54  shows the same when the zoom lens  71  is set at the telephoto extremity (which corresponds to the state shown in each of FIGS.  25  and  29 ), and  FIG. 55  shows the same when the zoom lens  71  is in the assemblable/disassemblable state (which corresponds to the state shown in each of FIGS.  26  and  30 ). As can be seen from  FIGS. 52 through 54 , the second external barrel  13  cannot be removed from the cam ring  11  in the optical axis direction when the zoom lens  71  is in between the wide-angle extremity and the telephoto extremity, or even in between the wide-angle extremity and the retracted position because some portions of the discontinuous inner flange  13   c  are engaged in at least a part of the discontinuous circumferential groove  11   c . Only when the third external barrel  15  and the helicoid ring  18  are rotated together to the respective assembling/disassembling angular positions as shown in  FIGS. 26 and 63 , the rotation of the third external barrel  15  causes the cam ring  11  to rotate to a specific rotational position thereof at which all the portions of the discontinuous inner flange  13   c  of the second external barrel  13  are exactly aligned with the three insertion/removable hole  11   r  or the three circumferential spaces among the three external protuberances  11   g , respectively. This makes it possible to remove the second external barrel  13  from the cam ring  11  from the front thereof as shown in  FIGS. 55 and 57 . 
   In addition, in the state shown in  FIG. 55  in which the zoom lens  71  is in the assemblable/disassemblable state, the set of three cam followers  31  on the first external barrel  12  are positioned close to the front open ends of the set of three outer cam grooves lib, respectively, so that the first external barrel  12  can be removed from the front of the zoom lens  71  as shown in FIG.  58 . In addition, the first lens group adjustment ring  2  can also be removed from the second external barrel  12  after the two set screws  64  are screwed off to remove the fixing ring  3  as shown in FIG.  2 . Thereafter, the first lens frame  1  that is supported by the first lens group adjustment ring  2  therein can also be removed from the first lens group adjustment ring  2  from the front thereof. 
   Although the first linear guide ring  14 , the helicoid ring  18 , the cam ring  11 , and some other elements in the cam ring  11  such as the second lens group moving frame  8  still remain in the stationary barrel  22  in the state shown in  FIG. 58 , the zoom lens  71  can be further disassembled as needed. 
   As can be seen from  FIGS. 57 and 58 , if the third external barrel  15  is removed with the zoom lens  71  being fully extended forward from the stationary barrel  22 , each of the three set screws  32   a  becomes accessible. Thereafter, if the set of three roller followers  32  are removed together with the three set screws  32   a  as shown in  FIG. 59 , a combination of the cam ring  11  and the second linear guide ring  10  can be removed from the first linear guide ring  14  from the rear thereof because no elements of the zoom lens  71  prevent the cam ring  11  from moving rearward in the optical axis direction relative to the first linear guide ring  14 . As shown in  FIGS. 15 and 59 , frond ends of each pair of first linear guide grooves  14   f , in which the pair of radial projections of the associated bifurcated projection  10   a  are engaged, are each formed as a closed end while rear ends of the same are each formed as an open end at the rear end of the first linear guide ring  14 . Accordingly, the combination of the cam ring  11  and the second linear guide ring  10  can be removed from the first linear guide ring  14  only from the rear thereof. Although the second linear guide ring  10  and the cam ring  11  are coupled to each other with the discontinuous outer edge of the ring portion  10   b  being engaged in the discontinuous circumferential groove  11   e  to be rotatable relative to each other about the lens barrel axis Z 0 , the second linear guide ring  10  and the cam ring  11  can be disengaged from each other as shown in  FIG. 3  when one of the second linear guide ring  10  and the cam ring  11  is positioned in a specific rotational position relative to the other. 
   When the third external barrel  15  and the helicoid ring  18  are rotated together to the respective assembling/disassembling angular positions as shown in  FIGS. 26 and 63 , the set of three front cam followers  8   b - 1  are removed from the set of three front inner cam grooves  11   a - 1  in the optical axis direction from the front of the cam ring  11  while the set of three rear cam followers  8   b - 2  are positioned in front open end sections  11   a - 2   x  of the set of three rear inner cam grooves  11   a - 2 , respectively. Therefore, the second lens group moving frame  8  can be removed from the cam ring  11  from the front thereof as shown in FIG.  3 . Since the front open end sections  11   a - 2   x  of the set of three rear inner cam grooves  11   a - 2  are formed as linear grooves extending in the optical axis direction, the second lens group moving frame  8  can be removed from the cam ring  11  from the front thereof regardless of whether the second lens group moving frame  8  is guided linearly in the optical axis direction by the second linear guide ring  10 , i.e., whether or not the set of three front cam followers  8   b - 1  and the set of three rear cam followers  8   b - 2  are engaged in the set of three front inner cam grooves  11   a - 1  and the set of three rear inner cam grooves  11   a - 2 , respectively. In the state shown in  FIG. 58  in which the cam ring  11  and the second linear guide ring  10  remain inside the first linear guide ring  14 , only the second lens group moving frame  8  can be removed. 
   The pivot shaft  33  and the second lens frame  6  can be removed from the second lens group moving frame  8  after the set screws  66  are unscrewed to remove the pair of second lens frame support plates  36  and  37  (see FIG.  3 ). 
   Aside from the elements positioned inside the cam ring  11 , the helicoid ring  18  can be removed from the stationary barrel  22 . In this case, after the CCD holder  21  is removed from the stationary barrel  22 , the helicoid ring  18  is rotated in the lens barrel retracting direction from the assembling/disassembling angular position to be removed from the stationary barrel  22 . This rotation of the helicoid ring  18  in the lens barrel retracting direction causes the set of three rotational sliding projections  18   b  to move back into the set of three inclined grooves  22   c  from the set of three rotational sliding grooves  22   d  so that the male helicoid  18   a  is engaged with the female helicoid  22   a , thus causing the helicoid ring  18  to move rearward while rotating about the lens barrel axis Z 0 . Upon the helicoid ring  18  moving rearward beyond the position thereof shown in  FIGS. 23 and 27 , the set of three rotational sliding projections  18   b  are respectively removed from the set of three inclined grooves  22   c  from rear open end sections  22   c -x thereof while the male helicoid  18   a  is disengaged from the female helicoid  22   a . Consequently, the helicoid ring  18 , together with the linear guide ring  14 , is removed from the stationary barrel  22  from the rear thereof. 
   The helicoid ring  18  and the linear guide ring  14  are engaged with each other by engagement of the first plurality of relative rotation guide projections  14   b  with the circumferential groove  18   g . Similar to the second plurality of relative rotation guide projections  14   c , the first plurality of relative rotation guide projections  14   b  are formed on the first linear guide ring  14  at irregular intervals in a circumferential direction thereof, and some of the first plurality of relative rotation guide projections  14   b  have different circumferential widths than another ones. The helicoid ring  18  is provided on an inner peripheral surface thereof with a plurality of insertion/removable grooves  18   h  via which the first plurality of relative rotation guide projections  14   b  can enter the helicoid ring  18  (the circumferential groove  18   g ) in the optical axis direction, respectively, only when the first linear guide ring  14  is positioned in a specific rotational position relative to the helicoid ring  18 . 
     FIGS. 48 through 51  show developed views of the first linear guide ring  14  and the helicoid ring  18 , showing the relationship of coupling therebetween in different states. Specifically,  FIG. 48  shows a state of coupling between the first linear guide ring  14  and the helicoid ring  18  when the zoom lens  71  is in the retracted state (which corresponds to the state shown in each of FIGS.  23  and  27 ),  FIG. 49  shows another state of coupling between the first linear guide ring  14  and the helicoid ring  18  when the zoom lens  71  is set at the wide-angle extremity (which corresponds to the state shown in each of FIGS.  24  and  28 ),  FIG. 50  shows the same when the zoom lens  71  is set at the telephoto extremity as shown in  FIGS. 25 and 29 , and  FIG. 51  shows another state of coupling between the first linear guide ring  14  and the helicoid ring  18  when the zoom lens  71  is in the assemblable/disassemblable state (which corresponds to the state shown in each of FIGS.  26  and  30 ). As can be seen from  FIGS. 48 through 51 , when the zoom lens  71  is in between the retracted position and the position in the assemblable/disassemblable state, in which the third external barrel  15  and the helicoid ring  18  are positioned in the respective assembling/disassembling angular positions as shown in  FIGS. 26 and 63 , all of the first plurality of relative rotation guide projections  14   b  cannot be inserted into or removed from the plurality of insertion/removable grooves  18   h  at the same time, respectively, which makes it impossible to disengage the helicoid ring  18  and the first linear guide ring  14  from each other in the optical axis direction. All the first plurality of relative rotation guide projections  14   b  can be inserted into or removed from the plurality of insertion/removable grooves  18   h  at the same time, respectively, only when the helicoid ring  18  is further rotated in the lens barrel retracting direction (downwards as viewed in  FIG. 48 ) to a specific rotational position beyond the retracted position of the helicoid ring  18  shown in FIG.  48 . After the helicoid ring  18  has been rotated to the specific rotational position, moving the helicoid  18  forward (leftward as viewed in  FIGS. 48 through 51 ) with respect to the first linear guide ring  14  causes the first plurality of relative rotation guide projections  14   b  to be removed from the plurality of insertion/removable grooves  18   h  to the rear of the circumferential groove  18   g , respectively. Alternatively, it is possible to modify the structure coupling between the first linear guide ring  14  and the helicoid ring  18  so that all the first plurality of relative rotation guide projections  14   b  can pass the helicoid ring  18  in the optical axis direction through the plurality of insertion/removable grooves  18   h  at the same time when the helicoid ring  18  and the linear guide ring  14  are positioned at the aforementioned respective rotational positions at which the helicoid ring  18  and the linear guide ring  14  can be removed from the stationary barrel  22 . 
   The second plurality of relative rotation guide projections  14   c , which are engaged in the circumferential groove  15   e  of the third external barrel  15 , are formed in front of the first plurality of relative rotation guide projections  14   b  on first linear guide ring  14  in the optical axis direction. As described above, the first plurality of relative rotation guide projections  14   b  are formed as circumferentially elongated projections at different circumferential positions on the first linear guide ring  14  while the second plurality of relative rotation guide projections  14   c  are formed as circumferentially elongated projections at different circumferential positions on the first linear guide ring  14 . More specifically, although the respective positions of the first plurality of relative rotation guide projections  14   b  are not coincident with those of the second plurality of relative rotation guide projections  14   c  in a circumferential direction of the first linear guide ring  14 , the first plurality of relative rotation guide projections  14   b  and the second plurality of relative rotation guide projections  14   c  are the same as each other in the number of projections, intervals of projections, and circumferential widths of corresponding projections as shown in FIG.  15 . Namely, there is a specific relative rotational position between the second plurality of relative rotation guide projections  14   c  and the plurality of insertion/removable grooves  18   h , in which the second plurality of relative rotation guide projections  14   c  and the plurality of insertion/removable grooves  18   h  can be disengaged from each other in the optical axis direction. If the helicoid ring  18  is moved forward from the first linear guide ring  14  in a state where the second plurality of relative rotation guide projections  14   c  and the plurality of insertion/removable grooves  18   h  are in such a specific relative rotational position, each relative rotation guide projections  14   c  can be inserted into the corresponding insertion/removable groove  18   h  from the front end thereof and subsequently removed from the same insertion/removable groove  18   h  from the rear end thereof so that the helicoid ring  18  can be removed from the first linear guide ring  14  from the front thereof. Accordingly, the front and rear ends of each insertion/removable groove  18   h  are respectively formed as open ends so that the associated relative rotation guide projections  14   c  can pass the helicoid ring  18  in the optical axis direction through the insertion/removable groove  18   h.    
   Namely, the helicoid ring  18  and the first linear guide ring  14  are not in a disengagable state until the helicoid ring  18  and the first linear guide ring  14  are removed from the stationary barrel  22  and relatively rotated by a predetermined amount of rotation. In other words, when disassembling the third external barrel  15 , the helicoid ring  18  and the first linear guide ring  14  are mutually engaged with each other while being supported inside the stationary barrel  22 . The assembly process is accordingly facilitated by disallowing the first linear guide ring  14  from being disengaged. 
   As can be understood from the foregoing, in the present embodiment of the zoom lens, the third external barrel  15 , which performs the rotating-advancing/rotating-retracting operation and the fixed-position rotating operation, can be easily removed from the zoom lens  71  by rotating the third external barrel  15  and the helicoid ring  18  together to the respective assembling/disassembling angular positions as shown in  FIGS. 26 and 63 , which are different from any of their respective positions in either of the zooming range and the retracting range, after the stop member  26  has been removed from the stationary barrel  22 . Moreover, a function of the three rotational sliding projections  18   b  for removing backlash between the third external barrel  15  and the stationary barrel  22  and backlash between the helicoid ring  18  and the stationary barrel  22  can be cancelled by removing the third external barrel  15  from the zoom lens  71 . Furthermore, when the zoom lens  71  is in the assemblable/disassemblable state, in which the third external barrel  15  can be inserted into or removed from the zoom lens  71 , the second external barrel  13 , the first external barrel  12 , the cam ring  11 , the second lens group moving frame  8  and other elements are also positioned at their respective assembling/disassembling positions to become removable from the zoom lens  71  one after another after the third external barrel  15  is removed from the zoom lens  71 . This results in an improvement in workability of disassembling the zoom lens  71 . 
   Although only a disassembling procedure of the zoom lens  71  has been discussed above, a reverse procedure to the above disassembling procedure can be performed as an assembling procedure of the zoom lens  71 . This also results in an improvement in workability of assembling the zoom lens  71 . 
   Another feature of the zoom lens  71  which is associated with the third external barrel  15  (and also the helicoid ring  18 ) will be hereinafter discussed with reference mainly to  FIGS. 60 through 72 . In  FIGS. 60 through 63 , some portions of the linear guide ring  14  and the third external barrel  15 , and the follower-biasing ring spring  17  for biasing the set of three roller followers  32  would not normally be visible (i.e., are supposed to be shown by hidden lines), but are shown by solid lines for the purpose of illustration.  FIGS. 64 through 66  show portions of the third external barrel  15  and the helicoid ring  18 , viewed from the inside thereof, and accordingly the direction of inclination of, e.g. the inclined lead slot portion  14   e - 3  appeared in  FIGS. 64 and 65 , is opposite to that shown in the other Figures. 
   As can be understood from the above descriptions, in the present embodiment of the zoom lens  71 , a rotatable barrel positioned immediately inside the stationary barrel  22  (namely, the first rotatable barrel when viewed from the side of the stationary barrel  22 ) is divided into two parts: the third external barrel  15  and the helicoid ring  18 . In the following descriptions, the third external barrel  15  and the helicoid ring  18  are referred to as a rotatable barrel KZ in some cases for clarity (e.g., see  FIGS. 23 through 26 ,  60  through  62 ). The function of the rotatable barrel KZ is to impart motion to the set of three roller followers  32  to rotate the set of three roller followers  32  about the lens barrel axis Z 0 . The cam ring  11  receives force, which makes the cam ring  11  rotate about the lens barrel axis Z 0  while moving in the optical axis direction, via the set of three roller followers  32  to move the first and second lens groups LG 1  and LG 2  in the optical axis direction in a predetermined moving manner. Engaging portions of the rotatable barrel KZ which are engaged with the set of three roller followers  32 , i.e., the set of three rotation transfer grooves  15   f  satisfy some conditions which will be hereinafter discussed. 
   First of all, the set of three rotation transfer grooves  15   f , in which the set of three roller followers  32  are engaged, need to have lengths corresponding to the range of movement of the set of three roller followers  32  in the optical axis direction. This is because each roller follower  32  is not only rotated about the lens barrel axis Z 0  between a retracted position shown in  FIG. 60 and a  position shown in  FIG. 62  which corresponds to the telephoto extremity of the zoom lens  71  via a position shown in  FIG. 61  which corresponds to the wide-angle extremity of the zoom lens  71 , but also moved in the optical axis direction relative to the rotatable barrel KZ by the associated inclined lead slot portion  14   e - 3  of the first linear guide ring  14 . 
   The third external barrel  15  and the helicoid ring  18  substantially operate as a one-piece rotatable barrel: the rotatable barrel KZ. This is because the third external barrel  15  and the helicoid ring  18  are prevented from rotating relative to each other by engagement of the three pairs of rotation transfer projections  15   a  with the three rotation transfer recesses  18   d , respectively. However, in the present embodiment of the zoom lens, since the third external barrel  15  and the helicoid ring  18  are provided as separate members for the purpose of assembling and disassembling the zoom lens  71 , there is provided a slight clearance between each pair of rotation transfer projections  15   a  and the associated rotation transfer recess  18   d  in a rotational direction (vertical direction as viewed in FIG.  66 ). More specifically, as shown in  FIG. 66 , the three pairs of rotation transfer projections  15   a  and the three rotation transfer recesses  18   d  are formed so that a circumferential space WD 1  between circumferentially-opposed two side surfaces  18   d -S of the helicoid ring  18  in each rotation transfer recess  18   d  that extend parallel to each other becomes slightly greater than a circumferential space WD 2  between opposite end surfaces  15   a -S of the associated pair of rotation transfer projections  15   a  that also extend parallel to each other. Due to this clearance, the third external barrel  15  and the helicoid ring  18  slightly rotate relative to each other about the lens barrel axis Z 0  when one of the third external barrel  15  and the helicoid ring  18  is rotated about the lens barrel axis Z 0  relative to the other. For instance, in the state shown in  FIG. 64 , if the helicoid ring  18  is rotated in the lens barrel advancing direction shown by an arrow AR 1  in  FIG. 65  (downwards as viewed in  FIGS. 64 and 65 ) with respect to the third external barrel  15 , the helicoid ring  18  rotates in the same direction by an amount of rotation “NR” with respect to the third external barrel  15  so that one of the circumferentially-opposed two side surfaces  18   d -S in each rotation transfer recess  18   d  comes into contact with corresponding one of the opposite end surfaces  15   a -S of the associated pair of rotation transfer projections  15   a  as shown in FIG.  65 . Therefore, the set of three rotation transfer grooves  15   f  must be formed on the third external barrel  15  to be capable of guiding the set of three roller followers  32  smoothly in the optical axis direction at all times regardless of the presence or absence of a variation in the relative rotational position between the third external barrel  15  and the helicoid ring  18  that is caused by the presence of the clearance between each pair of rotation transfer projections  15   a  and the associated rotation transfer recess  18   d . This clearance is exaggerated in the drawings for the purpose of illustration. 
   In the present embodiment of the zoom lens, the three pairs of rotation transfer projections  15   a  that extend rearward in the optical axis direction are formed on the third external barrel  15  as engaging portions thereof for engaging the third external barrel  15  with the helicoid ring  18 . This structure of the three pairs of rotation transfer projections  15   a  has been fully utilized for the formation of the set of three rotation transfer grooves  15   f  on the third external barrel  15 . More specifically, the major potion of each rotation transfer groove  15   f  is formed on an inner peripheral surface of the third external barrel  15  so that the circumferential positions of the three rotation transfer grooves  15   f  correspond to those of the three pairs of rotation transfer projections  15   a , respectively. In addition, the remaining rear end portion of each rotation transfer groove  15   f  is elongated rearward in the optical axis direction to be formed between opposed guide surfaces  15   f -S (see  FIG. 66 ) of the associated pair of rotation transfer projections  15   a.    
   No gaps or steps are formed in each rotation transfer groove  15   f  because each rotation transfer groove  15   f  is formed only on the third external barrel  15 , not formed as a groove extending over the third external barrel  15  and the helicoid ring  18 . Even if the relative rotational position between the third external barrel  15  and the helicoid ring  18  slightly varies due to the clearance between each pair of rotation transfer projections  15   a  and the associated rotation transfer recess  18   d , the opposed guide surfaces  15   f -S of each rotation transfer groove  15   f  remain invariant in shape. Therefore, the set of three rotation transfer grooves  15   f  are capable of guiding the set of three roller followers  32  smoothly in the optical axis direction at all times. 
   The set of three rotation transfer grooves  15   f  can be formed to have sufficient lengths in the optical axis direction by making most of the three pairs of rotation transfer projections  15   a  that project in the optical axis direction, respectively. As shown in  FIGS. 60 through 62 , a range of movement D 1  of the set of three roller followers  32  in the optical axis direction (see  FIG. 60 ) is greater than an axial length D 2  of an area on the inner peripheral surface of the third external barrel  15  (except for the three pairs of rotation transfer projections  15   a ) in the optical axis direction on which grooves extending in the optical axis direction can be formed. Specifically, in the state shown in  FIGS. 60 and 64  in which the zoom lens  71  is in the retracted state as shown in  FIG. 10 , each roller follower  32  has moved rearward to a point (retracted point) between the front and rear ends of the helicoid ring  18  in the optical axis direction. However, since each pair of rotation transfer projections  15   a  extends rearward to a point corresponding to the retracted point between the front and rear ends of the helicoid ring  18  in the optical axis direction because the three pairs of rotation transfer projections  15   a  need to remain engaged in the three rotation transfer recesses  18   d , respectively, the engagement of the set of three roller followers  32  with the set of three rotation transfer grooves  15   f  is maintained even if the set of three roller followers  32  are moved rearward to the respective retracted points. Accordingly, the set of three roller followers  32  can be guided in the optical axis direction in a range of movement extending over the third external barrel  15  and the helicoid ring  18  even if guiding portions (the set of three rotation transfer grooves  15   f ) which are engaged with the set of three roller followers  32  (to guide the set of three roller followers  32 ) are formed only on the third external barrel  15  of the rotatable barrel KZ. 
   Even though the circumferential groove  15   e  intersects each rotation transfer groove  15   f  on the inner peripheral surface of the third external barrel  15 , the circumferential groove  15   e  does not deteriorate the guiding function of the set of three rotation transfer grooves  15   f  because the depth of the circumferential groove  15   e  is smaller than that of each rotation transfer groove  15   f.    
     FIGS. 67 and 68  show a comparative example which is to be compared with the above described structure shown mainly in  FIGS. 64 through 66 . In this comparative example, a front ring  15 ′ (which corresponds to the third external barrel  15  of the present embodiment of the zoom lens) is provided with a set of three rotation transfer grooves  15   f ′ (only one of them is shown in  FIGS. 67 and 68 ) extending linearly in the optical axis direction, while a rear ring  18 ′ (which corresponds to the helicoid ring  18  of the present embodiment of the zoom lens) is provided with a set of three extension grooves  18   x  extending linearly in the optical axis direction. A set of three roller followers  32 ′ (which corresponds to the set of three roller followers  32  of the present embodiment of the zoom lens  71 ) are engaged in the set of three rotation transfer grooves  15   f ′ or the set of three extension grooves  18   x  so that each roller follower  32 ′ can move in the associated rotation transfer groove  15   f ′ and the associated extension groove  18   x  in the optical axis direction. Namely, the set of three roller followers  32 ′ are respectively movable in a set of three grooves extending over the front ring  15 ′ and the rear ring  18 ′. The front ring  15 ′ and the rear ring  18 ′ are engaged with each other via a plurality of rotation transfer projections  15   a ′ of the front ring  15 ′ and a corresponding plurality of rotation transfer grooves  18   d ′ of the rear ring  18 ′ in which the plurality of rotation transfer projections  15   a ′ are respectively engaged. The plurality of rotation transfer projections  15   a ′ are formed on a rear end surface of the front ring  15 ′ which faces a front surface of the rear ring  18 ′, while the plurality of rotation transfer grooves  18   d ′ are formed on the front surface of the rear ring  18 ′. There is a slight clearance between the plurality of rotation transfer projections  15   a ′ and the plurality of rotation transfer grooves  18   d ′ in a rotational direction (vertical direction as viewed in FIG.  68 ).  FIG. 67  shows a state where the set of three rotation transfer grooves  15   f ′ and the set of three extension grooves  18   x  are precisely aligned in the optical axis direction. 
   In the comparative example having the above described structure, in the state shown in  FIG. 67 , if the front ring  18 ′ is rotated in a direction shown by an arrow AR 1 ′ in  FIG. 68  (downwards as viewed in  FIGS. 67 and 68 ) with respect to the rear ring  18 ′, the rear ring  18 ′ slightly rotates in the same direction due to the aforementioned clearance between the plurality of rotation transfer projections  15   a ′ and the plurality of rotation transfer grooves  18   d ′. This causes a misalignment between the set of three rotation transfer grooves  15   f ′ and the set of three extension grooves  18   x . Therefore, in the state shown in  FIG. 68 , a gap is produced between a guide surface of each rotation transfer groove  15   f ′ and a corresponding guide surface of the associated extension groove  18   x . This gap may interfere with a movement of each roller follower  32 ′ in the associated rotation transfer groove  15   f ′ and the associated extension groove  18   x  in the optical axis direction, which cannot ensure a smooth movement of each roller follower  32 ′. If the gap becomes large, each roller follower  32 ′ may not be able to move between the associated rotation transfer groove  15   f ′ and the associated extension groove  18   x  across a border therebetween. 
   Supposing either the set of rotation transfer grooves  15   f ′ or the set of extension grooves  18   x  is omitted to prevent such an undesirable gap from being produced between a guide surface of each rotation transfer groove  15   f ′ and a corresponding guide surface of the associated extension groove  18   x , the other set of rotation transfer grooves  15   f ′ or extension grooves  18   x  may need to be elongated in the optical axis direction. Consequently, the length of either the front ring  15 ′ or the rear ring  18 ′ in the optical axis direction will increase. For instance, if it is desired to omit the set of extension grooves  18   x , each rotation transfer groove  15   f ′ must be elongated forward by a length corresponding to the length of each extension groove  18   x . This increases the dimensions of the zoom lens, specifically the length thereof. 
   In contrast to this comparative example, the present embodiment of the zoom lens, in which the three pairs of rotation transfer projections  15   a  that extend rearward in the optical axis direction are formed on the third external barrel  15  as engaging portions thereof for engaging the third external barrel  15  with the helicoid ring  18 , has the advantage that the set of three rotation transfer grooves  15   f  are respectively capable of guiding the set of three roller followers  32  smoothly in the optical axis direction at all times without any gaps being produced in the set of three rotation transfer grooves  15   f . Moreover, the present embodiment of the zoom lens has the advantage that each rotation transfer groove  15   f  can be formed to have a sufficient effective length without the third external barrel  15  being elongated forward in the optical axis direction. 
   Exerting a force to the set of three roller followers  32  in a direction to rotate the same about the lens barrel axis Z 0  via the set of three rotation transfer grooves  15   f  causes the cam ring  11  to rotate about the lens barrel axis Z 0  while rotating in the optical axis direction due to engagement of the set of three roller followers  32  with the lead slot portions  14   e - 3  of the set of three through-slots  14   e , respectively, when the zoom lens  71  is set in between the wide-angle extremity and the retracted position. When the zoom lens  71  is in the zooming range, the cam ring  11  rotates at the axial fixed position without moving in the optical axis direction due to engagement of the set of three roller followers  32  with the front circumferential slot portions  14   e - 1  of the set of three through-slots  14   e , respectively. Since the cam ring  11  rotates at the axial fixed position in the ready-to-photograph state of the zoom lens  71 , the cam ring  11  must be positioned precisely at a predetermined position in the optical axis direction to insure optical accuracy of movable lens groups of the zoom lens  71  such as the first lens group LG 1  and the second lens group LG 2 . Although the position of the cam ring  11  in the optical axis direction when the cam ring  11  rotates at the axial fixed position thereof is determined by the engagement of the set of three roller followers  32  with the front circumferential slot portions  14   e - 1  of the set of three through-slots  14   e , respectively, a clearance is provided between the set of three roller followers  32  and the front circumferential slot portions  14   e - 1  so that the set of three roller followers  32  can smoothly move in the front circumferential slot portions  14   e - 1  of the set of three through-slots  14   e , respectively. Accordingly, it is necessary to remove backlash between the set of three roller followers  32  and the set of three through-slots  14   e  which is caused by the clearance when the set of three roller followers  32  are engaged in the front circumferential slot portions  14   e - 1  of the set of three through-slots  14   e , respectively. 
   The follower-biasing ring spring  17  for removing the backlash is positioned inside the third external barrel  15 , and a structure supporting the follower-biasing ring spring  17  is shown in  FIGS. 33 ,  35 ,  63  and  69  through  72 . The frontmost inner flange  15   h  is formed on the third external barrel  15  to extend radially inwards from a front end of the inner peripheral surface of the third external barrel  15 . As shown in  FIG. 63 , the follower-biasing ring spring  17  is a non-flat annular member which is provided with a plurality of bends which are bent in the optical axis direction to be resiliently deformable in the optical axis direction. More specifically, the follower-biasing ring spring  17  is disposed so that the set of three follower pressing protrusions  17   a  are positioned at the rear end of the follower-biasing ring spring  17  in the optical axis direction. The follower-biasing ring spring  17  is provided with a set of three forwardly-projecting arc portions  17   b  which project forward in the optical axis direction. The three forwardly-projecting arc portions  17   b  and the three follower pressing protrusions  17   a  are alternately arranged to form the follower-biasing ring spring  17  as shown in  FIGS. 4 ,  14  and  63 . The follower-biasing ring spring  17  is disposed between the frontmost inner flange  15   h  and the plurality of relative rotation guide projections  15   d  in a slightly compressed state so as not to come off the third external barrel  15  from the inside thereof. If the set of three forwardly-projecting arc portions  17   b  are installed between the frontmost inner flange  15   h  and the plurality of relative rotation guide projections  15   d  with the set of three follower pressing protrusions  17   a  and the set of three rotation transfer grooves  15   f  being aligned in the optical axis direction, the set of three follower pressing protrusions  17   a  are engaged in respective front portions of the set of three rotation transfer grooves  15   f  to be supported thereby. When the first linear guide ring  14  is not attached to the third external barrel  15 , each follower pressing protrusion  17   a  is sufficiently apart from the frontmost inner flange  15   h  of the third external barrel  15  in the optical axis direction as clearly shown in  FIG. 72  to be movable to a certain degree in the associated rotation transfer groove  15   f.    
   When the first linear guide ring  14  is attached to the third external barrel  15 , the set of three forwardly-projecting arc portions  17   b  of the follower-biasing ring spring  17  are deformed by being pressed forward, toward the frontmost inner flange  15   h , by the front end of the linear guide ring  14  to make the shape of the set of three forwardly-projecting arc portions  17   b  become close to a flat shape. When the follower-biasing ring spring  17  is deformed in such a manner, the first linear guide ring  14  is biased rearward by the resiliency of the follower-biasing ring spring  17  to thereby fix the position of the first linear guide ring  14  with respect to the third external barrel  15  in the optical axis direction. At this time, a front guide surface in the circumferential groove  14   d  of the first linear guide ring  14  is pressed against respective front surfaces of the plurality of relative rotation guide projections  15   d , while respective rear surfaces of the second plurality of relative rotation guide projections  14   c  are pressed against a rear guide surface in the circumferential groove  15   e  of the third external barrel  15  in the optical axis direction, as clearly shown in FIG.  69 . At the same time, the front end of the first linear guide ring  14  is positioned between the frontmost inner flange  15   h  and the plurality of relative rotation guide projections  15   d  in the optical axis direction, while front surfaces the set of three forwardly-projecting arc portions  17   b  of the follower-biasing ring spring  17  are not entirely in pressing contact with the frontmost inner flange  15   h . Therefore, when the zoom lens  71  is in the retracted state, a slight space is secured between the set of three follower pressing protrusions  17   a  and the frontmost inner flange  15   h  so that each follower pressing protrusion  17   a  can move to a certain extent in the associated rotation transfer groove  15   f  in the optical axis direction. In addition, as shown in  FIGS. 35 and 69 , each follower pressing protrusion  17   a  which extends rearward that the tip thereof (rear end thereof in the optical axis direction) is positioned inside the front circumferential slot portion  14   e - 1  of the associated radial slot  14 . 
   In the state shown in  FIGS. 60 and 64  in which the zoom lens  71  is in the retracted state, the follower-biasing ring spring  17  do not contact with any elements other than the first linear guide ring  14 . At this time, although engaged in the set of three rotation transfer grooves  15   f , the set of three roller followers  32  stay away from the set of three follower pressing protrusions  17   a , respectively, because each roller follower  32  is engaged in the associated rear circumferential slot portion  14   e - 2  to be positioned in the vicinity of the rear end thereof. 
   Rotating the third external barrel  15  in the lens barrel advancing direction (upwards as viewed in FIGS.  60  and  69 ) causes the set of three rotation transfer groove  15   f  to push the set of three roller followers  32  upwards as viewed in  FIGS. 60 and 69 , respectively, to move each roller follower  32  in the associated through-slots  14   e  from the rear circumferential slot portion  14   e - 2  to the inclined lead slot portion  14   e - 3 . Since the inclined lead slot portion  14   e - 3  of each through-slot  14   e  extends in a direction having both a component in a circumferential direction of the first linear guide ring  14  and a component in the optical axis direction, each roller follower  32  gradually moves forward in the optical axis direction as the roller follower  32  moves in the inclined lead slot portion  14   e - 3  of the associated through-slot  14   e  toward the front circumferential slot portion  14   e - 1 . However, as long as the roller follower  32  is in the inclined lead slot portion  14   e - 3  of the associated through-slot  14   e , the roller follower  32  is still away from the associated pressing protrusion  17   a . This means that the set of three roller followers  32  are not at all biased by the set of three follower pressing protrusions  17   a , respectively. Nevertheless, no substantial problem arises even if backlash between the set of three roller followers  32  and the set of three through-slots  14   e  are removed thoroughly since the zoom lens  71  is in the retracted state or the transitional state from the retracted state to the ready-to-photograph state when each roller follower  32  is engaged in the rear circumferential slot portion  14   e - 2  or the inclined lead slot portion  14   e - 3  of the associated through-slot  14   e , respectively. If anything, the load on the zoom motor  150  decreases with decrease in frictional resistance to each roller follower  32 . 
   If the set of three roller followers  32  move from the inclined lead slot portions  14   e - 3  of the set of three through-slots  14   e  to the front circumferential slot portions  14   e - 1  of the same, respectively, by a further rotation of the third external barrel  15  in the lens barrel advancing direction, the first linear guide ring  14 , the third external barrel  15  and the set of three roller followers  32  are positioned as shown in  FIGS. 61 and 70  so that the zoom lens  71  is set at the wide-angle extremity. Since the tip of each follower pressing protrusion  17   a  is positioned inside the front circumferential slot portion  14   e - 1  of the associated radial slot  14  as described above, each roller follower  32  comes into contact with the associated follower pressing protrusion  17   a  upon entering the associated front circumferential slot portion  14   e - 1  (see  FIGS. 33 ,  61  and  70 ). This causes each follower pressing protrusion  17   a  to be pressed forward in the optical axis direction by the associated roller follower  32 , thus causing the follower-biasing ring spring  17  to be further deformed to make the shape of the set of three forwardly-projecting arc portions  17   b  become closer to a flat shape. At this time, each roller follower  32  is pressed against a rear guide surface in the associated front circumferential slot portion  14   e - 1  in the optical axis direction by the resiliency of the follower-biasing ring spring  17  to thereby remove backlash between the set of three roller followers  32  and the set of three through-slots  14   e , respectively. 
   Thereafter, even if the set of three roller followers  32  move in the front circumferential slot portions  14   e - 1  of the set of three through-slots  14   e  during a zooming operation between the positions shown in  FIGS. 61 and 70  in which the zoom lens  71  is set at the wide-angle extremity and the positions shown in  FIGS. 62 and 71  in which the zoom lens  71  is set at the telephoto extremity, each roller follower  32  remains in contact with the associated follower pressing protrusion  17   a  because each roller follower  32  does not move in the associated rotation transfer groove  15   f  in the optical axis direction when moving in the associated front circumferential slot portion  14   e - 1  that extend only in a circumferential direction of the first linear guide ring  14 . Therefore, in the zooming range of the zoom lens  71  in which photographing is possible, the set of three roller followers  32  are always biased rearward in the optical axis direction by the roller spring  17 , which achieves a stable positioning of the set of three roller followers  32  with respect to the first linear guide ring  14 . 
   Rotating the third external barrel  15  in the lens barrel retracting direction causes the first linear guide ring  14  and the set of three roller followers  32  to operate in the reverse manner to the above described operations. In this reverse operation, each roller follower  32  is disengaged from the associated follower pressing protrusion  17   a  upon passing a point (wide-angle extremity point) in the associated through-slot  14   e  which corresponds to the wide-angle extremity of the zoom lens  71  (the position of each roller follower  32  in the associated through-slot  14   e  in FIG.  61 ). From the wide-angle extremity point down to a point (retracted point) in the associated through-slot  14   e  which corresponds to the retracted position of the zoom lens  71  (the position of each roller follower  32  in the associated through-slot  14   e  in FIG.  60 ), the set of three roller followers  32  receive no pressure from the set of three follower pressing protrusions  17   a , respectively. If the set of three follower pressing protrusions  17   a  do not apply any pressure to the set of three roller followers  32 , the frictional resistance to each roller follower  32  becomes small when moving in the associated through-slot  14   e . Consequently, the load on the zoom motor  150  decreases with decrease in frictional resistance to each roller follower  32 . 
   As can be understood from the above descriptions, the set of three follower pressing protrusions  17   a , which are respectively fixed at the locations of the set of three roller followers  32  in the optical axis direction in the set of three rotation transfer grooves  15   f  when the zoom lens  71  is in the ready-to-photograph state, automatically bias the set of three roller followers  32  rearward to press the set of three roller followers  32  against rear guide surfaces of the front circumferential slot portions  14   e - 1  of the set of three through-slots  14   e  immediately after the set of three roller followers  32  which are guided by the inclined lead slot portions  14   e - 3  of the set of three through-slots  14   e  to move forward in the optical axis direction reach their respective photographing positions in a rotatable range at an axial fixed position (i.e., in the front circumferential slot portions  14   e - 1 ). With this structure, the backlash between the set of three roller followers  32  and the set of three through-slots  14   e  can be removed by a simple structure using a single biasing member: the follower-biasing ring spring  17 . Moreover, the follower-biasing ring spring  17  consumes little space in the zoom lens  71  since the follower-biasing ring spring  17  is a substantially simple annular member disposed along an inner peripheral surface and since the set of three follower pressing protrusions  17   a  are positioned in the set of three rotation transfer grooves  15   f , respectively. Accordingly, in spite of its small and simple structure, the follower-biasing ring spring  17  cam make the cam ring  11  positioned precisely at a predetermined fixed position in the optical axis direction with stability in the ready-to-photograph state of the zoom lens  71 . This insures optical accuracy of the photographing optical system such as the first lens group LG 1  and the second lens group LG 2 . Furthermore, the follower-biasing ring spring  17  can be removed easily because the set of three forwardly-projecting arc portions  17   b  are simply held and supported between the frontmost inner flange  15   h  and the plurality of relative rotation guide projections  15   d.    
   The follower-biasing ring spring  17  has not only a function of biasing the set of three roller followers  32  rearward in the optical axis direction to position the cam ring  11  precisely with respect to the first linear guide ring  14  in the optical axis direction, but also a function of biasing the first linear guide ring  14  rearward in the optical axis direction to give stability to positioning of the first linear guide ring  14  with respect to the third external barrel  15  in the optical axis direction. Although the second plurality of relative rotation guide projections  14   c  and the circumferential groove  15   e  are engaged with each other to be slightly movable relative to each other in the optical axis direction while the plurality of relative rotation guide projections  15   d  and the circumferential groove  14   d  are engaged with each other to be slightly movable relative to each other in the optical axis direction as shown in  FIGS. 69 through 72 , both backlash between the second plurality of relative rotation guide projections  14   c  and the circumferential groove  15   e  and backlash between the plurality of relative rotation guide projections  15   d  and the circumferential groove  14   d  are removed since the front end of the first linear guide ring  14  contacts with the follower-biasing ring spring  17  to be biased rearward in the optical axis direction by the follower-biasing ring spring  17 . Accordingly, in the case where three annular members: the cam ring  11 , the first linear guide ring  14  and the third external barrel  15  are regarded as a rotating-advancing/rotating-retracting unit, all the different backlashes arising in this whole rotating-advancing/rotating-retracting unit can be removed by a single biasing member: the follower-biasing ring spring  17 . This achieves a quite simple backlash removing structure. 
     FIGS. 73 through 75  show elements of a linear guide structure in section which guides the first external barrel  12  (which supports the first lens group LG 1 ) and the second lens group moving frame  8  (which supports the second lens group LG 2 ) linearly in the optical axis direction without rotating each of the first external barrel  12  and the second lens group moving frame  8  about the lens barrel axis Z 0 .  FIGS. 76 through 78  show the elements of the linear guide structure in oblique perspective.  FIGS. 73 ,  74  and  75  show the linear guide structure when the zoom lens  71  is set at the wide-angle extremity, when the zoom lens  71  is set at the telephoto extremity, and when the zoom lens  71  is in the retracted state, respectively. In each of the cross sectional views in  FIGS. 73 through 75 , the elements of the linear guide structure are crosshatched for the purpose of illustration. In addition, in each of the cross sectional views in  FIGS. 73 through 75 , among all the rotatable elements only the cam ring is crosshatched by dashed lines for the purpose of illustration. 
   The cam ring  11  is a double-side grooved cam ring that is provided on an outer peripheral surface thereof with the set of three outer cam grooves  11   b  for moving the first external barrel  12  in a predetermined moving manner, and that is provided on an inner peripheral surface of the cam ring  11  with the plurality of inner cam grooves  11   a  ( 11   a - 1  and  11   a - 2 ) for moving the second lens group moving frame  8  in a predetermined moving manner. Accordingly, the first external barrel  12  is positioned radially outside the cam ring  11  while the second lens group moving frame  8  is positioned radially inside the cam ring  11 . On the other hand, the first linear guide ring  14 , which is adopted for guiding each of the first external barrel  12  and the second lens group moving frame  8  linearly without rotating each of the first external barrel  12  and the second lens group moving frame  8  about the lens barrel axis Z 0 , is positioned radially outside the cam ring  11 . 
   In this linear guide structure having the above described positional relationship among the first linear guide ring  14 , the first external barrel  12  and the second lens group moving frame  8 , the first linear guide ring  14  directly guides the second external barrel  13  (which serves as a linear guide member for guiding the first external barrel  12  linearly in the optical axis direction without rotating the same about the lens barrel axis Z 0 ) and the second linear guide ring  10  (which serves as a linear guide member for guiding the second lens group moving frame  8  linearly in the optical axis direction without rotating the same about the lens barrel axis Z 0 ) linearly in the optical axis direction without rotating the same about the lens barrel axis Z 0 . The second external barrel  13  is positioned radially between the cam ring  11  and the first linear guide ring  14 , and guided linearly in the optical axis direction without rotating about the lens barrel axis Z 0  by engagement of the set of six radial projections  13   a , which are formed on an outer peripheral surface of the second external barrel  13 , with the set of six second linear guide grooves  14   g , respectively. Moreover, the second external barrel  13  guides the first external barrel  12  linearly in the optical axis direction without rotating the same about the lens barrel axis Z 0  by engagement of the set of three linear guide grooves  13   b , which are formed on an inner peripheral surface of the second external barrel  13 , with the set of three engaging protrusions  12   a  of the first external barrel  12 , respectively. On the other hand, as for the second linear guide ring  10 , to make the first linear guide ring  14  guide the second lens group moving frame  8  that is positioned inside the cam ring  11 , the ring portion  10   b  is positioned behind the cam ring  11 , the set of three bifurcated projections  10   a  are formed to project radially outwards from the ring portion  10   b  to be respectively engaged in the set of three pairs of first linear guide grooves  14   f , and the set of three linear guide keys  10   c  are formed to project forward from the ring portion  10   b  in the optical axis direction to be respectively engaged in the set of three guide grooves  8   a.    
   In the case of a linear guide structure having conditions similar to conditions of the linear guide structure shown in  FIGS. 73 through 75  that two linearly guided outer and inner movable elements (the first external barrel  12  and the second lens group moving frame  8 ) are respectively positioned outside and inside a double-side grooved cam ring (the cam ring  11 ) and that a primary linear guide member (the first linear guide ring  14 ) of the linear guide structure is positioned outside the cam ring, a secondary linear guide member serving as the outer movable element (which corresponds to the second external barrel  13 ) is disposed outside the cam ring, while a linearly guided movable member (which corresponds to the first external barrel  12 ) which is guided linearly in the optical axis direction without rotating by the secondary linear guide member is provided with a set of linear guide portions for guiding a movable member serving as the inner movable element (which corresponds to the second lens group moving frame  8 ) positioned inside the cam ring linearly in the optical axis direction without rotating the same in a conventional zoom lens. In other words, in the linear guide structure of such a conventional zoom lens, each of the aforementioned set of linear guide portions of the outer movable element extend radially inwards from the outside of the cam ring to the inside of the cam ring to be engaged with the inner movable element through a single path. According to this type of conventional linear guide structure, the resistance produced due to linear guiding operations of the outer and inner movable elements of the linear guide structure increases when a relative velocity in the optical axis direction between the two linearly guided movable elements that are respectively positioned outside and inside the cam ring is fast. In addition, since the inner movable element is indirectly guided linearly in the optical axis direction without rotating via the outer movable element, the inner movable element, in particular, is difficult to be guided linearly in the optical axis direction without rotating with a high degree of travel accuracy. 
   In contrast to such a conventional linear guide structure, according to the linear guide structure of the zoom lens  71  shown in  FIGS. 73 through 75 , the aforementioned resistance problem can be prevented from occurring by the structure wherein the second external barrel  13 , which serves as a linear guide member for guiding the first external barrel  12  (positioned outside the cam ring  11 ) linearly in the optical axis direction without rotating the same about the lens barrel axis Z 0 , is engaged with the set of six second linear guide grooves  14   g  while the second linear guide ring  10 , which serves as a linear guide member for guiding the second lens group moving frame  8  (positioned inside the cam ring  11 ) linearly in the optical axis direction without rotating the same about the lens barrel axis Z 0 , is engaged with the set of three pairs of first linear guide grooves  14   f  so that the second external barrel  13  and the second linear guide ring  10  are directly guided by the first linear guide ring  14  through two paths: a first path (inner path) extending from the set of three pairs of first linear guide grooves  14   f  to the set of three bifurcated projections  10   a  and a second path (outer path) extending from the set of six second linear guide grooves  14   g  to the set of six radial projections  13   a . Moreover, the first linear guide ring  14  that directly guides each of the second linear guide ring  10  and the second external barrel  13  linearly at the same time is, in effect, reinforced by the second linear guide ring  10  and the second external barrel  13 . This structure makes it easy for the linear guide structure to secure sufficient strength. 
   Furthermore, each pair of first linear guide grooves  14   f , which are adopted for guiding the second linear guide ring  10  linearly in the optical axis direction without rotating the same about the lens barrel axis Z 0 , are formed by using two opposed side walls between which the associated second linear guide groove  14   g  is formed. This structure is advantageous to make the linear guide structure simple, and does not impair the strength of the first linear guide ring  14  very much. 
   The relationship between the cam ring  11  and the second lens group moving frame  8  will be hereinafter discussed in detail. As described above, the plurality of inner cam grooves  11   a , which are formed on an inner peripheral surface of the cam ring  11 , consist of the set of three front inner cam grooves  11   a - 1  that are formed at different circumferential positions, and the set of three rear inner cam grooves  11   a - 2  that are formed at different circumferential positions behind the set of three front inner cam grooves  11   a - 1  in the optical axis direction. Each rear inner cam groove  11   a - 2  is formed as a discontinuous cam groove as shown in FIG.  17 . All the six cam grooves of the cam ring  11 : the set of three front inner cam grooves  11   a - 1  and the set of three rear inner cam grooves  11   a - 2  trace six reference cam diagrams “VT” having the same shape and size, respectively. Each reference cam diagram VT represents the shape of each cam groove of the set of three front inner cam grooves  11   a - 1  and the set of three rear inner cam grooves  11   a - 2 , and includes a lens-barrel operating section and a lens-barrel assembling/disassembling section, wherein the lens-barrel operating section consists of a zooming section and a lens-barrel retracting section. The lens-barrel operating section serves as a control section which controls movement of the second lens group moving frame  8  with respect to the cam ring  11 , and which is to be distinguished from the lens-barrel assembling/disassembling section that is used only when the zoom lens  71  is assembled or disassembled. The zooming section serves as a control section which controls the movement of the second lens group moving frame  8  with respect to the cam ring  11 , especially from a position of the second lens group moving frame  8  which corresponds to the wide-angle extremity of the zoom lens  71  to another position of the second lens group moving frame  8  which corresponds to the telephoto extremity of the zoom lens  71 , and which is to be distinguished from the lens-barrel retracting section. If each front inner cam groove  11   a - 1  and the rear inner cam groove  11   a - 2  positioned therebehind in the optical axis direction are regarded as a pair, it can be said that the cam ring  11  is provided, at regular intervals in a circumferential direction of the cam ring  11 , with three pairs of inner cam grooves  11   a  for guiding the second lens group LG 2 . 
   As can be seen in  FIG. 17 , the length of an axial range W 1  of the reference cam diagrams VT of the set of three front inner cam grooves  11   a - 1  in the optical axis direction (the horizontal direction as viewed in FIG.  17 ), which is equivalent to an axial range of the reference cam diagrams VT of the set of three rear inner cam grooves  11   a - 2  in the optical axis direction, is greater than a length W 2  of the cam ring  11  in the optical axis direction. The length of the zooming section included in the axial range W 1  of the reference cam diagrams VT of the set of three front inner cam grooves  11   a - 1  (or the rear inner cam grooves  11   a - 2 ) in the optical axis direction is represented by a length W 3  shown in  FIG. 17  which is alone substantially equivalent to the length W 2  of the cam ring  11 . This means that a set of cam grooves each having a sufficient length will not be obtained for the present embodiment of the cam ring  11  if designed according to a conventional method of formation of cam groove wherein a set of long cam grooves which entirely trace a corresponding set of long cam diagrams are formed on a peripheral surface of a cam ring. According to a cam mechanism of the present embodiment of the zoom lens, a sufficient range of movement of the second lens group moving frame  8  in the optical axis direction can be secured without increasing the length of the cam ring  11  in the optical axis direction. The detail of this cam mechanism will be discussed hereinafter. 
   Each front inner cam groove  11   a - 1  does not cover the entire range of the associated reference cam diagram VT while each rear inner cam groove  11   a - 2  does not cover the entire range of the associated reference cam diagram VT either. A range of each front inner cam groove  11   a - 1  which is included in the associated reference cam diagram VT is partly different from a range of each rear inner cam groove  11   a - 2  which is included in the associated reference cam diagram VT. Each reference cam diagram VT can be roughly divided into four sections: first through fourth sections VT 1  through VT 4 . The first section VT 1  extends in the optical axis direction. The second section VT 2  extends from a first inflection point VTh positioned at the rear end of the first section VT 1  to a second inflection point VTm positioned behind the first inflection point VTh in the optical axis direction. The third section VT 3  extends from the second inflection point VTm to a third inflection point VTn positioned in front of the second inflection point VTm in the optical axis direction. The fourth section VT 4  extends from the third inflection point VTn. The fourth section VT 4  is used only when the zoom lens  71  is assembled or disassembled, and is included in both each front inner cam groove  11   a - 1  and each rear inner cam groove  11   a - 2 . Each front inner cam groove  11   a - 1  is formed in the vicinity of the front end of the cam ring  11  not to include the entire part of the first section VT 1  and a part of the second section VT 2 , and is formed to include a front end opening R 1  at an intermediate point of the second section VT 2  so that the front end opening R 1  opens on a front end surface of the cam ring  11 . On the other hand, each rear inner cam groove  11   a - 2  is formed in the vicinity of the rear end of the cam ring  11  not to include adjoining portions of the second section VT 2  and the third section VT 3  on opposite sides of the second inflection point VTm. In addition, each rear inner cam groove  11   a - 2  is formed to include a front end opening R 4  (which corresponds to the aforementioned front open end section  11   a - 2   x ) at the front end of the first section VT 1  so that the front end opening R 4  opens on a front end surface of the cam ring  11 . A missing portion of each front inner cam groove  11   a - 1  which lies on the associated reference cam diagram VT is included in the associated rear inner cam groove  11   a - 2  that is positioned behind the front inner cam groove  11   a - 1  in the optical axis direction, whereas a missing portion of each rear inner cam groove  11   a - 2  which lies on the associated reference cam diagram VT is included in the associated front inner cam groove  11   a - 1  that is positioned in front of the rear inner cam groove  11   a - 2  in the optical axis direction. Namely, if each front inner cam groove  11   a - 1  and the associated rear inner cam groove  11   a - 2  are combined into a single cam groove, this signal cam groove will include the entire part of one reference cam diagram VT. In other words, one of each front inner cam groove  11   a - 1  and the associated rear inner cam groove  11   a - 2  is complemented by the other. The width of each front inner cam groove  11   a - 1  and the width of each rear inner cam groove  11   a - 2  are the same. 
   Meanwhile, as shown in  FIG. 19 , the plurality of cam followers  8   b , which are respectively engaged in the plurality of inner cam grooves  11   a , consist of the set of three front cam followers  8   b - 1  that are formed at different circumferential positions, and the set of three rear cam followers  8   b - 2  that are formed at different circumferential positions behind the set of three front cam followers  8   b - 1  in the optical axis direction, wherein each front cam follower  8   b - 1  and the rear cam follower  8   b - 2  positioned therebehind in the optical axis direction are provided as a pair in a manner similar to each pair of inner cam grooves  11   a . The space between the set of three front cam followers  8   b - 1  and the set of three rear cam followers  8   b - 2  in the optical axis direction is determined so that the set of three front cam followers  8   b - 1  are respectively engaged in the set of three front inner cam grooves  11   a - 1  and so that the set of three rear cam followers  8   b - 2  are respectively engaged in the set of three rear inner cam grooves  11   a - 2 . The diameter of each front cam follower  8   b - 1  and the diameter of each rear cam follower  8   b - 2  are the same. 
     FIG. 79  shows the positional relationship between the plurality of inner cam grooves  11   a  and the plurality of cam followers  8   b  when the zoom lens  71  is the retracted state as shown in FIG.  10 . When the zoom lens  71  is the retracted state, each front cam follower  8   b - 1  is positioned in the associated front inner cam groove  11   a - 1  in the vicinity of the third inflection point VTn thereof while each rear cam follower  8   b - 2  is positioned in the associated rear inner cam groove  11   a - 2  in the vicinity of the third inflection point VTn thereof. Since each front inner cam groove  11   a - 1  includes a portion thereof in the vicinity of the third inflection point VTn while each rear inner cam groove  11   a - 2  includes a portion thereof in the vicinity of the third inflection point VTn, each front cam follower  8   b - 1  and each rear cam follower  8   b - 2  are engaged in the associated front inner cam groove  11   a - 1  and the associated rear inner cam groove  11   a - 2 , respectively. 
   Rotating the cam ring  11  in the lens barrel advancing direction (upwards as viewed in  FIG. 79 ) in the retracted state shown in  FIG. 79  causes each front cam follower  8   b - 1  and each rear cam follower  8   b - 2  to be guided rearward in the optical axis direction to move on the third section VT 3  toward the second inflection point VTm by the associated front inner cam groove  11   a - 1  and the associated rear inner cam groove  11   a - 2 , respectively. In the middle of this movement of each cam follower  8   b , each rear cam follower  8   b - 2  is disengaged from the associated rear inner cam groove  11   a - 2  through a first rear end opening R 3  thereof which opens on a rear end surface of the cam ring  11  because each rear inner cam groove  11   a - 2  does not include adjoining portions of the second section VT 2  and the third section VT 3  on opposite sides of the second inflection point VTm. At this time, each front cam follower  8   b - 1  remains engaged in the associated front inner cam groove  11   a - 1  since each front inner cam groove  11   a - 1  includes a rear portion thereof in the optical axis direction which corresponds to the missing rear portion of each rear inner cam groove  11   a - 2  in the optical axis direction. On or after each rear cam follower  8   b - 2  being disengaged from the associated rear inner cam groove  11   a - 2  through the first rear end opening R 3  thereof, the second lens group moving frame  8  moves in the optical axis direction by rotation of the cam ring  11  only due to engagement of each front cam follower  8   b - 1  with the associated front inner cam groove  11   a - 1 . 
     FIG. 80  shows the positional relationship between the plurality of inner cam grooves  11   a  and the plurality of cam followers  8   b  when the zoom lens  71  is in the state shown below the photographing lens axis Z 1  in  FIG. 9  in which the zoom lens  71  is set at the wide-angle extremity. In this state shown below the photographing lens axis Z 1  in  FIG. 9 , each front cam follower  8   b - 1  is positioned in the second section VT 2  slightly beyond the second inflection point VTm. Although each rear cam follower  8   b - 2  is currently disengaged from the associated rear inner cam groove  11   a - 2  through the first rear end opening R 3  thereof as described above each rear cam follower  8   b - 2  remains positioned on the associated reference cam diagram VT because the associated front cam follower  8   b - 1  positioned in front of the rear cam follower  8   b - 2  remains engaged in the associated front inner cam groove  11   a - 1 . 
   Rotating the cam ring  11  in the lens barrel advancing direction (upward as viewed in  FIG. 80 ) in the state shown in  FIG. 80 , in which the zoom lens  71  is set at the wide-angle extremity, causes each front cam follower  8   b - 1  to be guided forward in the optical axis direction to move on the second section VT 2  toward the first section VT 1  by the associated front inner cam groove  11   a - 1  . With this forward movement of each front cam follower  8   b - 1 , each rear cam follower  8   b - 2  which is currently disengaged from the associated rear inner cam groove  11   a - 2  moves on the second section VT 2  toward the first section VT 1 , and shortly enters a second rear end opening R 2  formed on a rear end surface of the cam ring  11  to be re-engaged in the associated rear inner cam groove  11   a - 2 . On or after this re-engagement of each rear cam follower  8   b - 2  with the associated rear inner cam groove  11   a - 2 , each front cam follower  8   b - 1  and each rear cam follower  8   b - 2  are guided by the associated front inner cam groove  11   a - 1  and the associated rear inner cam groove  11   a - 2 , respectively. However, a shortly after the re-engagement of each rear cam follower  8   b - 2  with the associated rear inner cam groove  11   a - 2 , each front cam follower  8   b - 1  is disengaged from the associated front inner cam groove  11   a - 1  through the front end opening R 1  because a front end portion of each front inner cam groove  11   a - 1  which lies on the associated reference cam diagram VT is missing. At this time, each rear cam follower  8   b - 2  remains engaged in the associated rear inner cam groove  11   a - 2  since each rear inner cam groove  11   a - 2  includes a front end portion thereof in the optical axis direction which corresponds to the missing front end portion of each front inner cam groove  11   a - 1  in the optical axis direction. On or after each front cam follower  8   b - 1  being disengaged from the associated front inner cam groove  11   a - 1  through the front end opening R 1  thereof, the second lens group moving frame  8  moves in the optical axis direction by rotation of the cam ring  11  only due to engagement of each rear cam follower  8   b - 2  with the associated rear inner cam groove  11   a - 2 . 
     FIG. 81  shows the positional relationship between the plurality of inner cam grooves  11   a  and the plurality of cam followers  8   b  when the zoom lens  71  is in the state shown above the photographing lens axis Z 1  in  FIG. 9  in which the zoom lens  71  is set at the telephoto extremity. In this state shown above the photographing lens axis Z 1  in  FIG. 9 , each front cam follower  8   b - 1  is positioned in the second section VT 2  in the vicinity of the first inflection point VTh. Although each front cam follower  8   b - 1  is currently disengaged from the associated front inner cam groove  11   a - 1  through the front end opening R 1  thereof as described above, each front cam follower  8   b - 1  remains on the associated reference cam diagram VT because the associated rear cam follower  8   b - 2  positioned behind the front cam follower  8   b - 1  remains engaged in the associated rear inner cam groove  11   a - 2 . 
   Further rotating the cam ring  11  in the lens barrel advancing direction (upward as viewed in  FIG. 81 ) in the state shown in  FIG. 81 , in which the zoom lens  71  is set at the telephoto extremity, causes each rear cam follower  8   b - 2  to enter the first section VT 1  via the first inflection point VTh as shown in FIG.  82 . At this time, each front cam follower  8   b - 1  has been disengaged from the associated front inner cam groove  11   a - 1 , and merely each rear cam follower  8   b - 2  is engaged in a front end portion (the first section VT 1 ) of the associated rear inner cam groove  11   a - 2  which extends in the optical axis direction, so that the second lens group moving frame  8  can be removed from the cam ring  11  from the front thereof in the optical axis direction to remove each rear cam follower  8   b - 2  from the associated rear inner cam groove  11   a - 2  via the front end opening R 4 . Accordingly,  FIG. 82  shows a state where the cam ring  11  and the second lens group moving frame  8  are put together or removed from each other. 
   As described above, in the present embodiment of the zoom lens, each pair of cam grooves having the same reference cam diagram VT, i.e., each front inner cam groove  11   a - 1  and the associated rear inner cam groove  11   a - 2  are formed at different points in the optical axis direction on the cam ring  11 ; moreover, each front inner cam groove  11   a - 1  and the associated rear inner cam groove  11   a - 2  are formed so that one end of the front inner cam groove  11   a - 1  opens on a front end surface of the cam ring  11  without the front inner cam groove  11   a - 1  including the entire part of the associated reference cam diagram VT and so that one end of the rear inner cam groove  11   a - 2  opens on a rear end surface of the cam ring  11  without the rear inner cam groove  11   a - 2  including the entire part of the associated reference cam diagram VT; and furthermore, one of the front inner cam groove  11   a - 1  and the rear inner cam groove  11   a - 2  is complemented by the other to include the entire part of one reference cam diagram VT. In addition, only each rear cam follower  8   b - 2  is engaged in the associated rear inner cam groove  11   a - 2  when the second lens group moving frame  8  is positioned at a front limit for the axial movement thereof with respect to the cam ring  11  (which corresponds to the state shown above the photographing lens axis Z 1  in  FIG. 9  in which the zoom lens  71  is set at the telephoto extremity), while only each front cam follower  8   b - 1  is engaged in the associated front inner cam groove  11   a - 1  when the second lens group moving frame  8  is positioned at a rear limit for the axial movement thereof with respect to the cam ring  11  (which corresponds to a state shown below the photographing lens axis Z 1  in  FIG. 9  in which the zoom lens  71  is set at the wide-angle extremity). With this structure, a sufficient range of movement of the second lens group moving frame  8  in the optical axis direction which is greater than the range of movement of the cam ring  11  in the optical axis direction is achieved. Namely, the length of the cam ring  11  in the optical axis direction can be reduced without sacrificing the range of movement of the second lens group moving frame  8 , which supports the second lens group LG 2  via the second lens frame  6 , in the optical axis direction. 
   In a typical cam mechanism having a rotatable cam ring on which a set of cam grooves are formed and a driven member having a set of cam followers which are respectively engaged in the set of cam grooves, the amount of movement of each cam follower per unit of rotation of the cam ring decreases to thereby make it possible to move the driven member with a higher degree of positioning accuracy by rotation of the cam ring as the degree of inclination of each cam groove on the cam ring relative to the rotational direction of the cam ring becomes small, i.e., as the direction of extension of each cam groove becomes close to a circumferential direction of the cam ring. In addition, the degree of resistance to the cam ring when it rotates becomes smaller to thereby make the driving torque for rotating the cam ring smaller as the degree of inclination of each cam groove on the cam ring relative to the rotational direction of the cam ring becomes small. A reduction of the driving torque results in an increase in durability of elements of the cam mechanism and a decrease in power consumption of the motor for driving the cam ring, and makes it possible to adopt a small motor for driving the cam ring to downsize the lens barrel. Although it is known that the actual contours of the cam grooves are determined in consideration of various factors such as the effective area of an outer or inner peripheral surface of the cam ring and the maximum angle of rotation of the cam ring, it is generally the case that the cam grooves have the above described tendencies. 
   As described above, it can be said that the cam ring  11  is provided, at regular intervals in a circumferential direction of the cam ring  11 , with three pairs (groups) of inner cam grooves  11   a  for guiding the second lens group LG 2  if each front inner cam groove  11   a - 1  and the rear inner cam groove  11   a - 2  positioned therebehind in the optical axis direction are regarded as a pair (group). Similarly, it can be said that the second lens group moving frame  8  is provided, at regular intervals in a circumferential direction thereof, with three pairs (groups) of cam followers  8   b  if each front rear cam follower  8   b - 1  and the rear cam follower  8   b - 2 , positioned therebehind in the optical axis direction, are regarded as a pair (group). As for the reference cam diagrams VT of the plurality of inner cam grooves  11   a , provided only three of the reference cam diagrams VT are to be arranged on an inner peripheral surface of the cam ring  11  along a line thereon extending in a circumferential direction of the cam ring  11 , the three reference cam diagrams VT will not interfere with one another on the inner peripheral surface of the cam ring  11  though each reference cam diagram VT has an undulating shape. However, in the present embodiment of the zoom lens, in order to shorten the length of the cam ring  11  in the optical axis direction to thereby minimize the length of the zoom lens  71 , six reference cam diagrams VT need to be arranged on the inner peripheral surface of the cam ring  11  in total because the set of three front inner cam grooves  11   a - 1  and the corresponding set of three rear cam grooves (three discontinuous rear cam grooves)  11   a - 2 , six cam grooves in total, need to be formed separately on front and rear portions on the inner peripheral surface of the cam ring  11  in the optical axis direction, respectively. Although each of the six inner cam grooves  11   a - 1  and  11   a - 2  is shorter than the reference cam diagram VT, it is generally the case that the space for the inner cam grooves  11   a - 1  and  11   a - 2  on the cam ring  11  becomes tighter as the number of the cam grooves is great. Therefore, if the number of the cam grooves is great, it is difficult to form the cam grooves on the cam ring without making the cam grooves interfering with each other. To prevent this problem from occurring, it has been conventionally practiced to increase the degree of inclination of each cam groove relative to the rotational direction of the cam ring (i.e., to make the direction of extension of each cam groove close to a circumferential direction of the cam ring) or to increase the diameter of the cam ring to enlarge the area of a peripheral surface of the cam ring on which the cam grooves are formed. However, increasing the degree of inclination of each cam groove is not desirable in terms of the attainment of a high degree of positioning accuracy in driving a driven member driven by the cam ring and also a saving in the driving torque for rotating the cam ring, and increasing the diameter of the cam ring is not desirable either because the zoom lens will be increased in size. 
   In contrast to such conventional practices, according to the present embodiment of the zoom lens, the inventor of the present invention has found the fact that a substantial performance characteristics of the cam mechanism is maintained even if each front inner cam groove  11   a - 1  intersects one of the set of three rear inner cam grooves  11   a - 2 , as long as the reference cam diagrams VT of the six inner cam grooves  11   a  ( 11   a - 1  and  11   a - 2 ) are the same while one cam follower of each pair of cam followers (each front cam follower  8   b - 1  and the associated rear cam follower  8   b - 2 ) remains engaged in the associated inner cam groove  11   a - 1  or  11   a - 2  at the moment at which the other cam follower  8   b - 1  or  8   b - 2  passes through a point of intersection between the front inner cam groove  11   a - 1  and the rear inner cam groove  11   a - 2 . On the basis of this fact, each front inner cam groove  11   a - 1  and adjacent one of the set of three rear inner cam grooves  11   a - 2 , which are adjacent to each other in a circumferential direction of the cam ring  11 , are formed to intersect each other intentionally without changing the shape of each reference cam diagram VT and without increasing the diameter of the cam ring  11 . More specifically, if the three pairs of inner cam grooves  11   a  are respectively treated as a first pair of cam grooves G 1 , a second pair of cam grooves G 2  and a third pair of cam grooves G 3  as shown in  FIG. 17 , the front inner cam groove  11   a - 1  of the first pair G 1  and the rear inner cam groove  11   a - 2  of the second pair G 2 , which are adjacent to each other in a circumferential direction of the cam ring  11 , intersect each other, the first inner cam groove  11   a - 1  of the second pair G 2  and the rear inner cam groove  11   a - 2  of the third pair G 3 , which are adjacent to each other in a circumferential direction of the cam ring  11 , intersect each other, and the front inner cam groove  11   a - 1  of the third pair G 3  and the rear inner cam groove  11   a - 2  of the first pair G 1 , which are adjacent to each other in a circumferential direction of the cam ring  11 , intersect each other. 
   To make one cam follower of each pair of cam followers (each front cam follower  8   b - 1  and the associated rear cam follower  8   b - 2 ) remain properly engaged in the associated inner cam groove  11   a - 1  or  11   a - 2  at the moment at which the other cam follower  8   b - 1  or  8   b - 2  passes through the point of intersection between the front inner cam groove  11   a - 1  and the rear inner cam groove  11   a - 2 , the front inner cam groove  11   a - 1  and the rear inner cam groove  11   a - 2  of each pair of the first through third pairs of cam grooves G 1 , G 2  and G 3  are formed not only at different axial positions in the optical axis direction but also at different circumferential positions in a circumferential direction of the cam ring  11 . The positional difference in a circumferential direction of the cam ring  11  between the front inner cam groove  11   a - 1  and the rear inner cam groove  11   a - 2  of each pair of the first through third pairs of cam grooves G 1 , G 2  and G 3  is indicated by “HJ” in FIG.  17 . This positional difference HJ changes the point of intersection between the front inner cam groove  11   a - 1  and the rear inner cam groove  11   a - 2  in a circumferential direction of the cam ring  11 . Consequently, in each pair of the first through third pairs of cam grooves G 1 , G 2  and G 3 , the point of intersection is positioned in the vicinity of the second inflection point VTm on the third section VT 3  of the front inner cam groove  11   a - 1 , and also in the vicinity of the first inflection point VTh the front end opening R 4  (the front open end section  11   a - 2   x ) at the front end of the first section VT 1 . 
   As can be understood from the above descriptions, at the moment at which the set of three front cam followers  8   b - 1  pass through the points of intersection in the set of three front inner cam grooves  11   a - 1 , the set of three rear cam followers  8   b - 2  remain engaged in the set of three rear inner cam grooves  11   a - 2  so that the set of three front cam followers  8   b - 1  can pass through the points of intersection without being disengaged from the set of three front inner cam grooves  11   a - 1 , respectively (see FIG.  83 ), by forming the set of three front inner cam grooves  11   a - 1  and the corresponding set of three rear inner cam grooves  11   a - 2  in the above described manner. Although each front inner cam groove  11   a - 1  has the point of intersection therein between the zooming section and the lens-barrel retracting section, i.e. in the lens-barrel operating section, the lens barrel  71  can securely be advanced and retracted with the cam ring  11  regardless of the existence of a section of each front inner cam groove  11   a - 1  which includes the point of intersection therein. 
   Although each front cam follower  8   b - 1  is already disengaged from the associated front inner cam groove  11   a - 1  when each rear cam follower  8   b - 2  reaches the point of intersection in the rear inner cam groove  11   a - 2  as shown in  FIG. 82 , this point of intersection is positioned in the lens-barrel assembling/disassembling section, i.e., out of the lens-barrel operating section, so that each rear cam follower  8   b - 2  is not in a state where it receives torque from the cam ring  11 . Accordingly, as for the set of three rear inner cam grooves  11   a - 2 , a possibility of each rear cam follower  8   b - 2  being disengaged from the associated rear inner cam groove  11   a - 2  at the point of intersection therein does not have to be taken into consideration when the zoom lens  71  is in the ready-to-photograph state. 
   The point of intersection in each front inner cam groove  11   a - 1  is in a section thereof through which the associated front cam follower  8   b - 1  passes between a state shown in  FIG. 79  in which the zoom lens  71  is in the retracted state and a state shown in  FIG. 80  in which the zoom lens  71  is in the wide-angle extremity, while the point of intersection in each rear inner cam groove  11   a - 2  is in the lens-barrel assembling/disassembling section as described above. Therefore, either each front inner cam groove  11   a - 1  or each rear inner cam groove  11   a - 2  does not have the point of intersection therein in the zooming range between the wide-angle extremity and the telephoto extremity. This makes it possible to insure a high degree of positioning accuracy in driving the second lens group LG 2  during a zooming operation of the zoom lens  71  regardless of the existence of the point of intersection between cam grooves. 
   Namely, the timing of engagement or disengagement of each cam follower in or from the associated cam groove can be varied by adjusting the aforementioned positional difference b. Moreover, the point of intersection between two cam grooves ( 11   a - 1  and  11   a - 2 ) can be positioned in an appropriate section therein which does not affect any adverse effect on a zooming operation by adjusting the aforementioned positional difference b. 
   As can be understood from the above descriptions, in the present embodiment of the zoom lens, each front inner cam groove  11   a - 1  and each rear inner cam groove  11   a - 2  are successfully arranged on the inner peripheral surface of the cam ring  11  in a space-saving fashion without deteriorating the positioning accuracy in driving the second lens group LG 2  by making each front inner cam groove  11   a - 1  and adjacent one of the set of three rear inner cam grooves  11   a - 2 , which are adjacent to each other in a circumferential direction of the cam ring  11 , intersect each other intentionally and further by forming each front inner cam groove  11   a - 1  and the associated rear inner cam groove  11   a - 2  not only at different axial positions in the optical axis direction but also at different circumferential positions in a circumferential direction of the cam ring  11 . Accordingly, not only the length of the cam ring  11  in the optical axis direction but also the diameter of the cam ring  11  can be reduced. 
   The second lens group moving frame  8  is movable in the optical axis direction by a comparatively great amount of movement as compared with the length of the zoom lens by the above described structure of the cam ring  11 . However, it is conventionally the case that it is difficult to guide such a moving member the moving range of which is great linearly in a direction of an optical axis without rotating the moving member about the optical axis by a small linear guide structure. In the present embodiment of the zoom lens, the second lens group moving frame  8  can be guided linearly in the optical axis direction without rotating about the lens barrel axis Z 0  with reliability, without increasing the size of the second lens group moving frame  8 . 
   As can be seen from  FIGS. 73 through 75  and  79  through  82 , the second linear guide ring  10  does not move in the optical axis direction relative to the cam ring  11 . This is because the discontinuous outer edge of the ring portion  10   b  of the second linear guide ring  10  is engaged in the discontinuous circumferential groove  11   e  of the cam ring  11  to be rotatable about the lens barrel axis Z 0  relative to the cam ring  11  and to be immovable relative to the cam ring  11  in the optical axis direction. On the other hand, in the operating range of the zoom lens  71  from the retracted position to the telephoto extremity via the wide-angle extremity, the second lens group moving frame  8  is positioned at the rear limit for the axial movement thereof with respect to the cam ring  11  when the zoom lens  71  is set at a focal length in the vicinity of the wide-angle extremity, while the second lens group moving frame  8  is positioned at the front limit for the axial movement thereof with respect to the cam ring  11  when the zoom lens  71  is set at the telephoto extremity. More specifically, the second lens group moving frame  8  is positioned at the rear limit for the axial movement thereof with respect to the cam ring  11  when each front cam follower  8   b - 1  and each rear cam follower  8   b - 2  are positioned on the second inflection point VTm of the associated front inner cam groove  11   a - 1  and the second inflection point VTm of the associated rear inner cam groove  11   a - 2 , respectively, namely, when each front cam follower  8   b - 1  and each rear cam follower  8   b - 2  are each positioned in close vicinity of its wide-angle position between this wide-angle position and its retracted position. 
   As for the second linear guide ring  10 , the set of three linear guide keys  10   c  project forward in the optical axis direction from the ring portion  10   b , whereas the rear end of the second lens group moving frame  8  projects rearward, beyond the ring portion  10   b  of the second linear guide ring  10 , when the zoom lens  71  is set at the wide-angle extremity as shown in  FIGS. 73 and 80 . To allow the second lens group moving frame  8  having such a structure to move in the optical axis direction with respect to the second linear guide ring  10 , the ring portion  10   b  of the second linear guide ring  10  is provided with a central aperture  10   b -T (see  FIG. 88 ) which has a diameter allowing the second lens group moving frame  8  to pass therethrough. The set of three linear guide keys  10   c  are positioned to project forward through the central aperture  10   b -T. In other words, the set of three linear guide keys  10   c  are formed on the second linear guide ring  10  at radial positions not interfering with the ring portion  10   b . Front and rear ends of each guide groove  8   a  that is formed on the second lens group moving frame  8  are open on front and rear end surfaces of the second lens group moving frame  8  so that the associated linear guide key  10   c  can project forward and rearward from the front and the rear of the second lens group moving frame  8 , respectively. 
   Therefore, the second lens group moving frame  8  does not interfere with the ring portion  10   b  of the second linear guide ring  10  wherever the second lens group moving frame  8  is positioned relative to the second linear guide ring  10  in the optical axis direction. This makes it possible to utilize the full ranges of each linear guide key  10   c  and each guide groove  8   a  as sliding parts for guiding the second lens group moving frame  8  linearly without rotating the same about the lens barrel axis Z 0 . For instance, in the state shown in  FIGS. 84 and 85  showing the positional relationship between the second lens group moving frame  8  and the second linear guide ring  10  when the zoom lens  71  is set at the wide-angle extremity (i.e., when the second lens group moving frame  8  is positioned at its rear limit for the axial movement thereof with respect to the second linear guide ring  10 ), approximately a rear half of the second lens group moving frame  8  projects rearward from the ring portion  10 b through the central aperture  10   b -T in the optical axis direction, and a rear portion of each linear guide key  10   c  in the vicinity of the rear end thereof in the optical axis direction is engaged with a front portion of the associated guide groove  8   a  in the vicinity of the front end thereof in the optical axis direction. In addition, the front end of each linear guide key  10   c  projects forward from the associated guide groove  8   a . Assuming that each linear guide key  10   c  is not positioned radially inside the ring portion  10   b  but projects forward directly from the front of the ring portion  10   b  unlike the present embodiment of the zoom lens, the second lens group moving frame  8  will not be capable of moving rearward beyond the position thereof shown in  FIGS. 84 and 85  since the second lens group moving frame  8  will be prevented from moving rearward upon contacting with the ring portion  10   b.    
   Thereafter, if the zoom lens  71  changes its focal length from the wide-angle extremity to the telephoto extremity, a rear portion of the second lens group moving frame  8  which is positioned behind the ring portion  10   b  in the optical axis direction when the zoom lens  71  is set at the wide-angle extremity has been moved forward from the ring portion  10   b  through the central aperture  10   b -T in the optical axis direction so that the entire part of the second lens group moving frame  8  is positioned in front of the ring portion  10   b  as shown in  FIGS. 86 and 87 . As a result, the rear end of each linear guide key  10   c  projects rearward from the associated guide groove  8   a  so that only a front portion of each linear guide key  10   c  and a rear portion of the associated guide groove  8   a  are engaged with each other in the optical axis direction. During the movement of the second lens group moving frame  8  in the optical axis direction when the zoom lens  71  changes its focal length from the wide-angle extremity to the telephoto extremity, the set of three linear guide keys  10   c  remain engaged in the set of three guide grooves  8   a  so that the second lens group moving frame  8  is securely guided linearly in the optical axis direction without rotating about the lens barrel axis Z 0 . 
   In the case where only a linear guiding function between the second linear guide ring  10  and the second lens group moving frame  8  is considered, almost the entire portion of each linear guide key  10   c  in the optical axis direction and almost the entire portion of each guide groove  8   a  in the optical axis direction can be utilized theoretically as effective guide portions which can remain engaged with each other until just before being disengaged from each other. However, each of the respective effective guide portions is determined with a margin so as not to deteriorate the stability of engagement of the set of three linear guide keys  10   c  with the set of three guide grooves  8   a . For instance, in the state shown in  FIGS. 84 and 85  in which the zoom lens  71  is set at the wide-angle extremity, the relative position between the set of three linear guide keys  10   c  and the set of three guide grooves  8   a  shown in  FIGS. 84 and 85  corresponds to the wide-angle extremity of the zoom lens  71  to ensure a sufficient amount of engagement between the set of three linear guide keys  10   c  and the set of three guide grooves  8   a  though each guide groove  8   a  still has room for the associated linear guide key  10   c  to further move rearward in the optical axis direction. Although the second lens group moving frame  8  is positioned at the rear limit for the axial movement thereof with respect to the cam ring  11  when each front cam follower  8   b - 1  and each rear cam follower  8   b - 2  are positioned on the second inflection point VTm of the associated front inner cam groove  11   a - 1  and the second inflection point VTm of the associated rear inner cam groove  11   a - 2 , respectively, namely, when each front cam follower  8   b - 1  and each rear cam follower  8   b - 2  are each positioned in close vicinity of its wide-angle position between this wide-angle position and its retracted position as described above, a sufficient amount of engagement of the set of three linear guide keys  10   c  with the set of three guide grooves  8   a  is secured even when the second lens group moving frame  8  is positioned at such a rear limit for the axial movement thereof with respect to the cam ring  11 . In the state shown in  FIGS. 86 and 87  in which the zoom lens  71  is set at the telephoto extremity, the second lens group moving frame  8  can further move forward to the second linear guide ring  10  when the zoom lens  71  is in the assembling/disassembling state, each linear guide key  10   c  remains engaged in the associated guide groove  8   a  in the assembling/disassembling state (see FIG.  82 ). 
   As described above, to increase the maximum amount of movement of the second lens group moving frame  8  relative to the cam ring  11 , the plurality of cam followers  8   b  of the second lens group moving frame  8  include the set of three front cam followers  8   b - 1 , which are formed at different circumferential positions to be respectively engaged in the set of three front inner cam grooves  11   a - 1 , and a set of three rear cam followers  8   b - 2 , which are formed at different circumferential positions behind the set of three front cam followers  8   b - 1  to be respectively engaged in the set of three rear inner cam grooves  11   a - 2 . The set of three rear cam followers  8   b - 2  move rearward from the ring portion  10   b  when the zoom lens  71  is driven from the retracted position to the wide-angle extremity, and move forward from the ring portion  10   b  when the zoom lens  71  is driven from the wide-angle extremity to the telephoto extremity. The set of three rear cam followers  8   b - 2  are positioned behind the ring portion  10   b  when disengaged from the set of three rear inner cam grooves  11   a - 2  from the first rear end openings R 3  or the second rear end openings R 2 , respectively. The ring portion  10   b  is provided on an inner edge thereof at different circumferential positions with three radial recesses  10   e  through which the set of three rear cam followers  8   b - 2  can pass the ring portion  10   b  in the optical axis direction, respectively, (see FIGS.  88  and  89 ). 
   The three radial recesses  10   e  are formed on the ring portion  10   b  to be aligned with the set of three rear cam followers  8   b - 2  in the optical axis direction when engaged therewith, respectively. Therefore, at the time when each rear cam follower  8   b - 2  reaches the first rear end opening R 3  of the associated rear inner cam groove  11   a - 2  in the course of rearward movement of the rear cam follower  8   b - 2  with respect to the second linear guide ring  10  from the retracted position shown in  FIG. 79  toward a position shown in  FIG. 80  which corresponds to the wide-angle extremity of the zoom lens  71 , the three radial recesses  10   e  are also aligned with the three first rear end openings R 3  in the optical axis direction to allow the set of three rear cam followers  8   b - 2  to move rearward beyond the ring portion  10   b  through the three radial recesses  10   e  and the three first rear end openings R 3 , respectively. Thereafter, each rear cam follower  8   b - 2  changes the direction of movement thereof at the second inflection point VTm of the associated reference cam diagram VT to then move forward in the optical axis direction, and remains positioned behind the ring portion  10   b  until reaching the second rear end opening R 2  of the associated rear inner cam groove  11   a - 2  as shown in  FIGS. 80 and 85 . Upon each rear cam follower  8   b - 2  reaching the second rear end opening R 2  of the associated rear inner cam groove  11   a - 2  when moving forward further from the position shown in  FIG. 80  which corresponds to the wide-angle extremity of the zoom lens  71 , the three radial recesses  10   e  are aligned with the three second rear end openings R 2  in the optical axis direction this time to allow the set of three rear cam followers  8   b - 2  to enter the set of three rear inner cam grooves  11   a - 2  through the three radial recesses  10   e  and the three second rear end openings R 2 , respectively. Accordingly, the ring portion  10   b  of the second linear guide ring  10  does not interfere with movement of the set of three rear cam followers  8   b - 2  because the ring portion  10   b  is provided with the three radial recesses  10   e , through which the set of three rear cam followers  8   b - 2  can pass the ring portion  10   b  in the optical axis direction, respectively. 
   As can be understood from the above descriptions, according to the above described linear guide structure, the second lens group moving frame  8 , the moving range of which in the optical axis direction is comparatively great, can be securely guided linearly without rotating about the lens barrel axis Z 0  by the second linear guide ring  10  without the ring portion  10   b  interfering with the second lens group moving frame  8 . As can be seen from  FIGS. 79 through 82 , the present embodiment of the linear guide structure cannot be greater than a conventional linear guide structure because the length of each linear guide key  10   c  is smaller than the length of the cam ring  11  in the optical axis direction. 
   The support structure between the second linear guide ring  10  and the second lens group moving frame  8  that are positioned inside the cam ring  11  has been discussed above. The support structure between the first external barrel  12  and the second external barrel  13  that are positioned outside the cam ring  11  will be discussed hereinafter. 
   The cam ring  11  and the first external barrel  12  are arranged concentrically about the lens barrel axis Z 0 . The first external barrel  12  moves in the optical axis direction in a predetermined moving manner by engagement of the set of three cam followers  31 , which project radially inwards from the first external barrel  12 , with the set of three outer cam grooves  11   b , which are formed on an outer peripheral surface of the cam ring  11 .  FIGS. 90 through 100  show positional relationships between the set of three cam followers  31  and the set of three outer cam grooves  11   b . In  FIGS. 90 through 100 , the first external barrel  12  is shown by one-dot chain lines while the second external barrel  13  is shown by two-dot chain lines. 
   As shown in  FIG. 16 , each outer cam groove  11   b , which is formed on an outer peripheral surface of the cam ring  11 , is provided at one end (front end) thereof with a front end opening section  11   b -X which is open on a front end surface of the cam ring  11 , and is provided at the other end (rear end) thereof with a rear end opening section  11   b -Y which is open on a rear end surface of the cam ring  11 . Accordingly, the opposite ends of each outer cam groove  11   b  are respectively formed as open ends. Each outer cam groove  11   b  is provided between the front end opening section  11   b -X and the rear end opening section  11   b -Y with an inclined lead section  11   b -L which extends linearly obliquely from the rear end opening section  11   b -Y toward the front of the optical axis direction, and a curved section  11   b -Z which is positioned between the inclined lead section  11   b -L and the front end opening section  11   b -X to be curved rearward (downward as viewed in  FIG. 16 ) in the optical axis direction. A zooming section for changing the focal length of the zoom lens  71  before picture taking is included in the curved section  11   b -Z of each outer cam groove  11   b . As shown in  FIGS. 94 through 100 , the set of three cam followers  31  can be inserted into and removed from the set of three outer cam grooves  11   b  through the front end opening sections  11   b -X thereof, respectively. When the zoom lens  71  is set at the telephoto extremity, each cam follower  31  is positioned in the associated curved section  11   b -Z in the vicinity of the front end opening section  11   b -X as shown in  FIGS. 93 and 99 . When the zoom lens  71  is set at the wide-angle extremity, each cam follower  31  is positioned in the associated curved section  11   b -Z in the vicinity of inclined lead section  11   b -L as shown in  FIGS. 92 and 98 . 
   In the state shown in  FIGS. 90 and 95  in which the zoom lens  71  is in the retracted state, each cam follower  31  is in the associated rear end opening section  11   b -Y. The width of the rear end opening section  11   b -Y of each outer cam groove  11   b  is greater than the width of the inclined lead section  11   b -L and the width of the curved section  11   b -Z in a circumferential direction of the cam ring  11  so that each cam follower  31  is allowed to move in a circumferential direction of the cam ring  11  to some extent in the associated rear end opening section  11   b -Y. Although the rear end opening section  11   b -Y of each outer cam groove  11   b  is open at the rear of the cam ring  11 , the set of three cam followers  31  do not come off the set of three outer cam grooves  11   b  through the three rear end opening sections  11   b -Y, respectively, because the cam ring  11  is provided with at least one stop portion which determines a rear limit for the axial movement of the first external barrel  12  with respect to the cam ring  11 . 
   More specifically, the cam ring  11  is provided, at the front end thereof at different circumferential positions, with a set of three front projecting portions  11   f  which project forward in the optical axis direction as shown in FIG.  16 . The aforementioned set of three external protuberances  11   g , which are formed on the cam ring  11  to project radially outwards, are formed behind the set of three front projecting portions  11   f  in the optical axis direction, respectively. Each external protuberance  11   g  is provided with a corresponding section of the discontinuous circumferential groove  11   c . The set of three roller followers  32  are fixed onto the set of three external protuberances  11   g  by the three set screws  32   a , respectively. The set of three front projecting portions  11   f  are provided at the front ends thereof with a set of three front stop surfaces  11   s - 1 , respectively, which lie in a plane orthogonal to the photographing optical axis Z 1 . The set of three external protuberances  11   g  are provided at the front ends thereof with a set of three rear stop surfaces  11   s - 2  which lie in a plane orthogonal to the photographing optical axis Z 1 . On the other hand, as shown in  FIG. 21 , the first external barrel  12  is provided on an inner peripheral surface thereof with a set of three protuberances, and a set of three front stop surfaces  12   s - 1  are provided at the rear end surface of the protuberances to correspond (oppose) to the set of three front stop surfaces  11   s - 1  so that the set of three front stop surfaces  12   s - 1  can come into contact with the set of three front stop surfaces  11   s - 1 , respectively. The first external barrel  12  is provided at the rear end thereof with a set of three rear stop surfaces  12   s - 2  to correspond to the set of three rear stop surfaces  11   s - 2  so that the set of three rear stop surfaces  12   s - 2  can come into contact with the set of three rear stop surfaces  11   s - 2 , respectively. Each front stop surface  12   s - 1  and each rear stop surface  12   s - 2  are parallel to each front stop surface  11   s - 1  and each rear stop surface  11   s - 2 , respectively. The space between the set of three front stop surfaces  11   s - 1  and the set of three rear stop surfaces  11   s - 2  is the same as the space between the set of three front stop surfaces  12   s - 1  and the set of three rear stop surfaces  12   s - 2 . 
   When the zoom lens  71  is in the retracted state, each front stop surface  12   s - 1  comes very close to the associated front stop surface  11   s - 1  while each rear stop surface  12   s - 2  comes very close to the associated rear stop surface  11   s - 2  so that the first external barrel  12  does not further move rearward beyond the position thereof shown in  FIGS. 90 and 95 . In the lens barrel retracting operation of the zoom lens  71 , the first external barrel  12  stops moving rearward immediately before each front stop surface  12   s - 1  and each rear stop surface  12   s - 2  comes into contact with the associated front stop surface  11   s - 1  and the associated rear stop surface  11   s - 2 , respectively, because the first external barrel  12  stops being driven in the optical axis direction by the cam ring  11  via the set of three cam followers  31  at the time when the set of three cam followers  31  respectively enter the rear end opening sections  11   b -Y of the set of three outer cam grooves  11   b  due to a wide circumferential width of each rear end opening section  11   b -Y. The space between the set of three front stop surfaces  11   s - 1  and the set of three front stop surfaces  12   s - 1  in the retracted state of the zoom lens  71  is predetermined at approximately 0.1 mm. Likewise, the space between the set of three rear stop surfaces  11   s - 2  and the set of three rear stop surfaces  12   s - 2  in the retracted state of the zoom lens  71  is also predetermined at approximately 0.1 mm. However, in an alternative embodiment, the first external barrel  12  can be allowed to retract by inertia so that the front stop surfaces  11   s - 1  and  12   s - 1  and the rear stop surfaces  11   s - 2  and  12   s - 2  contact each other, respectively. 
   The first external barrel  12  is provided on an inner peripheral surface thereof with an inner flange  12   c  which projects radially inwards. The set of three front stop surfaces  12   s - 1  are positioned in front of the inner flange  12   c  in the optical axis direction. The inner flange  12   c  of the first external barrel  12  is provided with a set of three radial recesses  12   d  through which the set of three front projecting portions  11   f  can pass the inner flange  12   c  in the optical axis direction, respectively. When the set of three front stop surfaces  11   s - 1  approach the set of three front stop surfaces  12   s - 1 , the set of three front projecting portions  11   f  passes the inner flange  12   c  through the set of three radial recesses  12   d.    
   Although each of the cam ring  11  and the first external barrel  12  is provided, at front and rear portions thereof in the optical axis direction, with a set of front stop surfaces ( 11   s - 1  or  12   s - 1 ) and a set of rear stop surfaces ( 11   s - 2  or  12   s - 2 ) in the present embodiment of the zoom lens, each of the cam ring  11  and the first external barrel  12  can be provided with only one of the set of front stop surfaces or the set of rear stop surfaces to determine the rear limit for the axial movement of the first external barrel  12  with respect to the cam ring  11 . Conversely, each of the cam ring  11  and the first external barrel  12  can be provided with one or more additional sets of stop surfaces. For instance, in addition to the front stop surfaces  11   s - 1  and  12   s - 1  and the rear stop surfaces  11   s - 2  and  12   s - 2 , three front end surfaces  11   h  each of which are formed between two adjacent front projecting portions  11   f  can be made to be capable of coming into contact with a rear surface  12   h  of the inner flange  12   c  to determine the rear limit for the axial movement of the first external barrel  12  with respect to the cam ring  11 . Note that the front projecting portions  11   f  do not contact with the rear surface  12   h , in the illustrated embodiment. 
   In each of the three outer cam grooves  11   b , the entire section thereof except for the front end opening section  11   b -X serving as a lens-barrel assembling/disassembling section serves as a lens-barrel operating section consisting of a zooming section and a lens-barrel retracting section. Namely, a specific section of each of the three outer cam grooves  11   b  which extends from the position of the associated cam follower  31  in the outer cam groove  11   b  shown in  FIGS. 90 and 95  (i.e., the rear end opening section  11   b -Y), where the zoom lens  71  is in the retracted state, to that shown in  FIGS. 93 and 99 , where the zoom lens  71  is set at the telephoto extremity serves as a lens-barrel operating section consisting of a zooming section and a lens-barrel retracting section. In the present embodiment of the zoom lens, the rear end opening section  11   b -Y of each outer cam groove  11   b  is formed as an opening which is open at the rear of the cam ring  11 . This structure makes it unnecessary to form any rear end wall having a certain thickness on a portion of the cam ring  11  behind each rear end opening section  11   b -Y, thus reducing the length of the cam ring  11  in the optical axis direction. In a conventional cam ring having cam grooves thereon, at least the terminal end of an operating section of each cam groove (one end of each cam groove if the other end is an open end for the insertion of the associated cam groove in the cam groove) has to be formed as a closed end which requires the cam ring to have a end wall having a certain thickness to close the terminal end of the operating section of each cam groove. This kind of end wall does not have to be formed on the cam ring  11  of the present embodiment of the zoom lens, which is advantageous to downsize the cam ring  11 . 
   The reason why the rear end of each outer cam groove  11   b  is successfully formed as an open end such as the rear end opening section  11   b -Y is that the rear limit for the axial movement of the first external barrel  12  with respect to the cam ring  11  is determined by the front stop surfaces ( 11   s - 1  and  12   s - 1 ) and the rear stop surfaces ( 11   s - 2  and  12   s - 2 ) which are provided independent of the set of three outer cam grooves  11   b  and the set of three cam followers  31 . Providing the cam ring  11  and the first external barrel  12  with such stop surfaces as the front and rear stop surfaces ( 11   s - 1 ,  12   s - 1 ,  11   s - 2  and  12   s - 2 ) that operate independently of the set of three outer cam grooves  11   b  and the set of three cam followers  31 , eliminates a possibility of each cam follower  31  becoming incapable of being re-engaged in the associated outer cam groove  11   b  through the rear end opening section  11   b -Y thereof if each cam follower  31  should be disengaged therefrom. 
   When the set of three cam followers  31  are respectively positioned in the rear end opening sections  11   b -Y of the set of three outer cam grooves  11   b , the optical elements of the zoom lens  71  are not required to have a high degree of positioning accuracy because the zoom lens  71  is in the retracted state as shown in FIG.  10 . Due to this reason, there is no substantial problem even if each rear end opening section  11   b -Y has a wide circumferential width so that each cam follower  31  is loosely engaged in the associated rear end opening section  11   b -Y. Conversely, the lens-barrel retracting section of the lens-barrel operating section of each outer cam groove  11   b  is successfully formed as an open end such as the rear end opening section  11   b -Y because the lens-barrel retracting section of the lens-barrel operating section of each outer cam groove  11   b , in which the associated cam follower  31  is allowed to be loosely engaged, is formed at the terminal end of the outer cam groove  11   b  and further because the entire cam contour of each outer cam groove  11   b  is determined so that the terminal end thereof is positioned at the rearmost position of the outer cam groove  11   b  in the optical axis direction. 
   To make each cam follower  31  move from the rear end opening section  11   b -Y, in which the cam follower  31  is loosely engaged, to the inclined lead section  11   b -L of the associated outer cam groove  11   b  with reliability, the cam ring  11  is provided at different circumferential positions with a set of three beveled lead surfaces  11   t  while the first external barrel  12  is provided at different circumferential positions with a set of three beveled lead surfaces  12   t . The set of three beveled lead surfaces  11   t  are formed to adjoin the set of three front stop surfaces  11   s - 1  on the set of three front projecting portions  11   f  so that the set of three beveled lead surfaces  11   t  and the set of three front stop surfaces  11   s - 1  become a set of three continuous surfaces, respectively. The first external barrel  12  is provided at different circumferential positions with a set of three rear end protrusions  12   f  each having a substantially isosceles triangle shape. The set of three engaging protrusions  12   a  are formed on the set of three rear end protrusions  12   f , respectively. One of the two equal sides of each rear end protrusion  12   f  is formed as one of the three beveled lead surfaces  12   t . As shown in  FIGS. 95 through 100 , each beveled lead surface  11   t  and each beveled lead surface  12   t  extend parallel to the inclined lead section  11   b -L. 
   In the state shown in  FIGS. 90 and 95  in which the zoom lens  71  is in the retracted state, an edge ED 1  of each of the three inner flanges  12   c  is positioned to be opposed to the adjacent beveled lead surface  11   t  in a circumferential direction, and also an edge ED 2  of each of the three external protuberances  11   g  is positioned to be opposed to the adjacent beveled lead surface  12   t  in a circumferential direction. In addition, in the same state shown in  FIGS. 90 and 95 , the edge ED 1  of each inner flange  12   c  is slightly apart from the adjacent beveled lead surface  11   t  while the edge ED 2  of each external protuberance  11   g  is slightly apart from the adjacent beveled lead surface  12   t . In this state shown in  FIGS. 90 and 95 , a rotation of the cam ring  11  in the lens barrel advancing direction (upwards as viewed in  FIGS. 91 and 96 ) causes each beveled lead surface  11   t  to come into contact with the edge ED 1  of the adjacent inner flange  12   c , and at the same time causes each beveled lead surface  12   t  to come into contact with the edge ED 2  of the associated external protuberance  11   g  as shown in  FIGS. 91 and 96 . Accordingly, at an initial stage of rotation of the cam ring  11  from the state shown in  FIG. 95 , in which the three edges ED 1  and the three edges ED 2  are respectively apart from the three beveled lead surfaces  11   t  and the three beveled lead surfaces  12   t , to the state shown in  FIG. 96 , in which the three edges ED 1  and the three edges ED 2  are respectively in contact with the three beveled lead surfaces  11   t  and the three beveled lead surfaces  12   t , each cam follower  31  moves solely within the associated rear end opening section  11   b -Y in a circumferential direction of the cam ring  11 , so that the first external barrel  12  is not moved in the optical axis direction with respect to the cam ring  11  by rotation of the cam ring  11 . 
   In the state shown in  FIGS. 91 and 96 , in which the three edges ED 1  and the three edges ED 2  are respectively in contact with the three beveled lead surfaces  11   t  and the three beveled lead surfaces  12   t , each cam follower  31  is positioned at the insertion end of the inclined lead section  11   b -L of the associated outer cam groove  11   b . A further rotation of the cam ring  11  causes each edge ED 1  to slide on the associated beveled lead surface  11   t  and at the same time causes each edge ED 2  to slide on the associated beveled lead surface  12   t  so that the first external barrel  12  is pushed forward with respect to the cam ring  11  by the three beveled lead surfaces  11   t  in accordance with the sliding movements of the three edges ED 1  and the three edges ED 2  on the three beveled lead surfaces  11   t  and the three beveled lead surfaces  12   t , respectively. Since each beveled lead surface  11   t  and each beveled lead surface  12   t  extend parallel to the inclined lead section  11   b -L, the force acting on the first external barrel  12  by the rotation of the cam ring  11  via the three beveled lead surfaces  11   t  causes each cam follower  31  to move into the inclined lead section  11   b -L of the associated outer cam groove  11   b  from the rear end opening section  11   b -Y thereof. After each cam follower  31  completely enters the inclined lead section  11   b -L of the associated outer cam groove  11   b  as shown in  FIG. 97 , each beveled lead surface  11   t  and each beveled lead surface  12   t  are disengaged from the associated edge ED 1  and the associated edge ED 2 , respectively, and accordingly, the first external barrel  12  is guided linearly in the optical axis direction due only to the engagement of the set of three cam followers  31  with the set of three outer cam grooves  11   b , respectively. 
   Accordingly, in the lens barrel advancing operation of the zoom lens  71  which commences from the retracted state shown in  FIG. 10 , providing the cam ring  11  and the first external barrel  12  with the three beveled lead surfaces  11   t  and the three beveled lead surfaces  12   t , whose functions are similar to those of the three inclined lead section  11   b -L, and further providing the first external barrel  12  with the three edge ED 2  and the three ED 1 , whose functions are similar to those of the three cam followers  31 , respectively, make it possible to have each cam follower  31  enter the inclined lead section  11   b -L of the associated outer cam groove  11   b  properly to move therein toward the associated curved section  11   b -Z even from a state as shown in  FIG. 95  where each cam follower  31  is loosely engaged in the associated rear end opening section  11   b -Y. This prevents the zoom lens  71  from malfunctioning. 
   Although each of the cam ring  11  and the first external barrel  12  is provided with a set of three beveled lead surfaces ( 11   t  or  12   t ) in the present embodiment of the zoom lens, only one of the cam ring  11  and the first external barrel  12  can be provided with a set of three beveled lead surfaces ( 11   t  or  12   t ), or each of the cam ring  11  and the first external barrel  12  can be provided with more than one set of three beveled lead surfaces. 
     FIG. 101  shows another embodiment of the structure shown in  FIG. 95 , in which the zoom lens  71  is in the retracted state. Elements shown in  FIG. 101  which are similar to those shown in  FIG. 95  are designated by the same reference numerals each of which the mark (′) is appended to. 
   Each outer cam groove  11   b ′ is provided at the rear end of each inclined lead section  11   b -L′ with a rear end opening  11   b -K instead of the rear end opening section  11   b -Y of the cam ring  11  shown in FIG.  95 . Unlike each rear end opening section  11   b -Y, each rear end opening  11   b -K is formed as a simple end opening of the associated outer cam groove  11   b . Performing the lens barrel retracting operation in a state where the zoom lens is set at the wide-angle extremity causes each cam follower  31 ′ to move rearward (rightward as viewed in  FIG. 101 ) in the associated inclined lead section  11   b -L′, and subsequently causes each cam follower  31 ′ to come out of the associated outer cam groove  11   b ′ through the rear end opening  11   b -K thereof upon the zoom lens reaching the retracted position thereof. If each cam follower  31 ′ comes out of the associated outer cam groove  11   b ′ through the rear end opening  11   b -K thereof, the first external barrel  12 ′ stops being driven by the cam ring  11 ′ via the set of three cam followers  31 ′ and therefore stops moving rearward. At this time, the first external barrel  12 ′ is prevented from further moving rearward because each front stop surface  12   s - 1 ′ and each rear stop surface  12   s - 2 ′ are positioned very close to the associated front stop surface  11   s - 1 ′ and the associated rear stop surface  11   s - 2 ′, respectively. Therefore, the first external barrel  12 ′ is prevented from moving rearward overly even if each cam follower  31 ′ comes out of the associated outer cam groove  11   b ′ through the rear end opening  11   b -K thereof. In this embodiment shown in  FIG. 101 , similar to the embodiment shown in  FIG. 95 , the space between the set of three front stop surfaces  11   s - 1 ′ and the set of three front stop surfaces  12   s - 1 ′ in the retracted state of the zoom lens is desirably at approximately 0.1 mm. Likewise, the space between the set of three rear stop surfaces  11   s - 2 ′ and the set of three rear stop surfaces  12   s - 2 ′ in the retracted state of the zoom lens is desirably at approximately 0.1 mm. However, in an alternative embodiment, the first external barrel  12 ′ can be allowed to retract by inertia so that the front stop surfaces  11   s - 1 ′ and  12   s - 1 ′ and the rear stop surfaces  11   s - 2 ′ and  12   s - 2 ′ contact each other, respectively. 
   According to the structure shown in  FIG. 101  in which each cam follower  31 ′ comes out of the associated outer cam groove  11   b ′ in the retracted state of the zoom lens  71 , it is possible to further downsize the cam ring  11 , because each outer cam groove  11   b ′ does not have to be provided with any accommodation section, which corresponds to each rear end opening section  11   b -Y of the cam ring  11 , for accommodating the associated cam follower therein when the zoom lens is in the retracted position. 
   In the retracted state shown in  FIG. 101 , the edge ED 1 ′ of each of the three inner flanges  12   c ′ is in contact with the beveled lead surface  11   t ′ of the associated front projecting portions  11   f ′ while the edge ED 2 ′ of each of the three external protuberances  11   g ′ is in contact with the beveled lead surface  12   t ′ of the associated rear projecting portions  12   f ′. Each beveled lead surface  11   t , and each beveled lead surface  12   t ′ extend parallel to the inclined lead section  11   b -L′. Due to this structure, rotating the cam ring  11 ′ in the retracted state shown in  FIG. 101  causes the first external barrel  12 ′ to be pushed forward with respect to the cam ring  11 ′, and subsequently causes each cam follower  31 ′ which is currently positioned outside the associated outer cam groove  11   b ′ to move into the inclined lead section  11   b -L′ of the associated outer cam groove  11   b ′ from the rear end opening  11   b -K thereof. Thereafter, a further rotation of the cam ring  11 ′ in the lens barrel advancing direction causes each cam follower  31 ′ to move into the associated curved section  11   b -Z′ in the associated outer cam groove  11   b ′. Thereafter, each cam follower  31 ′ moves in the associated outer cam groove  11   b ′ to perform a zooming operation in accordance with rotation of the cam ring  11 ′. Moving each cam follower  31 ′ to the front end opening sections  11   b -X of the associated outer cam groove  11   b  makes it possible to remove the first external barrel  12 ′ from the cam ring  11 ′. 
   As can be understood from the foregoing, also in the embodiment shown in  FIG. 101 , the rear limit for the axial movement of the first external barrel  12 ′ with respect to the cam ring  11  can be surely determined, while each cam follower  31 ′ can properly enter the inclined lead section  11   b -L′ of the associated outer cam groove  11   b ′ even though each cam follower  31 ′ comes out of the associated outer cam groove  11   b ′ through the rear end opening  11   b -K thereof when the zoom lens is retracted into the camera body. 
   The structure of the zoom lens  71  which accommodates the zoom lens  71  in the camera body  72  as shown in  FIG. 9  upon a main switch (not shown) of the digital camera  70  being turned OFF, which incorporates the structure retracting the second lens frame  6  (the second lens group LG 2 ) to the radially retracted position, will be hereinafter discussed in detail. In the following descriptions the terms “vertical direction” and “horizontal direction” mean the vertical direction and the horizontal direction as viewed from front or rear of the digital camera  70  such as the vertical direction of FIG.  110  and the horizontal direction of  FIG. 111 , respectively. In addition, the term “forward/backward direction” corresponds to the optical axis direction (i.e., a direction parallel to the photographing optical axis Z 1 ). 
   The second lens group LG 2  is supported by the second lens group moving frame  8  via peripheral elements shown in FIGS.  102 . The second lens frame  6  is provided with a cylindrical lens holder portion  6   a , a pivoted cylindrical portion  6   b , a swing arm portion  6   c  and an engaging protrusion  6   e . The cylindrical lens holder portion  6   a  directly holds and supports the second lens group L 2 . The swing arm portion  6   c  extends in a radial direction of the cylindrical lens holder portion  6   a  to connect the cylindrical lens holder portion  6   a  to the pivoted cylindrical portion  6   b . The engaging protrusion  6   e  is formed on the cylindrical lens holder portion  6   a  to extend in a direction away from the swing arm portion  6   c . The pivoted cylindrical portion  6   b  is provided with a through hole  6   d  extending in a direction parallel to the optical axis of the second lens group LG 2 . The pivoted cylindrical portion  6   b  is provided at front and rear ends thereof, on front and rear sides of a portion of the pivoted cylindrical portion  6   b  which is connected to the swing arm portion  6   c , with a front spring support portion  6   f  and a rear spring support portion  6   g , respectively. The front spring support portion  6   f  is provided, on an outer peripheral surface thereof in the vicinity of the front end of the front spring support portion  6   f , with a front spring hold projection  6   h . The rear spring support portion  6   g  is provided, on an outer peripheral surface thereof in the vicinity of the rear end of the rear spring support portion  6   g , with a rear spring hold projection  6   i . The pivoted cylindrical portion  6   b  is provided on an outer peripheral surface thereof with a position control arm  6   j  extending in a direction away from the swing arm portion  6   c . The position control arm  6   j  is provided with a first spring engaging hole  6   k , and the swing arm portion  6   c  is provided with a second spring engaging hole  6   p  (see FIGS.  118  through  120 ). 
   The second lens frame  6  is provided with a rear projecting portion  6   m  which projects rearward in the optical axis direction from the swing arm portion  6   c . The rear projecting portion  6   m  is provided at the rear end thereof with a contacting surface  6   n  which lies in a plane orthogonal to the optical axis of the second lens group LG 2 , i.e., to the photographing optical axis Z 1 . Although a light shield ring  9  is fixed as shown in  FIGS. 104 ,  105 ,  128  and  129 , the contacting surface  6   n  is positioned behind the second lens group light shield ring in the optical axis direction. Namely, the contacting surface  6   n  is positioned behind the rearmost position of the second lens group LG 2  in the optical axis direction. 
   The front second lens frame support plate  36  is a vertically-elongated narrow plate having a narrow width in horizontal direction. The front second lens frame support plate  36  is provided with a first vertically-elongated hole  36   a , a pivot hole  36   b , a cam-bar insertable hole  36   c , a screw insertion hole  36   d , a horizontally-elongated hole  36   e  and a second vertically-elongated hole  36   f , in this order from top to bottom of the front second lens frame support plate  36 . All of these holes  36   a  through  36   f  are through holes which penetrate the front second lens frame support plate  36  in the optical axis direction. The front second lens frame support plate  36  is provided on an outer edge thereof in the vicinity of the first vertically-elongated hole  36   a  with a spring engaging recess  36   g.    
   Similar to the front second lens frame support plate  36 , the rear second lens frame support plate  37  is also a vertically-elongated narrow plate having a narrow width in horizontal direction. The rear second lens frame support plate  37  is provided with a first vertically-elongated hole  37   a , a pivot hole  37   b , a cam-bar insertable hole  37   c , a screw hole  37   d , a horizontally-elongated hole  37   e  and a second vertically-elongated hole  37   f , in this order from top to bottom of the rear second lens frame support plate  37 . All of these holes  37   a  through  37   f  are through holes which penetrate through the rear second lens frame support plate  37  in the optical axis direction. The rear second lens frame support plate  37  is provided on an inner edge of the cam-bar insertable hole  37   c  with a guide key insertable recess  37   g . The through holes  36   a  through  36   f  of the front second lens frame support plate  36  and the through holes  37   a  through  37   f  of the rear second lens frame support plate  37  are aligned in the optical axis direction, respectively. 
   The set screw  66  is provided with a threaded shaft portion  66   a  and a head portion fixed to an end of the threaded shaft portion  66 . The head portion is provided with a cross-slot  66   b  into which the tip of a Phillips screwdriver (not shown) serving as an adjusting tool can be inserted. The screw insertion hole  36   d  of the front second lens frame support plate  36  has a diameter by which the threaded shaft portion  66   a  of the set screw  66  is insertable. The threaded shaft portion  66   a  of the set screw  66  can be screwed through the screw hole  37   d  of the rear second lens frame support plate  37  to fix the front second lens frame support plate  36  and the rear second lens frame support plate  37  to the second lens group moving frame  8 . 
   The zoom lens  71  is provided between the front second lens frame support plate  36  and the rear second lens frame support plate  37  with a first eccentric shaft  34 X which extends in the optical axis direction. The first eccentric shaft  34 X is provided with a large diameter portion  34 X-a, and is provided at front and rear ends of the large diameter portion  34 X-a with a front eccentric pin  34 X-b and a rear eccentric pin  34 X-c which project forward and rearward in the optical axis direction, respectively. The front eccentric pin  34 X-b and the rear eccentric pin  34 X-c have the common axis eccentric to the axis of the large diameter portion  34 X-a. The front eccentric pin  34 X-b is provided at the front end thereof with a recess  34 X-d into which the tip of a flatblade screwdriver (not shown) serving as an adjusting tool can be inserted. 
   The zoom lens  71  is provided between the front second lens frame support plate  36  and the rear second lens frame support plate  37  with a second eccentric shaft  34 Y which extends in the optical axis direction. The structure of the second eccentric shaft  34 Y is the same as the structure of the first eccentric shaft  34 X. Namely, the second eccentric shaft  34 Y is provided with a large diameter portion  34 Y-a, and is provided at front and rear ends of the large diameter portion  34 Y-a with a front eccentric pin  34 Y-b and a rear eccentric pin  34 Y-c which projects forward and rearward in the optical axis direction, respectively. The front eccentric pin  34 Y-b and the rear eccentric pin  34 Y-c have the common axis eccentric to the axis of the large diameter portion  34 Y-a. The front eccentric pin  34 Y-b is provided at the front end thereof with a recess  34 Y-d into which the tip of a flatblade screwdriver (not shown) serving as an adjusting tool can be inserted. 
   The bore diameter of a rear end portion of the through hole  6   d  that penetrates the second lens frame  6  is increased to form a spring-accommodation large diameter hole  6 Z (see  FIG. 126 ) so that the compression coil spring  38  is accommodated in the spring-accommodation large diameter hole  6 Z. The front torsion coil spring  39  and a rear torsion coil spring  40  are fitted on the front spring support portion  6   f  and the rear spring support portion  6   g , respectively. The front torsion coil spring  39  is provided with a front spring end  39   a  and a rear spring end  39   b , and the rear torsion coil spring  40  is provided with a front stationary spring end  40   a  and a rear movable spring end  40   b.    
   The pivot shaft  33  is fitted in the through hole  6   d  from the rear end thereof so that the pivoted cylindrical portion  6   b  of the second lens frame  6  can freely rotate on the pivot shaft  33  with no play in radial directions. The diameters of front and rear ends of the pivot shaft  33  correspond to the pivot hole  36   b  of the front second lens frame support plate  36  and the pivot hole  37   b  of the rear second lens frame support plate  37  so that the front and rear ends of the pivot shaft  33  are fitted in the pivot hole  36   b  and the pivot hole  37   b  to be supported by the front second lens frame support plate  36  and the rear second lens frame support plate  37 , respectively. In a state where the pivot shaft  33  is fitted in the through hole  6   d , the axis of the pivot shaft  33  extends parallel to the optical axis of the second lens group LG 2 . As shown in  FIG. 113 , the pivot shaft  33  is provided in the vicinity of the rear end thereof with a flange  33   a  which is inserted in the spring-accommodation large diameter hole  6 Z to contact with the rear end of the compression coil spring  38  that is accommodated in the spring-accommodation large diameter hole  6 Z. 
   As clearly shown in  FIGS. 106 and 107 , the second lens group moving frame  8  is an annular member having a through internal space  8   n  which penetrates the second lens group moving frame  8  in the optical axis direction. The second lens group moving frame  8  is provided, on an inner peripheral surface thereof at a substantially center thereof in the optical axis direction, with a central inner flange  8   s . The inner edge of the central inner flange  8   s  forms a vertically-elongated opening  8   t  in which the second lens frame  6  is swingable. The shutter unit  76  is fixed to a front surface of the central inner flange  8   s . The second lens group moving frame  8  is provided on an inner peripheral surface thereof behind the central inner flange  8   s  in the optical axis direction with a first radial recess  8   q  (see  FIGS. 111 and 112 ) which is recessed radially outwards (upwards as viewed in  FIG. 111 ) to correspond to the shape of an outer peripheral surface of the cylindrical lens holder portion  6   a  of the second lens frame  6  so that the cylindrical lens holder portion  6   a  can partly enter the radial recess  8   q . The second lens group moving frame  8  is further provided on an inner peripheral surface thereof behind the central inner flange  8   s  with a second radial recess  8   r  (see  FIGS. 111 and 112 ) which is recessed radially outwards to correspond to the shape of an outer edge of the engaging protrusion  6   e  of the second lens frame  6  so that the engaging protrusion  6   e  can partly enter the second radial recess  8   r.    
   As shown in  FIGS. 106 and 107 , the second lens group moving frame  8  is provided on a front end surface thereof (specifically, a right portion of the front end surface of the second lens group moving frame  8 , on the right hand side of the vertically-elongated opening  8   t , as viewed from front of the second lens group moving frame  8 ) with a vertically-elongated front fixing surface  8 c to which the front second lens frame support plate  36  is fixed. The front fixing surface  8   c  is hatched in  FIGS. 106 and 107  for the purpose of illustration. The front fixing surface  8   c  does not overlap the vertically-elongated opening  8   t  in the optical axis direction, and lies in a plane orthogonal to the lens barrel axis Z 0  (the photographing optical axis Z 1 , the optical axis of the second lens group LG 2 ). The front fixing surface  8   c  is positioned in front of the shutter unit  76  in the optical axis direction. The front fixing surface  8   c  is formed to be exposed to the front of the second lens group moving frame  8 . The second lens group moving frame  8  is provided at the front end thereof with a set of three extensions  8   d  extending forward in the optical axis direction. The set of three extensions  8   d  are formed as extensions of the second lens group moving frame  8  which extend forward from the front end of the second lens group moving frame  8 . The set of three front cam followers  8   b - 1  are formed on outer peripheral surfaces of the set of three extensions  8   d , respectively. The second lens group moving frame  8  is provided on a rear end surface thereof (specifically, a left portion of the rear end surface of the second lens group moving frame  8 , on the left hand side of the vertically-elongated opening  8   t , as viewed from rear of the second lens group moving frame  8 ) with a vertically-elongated rear fixing surface  8   e  to which the rear second lens frame support plate  37  is fixed. The rear fixing surface  8   e  is positioned on the opposite side of the central inner flange  8   s  from the front fixing surface  8   c  in the optical axis direction to be parallel to the front fixing surface  8   c . The rear fixing surface  8   e  is formed as a part of the rear end surface of the second lens group moving frame  8 ; namely, the rear fixing surface  8   e  is flush with the rear end surface of the second lens group moving frame  8 . 
   The second lens group moving frame  8  is provided with a first eccentric shaft support hole  8   f , a pivoted cylindrical portion receiving hole  8   g , a screw insertion hole  8   h  and a second eccentric shaft support hole  8   i , in this order from top to bottom of the second lens group moving frame  8 . All of these holes  8   f ,  8   g ,  8   h  and  8   i  are through holes which penetrate the second lens group moving frame  8  in the optical axis direction between the front fixing surface  8   c  and the rear fixing surface  8   e . The through holes  8   f ,  8   h  and  8   i  of the second lens group moving frame  8  are aligned with the through holes  36   a ,  36   d  and  36   e  of the front second lens frame support plate  36 , respectively, and also aligned with the through holes  37   a ,  37   d  and  37   e  of the rear second lens frame support plate  37  in the optical axis direction, respectively. The second lens group moving frame  8  is provided on an inner peripheral surface thereon in the pivoted cylindrical portion receiving hole  8   g  with a key way  8   p  extending in the optical axis direction. The key way  8   p  penetrates the second lens group moving frame  8  in the optical axis direction between the front fixing surface  8   c  and the rear fixing surface  8   e . The diameter of the first eccentric shaft support hole  8   f  is determined so that the large diameter portion  34 X-a is rotatably fitted in the first eccentric shaft support hole  8   f , and the diameter of the second eccentric shaft support hole  8   i  is determined so that the large diameter portion  34 Y-a is rotatably fitted in the second eccentric shaft support hole  8   i  (see FIG.  113 ). On the other hand, the diameter of the screw insertion hole  8   h  is determined so that the threaded shaft portion  66   a  is inserted in the screw insertion hole  8   h  with a substantial gap between the threaded shaft portion  66   a  and an inner peripheral surface of the screw insertion hole  8   h  (see FIG.  113 ). The second lens group moving frame  8  is provided on the front fixing surface  8   c  and the rear fixing surface  8   e  with a front boss  8   j  and a rear boss  8   k  which project forward and rearward in the optical axis direction, respectively. The front boss  8   j  and the rear boss  8   k  have a common axis extending in the optical axis direction. The second lens group moving frame  8  is provided below the vertically-elongated opening  8   t  with a through hole  8   m  which penetrates through the central inner flange  8   s  in the optical axis direction so that the rotation limit shaft  35  can be inserted into the vertically-elongated opening  8   t.    
   The rotation limit shaft  35  is provided with a large diameter portion  35   a , and is provided at a rear end thereof with an eccentric pin  35   b  which projects rearward in the optical axis direction. The axis of the eccentric pin  35   b  is eccentric to the axis of the large diameter portion  35 . The rotation limit shaft  35  is provided at a front end thereof with a recess  35   c  into which the tip of a flatblade screwdriver (not shown) serving as an adjusting tool can be inserted. 
     FIGS. 108 through 112  show a state where the above described assemble parts shown in  FIGS. 102 through 107  are put together, viewed from different angles. A manner of putting the assembled parts together will be discussed hereinafter. 
   First, the front torsion coil spring  39  and the rear torsion coil spring  40  are fixed to the second lens frame  6 . At this time, a coil portion of the front torsion coil spring  39  is fitted on the front spring support portion  6   f  of the pivoted cylindrical portion  6   b  with the rear spring end  39   b  being engaged with a portion of the second lens frame  6  between the pivoted cylindrical portion  6   b  and the swing arm portion  6   c  (see FIG.  104 ). The front spring end  39   a  of the front torsion coil spring  39  is not engaged with any part of the second lens frame  6 . A coil portion of the rear torsion coil spring  40  is fitted on the rear spring support portion  6   g  of the pivoted cylindrical portion  6   b  with the front stationary spring end  40   a  and the rear movable spring end  40   b  being inserted into the second spring engaging hole  6   p  of the swing arm portion  6   c  and the first spring engaging hole  6   k  of the position control arm  6   j , respectively. The front stationary spring end  40   a  is fixed to the second spring engaging hole  6   p  while the rear movable spring end  40   b  is allowed to move in the first spring engaging hole  6   k  in a range “NR 1 ” shown in FIG.  120 . In a free state, the rear torsion coil spring  40  is supported by the second lens frame  6  thereon with the front stationary spring end  40   a  and the rear movable spring end  40   b  being slightly pressed to move in opposite directions approaching each other so that the rear movable spring end  40   b  is in pressing contact with an inner wall surface of the position control arm  6   j  in the first spring engaging hole  6   k  (see FIG.  120 ). The front torsion coil spring  39  is prevented from coming off the front spring support portion  6   f  from the front end thereof in the optical axis direction by the front spring hold projection  6   h , while the rear torsion coil spring  40  is prevented from coming off the rear spring support portion  6   g  from the rear end thereof in the optical axis direction by the rear spring hold projection  6   i.    
   Aside from the installation of the front torsion coil spring  39  and the rear torsion coil spring  40 , the pivot shaft  33  is inserted into the through hole  6   d  after the compression coil spring  38  is inserted into the spring-accommodation large diameter hole  6 Z that is formed in the rear end portion of the rear spring support portion  6   g . At this time, the flange  33   a  of the pivot shaft  33  enters the rear spring support portion  6   g  to contact with the rear end of the compression coil spring  38 . The axial length of the pivot shaft  33  is greater than the axial length of the pivoted cylindrical portion  6   b  so that the opposite ends of the pivot shaft  33  project from the front and rear ends of the pivoted cylindrical portion  6   b , respectively. 
   Concurrent with the above described installation operations to the pivoted cylindrical portion  6   b , the first eccentric shaft  34 X and the second eccentric shaft  34 Y are inserted into the first eccentric shaft support hole  8   f  and the second eccentric shaft support hole  8   i , respectively. As shown in  FIG. 113 , the diameter of a front end portion (left end portion as viewed in  FIG. 113 ) of the large diameter portion  34 X-a of the first eccentric shaft  34 X is greater than the diameter of the remaining portion of the large diameter portion  34 X-a, and the inner diameter of a corresponding front end portion (left end portion as viewed in  FIG. 113 ) of the first eccentric shaft support hole  8   f  is greater than the inner diameter of the remaining portion of the first eccentric shaft support hole  8   f . Likewise, the diameter of a front end portion (left end portion as viewed in  FIG. 113 ) of the large diameter portion  34 Y-a of the second eccentric shaft  34 Y is greater than the diameter of the remaining portion of the large diameter portion  34 Y-a, and the inner diameter of a corresponding front end portion (left end portion as viewed in  FIG. 113 ) of the second eccentric shaft support hole  8   i  is greater than the inner diameter of the remaining portion of the second eccentric shaft support hole  8   i . Therefore, when inserted into the first eccentric shaft support hole  8   f  from the front end thereof (the left end as viewed in FIG.  113 ), the first eccentric shaft  34 X is prevented from being further inserted into the first eccentric shaft support hole  8   f  upon the stepped portion between the large diameter portion  34 X-a and the remaining portion of the first eccentric shaft  34 X contacting with the bottom of the large-diameter front end portion of the first eccentric shaft support hole  8   f  as shown in FIG.  113 . Likewise, when inserted into the second eccentric shaft support hole  8   i  from the front end thereof (the left end as viewed in FIG.  113 ), the second eccentric shaft  34 Y is prevented from being further inserted into the second eccentric shaft support hole  8   i  upon the stepped portion between the large diameter portion  34 Y-a and the remaining portion of the first eccentric shaft  34 Y contacting with the bottom of the large-diameter front end portion of the second eccentric shaft support hole  8   i  as shown in FIG.  113 . In this state, the front eccentric pin  34 X-b and the front eccentric pin  34 Y-b project forward in the optical axis direction from the front fixing surface  8   c  while the rear eccentric pin  34 X-c and the eccentric pin  34 Y-c project rearward in the optical axis direction from the rear fixing surface  8   e.    
   Subsequently, the front second lens frame support plate  36  and the rear second lens frame support plate  37  are fixed to the front fixing surface  8   c  and the rear fixing surface  8   e , respectively, while the front end of the pivot shaft  33 , which projects from the front end of the front spring support portion  6   f  of the pivoted cylindrical portion  6   b , is fitted into the pivot hole  36   b  of the front second lens frame support plate  36  and at the same time the rear end of the pivot shaft  33  is fitted into the pivot hole  37   b  of the rear second lens frame support plate  37 . At this time, the front eccentric pin  34 X-b, the front eccentric pin  34 Y-b and the front boss  8   j  which project forward from the front fixing surface  8   c  are inserted into the first vertically-elongated hole  36   a , the horizontally-elongated hole  36   e  and the second vertically-elongated hole  36   f , respectively, and also the rear eccentric pin  34 X-c, the rear eccentric pin  34 Y-c and the rear boss  8   k  which project rearward from the rear fixing surface  8   e  are inserted into the first vertically-elongated hole  37   a , the horizontally-elongated hole  37   e  and the second vertically-elongated hole  37   f , respectively. The front eccentric pin  34 X-b is movable and immovable in the first vertically-elongated hole  36   a  in the lengthwise direction and the widthwise direction thereof (vertically and horizontally as viewed in FIG.  110 ), respectively, the front eccentric pin  34 Y-b is movable and immovable in the horizontally-elongated hole  36   e  in the lengthwise direction and the widthwise direction thereof (horizontally and vertically as viewed in FIG.  110 ), respectively, and the front boss  8   j  is movable and immovable in the second vertically-elongated hole  36   f  in the lengthwise direction and the widthwise direction thereof (vertically and horizontally as viewed in FIG.  110 ), respectively. Likewise, the rear eccentric pin  34 X-c is movable and immovable in the first vertically-elongated hole  37   a  in the lengthwise direction and the widthwise direction thereof (vertically and horizontally as viewed in FIG.  111 ), respectively, the rear eccentric pin  34 Y-c is movable and immovable in the horizontally-elongated hole  37   e  in the lengthwise direction and the widthwise direction thereof (horizontally and vertically as viewed in FIG.  111 ), respectively, and the rear boss  8   k  is movable and immovable in the second vertically-elongated hole  37   f  in the lengthwise direction and the widthwise direction thereof (vertically and horizontally as viewed in FIG.  111 ), respectively. 
   Lastly, the threaded shaft portion  66   a  of the set screw  66  is inserted into the screw insertion hole  36   d  and the screw insertion hole  8   h , and is screwed through the screw hole  37   d  to fix the front second lens frame support plate  36  and the rear second lens frame support plate  37  to the second lens group moving frame  8 . In this state, screwing down the set screw  66  with the set screw  66  being engaged in the screw hole  37   d  causes the front second lens frame support plate  36  and the rear second lens frame support plate  37  to be pressed against the front fixing surface  8   c  and the rear fixing surface  8   e , respectively, so that the front second lens frame support plate  36  and the rear second lens frame support plate  37  are fixed to the second lens group moving frame  8  with a spacing therebetween which corresponds to the spacing between the front fixing surface  8   c  and the rear fixing surface  8   e  in the optical axis direction. As a result, the first eccentric shaft  34 X and the second eccentric shaft  34 Y are prevented from coming off the second lens group moving frame  8  by the front second lens frame support plate  36  and the rear second lens frame support plate  37 . The front end of the pivoted cylindrical portion  6   b  is pressed against the front second lens frame support plate  36  because the flange  33   a  of the pivot shaft  33  contacts with the rear second lens frame support plate  37  to be prevented from moving rearward beyond the rear second lens frame support plate  37  so that the pivot shaft  33  is biased forward in the optical axis direction by the spring force of the compression coil spring  38  which is compressed in the spring-accommodation large diameter hole  6 Z of the rear spring support portion  6   g . This maintains the position of the second lens frame  6  relative to the second lens group moving frame  8  in the optical axis direction. In a state where the rear second lens frame support plate  37  is fixed to the second lens group moving frame  8 , the guide key insertable recess  37   g  communicates with the key way  8   p  in the optical axis direction (see FIG.  112 ). 
   After the front second lens frame support plate  36  is fixed to the second lens group moving frame  8 , the front spring end  39   a  of the front torsion coil spring  39  is placed into the spring engaging recess  36   g . The rear spring end  39   b  of the front torsion coil spring  39  has been engaged with a portion of the second lens frame  6  between the pivoted cylindrical portion  6   b  and the swing arm portion  6   c  as mentioned above. Placing the front spring end  39   a  into the spring engaging recess  36   g  causes the front torsion coil spring  39  to be twisted, thus causing the second lens frame  6  to be biased to rotate about the pivot shaft  33  in a counterclockwise direction as viewed from front of the second lens frame  6  (counterclockwise as viewed in FIG.  114 ). 
   Aside from the installation of the second lens frame  6 , the rotation limit shaft  35  is inserted into the through hole  8   m  of the second lens group moving frame  8  from the front end of the through hole  8   m . An inner peripheral surface in the through hole  8   m  is formed to prevent the rotation limit shaft  35  from being further inserted into the through hole  8   m  from the position of the rotation limit shaft  35  shown in Figures and  108  and  109 . In this state where the rotation limit shaft  35  is properly inserted into the through hole  8   m , the eccentric pin  35   b  of the rotation limit shaft  35  projects rearward from the rear end of the through hole  8   m  as shown in FIG.  109 . 
   In a state where the second lens frame  6  is properly mounted to the second lens group moving frame  8  in the above described manner, the second lens frame  6  can swing about the pivot shaft  33 . The pivoted cylindrical portion receiving hole  8   g  of the second lens group moving frame  8  is sufficiently large so that the pivoted cylindrical portion  6   b  and the swing arm portion  6   c  may not interfere with the inner edge in the pivoted cylindrical portion receiving hole  8   g  when the second lens frame  6  swings. Since the pivot shaft  33  extends parallel to the photographing optical axis Z 1  and the optical axis of the second lens group LG 2 , the second lens group LG 2  swings about the pivot shaft  33  while the optical axis thereof remaining parallel to the photographing optical axis Z 1  when the second lens frame  6  swings. One end of the range of rotation of the second lens frame  6  about the pivot shaft  33  is determined by the engagement of the tip of the engaging protrusion  6   e  with the eccentric pin  35   b  as shown in FIG.  111 . The front torsion coil spring  39  biases the second lens frame  6  to rotate in a direction to bring the tip of the engaging protrusion  6   e  into contact with the eccentric pin  35   b.    
   Subsequently, the shutter unit  76  is fixed to the second lens group moving frame  8  to obtain a sub-assembly shown in  FIGS. 108 through 112 . As can be seen in  FIGS. 108 through 112 , the shutter unit  76  is fixed to the front of the central inner flange  8   s . In this state where the shutter unit  76  is fixed to the front of the central inner flange  8   s , the front fixing surface  8   c  is positioned in front of the shutter S and the adjustable diaphragm A in the shutter unit  76  in the optical axis direction. A front portion of the cylindrical lens holder portion  6   a  of the second lens frame  6  is positioned in the vertically-elongated opening  8   t , and is also positioned immediately behind the shutter unit  76  regardless of variation of the position of the second lens frame  6  relative to the second lens group moving frame  8  as can be see in  FIGS. 111 and 112 . 
   In a state where the second lens group moving frame  8  and the second linear guide ring  10  are coupled to each other, the flexible PWB  77  that extends from the shutter unit  76  is installed as shown in FIG.  125 . As described above, the wide linear guide key  10   c -W of the second linear guide ring  10  is engaged in the wide guide groove  8   a -W. The flexible PWB  77 , the wide guide groove  8   a -W and the wide linear guide key  10   c -W in a radial direction of the lens barrel axis Z 0  are positioned in the same position in a circumferential direction of the zoom lens  71 . Namely, the flexible PWB  77 , the wide guide groove  8   a -W and the wide linear guide key  10   c -W are aligned in a radial direction perpendicular to the optical axis direction. As shown in  FIG. 125 , the flexible PWB  77  includes a first straight portion  77   a , a loop-shaped turning portion  77   b , a second straight portion  77   c  and a third straight portion  77   d  in this order from the side of the shutter unit  76 . A bend of the flexible PWB  77  is formed between the second straight portion  77   c  and the third straight portion  77   d  in the vicinity of the front end of the wide linear guide key  10   c -W. From the side of the shutter unit  76  (the left side as viewed in FIG.  125 ), firstly the first straight portion  77   a  extends rearward in the optical axis direction from the shutter unit  76 , and subsequently the flexible PWB  77  bends radially outwards to extend forward so that the loop-shaped turning portion  77   b  is formed in the vicinity of the rear end of the second lens group moving frame  8  and so that the second straight portion  77   c  extends forward in the optical axis direction along an inner surface of the wide linear guide key  10   c -W. Subsequently, the flexible PWB bends radially outwards to extend rearward so that the third straight portion  77   d  extends rearward in the optical axis direction along an outer surface of the wide linear guide key  10   c -W. Subsequently, the tip of the third straight portion  77   d  (the tip of the flexible PWB) passes through the radial through hole  10   d  to extend rearward, is further passed through a hole  22   q  (see  FIGS. 4 and 40 ) to extend through to the outer side of the stationary barrel  22 , to be connected to the control circuit  140  via a main circuit board (not shown). The third straight portion  77   d  is partly fixed to the outer surface of the wide linear guide key  10   c -W by a fixing means such as a double-faced tape (not shown) so that the size of the loop-shaped turning portion  77   b  becomes variable in accordance with relative axial movement between the second lens group moving frame  8  and the second linear guide ring  10 . 
   The AF lens frame  51 , which is positioned behind the second lens group moving frame  8 , is made of an opaque material, and is provided with a forwardly-projecting lens holder portion  51   c , a first arm portion  51   d  and a second arm portion  51   e . The first arm portion  51   d  and the second arm portion  51   e  are positioned on radially opposite sides of the forwardly-projecting lens holder portion  51   c . The forwardly-projecting lens holder portion  51   c  is positioned in front of the first arm portion  51   d  and the second arm portion  51   e  in the optical axis direction. The pair of guide holes  51   a  and  52   a , in which the pair of AF guide shafts  52  and  53  are respectively fitted, are formed on the first arm portion  51   d  and the second arm portion  51   e , respectively. The forwardly-projecting lens holder portion  51   c  is formed in a box shape (rectangular ring shape) including a substantially square-shaped front end surface  51   c   1  and four side surfaces  51   c   3 ,  51   c   4 ,  51   c   5  and  51   c   6 . The front end surface  51   c   1  lies in a plane orthogonal to the photographing optical axis Z 1 . The four side surfaces  51   c   3 ,  51   c   4 ,  51   c   5  and  51   c   6  extend rearward in a direction substantially parallel to the photographing optical axis Z 1 , toward the CCD image sensor  60 , from the four sides of the front end surface  51   c   1 . The rear end of the forwardly-projecting lens holder portion  51   c  is formed as an open end which is open toward the low-pass filter LG 4  the CCD image sensor  60 . The forwardly-projecting lens holder portion  51   c  is provided on the front end surface  51   c   1  thereof with a circular opening  51   c   2  the center of which is coincident with the photographing optical axis Z 1 . The third lens group LG 3  is positioned inside the circular opening  51   c   2 . The first arm portion  51   d  and the second arm portion  51   e  extend from the forwardly-projecting lens holder portion  51   c  radially in opposite directions away from each other. More specifically, the first arm portion  51   d  extends from a corner of the forwardly-projecting lens holder portion  51   c  between the two side surfaces  51   c   3  and  51   c   6  radially in a lower-rightward direction as viewed from front of the AF lens frame  51 , while the second arm portion  51 e extends from another corner of the forwardly-projecting lens holder portion  51   c  between the two side surfaces  51   c   4  and  51   c   5  radially in a upper-leftward direction as viewed from front of the AF lens frame  51  as shown in FIG.  130 . As can be seen in  FIGS. 128 and 129 , the first arm portion  51   d  is fixed to the rear end of the corner of the forwardly-projecting lens holder portion  51   c  between the two side surfaces  51   c   3  and  51   c   6  while the second arm portion  51   e  is fixed to the rear end of the corner of the forwardly-projecting lens holder portion  51   c  between the two side surfaces  51   c   4  and  51   c   5 . 
   As shown in  FIG. 9 , radially outwards ends of the first arm portion  51   d  and the second arm portion  51   e  are positioned radially outside a cylindrical wall  22   k  of the stationary barrel  22 . The pair of guide holes  51   a  and  52   a  are respectively formed on radially outer ends of the first arm portion  51   d  and the second arm portion  51   e  which are positioned outside the cylindrical wall  22   k . Accordingly, the AF guide shaft  52 , which is fitted in the guide hole  51   a  and serves as a main guide shaft for guiding the AF lens frame  51  in the optical axis direction with a high positioning accuracy, is positioned outside the cylindrical wall  22   k , while the AF guide shaft  53 , which is loosely fitted in the guide hole  51   b  to serve as an auxiliary guide shaft for secondarily guiding the AF lens frame  51  in the optical axis direction is also positioned outside the cylindrical wall  22   k . As shown in  FIG. 9 , the cylindrical wall  22   k  is provided on the outer peripheral surface thereof with two radial projections  22   t   1  and  22   t   2  provided at different circumferential positions. A shaft-supporting hole  22   v   1  is formed on the rear surface of the radial projection  22   t   1 . Similarly, a shaft-supporting hole  22   v   2  is formed on the rear surface of the radial projection  22   t   2 . The CCD holder  21  is provided on the front surface thereof with two shaft-supporting holes  21   v   1  and  21   v   2  which oppose the shaft-supporting holes  22   v   1  and  22   v   2  in the optical axis direction, respectively. The front end and the rear end of the AF guide shaft  52  are supported by (fixed to) the shaft-supporting hole  22   v   1  and the shaft-supporting hole  21   v   1 , respectively. The front end and the rear end of the AF guide shaft  53  are supported by (fixed to) the shaft-supporting hole  22   v   2  and the shaft-supporting hole  21   v   2 , respectively. 
   The cylindrical wall  22   k  is provided with two cutout portions  22   m  and  22   n  (see  FIG. 11 ) which are cut out along the AF guide shafts  52  and  53  to prevent the second arm portion  51   e  and the first arm portion  51   d  from interfering with the cylindrical wall  22   k  when the AF lens frame  51  moves in the optical axis direction. As shown in  FIGS. 122 and 130 , the pair of guide holes  51   a  and  52   a  are positioned on radially opposite sides of the photographing optical axis Z 1 , and accordingly, the pair of AF guide shafts  52  and  53  are positioned on radially opposite sides of the photographing optical axis Z 1 . 
   The AF lens frame  51  can move rearward in the optical axis direction to a point (rear limit for the axial movement of the AF lens frame  51 ) at which the forwardly-projecting lens holder portion  51   c  comes into contact with the filter holder portion  21   b  (see  FIG. 10 ) formed on a front surface of the CCD holder  21 . In other words, the CCD holder  21  includes a stop surface (front surface of the filter holder portion  21   b ) which determines rear limit for the axial movement of the AF lens frame  51 . In a state where the forwardly-projecting lens holder portion  51   c  is in contact with the filter holder portion  21   b , the front end of the position-control cam bar  21   a , which projects forward from the CCD holder  21 , is positioned in front of the AF lens frame  51  in the optical axis direction (see  FIGS. 121 ,  123  and  124 ). The cam-bar insertable hole  36   c  of the front second lens frame support plate  36  and the cam-bar insertable hole  37   c  of the rear second lens frame support plate  37  are positioned on an axis of the position-control cam bar  21   a . Namely, the cam-bar insertable hole  36   c , the cam-bar insertable hole  37   c  and the position-control cam bar  21   a  are aligned in the optical axis direction. 
   As shown in  FIGS. 103 and 104 , the position-control cam bar  21   a  is provided at a front end thereof with the aforementioned retracting cam surface  21   c  which is inclined with respect to the optical axis direction, and is further provided along an inner side edge of the position-control cam bar  21   a  with a removed-position holding surface  21   d  which extends rearward from the retracting cam surface  21   c  in the optical axis direction. As can be seen in  FIGS. 118 through 120  and  122 , in which the position-control cam bar  21   a  is viewed from front thereof, the position-control cam bar  21   a  has a certain width in a substantially radial direction of the photographing optical axis Z 1 . The retracting cam surface  21   c  is formed as an inclined surface which is inclined forward in a direction from the radially inner side to the radially outer side of the position-control cam bar  21   a  (i.e., from a side closer to the photographing optical axis Z 1  to a side farther from the photographing optical axis Z 1 ), substantially along a widthwise direction of the retracting cam surface  21   c . In other words, the retracting cam surface  21   c  is formed as an inclined surface which is inclined forward in a direction away from the photographing optical axis Z 1 . In  FIGS. 118 through 120 , the retracting cam surface  21   c  is hatched for the purpose of illustration. Moreover, the position-control cam bar  21   a  is formed so that an upper surface and a lower surface of the position-control cam bar  21   a  become a concave surface and a convex surface, respectively, to prevent the position-control cam bar  21   a  from interfering with the pivoted cylindrical portion  6   b  of the second lens frame  6 . In other words, the position-control cam bar  21   a  is formed as a portion a cylinder centered about the pivot shaft  33  of the second lens group  6 , and the retracting cam surface  21   c  is a lead surface which is formed on the periphery (edge surface) of this cylinder. . The position-control cam bar  21   a  is provided on a lower surface thereof with a guide key  21   e  which is elongated in the optical axis direction. The guide key  21   e  extends from the rear end of the position-control cam bar  21   a  to an intermediate point thereon behind the front end of the position-control cam bar  21   a . Therefore, no part of the guide key  21   e  is formed on the position-control cam bar  21   a  in the vicinity of the front end thereof. The guide key  21   e  is formed to have a cross section shape allowed to enter the guide key insertable recess  37   g  in the optical axis direction. 
   Operations of the second lens group LG 2 , the third lens group LG 3  and other associated elements, which are supported by the above described accommodating structure including a structure retracting the second lens frame  6  to the radially retracted position thereof, will be hereinafter discussed. The position of the second lens group moving frame  8  with respect to the CCD holder  21  in the optical axis direction is determined by a combination of the axial movement of the cam ring  11  by the cam diagrams of the plurality of inner cam grooves  11   a  ( 11   a - 1  and  11   a - 2 ) and the axial movement of the cam ring  11  itself. The second lens group moving frame  8  is positioned farthest from the CCD holder  21  when the zoom lens  71  is set at about the wide-angle extremity as shown above the photographing optical axis Z 1  in  FIG. 9 , and is positioned closest to the CCD holder  21  when the zoom lens  71  is in the retracted state as shown in FIG.  10 . The second lens frame  6  is retracted to the radially retracted position thereof by utilizing the retracting rearward movement of the second lens group moving frame  8  from the frontmost axial potion thereof (wide-angle extremity) to the rearmost axial position thereof (retracted position). 
   In the zooming range between the wide-angle extremity and the telephoto extremity, the second lens frame  6  is held still at a fixed position by the engagement of the tip of the engaging protrusion  6   e  with the eccentric pin  35   b  of the rotation limit shaft  35  as shown in FIG.  111 . At this time, the optical axis of the second lens group LG 2  is coincident with the photographing optical axis Z 1 , so that the second lens frame  6  is in a photographing position thereof. When the second lens frame  6  is in a photographing position thereof as shown in  FIG. 111 , a part of the position control arm  6   j  and the rear movable spring end  40   b  of the rear torsion coil spring  40  are exposed to the rear of the second lens group moving frame  8  through the cam-bar insertable hole  37   c.    
   Upon the main switch of the digital camera  70  being turned OFF in the ready-to-photograph state of the zoom lens  71 , the control circuit  140  drives the AF motor  160  in the lens barrel retracting direction to move the AF lens frame  51  rearward, toward the CCD holder  21  to a rearmost position (retracted position) thereof as shown in  FIGS. 121 ,  123  and  124 . The forwardly-projecting lens holder portion  51   c  holds the third lens group LG 3  therein in the vicinity of the front end surface  51   c   1 . The space immediately behind the third lens group LG 3  is provided as an open space surrounded by the four side surfaces  51   c   3 ,  51   c   4 ,  51   c   5  and  51   c   6  so that the low-pass filter LG 4  and the CCD image sensor  60 , which are supported by the CCD holder  21  (the filter holder portion  21   b ), can enter the space immediately behind the third lens group LG 3  so as to reduce the space between the third lens group LG 3  and the low-pass filter LG 4  when the AF lens frame  51  is retracted to the rearmost position. In a state where the AF lens frame  51  is in the rearmost position as shown in  FIG. 10 , the front end of the position-control cam bar  21   a  is positioned in front of the AF lens frame  51  in the optical axis direction. 
   Subsequently, the control circuit  140  drives the zoom motor  150  in the lens barrel retracting direction to perform the above described lens barrel retracting operation. Keep driving the zoom motor  150  in the lens barrel retracting direction beyond the wide-angle extremity of the zoom lens  71  causes the cam ring  11  to move rearward in the optical axis direction while rotating about the lens barrel axis Z 0  due to engagement of the set of three roller followers  32  with the set of three through-slots  14 e, respectively. As can be understood from the relationship shown in  FIG. 17  between the plurality of inner cam grooves  11   a  and the plurality of cam followers  8   b , even though the second lens group moving frame  8  is positioned closer to the front of the zoom lens  71  in the optical axis direction relative to the cam ring  11  when the zoom lens  71  is in the retracted position than that when the zoom lens  71  is in the wide-angle extremity, the second lens group moving frame  8  comes near the CCD holder  21  when the zoom lens  71  is in the retracted state because the amount or rearward movement of the cam ring  11  relative to the stationary barrel  22  is greater than the amount of forward movement of the second lens group moving frame  8  in the cam ring  11  relative to the cam ring  11  in the lens barrel retracting operation. 
   A further retracting movement of the second lens group moving frame  8  together with the second lens frame  6  causes the front end of the position-control cam bar  21   a  to enter the cam-bar insertable hole  37   c  (see FIG.  105 ). As described above, a part of the position control arm  6   j  and the rear movable spring end  40   b  of the rear torsion coil spring  40  are exposed to the rear of the second lens group moving frame  8  through the cam-bar insertable hole  37   c  as shown in FIG.  111 .  FIG. 118  shows the positional relationship at this time among the position control arm  6   j , the rear movable spring end  40   b  and the position-control cam bar  21   a , viewed from the front of the zoom lens  71 . The rear movable spring end  40   b  is positioned closer to the position-control cam bar  21   a  than the position control arm  6   j  (except for a protrusion formed thereon for the formation of the first spring engaging hole  6   k ) in a radial direction of the photographing optical axis Z 1 . On the other hand, the retracting cam surface  21   c  is formed as an inclined surface which is inclined forward in a direction away from the photographing optical axis Z 1 . A frontmost portion of the retracting cam surface  21   c  is positioned immediately behind the rear movable spring end  40   b  of the rear torsion coil spring  40  in the state shown in  FIG. 118. A  rearward movement of the second lens frame  6  together with the second lens group moving frame  8  toward the CCD holder  21  with the positional relationship shown in  FIG. 118  being maintained causes the retracting cam surface  21   c  to come into contact with the rear movable spring end  40   b , not the position control arm  6   j  of the second lens frame  6 .  FIG. 123  shows the position of the second lens frame  6  at the time immediately before the rear movable spring end  40   b  comes into contact with the retracting cam surface  21   c.    
   A further rearward movement of the second lens frame  6  together with the second lens group moving frame  8  with the rear movable spring end  40   b  remaining in contact with the retracting cam surface  21   c  causes the rear movable spring end  40   b  to slide on the retracting cam surface  21   c  in a clockwise direction as viewed in  FIG. 118  in accordance with the shape of the retracting cam surface  21   c . This clockwise rotation of the rear movable spring end  40   b  is transferred to the second lens group  6  via the front stationary spring end  40   a . The spring force (rigidity) of the rear torsion coil spring  40  is predetermined to be capable of transferring a torque from the rear movable spring end  40   b  to the second lens group  6  via the front stationary spring end  40   a  without the front stationary spring end  40   a  and the rear movable spring end  40   b  being further pressed to move in opposite directions approaching each other than those shown in  FIGS. 118 through 120 . Namely, the resiliency of the rear torsion coil spring  40  is determined to be greater than that of the front torsion coil spring  39  at the time the front torsion coil spring  39  holds the second lens frame  6  in the photographing position. 
   Upon receiving a turning force from the retracting cam surface  21   c  via the rear torsion coil spring  40 , the second lens group  6  rotates about the pivot shaft  33  against the spring force of the front torsion coil spring  39  from the photographing position shown in  FIG. 111  toward the radially retracted position shown in  FIG. 112  in accordance with the retracting movement of the second lens group moving frame  8 . With this rotation of the second lens group  6 , the rear movable spring end  40   b  of the rear torsion coil spring  40  slides on the retracting cam surface  21   c  from the position shown in  FIG. 118  to the position shown in FIG.  119 . Upon the second lens frame  6  rotating to the radially retracted position shown in  FIG. 112 , the rear movable spring end  40   b  moves from the retracting cam surface  21   c  to the removed-position holding surface  21   d  to be engaged therewith. Thereafter, the second lens frame  6  is not rotated about the pivot shaft  33  in a direction to the radially retracted position by a retracting movement of the second lens group moving frame  8 . In a state where the second lens frame  6  is held in the radially retracted position as shown in  FIG. 112 , an outer peripheral portion of the cylindrical lens holder portion  6   a  enters the radial recess  8   q  while an outer edge of the engaging protrusion  6   e  enters the second radial recess  8   r  of the second lens group moving frame  8 . 
   After the second lens frame  6  reaches the radially retracted position, the second lens group moving frame  8  continues to move rearward until reaching the retracted position shown in FIG.  10 . During this rearward movement of the second lens group moving frame  8 , the second lens group  6  moves rearward together with the second lens group moving frame  8  to the position shown in  FIG. 124  with the second lens group  6  held in the radially retracted position, in which the rear movable spring end  40   b  remains in engaged with the retracting cam surface  21   c . At this time, the front end of the position-control cam bar  21   a  projects forward from the cam-bar insertable hole  37   c  through the cam-bar insertable hole  36   c  and the pivoted cylindrical portion receiving hole  8   g.    
   As shown in  FIGS. 10 and 124 , in the retracted state of the zoom lens  71 , the cylindrical lens holder portion  6   a  of the second lens frame  6  has moved into the space immediately above the forwardly-projecting lens holder portion  51   c , the forwardly-projecting lens holder portion  51   c  has moved into that space in the second lens group moving frame  8  in which the second lens group LG 2  is positioned in the ready-to-photograph state of the zoom lens  71 , and the third lens group LG 3  is positioned immediately behind the shutter unit  76 . In addition, the low-pass filter LG 4  and the CCD image sensor  60  have entered the forwardly-projecting lens holder portion  51   c  from the rear thereof by a rearward movement of the forwardly-projecting lens holder portion  51   c , and accordingly, the space between the third lens group LG 3  and the low-pass filter LG 4  and also the space between the third lens group LG 3  and the CCD image sensor  60  in the optical axis direction are smaller in the retracted state of the zoom lens  71  than those in the ready-to-photograph state of the zoom lens  71  as can be seen by making a comparison between  FIGS. 9 and 10 . Namely, in the retracted state of the zoom lens  71 , the second lens group LG 2  is positioned in the space radially outside the space in which the third lens group LG 3 , the low-pass filter LG 4  and the CCD image sensor  60  are positioned. In a conventional photographing lens barrel including a plurality of optical elements in which one or more movable optical elements thereof are moved only along a photographing optical axis, it is impossible to make the length of the photographing lens barrel smaller than the sum of the thicknesses of all the plurality of optical elements. However, according to the accommodating structure of the zoom lens  71 , it is substantially unnecessary to secure any space for accommodating the second lens group LG 2  on the photographing optical axis Z 1 . This makes it possible to make the length of the zoom lens  71  smaller than the sum of the thicknesses of all the plurality of optical elements of the zoom lens  71 . 
   In the present embodiment of the zoom lens, the AF lens frame  51  has various features in its shape and supporting structure that make it possible to retract the zoom lens  71  in the camera body  72  in a highly space-saving fashion. Such features will be hereinafter discussed in detail. 
   The AF guide shaft  52  ,which serves as a main guide shaft for guiding the AF lens frame  51  in the optical axis direction with a high positioning accuracy, and the AF guide shaft  53 , which serves as an auxiliary guide shaft for secondarily guiding the AF lens frame  51  in the optical axis direction, are positioned outside cylindrical wall  22   k  of the stationary barrel  22  on radially opposite sides of the photographing optical axis Z 1  (at positions not interfering with any of the movable lens groups of the zoom lens  71 ). This structure of the AF lens frame  51  contributes to a reduction of the length of the zoom lens  71  when the zoom lens  71  is retracted into the camera body  72  because neither the AF guide shaft  52  nor the AF guide shaft  53  becomes an obstruction which interferes with one or more of the first through third lens groups LG 1 , LG 2  and LG 3  and the low-pass filter LG 4 . 
   In other words, according to such a structure of the AF lens frame  51 , since the pair of AF guide shafts  52  and  53  can be disposed freely without being subject to constraints by moving parts positioned in the stationary barrel  22  such as the second lens frame  6 , the effective length of each of the AF guide shafts  52  and  53  for guiding the AF lens frame  51  in the optical axis direction can be made long enough to guide the AF lens frame  51  in the optical axis direction with a high positioning accuracy. As can be seen in  FIGS. 9 and 10 , the LCD panel  20  is positioned immediately behind the zoom lens barrel  71  (on a rearward extension line of the optical axis Z 1 ) while the pair of AF guide shafts  52  and  53  are positioned outside the LCD panel  20  in radial directions of the lens barrel axis Z 0 . This arrangement achieves the pair of AF guide shafts  52  and  53  having long axial lengths which are largely extended even toward the rear of the camera body  72  without interfering with the LCD panel  20  that is comparatively large in dimension. In practice, the rear end of the AF guide shaft  52  is extended to a position below the LCD panel  20  in the camera body  72  as shown in FIG.  9 . 
   Additionally, an annular space which is surrounded by the outer peripheral surface of the forwardly-projecting lens holder portion  51   c , the first arm portion  51   d , the second arm portion  51   e  and the inner peripheral surface of the stationary barrel  22  (the AF guide shafts  52  and  53 ) is secured due to the structure wherein the AF lens frame  51  is shaped so that the first arm portion  51   d  extends radially outwards from the rear end of the corner of the forwardly-projecting lens holder portion  51   c  between the two side surfaces  51   c   3  and  51   c   6  and so that the second arm portion  51   e  extends radially outwards from the rear end of the corner of the forwardly-projecting lens holder portion  51   c  between the two side surfaces  51   c   4  and  51   c   5 . This annular space is used to accommodate not only the second lens group LG 2  but also rear end portions of annular members such as the first through third external barrels  12 ,  13  and  15  and the helicoid ring  18  to maximize the utilization of the internal space of the camera body  72 . Moreover, the annular space contributes to a further retraction of the zoom lens  71  in the camera body  72  (see FIG.  10 ). If the AF lens frame  51  does not have the above described space-saving structure, e.g., if each of the first and second arm portions  51   d  and  51   e  is formed on the forwardly-projecting lens holder portion  51   c  to extend radially from an axially intermediate portion or an axially front end portion thereof unlike the present embodiment of the zoom lens, such elements as the second lens group L 2  cannot be retracted to their respective positions shown in FIG.  10 . 
   In addition, in the present embodiment of the zoom lens, the AF lens frame  51  is constructed so that the third lens group LG 3  is supported by the forwardly-projecting lens holder portion  51   c  in a front end space thereof and so that the low-pass filter LG 4  and the CCD image sensor  60  are accommodated in the space in the rear of the forwardly-projecting lens holder portion  51   c  in the retracted state of the zoom lens  71 . This further maximizes the utilization of the internal space of the zoom lens  71 . 
   Upon the main switch of the digital camera  70  being turned ON in the retracted state of the zoom lens  71 , the control circuit  140  drives the AF motor  160  in the lens barrel advancing direction so that the above described moving parts operate in the reverse manner to the above described retracting operations. The cam ring  11  advances while rotating relative to the first linear guide ring  14  and at the same time the second lens group moving frame  8  and the first external barrel  12  advance together with the cam ring  11  without rotating relative to the first linear guide ring  14 . At an initial stage of the advancement of the second lens group moving frame  8 , the second lens frame  6  remains in the radially retracted position since the rear movable spring end  40   b  is still engaged with the removed-position holding surface  21   d . A further forward movement of the second lens group moving frame  8  causes the rear movable spring end  40   b  to firstly reach the front end of the position-control cam bar  21   a  and subsequently be disengaged from the removed-position holding surface  21   d  to be engaged-with the retracting cam surface  21   c  as shown in FIG.  120 . At this stage, the cylindrical lens holder portion  6   a  of the second lens frame  6  has moved ahead of the forwardly-projecting lens holder portion  51   c  in the optical axis direction, so that the cylindrical lens holder portion  6   a  does not interfere with the forwardly-projecting lens holder portion  51   c  even if the second lens frame  6  commences to rotate about the pivot shaft  33  in a direction to the photographing position. A further forward movement of the second lens group moving frame  8  causes the rear movable spring end  40   b  to slide on the retracting cam surface  21   c  so that the second lens frame  6  starts rotating from the radially retracted position to the photographing position by the spring force of the front torsion coil spring  39 . 
   A further forward movement of the second lens group moving frame  8  firstly causes the rear movable spring end  40   b  to keep sliding on the retracting cam surface  21   c  in a direction away from the removed-position holding surface  21   d  (left to right as viewed in FIG.  118 ), and subsequently causes the rear movable spring end  40   b  to be disengaged from the retracting cam surface  21   c  upon the rear movable spring end  40   b  moving to a predetermined point on the retracting cam surface  21   c . At this time, the relative position between the rear movable spring end  40   b  and the retracting cam surface  21   c  as viewed from front of the second lens frame  6  corresponds to that shown in FIG.  118 . As a result, the second lens frame  6  becomes totally free from the constraint of the position-control cam bar  21   a . Consequently, the second lens frame  6  is held in the photographing position as shown in  FIG. 111  with the tip of the engaging protrusion  6   e  being in pressing contact with the eccentric pin  35   b  of the rotation limit shaft  35  by the spring force of the front torsion coil spring  39 . Namely, the optical axis of the second lens group LG 2  coincides with the photographing optical axis Z 1 . The second lens frame  6  finishes rotating from the radially retracted position to the photographing position by the time the zoom lens  71  has been extended to the wide-angle extremity when the main switch of the digital camera  70  is turned ON. 
   Although the AF lens frame  51  moves forward from its rearmost position when the zoom lens  71  changes from the retracted state shown in  FIG. 10  to the ready-to-photograph state shown in  FIG. 9 , the forwardly-projecting lens holder portion  51   c  still covers the front of the low-pass filter LG 4  and the CCD image sensor  60  even in the ready-to-photograph state shown in  FIG. 9  so that the front end surface  51   c   1  and the four side surfaces  51   c   3 ,  51   c   4 ,  51   c   5  and  51   c   6  can prevent unnecessary light such as stray light from being incident on the low-pass filter LG 4  and the CCD image sensor  60  through any part other than the third lens group LG 3 . Accordingly, the forwardly-projecting lens holder portion  51   c  of the AF lens frame  51  serves as not only a member for supporting the third lens group LG 3  but also a member for accommodating the low-pass filter LG 4  and the CCD  60  in the retracted state of the zoom lens  71 , and also a light shield member for preventing unnecessary light such as stray light from being incident on the low-pass filter LG 4  and the CCD image sensor  60  in the ready-to-photograph state of the zoom lens  71 . 
   In general, a structure supporting a movable lens group of a photographing lens system must be precise so as not to deteriorate the optical performance of the photographing lens system. In the present embodiment of the zoom lens, each of the second lens frame  6  and the pivot shaft  33 , in particular, is required to have high dimensional accuracy which is several orders of magnitude higher than those of simple movable elements since the second lens group LG 2  is driven to not only move along the photographing optical axis Z 1  but also rotate to retract to the radially retracted position. For instance, with the shutter unit  76  (having exposure control devices such as the shutter S and the diaphragm A) provided inside the second lens group moving frame  8 , if a pivot shaft corresponding to the pivot shaft  33  is provided in front of or behind the shutter unit  76 , the length of the pivot shaft would be limited, or would make the pivot shaft act as a cantilever type pivot shaft. Nevertheless, since it is necessary to secure a minimum clearance allowing the pivot shaft (such as the pivot shaft  33 ) and a through hole (such as the through hole  6   d ) into which the pivot shaft is fitted to rotate relative to each other, such a clearance may cause the axis of the through hole to tilt relative to the axis of the pivot shaft if the pivot shaft is a short shaft or a cantilever pivot shaft. Even if within tolerance in a conventional lens supporting structure, such a tilt must be prevented from occurring in the present embodiment of the zoom lens because each of the second lens frame  6  and the pivot shaft  33  is required to have a very high dimensional accuracy. 
   In the above described retracting structure for the second lens frame  6 , since it can be seen in  FIGS. 108 ,  109  and  113  that the front second lens frame support plate  36  and the rear second lens frame support plate  37  are respectively fixed to the front fixing surface  8   c  and the rear fixing surface  8   e , which are respectively positioned on front and rear of the shutter unit  76  in the optical axis direction, and that the pivot shaft  33  is disposed to extend between the front second lens frame support plate  36  and the rear second lens frame support plate  37 , both the front end and the rear end of the pivot shaft  33  are supported by the front second lens frame support plate  36  and the rear second lens frame support plate  37 , respectively. Accordingly, the axis of the pivot shaft  33  does not easily tilt with respect to the axis of the through hole  6   d  of the second lens frame  6 . Moreover, the pivot shaft  33  can be lengthened regardless of the shutter unit  76  (without interfering with the shutter unit  76 ) since the front second lens frame support plate  36 , the rear second lens frame support plate  37  and the pivoted cylindrical portion receiving hole  8   g , which serve as elements of the structure supporting the pivot shaft  33 , are positioned not to overlap the shutter unit  76 . In fact, the pivot shaft  33  is elongated so that the length thereof becomes close to the length of the second lens group moving frame  8  in the optical axis direction. In accordance with the length of the pivot shaft  33 , the pivoted cylindrical portion  6   b  is elongated in the optical axis direction. Namely, a wide range of engagement in the axial direction is secured between the pivoted cylindrical portion  6   b  and the pivot shaft  33 . With this structure, there is little possibility of the second lens frame  6  from tilting with respect to the pivot shaft  33 , which makes it possible to rotate the second lens frame  6  about the pivot shaft  33  with a high degree of positioning accuracy. 
   The front boss  8   j  and the rear boss  8   k  that project from the front fixing surface  8   c  and the rear fixing surface  8   e  determine the position of the front second lens frame support plate  36  and the position of the rear second lens frame support plate  37 , respectively, and the front and rear second lens frame support plates  36  and  37  are firmly fixed to the second lens group moving frame  8  by the common set screw  66 . With this structure, the front and rear second lens frame support plates  36  and  37  are positioned relative to the second lens group moving frame  8  with a high degree of positioning accuracy. Therefore, the pivot pin  33  is also positioned relative to the second lens group moving frame  8  with a high degree of positioning accuracy. 
   In the present embodiment of the zoom lens, the set of three extensions  8   d  are formed on the front end surface of the second lens group moving frame  8  in front of the front fixing surface  8   c , whereas the rear fixing surface  8   e  is flush with the rear end surface of the second lens group moving frame  8 . Namely, the front fixing surface  8   c  is not formed on the frontmost end surface of the second lens group moving frame  8 . However, if the second lens group moving frame  8  is formed as a simple cylindrical member having no projections such as the set of three extensions  8   d , the front and rear second lens frame support plates  36  and  37  can be fixed to frontmost and rearmost end surfaces of the simple cylindrical member, respectively. 
   In the above described retracting structure for the second lens frame  6 , if the range of movement of the second lens group moving frame  8  in the optical axis direction from the position corresponding to the wide-angle extremity to the retracted position is fully used to rotate the second lens frame  6  about the pivot shaft  33  from the photographing position to the radially retracted position, the second lens frame  6  will interfere with the forwardly-projecting lens holder portion  51   c  of the AF lens frame  51  on the way to the radially retracted position. To prevent this problem from occurring, in the above described retracting structure for the second lens frame  6 , the second lens frame  6  finishes rotating to the radially retracted position within an axial range of movement sufficiently shorter than the range of movement of the second lens group moving frame  8  in the optical axis direction, and subsequently the cylindrical lens holder portion  6   a  of the second lens frame  6  moves rearward in parallel in the optical axis direction to the space immediately above the forwardly-projecting lens holder portion  51   c . Therefore, the space for the parallel displacement of the cylindrical lens holder portion  6   a  to the space immediately above the forwardly-projecting lens holder portion  51   c  must be secured in the zoom lens  71 . In order for the second lens frame  8  to secure a sufficient range of rotation from the photographing position to the radially retracted position within a short range of movement in the optical axis direction, it is necessary to increase the inclination of the retracting cam surface  21   c , that is formed on the front end of the position-control cam bar  21   a  of the CCD holder  21 , with respect to the direction of movement of the second lens group moving frame  8 , i.e., with respect to the optical axis direction. While the retracting cam surface  21 c that is formed in such a manner presses the rear movable spring end  40   b  during the rearward movement of the second lens group  8 , a great reaction force is exerted on the position-control cam bar  21   a  and the second lens group moving frame  8 ; such a reaction force is greater than that in the case where a cam surface (which corresponds to the cam surface  21   c ) the inclination of which with respect to the direction of movement of the second lens group moving frame  8  is small presses the rear movable spring end  40   b  during the rearward movement of the second lens group  8 . 
   The position-control cam bar  21   a  is a fixed member just like the stationary barrel  22 , whereas the second lens group moving frame  8  is a linearly movable member; the second lens group moving frame  8  is guided linearly without rotating about the lens barrel axis Z 0  indirectly by the stationary barrel  22  via such intermediate members as the first and second linear guide rings  14  and  10 , not directly by the stationary barrel  22 . A clearance exits in each of the following two engagements: the engagement of the second lens group moving frame  8  with the second linear guide ring  10  and the engagement of the second linear guide ring  10  with the second linear guide ring  14 . Due to this reason, it has to be taken into account that such clearances may cause the second lens group moving frame  8  and the CCD holder  21  to become misaligned in the plane orthogonal to the lens barrel axis Z 0  to thereby exert an averse effect on the retracting operation for the second Lens frame  6  from the photographing position to the radially retracted position if a great reaction force is exerted on the position-control cam bar  21   a  and the second lens group moving frame  8 . For instance, if the second lens frame  6  rotates beyond an original radial-outer limit thereof (see  FIG. 112 ) for the rotational movement of the second lens frame  6  about the pivot shaft  33  when rotated from the photographing position to the radially retracted position, the cylindrical lens holder portion  6   a  may interfere with an inner peripheral surface of the second lens group moving frame  8 . Likewise, if the second lens frame  6  stops rotating before the original radial-outer limit when rotated from the photographing position to the radially retracted position, i.e., if the second lens frame  6  does not rotate to the original radial-outer limit when rotated from the photographing position to the radially retracted position, the cylindrical lens holder portion  6   a  may interfere with the AF lens frame  51  and others. 
   The position-control cam bar  21   a  and the second lens group moving frame  8  are prevented from being misaligned by inserting the guide key  21   e  into the guide key insertable recess  37   g  to hold the second lens frame  6  precisely in the radially retracted position when the second lens frame  6  rotates from the photographing position to the radially retracted position (see FIG.  106 ). Specifically, when the second lens group moving frame  8  is in the process of retracting toward the retracted position with the second lens frame  6  having been held in the radially retracted position by the engagement of the rear movable spring end  40   b  of the rear torsion coil spring  40  with the removed-position holding surface  21   d , the guide key  21   e  enters the key way  8   p  of the second lens group moving frame  8  from the rear end thereof through the guide key insertable recess  37   g . Since the guide key  21   e  and the key way  8   p  are an elongated projection and an elongated groove which extend in the optical axis direction, the guide key  21   e  is freely movable relative to the key way  8   p  in the optical axis direction and prevented from moving in a widthwise direction of the key way  8   p  when the guide key  21   e  is engaged in the key way  8   p . Due to this structure, even if a comparatively great reaction force is exerted on the second lens group moving frame  8  while the retracting cam surface  21   c  presses the rear movable spring end  40   b , the engagement of the guide key  21   e  with the key way  8   p  prevents the second lens group moving frame  8  and the position-control cam bar  21   a  from being misaligned in the plane orthogonal to the lens barrel axis Z 0 . Consequently, the second lens frame  6  is held precisely in the radially retracted position when the second lens frame  6  rotates from the photographing position to the radially retracted position. 
   Although the guide key  21   e  commences to be engaged in the key way  8   p  after the second lens frame  6  has been rotated to the radially retracted position in the present embodiment of the zoom lens, the guide key  21   e  can commence to be engaged in the key way  8   p  before the second lens frame  6  has been rotated to the radially retracted position or during the retracting movement of the second lens frame  6  toward the radially retracted position. In short, the second lens group moving frame  8  and the position-control cam bar  21   a  have only to be precisely aligned at the time when the second lens frame  6  is held in the radially retracted position after all. The timing of commencement of the engagement between the guide key  21   e  with the key way  8   p  can be freely determined by, e.g., changing the axial range of formation of the guide key  21   e  in the optical axis direction. 
   It is possible that the guide key  21   e  and the key way  8   p  be replaced by a key way corresponding to the key way  8   p  and a guide key corresponding to the guide key  21   e , respectively. 
   Although the guide key  21   e  is formed on the position-control cam bar  21   a  which includes the retracting cam surface  21   c  in the above illustrated embodiment, an element corresponding to the guide key  21   e  can be formed on any portion on the CCD holder  21  other than the position-control cam bar  21   a . However, from a structural point of view, it is desirable that the guide key  21   e  be formed together with the retracting cam surface  21   c  on the position-control cam bar  21   a . In addition, to align the second lens group moving frame  8  and the position-control cam bar  21   a  precisely, it is desirable that the guide key  21   e  be formed on the position-control cam bar  21   a  which serves as an engaging portion which is engageable with the second lens frame  6  through the side second lens group moving frame  8 . 
   Not only the aforementioned reaction force which is exerted on the second lens group moving frame  8  while the retracting cam surface  21   c  presses the rear movable spring end  40   b , but also the positioning accuracy of each element of the retracting structure for the second lens frame  6  exert an adverse influence on the operating accuracy of the second lens frame  6 . As described above, it is undesirable if the range of rotation of the second lens frame  6  about the pivot shaft R 3  from the photographing position to the radially retracted position is either excessive or insufficient. However, if a force which may retract the second lens frame  6  beyond the radially retracted position shown in  FIG. 112  is applied to the second lens frame  6 , a mechanical stress is applied to the retracting structure for the second lens frame  6  because cylindrical lens holder portion  6   a  and the engaging protrusion  6   e  are brought very close to an inner peripheral surface of the second lens group moving frame  8  in the retracted state of the zoom lens  71  to achieve a space-saving retracting structure for the second lens frame  6  (see FIG.  112 ). Accordingly, it is required to prevent such a mechanical stress from being applied to the retracting structure for the second lens frame  6 . 
   To prevent such mechanical stress from being applied to the retracting structure for the second lens frame  6 , rather than the position control arm  6   j  of the pivoted cylindrical portion, the rear movable spring end  40   b  of the rear torsion coil spring  40  serves as a portion which is to be engageable with the retracting cam surface  21   c  and the removed-position holding surface  21   d  when the second lens frame  6  retracts from the photographing position to the radially retracted position so that a slight error in movement of the second lens group  6  is absorbed by a resilient deformation of the rear torsion coil spring  40 . Although the rear torsion coil spring  40  transfers a torque from the rear movable spring end  40   b  to the second lens group  6  via the front stationary spring end  40   a  without the front stationary spring end  40   a  and the rear movable spring end  40   b  being further pressed to move in opposite directions approaching each other than those shown in  FIGS. 118 through 120  as mentioned above in a normal retracting operation of the zoom lens  71 , the rear movable spring end  40   b  is further pressed to move in a direction approaching the front stationary spring end  40   a  than the rear movable spring end  40   b  shown in  FIGS. 118 through 120  within the range q 1  shown in  FIG. 120  if the position-control cam bar  21   a  slightly deviates leftward, as viewed in  FIG. 120  from the original position shown in  FIG. 120 , since the rear movable spring end  40   b  is allowed to move in the first spring engaging hole  6   k  in the range q 1  as mentioned above. Accordingly, such a movement of the rear movable spring end  40   b  within the range NR 1  can absorb the deviation of the position-control cam bar  21   a  from the original position thereof. Namely, even if the position-control cam bar  21   a  further presses the rear movable spring end  40   b  in a state where the cylindrical lens holder portion  6   a  and the engaging protrusion  6   e  are in contact with an inner peripheral surface of the second lens frame moving frame  8  (in a state where an outer peripheral portion of the cylindrical lens holder portion  6   a  and an outer edge of the engaging protrusion  6   e  have entered the radial recess  8   q  and the second radial recess  8   r , respectively), an excessive mechanical stress is prevented from being applied to the retracting structure for the second lens frame  6  by a resilient deformation of the rear torsion coil spring  40 . 
   In the retracting structure for the second lens frame  6 , when the second lens frame  6  is in the radially retracted position as shown in  FIG. 112 , a radially outside surface of the swing arm portion  6   c  is positioned to adjoin the bottom of the wide guide groove  8   a -W to partly close the bottom of the wide guide groove  8   a -W. In other words, the bottom of the wide guide groove  8   a -W is formed on the radially outside of an intermediate point of a line extending between the axis of the pivot shaft  33  and the retracted optical axis Z 2  of the second lens group LG 2 , and a part of the flexible PWB  77  is positioned in the wide guide groove  8   a -W. Due to this structure, the swing arm portion  6   c  supports this part of the flexible PWB  77  from inside the second lens group moving frame  8  as shown in  FIG. 112  when the second lens frame  6  is positioned in the radially retracted position.  FIG. 126  shows the flexible PWB  77  and the second lens frame  6  by solid lines when the second lens frame  6  is positioned in the radially retracted position, and shows the second lens frame  6  by two-dot chain lines when the second lens frame  6  is positioned in the photographing position. It can be understood from  FIG. 126  that the swing arm portion  6   c  prevents the flexible PWB  77  from curving radially inwards by pushing the first straight portion  77   a  and the loop-shaped turning portion  77   b  of the flexible PWB  77  radially outwards 
   Specifically, the swing arm portion  6   c  is provided on a radially outer surface thereof with a straight flat surface  6   q , and is further provided immediately behind the straight flat surface  6   q  with an oblique surface  6   r . The rear projecting portion  6   m  projects rearward in the optical axis direction from a portion of the swing arm portion  6   c  immediately behind the straight flat surface  6   q  (see FIG.  105 ). In the retracted state of the zoom lens  71 , the straight flat surface  6   q  pushes the first straight portion  77   a  radially outwards while the oblique surface  6   r  and the rear projecting portion  6   m  push the loop-shaped turning portion  77   b  radially outwards. The oblique surface  6   r  is inclined to correspond to a curve of the loop-shaped turning portion  77   b.    
   In typical retractable lenses, in the case where a flexible PWB extends between a movable element guided in an optical axis direction and a fixed element, the flexible PWB needs to be sufficiently long to cover the full range of movement of the movable element. Therefore, the flexible PWB tends to sag when the amount of advancement of the movable element is minimum, i.e., when the retractable lens is in the retracted state. Such a tendency of the flexible PWB is especially strong in the present embodiment of the zoom lens because the length of the zoom lens  71  is greatly reduced in the retracted state thereof by retracting the second lens group so that it is positioned on the retracted optical axis Z 2  and also by adopting a three-stage telescoping structure for the zoom lens  71 . Since interference of any sag of the flexible PWB with internal elements of the retractable lens or jamming of a sagging portion of the flexible PWB into internal elements of the retractable lens may cause a failure of the retractable lens, it is necessary for the retractable lens to be provided with a structure preventing such problems associated with the flexible PWB from occurring. However, this preventing structure is generally complicated in conventional retractable lenses. In the present embodiment of the zoom lens  71 , in the view of the fact that the flexible PWB  77  tends to sag when the zoom lens  71  is in the retracted state, the loop-shaped turning portion  77   b  is pushed radially outwards by the second lens frame  6  positioned in the radially retracted position, which reliably prevents the flexible PWB  77  from sagging with a simple structure. 
   In the retracting structure for the second lens frame  6  in the present embodiment of the zoom lens, the moving path of the second lens frame  6  from the photographing position to the radially retracted position extends obliquely from a point (front point) on the photographing optical axis Z 1  to a point (rear point) behind the front point and above the photographing optical axis Z 1  because the second lens frame  6  moves rearward in the optical axis direction while rotating about the pivot shaft  33 . On the other hand, the AF lens frame  51  is provided thereon between the front end surface  51   c   1  and the side surface  51   c   5  with a recessed oblique surface  51   h . The recessed oblique surface  51   h  is inclined in a radially outward direction from the photographing optical axis Z 1  from front to rear of the optical axis direction. The edge of the forwardly-projecting lens holder portion  51   c  between the front end surface  51   c   1  and the side surface  51   c   5  is cut out along a moving path of the cylindrical lens holder portion  6   a  so as to form the recessed oblique surface  51   h . Moreover, the recessed oblique surface  51   h  is formed as a concave surface which corresponds to the shape of an associated outer surface of the cylindrical lens holder portion  6   a.    
   As described above, the AF lens frame  51  moves rearward to the rear limit for the axial movement thereof (i.e., the retracted position), at which the AF lens frame  51  (forwardly-projecting lens holder portion  51   c ) comes into contact with the filter holder portion  21   b  (stop surface), before the commencement of retracting movement of the second lens frame  6  from the photographing position to the radially retracted position. In the state shown in  FIG. 123  in which the AF lens frame  51  is in contact with the filter holder portion  21   b  while the second lens frame  6  has not commenced to retract from the photographing position to the radially retracted position, if the second lens frame  6  starts moving rearward in the optical axis direction while rotating about the pivot shaft  33  to retract to the radially retracted position, the rear end of the cylindrical lens holder portion  6   a  firstly moves obliquely rearward while approaching the recessed oblique surface  51   h , and subsequently further moves obliquely rearward while just missing (passing closely across) the recessed oblique surface  51   h  to finally reach a fully retracted position shown in FIG.  124 . Namely, the retracting operation for the second lens frame  6  from the photographing position to the radially retracted position can be performed at a closer point to the AF lens frame  51  in the optical axis direction substantially by the amount by which the oblique surface  51   h  is recessed. 
   If the recessed oblique surface  51   h  or a similar surface is not formed on the AF lens frame  51 , the retracting operation for the second lens frame  6  from the photographing position to the radially retracted position has to be completed at an earlier stage than that in the illustrated embodiment to prevent the cylindrical lens holder portion  6   a  from interfering with the AF lens frame  51 . To this end, it is necessary to increase the amount of rearward movement of the second lens group moving frame  8  or the amount of projection of the position-control cam bar  21   a  from the CCD holder  22 ; this runs counter to further miniaturization of the zoom lens  71 . If the amount of rearward movement of the second lens group moving frame  8  is fixed, the inclination of the retracting cam surface  21   c  with respect to the photographing axis direction has to be increased. However, if this inclination is excessively large, the reaction force which is exerted on the position-control cam bar  21   a  and the second lens group moving frame  8  while the retracting cam surface  21   c  presses the rear movable spring end  40   b  is increased. Accordingly, it is undesirable that the inclination of the retracting cam surface  21   c  be increased to prevent a jerky motion from occurring in the retracting operation for the second lens frame  6 . In contrast, in the present embodiment of the zoom lens, the retracting movement of the second lens frame  6  from the photographing position to the radially retracted position can be performed even after the AF lens frame  51  has retracted at a point very close to the AF lens frame  51  due to the formation of the recessed oblique surface  51   h . Therefore, even if the amount of rearward movement of the second lens group moving frame  8  is limited, the retracting cam surface  21   c  does not have to be shaped to be inclined largely with respect to the optical axis direction. This makes it possible to achieve further miniaturization of the zoom lens  71  with a smoothing of the retracting movement of the second lens group moving frame  8 . Similar to the AF lens frame  51 , the CCD holder  21  is provided on a top surface thereof behind the recessed oblique surface  51   h  with a recessed oblique surface  21   f  the shape of which is similar to the shape of the recessed oblique surface  51   h . The recessed oblique surface  51 h and the recessed oblique surface  21   f  are successively formed along a moving path of the cylindrical lens holder portion  6   a  to be shaped like a single oblique surface. Although the AF lens frame  51  serves as a movable member guided in the optical axis direction in the illustrated embodiment, a lens frame similar to the AF lens frame  51  can be provided with a recessed oblique surface corresponding to the recessed oblique surface  51   h  to incorporate features similar to the above described features of the recessed oblique surface  51   h  even if the lens frame similar to the AF lens frame  51  is of a type which is not guided in an optical axis direction. 
   As can be understood from the above descriptions, the retracting structure for the second lens frame  6  is designed so that the second lens frame  6  does not interfere with the AF lens frame  51  when moving rearwards while retracting radially outwards to the radially retracted position in a state where the AF lens frame  51  has retracted to the rear limit (the retracted position) for the axial movement of the AF lens frame  51  as shown in  FIGS. 123 and 124 . In this state, upon the main switch being turned OFF, the control circuit  140  drives the AF motor  160  in the lens barrel retracting direction to move the AF lens frame  51  rearward the retracted position thereof. However, if the AF lens frame  51  does not retract to the retracted position accidentally for some reason upon the main switch being turned OFF, the AF lens frame  51  may interfere with the moving path of the second lens group  6  which is in the middle of moving rearward together with the second lens group moving frame  8  while rotating to the radially retracted position (see FIGS.  127  and  129 ). 
   To prevent such a problem from occurring, the zoom lens  71  is provided with a fail-safe structure. Namely, the second lens frame  6  is provided on the swing arm portion  6   c  with the rear projecting portion  6   m  that projects rearward, beyond the rear end of the second lens group LG 2 , in the optical axis direction, while the AF lens frame  51  is provided, on that portion of the front end surface  51   c   1  of the forwardly-projecting lens holder portion  51   c  which faces the rear projecting portion  6   m , with a rib-like elongated protrusion  51   f  which projects forward from the front end surface  51   c   1  (see  FIGS. 123 ,  124  and  127  through  130 ). As shown in  FIG. 130 , the elongated protrusion  51   f  is elongated vertically, and is formed to lie in a plane orthogonal to the photographing optical axis Z 1  to correspond to the range of rotation of the rear projecting portion  6   m  (the contacting surface  6 n) about the pivot shaft  33  at the rotation of the second lens frame  6  from the photographing position to the radially retracted position. The rear projecting portion  6   m  and the rib-like elongated protrusion  51   f  are elements of the aforementioned fail-safe structure. 
   With the fail-safe structure, even if the second lens frame  6  starts retracting to the radially retracted position in a state where the AF lens frame  51  does not retract to the retracted position and stops short of the retracted position accidentally upon the main switch being turned OFF, the contacting surface  6   n  of the rear projecting portion  6   m  surely comes into contact with the rib-like elongated protrusion  51   f  of the AF lens frame  51  first. This prevents the second lens group LG 2  from coming into collision with the AF lens frame  51  to get scratched and damaged thereby even if such a malfunction occurs. In other words, since the moving path of the rear projecting portion  6   m  does not overlap the third lens group LG 3  in the optical axis direction at any angular positions of the second lens frame  6 , there is no possibility of any portions of the second lens group  6  other than the rear projecting portion  6   m  coming into contact with the third lens group LG 3  to scratch the third lens group LG 3 . Accordingly, since the rear projecting portion  6   m  and the elongated protrusion  51   f  are only the portions at which the second lens group LG 2  and the AF lens frame  51  can contact with each other, the optical performances of the second lens group LG 2  and the third lens group LG 3  are prevented from deteriorating even if the AF lens frame  51  stops short of the retracted position accidentally upon the main switch being turned OFF. If such a malfunction occurs, it is possible for the second lens frame  6  in the process of moving rearward while rotating to the radially retracted position to push back the AF lens frame  51  forcefully, via the rear projecting portion  6   m , which stops short of the retracted position. 
   Note that although in the illustrated embodiment, the contacting surface  6   n  and the rib-like elongated protrusion  51   f  are (possible) contact surfaces, an alternative embodiment can be applied wherein (possible) contact surfaces of the second lens group frame  6  and the AF lens frame  51  differ from that of the illustrated embodiment. For example, a projection like that of the rear projecting portion  6   m  can be provided on the AF lens frame  51 . Namely, an appropriate position can be provided whereby the above-mentioned projection and another member contact each other before the second lens group LG 2  and the third lens group L 3  contact any other members. 
   The contacting surface  6   n  lies in a plane orthogonal to the photographing optical axis Z 1 , whereas the front surface of the elongated protrusion  51   f  is formed as an inclined contacting surface  51   g  which is inclined to a plane orthogonal to the optical axis of the photographing optical axis Z 1  by an angle of NR 2  as shown in FIG.  128 . The inclined contacting surface  51   g  is inclined toward the rear of the optical axis direction in the direction of movement of the rear projecting portion  6   m  from a position when the second lens frame  6  is in the photographing position to a position when the second lens frame  6  is in the radially retracted position (upwards as viewed in FIGS.  128  through  130 ). Unlike the illustrated embodiment, if the front surface of the elongated protrusion  51   f  is formed as a mere flat surface parallel to the contacting surface, the frictional resistance produced between the elongated protrusion  51   f  and the contacting surface  6   n  becomes great to impede a smooth movement of the second lens frame  6  in the event that the contacting surface  6   n  comes into contact with the elongated protrusion  51   f  when the second lens frame  6  is in the process of moving rearward while rotating to the radially retracted position. In contrast, according to the present embodiment of the fail-safe structure, even if the contacting surface  6   n  comes into contact with the elongated protrusion  51   f  when the second lens frame  6  is in the middle of moving rearward while rotating to the radially retracted position, a great frictional resistance is not produced between the elongated protrusion  51   f  and the contacting surface  6   n  because of the inclination of the elongated protrusion  51   f  with respect to the contacting surface  6   n . This makes it possible to retract the zoom lens  71  with reliability with less frictional force produced between the elongated protrusion  51   f  and the contacting surface  6   n  even if the aforementioned malfunction occurs. In the present embodiment of the fail-safe structure, the angle of inclination NR 2  shown in  FIG. 128  is set at three degrees as a desirable angle of inclination. 
   It is possible that the elongated protrusion  51   f  be formed so that the recessed oblique surface  51   h  can come into contact with the light shield ring  9 , that is fixed to the rear end of the cylindrical lens holder portion  6   a , to serve just like the inclined contacting surface  51   g  of the above illustrated embodiment of the fail-safe structure in the case where the AF lens frame  51  stops short of the retracted position accidentally to a lesser extent than the rear projecting portion  6   m  comes into contact with the elongated protrusion  51   f.    
   In the retracted position for the second lens frame  6 , the position of the optical axis of the second lens group LG 2  can be adjusted in directions lying in a plane orthogonal to the photographing optical axis Z 1  in such a case where the optical axis of the second lens group LG 2  is not precisely coincident with the photographing optical axis Z 1  even though the second lens group LG 2  is in the photographing position. Such an adjustment is carried out by two positioning devices: a first positioning device for adjusting the positions of the front and rear second lens frame support plates  36  and  37  relative to the second lens group moving frame  8 , and a second positioning device for adjusting the point of engagement of the eccentric pin  35   b  of the rotation limit shaft  35  with the engaging protrusion  6   e  of the second lens frame  6 . The first eccentric shaft  34 X and the second eccentric shaft  34 Y are elements of the first positioning device; the positions of the front and rear second lens frame support plates  36  and  37  relative to the second lens group moving frame  8  are adjusted by rotating the first eccentric shaft  34 X and the second eccentric shaft  34 Y. The rotation limit shaft  35  is a element of the second positioning device; the point of engagement of the eccentric pin  35   b  with the engaging protrusion  6   e  is adjusted by rotating the rotation limit shaft  35 . 
   First, the first positioning device for adjusting the positions of the front and rear second lens frame support plates  36  and  37  relative to the second lens group moving frame  8  will be discussed hereinafter. As described above, the front eccentric pin  34 X-b of the first eccentric shaft  34 X is inserted into the first vertically-elongated hole  36   a  to be movable and immovable in the first vertically-elongated hole  36   a  in the lengthwise direction and the widthwise direction thereof, respectively, while the rear eccentric pin  34 Y-b of the second eccentric shaft  34 Y is inserted into the horizontally-elongated hole  36   e  to be movable and immovable in the horizontally-elongated hole  36   e  in the lengthwise direction and the widthwise direction thereof, respectively, as shown in  FIGS. 110 ,  114  and  115 . The lengthwise direction of the first vertically-elongated hole  36   a , which corresponds to the vertical direction of the digital camera  70 , is orthogonal to the lengthwise direction of the horizontally-elongated hole  36   e , which corresponds to the horizontal direction of the digital camera  70  as shown in  FIGS. 110 ,  114  and  115 . In the following descriptions, the lengthwise direction of the first vertically-elongated hole  36   a  is referred to as “Y-direction” while the lengthwise direction of the horizontally-elongated hole  36   e  is referred to as “X-direction”. 
   The lengthwise direction of the first vertically-elongated hole  37   a  is parallel to the lengthwise direction of the first vertically-elongated hole  36   a . Namely, the first vertically-elongated hole  37   a  is elongated in the Y-direction. The first vertically-elongated hole  36   a  and the first vertically-elongated hole  37   a  are formed at opposed positions on the front and rear second lens frame support plates  36  and  37  in the optical axis direction. The lengthwise direction of the horizontally-elongated hole  37   e  is parallel to the lengthwise direction of the horizontally-elongated hole  36   e . Namely, the horizontally-elongated hole  37   e  is elongated in the X-direction. The horizontally-elongated hole  36   e  and the horizontally-elongated hole  37   e  are formed at opposed positions on the front and rear second lens frame support plates  36  and  37  in the optical axis direction. Similar to the front eccentric pin  34 X-b, the rear eccentric pin  34 X-c is movable and immovable in the first vertically-elongated hole  37   a  in the Y-direction and X-direction, respectively. The front eccentric pin  34 Y-b is movable and immovable in the horizontally-elongated hole  37   e  in the X-direction and Y-direction, respectively. 
   Similar to the pair of first vertically-elongated holes  36   a  and  37   a  and the pair of horizontally-elongated holes  36   e  and  37   e , the lengthwise direction of the second vertically-elongated hole  36   f  is parallel to the lengthwise direction of the second vertically-elongated hole  37   f , while the second vertically-elongated hole  36   f  and the second vertically-elongated hole  37   f  are formed at opposed positions on the front and rear second lens frame support plates  36  and  37  in the optical axis direction. The pair of the second vertically-elongated holes  36   f  and  37   f  are each elongated in the Y-direction to extend parallel to the pair of first vertically-elongated holes  36   a  and  37   a . The front boss  8   j , which is engaged in the second vertically-elongated hole  36   f , is movable and immovable in the second vertically-elongated hole  36   f  in the Y-direction and X-direction, respectively. Similar to the front boss  8   j , the rear boss  8   k , which is engaged in the second vertically-elongated hole  37   f , is movable and immovable in the second vertically-elongated hole  37   f  in the Y-direction and X-direction, respectively. 
   As shown in  FIG. 113 , the large diameter portion  34 X-a is inserted into the first eccentric shaft support hole  8   f  so as not to move in radial directions thereof, and is accordingly rotatable about the axis (adjustment axis PX) of the large diameter portion  34 X-a. Likewise, the large diameter portion  34 Y-a is inserted into the second eccentric shaft support hole  8   i  so as not to move in radial directions thereof, and is accordingly rotatable on the axis (adjustment axis PY 1 ) of the large diameter portion  34 Y- a.    
   The front eccentric pin  34 Y-b and the rear eccentric pin  34 Y-c have the common axis eccentric to the axis of the large diameter portion  34 Y-a as mentioned above. Therefore, a rotation of the second eccentric shaft  34 Y on the adjustment axis PY 1  causes the front and rear eccentric pins  34 Y-b and  34   b - c  to revolve about the adjustment axis PY 1 , i.e., rotate in a circle about the adjustment axis PY 1 , thus causing the front eccentric pin  34 Y-b to push the front second lens frame support plate  36  in the Y-direction while moving in the X-direction and at the same time causing the rear eccentric pin  34 Y-c to push the rear second lens frame support plate  37  in the Y-direction while moving in the X-direction. At this time, the front second lens frame support plate  36  moves linearly in the Y-direction while guided in the same direction by the front eccentric pin  34 Y-b and the front boss  8   j  since both the first vertically-elongated hole  36   a  and the second vertically-elongated hole  36   f  are elongated in the Y-direction, and at the same time, the rear second lens frame support plate  37  moves linearly in the Y-direction while guided in the same direction by the rear eccentric pin  34 Y-c and the rear boss  8   k  since both the first vertically-elongated hole  37   a  and the second vertically-elongated hole  37   f  are elongated in the Y-direction. Consequently, the position of the second lens frame  6  relative to the second lens group moving frame  8  on the front fixing surface  8   c  thereof varies to adjust the position of the optical axis of the second lens group LG 2  in the Y-direction. 
   The front eccentric pin  34 X-b and the rear eccentric pin  34 X-c have the common axis eccentric to the axis of the large diameter portion  34 X-a as mentioned above. Therefore, a rotation of the first eccentric shaft  34 X on the adjustment axis PX causes the front and rear eccentric pins  34 X-b and  34 X-c to revolve about the adjustment axis PX, i.e., rotate in a circle about the adjustment axis PX, thus causing the front eccentric pin  34 X-b to push the front second lens frame support plate  36  in the X-direction while moving in the Y-direction and at the same time causing the rear eccentric pin  34 X-c to push the rear second lens frame support plate  37  in the X-direction while moving in the Y-direction. At this time, although the front eccentric pin  34 Y-b and the rear eccentric pin  34 Y-c are respectively movable in the horizontally-elongated hole  36   e  and the horizontally-elongated hole  37   e  in the X-direction, the front second lens frame support plate  36  swings about a fluctuating axis (not shown) extending substantially parallel to the common axis of the front and rear bosses  8   j  and  8   k  in the vicinity of this common axis since the second vertically-elongated hole  36   f  is immovable in the X-direction relative to the front boss  8   j  and at the same time the rear second lens frame support plate  37  swings about the fluctuating axis since the second vertically-elongated hole  37   f  is immovable in the X-direction relative to the rear boss  8   k . The position of the fluctuating axis corresponds to the following two resultant positions: a front resultant position between the position of the horizontally-elongated hole  36   e  relative to the front eccentric pin  34 Y-b and the position of the second vertically-elongated hole  36   f  relative to the front boss  8   j , and a rear resultant position between the position of the horizontally-elongated hole  37   e  relative to the rear eccentric pin  34 Y-b and the position of the second vertically-elongated hole  37   f  relative to the rear boss  8   k . Therefore, the fluctuating axis fluctuates in parallel to itself by a swing of the front and rear second lens frame support plates  36  and  37  about the fluctuating axis. A swing of the front and rear second lens frame support plates  36  and  37  about the fluctuating axis causes the pivot shaft  33  to move substantially linearly in the X-direction. Therefore, the second lens group LG 2  moves in the X-direction by a rotation of the first eccentric shaft  34 X on the adjustment axis PX. 
     FIG. 116  shows another embodiment of the first positioning device for adjusting the positions of the front and rear second lens frame support plates  36  and  37  relative to the second lens group moving frame  8 . This embodiment of the first positioning device is different from the above described first positioning device in that a front obliquely-elongated hole  36   f ′ and a rear obliquely-elongated hole  37   f ′ in which the front boss  8   j  and the rear boss  8   k  are engaged are formed on the front and rear second lens frame support plates  36  and  37  instead of the second vertically-elongated hole  36   f  and the second vertically-elongated hole  37   f , respectively. The front obliquely-elongated hole  36   f , and the rear obliquely-elongated hole  37   f ′ extend parallel to each other obliquely to both X-direction and Y-direction, and are aligned in the optical axis direction. Since each of the front obliquely-elongated hole  36   f ′ and the rear obliquely-elongated hole  37   f ′ includes both a component in the X-direction and a component in the Y-direction, a rotation of the second eccentric shaft  34 Y on the adjustment axis PY 1  causes the front obliquely-elongated hole  36   f ′ and the rear obliquely-elongated hole  37   f ′ to move in the Y-direction while moving in the X-direction slightly relative to the front boss  8   j  and the rear boss  8   k , respectively. Consequently, the front and rear second lens frame support plates  36  and  37  move in the Y-direction while the respective lower end portions thereof swing slightly in the X-direction. On the other hand, a rotation of the first eccentric shaft  34 X on the adjustment axis PX causes the front and rear second lens frame support plates  36  and  37  to move in the X-direction while moving (swinging) slightly in the Y-direction. Accordingly, the position of the optical axis of the second lens group LG 2  can be adjusted in directions lying in a plane orthogonal to the photographing optical axis Z 1  by a combination of an operation of the first eccentric shaft  34 X and an operation of the second eccentric shaft  34 Y. 
   The set screw  66  needs to be loosened before the position of the optical axis of the second lens group LG 2  is adjusted by operating the first eccentric shaft  34 X and the second eccentric shaft  34 Y. The set screw  66  is tightened after the adjustment operation is completed. Thereafter, the front and rear second lens frame support plates  36  and  37  are tightly fixed to the front fixing surface  8   c  and the rear fixing surface  8   e  to be held at their respective adjusted positions. Therefore, the pivot shaft  33  is also held at its adjusted position. Consequently, the position of the optical axis of the second lens group LG 2  is held at its adjusted position since the position of the optical axis of the second lens group LG 2  depends on the position of the pivot shaft  33 . As a result of the optical axis position adjustment operation, the set screw  66  has been moved radially from the previous position thereof; however, this presents no problem because the set screw  66  does not move radially to such an extent so as to interfere with the second lens group moving frame  8  by the optical axis position adjustment operation since the threaded shaft portion  66   a  is loosely fitted in the screw insertion hole  8   h  as shown in FIG.  113 . 
   A two-dimensional positioning device which incorporates a first movable stage movable linearly along a first direction and a second movable stage movable linearly along a second direction perpendicular to the first direction, wherein an object the position of which is to be adjusted is mounted on the second movable stage, is known in the art. The structure of this conventional two-dimensional positioning device is generally complicated. In contrast, the above illustrated first positioning device for adjusting the positions of the front and rear second lens frame support plates  36  and  37  relative to the second lens group moving frame  8  is simple because each of the front second lens frame support plate  36  and the rear second lens frame support plate  37  is supported on a corresponding single flat surface (the front fixing surface  8   c  or the rear fixing surface  8   e ) to be movable thereon in both X-direction and Y-direction, which makes it possible to achieve a simple two-dimensional positioning device. 
   Although the above illustrated first positioning device includes two support plates (the pair of second lens frame support plates  36  and  37 ) for supporting the second lens frame  6 , which are positioned separately from each other in the optical axis direction to increase a stability of the structure supporting the second lens frame  6 , it is possible for the second lens frame  6  to be supported with only one of the two support plates. In this case, the first positioning device has only to be provided on the one support plate. 
   Nevertheless, in the above illustrated embodiment of the first positioning device, the front second lens frame support plate  36  and the rear second lens frame support plate  37  are arranged on front and rear sides of the second lens group moving frame  8 , each of the first and second eccentric shafts  34 X is provided at the front and rear ends thereof with a pair of eccentric pins ( 34 X-b and  34 X-c), respectively, and the second lens group moving frame  8  is provided on front and rear sides thereof with a pair of bosses ( 8   j  and  8   k ), respectively. With this arrangement, a rotation of either eccentric shafts  34 X or  34 Y causes the pair of second lens frame support plates  36  and  37  to move in parallel as one-piece member. Specifically, rotating the first eccentric shaft  34 X with a screwdriver engaged in the recess  34 X-d causes the front and rear eccentric pins  34 X-b and  34 X-c to rotate together by the same amount of rotation in the same rotational direction, thus causing the pair of second lens frame support plates  36  and  37  to move in parallel as an integral member in the X-direction. Likewise, rotating the second eccentric shaft  34 Y with a screwdriver engaged in the recess  34 Y-d causes the front and rear eccentric pins  34 Y-b and  34 Y-c to rotate together by the same amount of rotation in the same rotational direction, thus causing the pair of second lens frame support plates  36  and  37  to move in parallel as an integral member in the Y-direction. When the first and second eccentric shafts  34 X and  34 Y are each rotated with a screwdriver engaged in the recesses  34 X-d and  34 Y-d, respectively, the rear second lens frame support plate  37  properly follows the movement of the front second lens frame support plate  36  without being warped. Accordingly, the optical axis of the second lens group LG 2  does not tilt by an operation of the first positioning device, which makes it possible to adjust the position of the optical axis of the second lens group LG 2  two-dimensionally in directions lying in a plane orthogonal to the photographing optical axis Z 1  with a high degree of precision. 
   Since the first and second eccentric shafts  34 X and  34 Y are supported and held between the front second lens frame support plate  36  and the rear second lens frame support plate  37  disposed on front and rear sides of the shutter unit  76 , each of the first and second eccentric shafts  34 X and  34 Y is elongated so that the length thereof becomes close to the length of the second lens group moving frame  8  in the optical axis direction, just as the length of the pivot shaft  33 . This prevents the second lens group moving frame  8  from tilting, which accordingly makes it possible to adjust the position of the optical axis of the second lens group LG 2  two-dimensionally in directions lying in a plane orthogonal to the photographing optical axis Z 1  with a higher degree of precision. 
   The second positioning device for adjusting the point of engagement of the eccentric pin  35   b  of the rotation limit shaft  35  with the engaging protrusion  6   e  of the second lens frame  6  will be hereinafter discussed. As shown in  FIGS. 111 and 112 , the large diameter portion  35   a  of the rotation limit shaft  35  is rotatably fitted in the through hole  8   m  with the eccentric pin  35   b  projecting rearward from the rear end of the through hole  8   m . Note that the large diameter portion  35   a  of the rotation limit shaft  35  does not rotate by itself with respect to the through hole  8   m , however, if a predetermined amount of force is applied, it is possible for the large diameter portion  35   a  to be rotated. 
   As shown in  FIG. 109 , the eccentric pin  35   b  is positioned at one end of the moving path of the tip of the engaging protrusion  6   e  of the second lens frame  6 . The eccentric pin  35   b  projects rearward from the rear end of the large diameter portion  35   a  so that the axis of the eccentric pin  35   b  is eccentric from the axis of the large diameter portion  35   a  as shown in FIG.  117 . Therefore, a rotation of the eccentric pin  35   b  on an axis thereof (adjustment axis PY 2 ) causes the eccentric pin  35   b  to revolve about the adjustment axis PY 2 , thus causing the eccentric pin  35   b  to move in the Y-direction. Since the eccentric pin  35   b  of the rotation limit shaft  35  serves as an element for determining the photographing position of the second lens frame  6 , a displacement of the eccentric pin  35   b  in the Y-direction causes the second lens group LG 2  to move in the Y-direction. Therefore, the position of the optical axis of the second lens group LG 2  can be adjusted in the Y-direction by an operation of the rotation limit shaft  35 . Accordingly, the position of the optical axis of the second lens group LG 2  can be adjusted in the Y-direction by the combined use of the rotation limit shaft  35  and the second eccentric shaft  34 Y. It is desirable that the rotation limit shaft  35  be operated secondarily in a particular case where the range of adjustment of the second eccentric shaft  34 Y is insufficient. 
   As shown in  FIG. 110 , the recess  34 X-d of the first eccentric shaft  34 X, the recess  34 Y-d of the second eccentric shaft  34 Y and the recess  35   c  of the rotation limit shaft  35  are all exposed to the front of the second lens group moving frame  8 . In addition, the head of the set screw  66  that is provided with the cross slot  66   b  is exposed to the front of the second lens group moving frame  8 . Due to this structure, the position of the optical axis of the second lens group LG 2  can be adjusted two-dimensionally with the above described first and second positioning devices from the front of the second lens group moving frame  8 , i.e., all the operating members of the first and second positioning devices are accessible from the front of the second lens group moving frame  8 . On the other hand, the first external barrel  12 , that is positioned radially outside the second lens group moving frame  8 , is provided on an inner peripheral surface thereof with the inner flange  12   c  which projects radially inwards to close the front of the second lens group moving frame  8  in cooperation with the fixing ring  3 . 
   As shown in  FIGS. 131 and 132 , the first external barrel  12  is provided on the inner flange  12   c  with four screwdriver insertion holes  12   g   1 ,  12   g   2 ,  12   g   3  and  12   g   4  which penetrate the inner flange  12   c  in the optical axis direction so that the recess  34 X-d, the recess  34 Y-d, the recess  35   c  and the cross slot  66   b  are exposed to the front of the first external barrel  12 , respectively. A screwdriver can be brought into engagement with the recess  34 X-d, the recess  34 Y-d, the recess  35   c  and the cross slot  66   b  from the front of the second lens group moving frame  8  through the four screwdriver insertion holes  12   g   1 ,  12   g   2 ,  12   g   3  and  12   g   4 , respectively, without removing the first external barrel  12  from the front of the second lens group moving frame  8 . As shown in  FIGS. 2 ,  131  and  132 , portions of the fixing ring  3  which are aligned with the screwdriver insertion holes  12   g   2 ,  12   g   3  and  12   g   4  are cut out so as not to interfere with the screwdriver. The respective front ends of the four screwdriver insertion holes  12   g   1 ,  12   g   2 ,  12   g   3  and  12   g   4  are exposed to the front of the zoom lens  71  by removing the lens barrier cover  101  and the aforementioned lens barrier mechanism positioned immediately behind the lens barrier cover  101 . Due to this structure, the position of the optical axis of the second lens group LG 2  can be adjusted two-dimensionally with the above described first and second positioning devices from the front of the second lens group moving frame  8  without dismounting components of the zoom lens  71  except for substantially the lens barrier mechanism, i.e., in substantially finished form. Accordingly, the position of the optical axis of the second lens group LG 2  can be easily adjusted two-dimensionally with the first and second positioning devices in a final assembling process even if the degree of deviation of the second lens group LG 2  is out of tolerance during assembly. This results in an improvement in workability of the assembly process. 
   The structure accommodating the second lens group LG 2  and other optical elements behind the second lens group LG 2  in the camera body  72  upon the main switch of the digital camera  70  being turned OFF has mainly been discussed above. Improvements in the structure of the zoom lens  71  which accommodates the first lens group LG 1  upon the main switch of the digital camera  70  being turned OFF will be hereinafter discussed in detail. 
   As shown in  FIG. 2 , the inner flange  12   c  of the first external barrel  12  is provided at radially opposite positions thereon with respect to the photographing optical axis Z 1  with a pair of first guide grooves  12   b , respectively, while the first lens group adjustment ring  2  is provided on an outer peripheral surface thereof with a corresponding pair of guide projections  2   b  which project radially outwards in opposite directions away from each other to be slidably fitted in the pair of first guide grooves  12   b , respectively. Only one guide projection  2   b  and the associated first guide groove  12   b  appear in  FIGS. 9 ,  141  and  142 . The pair of first guide grooves  12   b  extend parallel to the photographing optical axis Z 1  so that the combination of the first lens frame  1  and the first lens group adjustment ring  2  is movable in the optical axis direction with respect to the first external barrel  12  by engagement of the pair of guide projections  2   b  with the pair of first guide grooves  12   b.    
   The fixing ring  3  is fixed to the first external barrel  12  by the two set screws  64  to close the front of the pair of guide projections  2   b . The fixing ring  3  is provided at radially opposite positions thereon with respect to the photographing optical axis Z 1  with a pair of spring receiving portions  3   a , so that a pair of compression coil springs  24  are installed in a compressed manner between the pair of spring receiving portions  3   a  and the pair of guide projections  2   b , respectively. Therefore, the first lens group adjustment ring  2  is biased rearward in the optical axis direction with respect to the first external barrel  12  by the spring force of the pair of compression coil springs  24 . 
   In an assembly process of the digital camera  70 , the position of the first lens frame  1  relative to the first lens group adjustment ring  2  in the optical axis direction can be adjusted by changing the position of engagement of the male screw thread  1   a  relative to the female screw thread  2   a  of the first lens group adjustment ring  2 . This adjusting operation can be carried out in a state where the zoom lens  71  is set at the ready-to-photograph state as shown in FIG.  141 . Two-dot chain lines shown in  FIG. 141  show movements of the first lens frame  1  together with the first lens group LG 1  with respect to the first external barrel  12  in the optical axis direction. On the other hand, when the zoom lens  71  is retracted to the retracted position as shown in  FIG. 10 , the first external barrel  12 , together with the fixing ring  3 , can further move rearward relative to the first lens frame  1  and the first lens group adjustment ring  2  while compressing the pair of compression coil springs  24  even after the first lens frame  1  has fully retracted to a point at which the first lens frame  1  contacts with a front surface of the shutter unit  76  to thereby be prevented from further moving rearward (see FIG.  142 ). Namely, when the zoom lens  71  is retracted to the retracted position, the first external barrel  12  is retracted to be accommodated in such a manner as to reduce an axial margin (axial space) for positional adjustment of the first lens frame  1  in the optical axis direction. This structure makes it possible for the zoom lens  71  to be fully retracted deeper into the camera body  72 . Conventional telescoping lens barrels in which a lens frame (which corresponds to the first lens frame  1 ) is directly fixed to an external lens barrel (which corresponds to the first external barrel  12 ) by screw threads (similar to the female screw thread  2   a  and the male screw thread  1   a ) without any intermediate member (which corresponds to the first lens group adjustment ring  2 ) interposed between the lens frame and the external lens barrel are known in the art. In this type of telescoping lens barrels, since the amount of retracting movement of the external lens barrel into a camera body is the same as that of the lens frame, the external lens barrel cannot be further moved rearward relative to the lens frame, unlike the first external barrel  12  of the present embodiment of the zoom lens. 
   The first lens frame  1  is provided at the rear end thereof with an annular end protrusion  1   b  (see  FIGS. 133 ,  134 ,  141  and  142 ), the rear end of which is position behind the rearmost point on the rear surface of the first lens group LG 1  in the optical axis direction, so that the rear end of the annular end protrusion  1   b  comes into contact with a front surface of the shutter unit  76  to prevent the rear surface of the first lens group LG 1  from contacting with the shutter unit  76  and being damaged thereby when the zoom lens  71  is retracted to the retracted position. 
   More than two guide projections, each corresponding to each of the two guide projections  2   b , can be formed on the first lens group adjustment ring  2  at any positions on an outer peripheral surface thereof, and also the shape of each guide projection is optional. According to the number of the guide projections of the first lens group adjustment ring  2 , the fixing ring  3  can be provided with more than two spring receiving portions each corresponding to each of the two spring receiving portions  3   a , and also the shape of each spring receiving portion is optional. In addition, the pair of spring receiving portions  3   a  is not essential; the pair of compression coil springs  24  can be installed in a compressed manner between corresponding two areas on a rear surface of the fixing ring  3  and the pair of guide projections  2   b , respectively. 
   The first lens group adjustment ring  2  is provided on an outer peripheral surface thereof, at the front end of the outer peripheral surface at substantially equi-angular intervals about the photographing optical axis Z 1 , with a set of four engaging projections  2   c  (see  FIG. 2 ) which are engageable with a front surface  3   c  of the fixing ring  3 . The rear limit for the axial movement of the first lens group adjustment ring  2  with respect to the fixing ring  3  (i.e., with respect to the first external barrel  12 ) is determined by engagement (bayonet engagement) of the set of four engaging projections  2   c  with the front surface  3   c  of the fixing ring  3  (see FIGS.  9  and  141 ). The set of four engaging projections  2   c  serve as a set of bayonets. 
   Specifically, the fixing ring  3  is provided on an inner edge thereof with a set of four recesses  3   b  (see  FIG. 2 ) to correspond to the set of four engaging projections  2   c , respectively. The set of four engaging projections  2   c  can be inserted into the set of four recesses  3   b  from behind, respectively, and are engaged with the front surface  3   c  of the fixing ring  3  by rotating one of the first lens group adjustment ring  2  and the fixing ring  3  relative to the other clockwise or counterclockwise after the set of four engaging projections  2   c  are inserted into the set of four recesses  3   b  from behind. After this operation rotating one of the first lens group adjustment ring  2  and the fixing ring  3  relative to the other, a rear end surface  2   c   1  of each engaging projection  2   c  is pressed against the front surface  3   c  (a surface of the fixing ring  3  which can be seen in  FIG. 2 ) of the fixing ring  3  by the spring force of the pair of compression coil springs  24 . This firm engagement of the set of four engaging projections  2   c  with the front surface  3   c  of the fixing ring  3  prevents the combination of the first lens frame  1  and the first lens group adjustment ring  2  from coming off the first external barrel  12  from the rear thereof, and accordingly determines the rear limit for the axial movement of the first lens group adjustment ring  2  with respect to the first external barrel  12 . 
   When the zoom lens  71  is fully retracted into the camera body  72  as shown in  FIGS. 10 and 142 , the rear surfaces  2   c   1  of the set of four engaging projections  2   c  are disengaged from the front surface  3   c  of the fixing ring  3  because the first lens group adjustment ring  2  has moved forward slightly with respect to the first external barrel  12  from the position of the first lens group adjustment ring  2  shown in  FIG. 141  by further compressing the pair of compression coil springs  24 . However, once the zoom lens  71  enters the ready-to-photograph state as shown in  FIG. 141 , the rear surfaces  2   c   1  are re-engaged with the front surface  3   c . Accordingly, the rear surfaces  2   c   1  of the four engaging projections  2   c  and the front surface  3   c  serve as reference surfaces for determining the position of the first lens group LG 1  with respect to the first external barrel  12  in the optical axis direction in the ready-to-photograph state of the zoom lens barrel  71 . With this structure, even if the axial position of the first lens group LG 1  with respect to the first external barrel  12  changes when the zoom lens  71  is retracted into the camera body  72 , the first lens group LG 1  automatically returns to its original position by the action of the pair of compression coil springs  24  as soon as the zoom lens  71  is ready to photograph. 
   At least two and any number other than four engaging projections each corresponding to each of the four engaging projections  2   c  can be formed on the first lens group adjustment ring  2  at any position on an outer peripheral surface thereof. According to the number of the engaging projections of the first lens group adjustment ring  2 , the fixing ring  3  can be provided with at least two and any number other than four recesses each corresponding to each of the four recesses  3   b . Moreover, the shape of each engaging projection of the first lens group adjustment ring  2  and also the shape of each spring receiving portion of the fixing ring  3  are optional as long as each engaging projection of the first lens group adjustment ring  2  is insertable into the corresponding recess of the fixing ring  3 . 
   As has been described above, when the zoom lens  71  changes from the ready-to-photograph state to the retracted state, the cylindrical lens holder portion  6   a  of the second lens frame  6 , which holds the second lens group LG 2 , rotates about the pivot pin  33  in a direction away from the photographing optical axis Z 1  inside the second lens group moving frame  8 , while the AF lens frame  51  which holds the third lens group LG 3  enters the space in the second lens group moving frame  8  from which the lens holder portion  6   a  has retracted (see  FIGS. 134 ,  136  and  137 ). In addition, when the zoom lens  71  changes from the ready-to-photograph state to the retracted state, the first lens frame  1  that holds the first lens group LG 1  enters the second lens group moving frame  8  from the front thereof (see FIGS.  133  and  135 ). Accordingly, the second lens group moving frame  8  has to be provided with two internal spaces: a front internal space immediately in front of the central inner flange  8   s  in which the first lens frame  1  is allowed to move in the optical axis direction, and a rear internal space immediately behind the central inner flange  8   s  in which the second lens frame  6  is allowed to retract along a plane orthogonal to the photographing optical axis Z 1  and in which the AF lens frame  51  is allowed to move in the optical axis direction. In the present embodiment of the zoom lens, the shutter unit  76 , specifically an actuator thereof, is disposed inside the second lens group moving frame  8 , which accommodates more than one lens group therein, in a space-saving manner to maximize the internal space of the second lens group moving frame  8 . 
     FIG. 140  shows the elements of the shutter unit  76 . The shutter unit  76  is provided with a base plate  120  having a central circular aperture  120   a  with its center on the photographing optical axis Z 1 . The base plate  120  is provided on a front surface thereof (a surface which can be seen in  FIG. 140 ) above the circular aperture  120   a  with a shutter-actuator support portion  120   b  formed integral with the base plate  120 . The shutter-actuator support portion  120   b  is provided with a substantially cylindrical accommodation recess  120   b   1  in which the shutter actuator  131  is accommodated. After the shutter actuator  131  is embedded in the accommodation recess  120   b   1 , a holding plate  121  is fixed to the shutter-actuator support portion  120   b  so that the shutter actuator  131  is supported by the base plate  120  on the front thereof. 
   The shutter unit  76  is provided with a diaphragm-actuator support member  120   c  which is fixed to the back of the base plate  120  on the right side of the cylindrical recess  120   b   1  as viewed from the rear of the base plate  120 . The shutter unit  76  is provided with a diaphragm-actuator support cover  122  having a substantially cylindrical accommodation recess  122   a  in which the diaphragm actuator  132  is accommodated. The diaphragm-actuator support cover  122  is fixed to the back of the diaphragm-actuator support member  120   c . After the diaphragm actuator  132  is embedded in the accommodation recess  122   a , the diaphragm-actuator support cover  122  is fixed to the back of the diaphragm-actuator support member  120   c  so that the diaphragm actuator  132  is supported by the diaphragm-actuator support member  120   c  on the back thereof. The shutter unit  76  is provided with a cover ring  123  which is fixed to the diaphragm-actuator support cover  122  to cover an outer peripheral surface thereof. 
   The holding plate  121  is fixed to the shutter-actuator support portion  120   b  by a set screw  129   a . The diaphragm-actuator support member  120   c  is fixed to the back of the base plate  120  by set screw  129   b . Furthermore, the diaphragm-actuator support member  120   c  is fixed to the holding plate  121  by a set screw  129   c . A lower end portion of the diaphragm-actuator support member  120   c  which is provided with a screw hole into which the set screw  129   b  is screwed is formed as a rearward-projecting portion  120   c   1 . 
   The shutter S and the adjustable diaphragm A are mounted to the rear of the base plate  120  immediately beside the diaphragm-actuator support member  120   c . The shutter S is provided with a pair of shutter blades S 1  and S 2 , and the adjustable diaphragm A is provided with a pair of diaphragm blades A 1  and A 2 . The pair of shutter blades S 1  and S 2  are pivoted on a first pair of pins (not shown) projecting rearward from the back of the base plate  120 , respectively, and the pair of diaphragm blades A 1  and A 2  are pivoted on a second pair of pins (not shown) projecting rearward from the back of the base plate  120 , respectively. These first and second pairs of pints do no appear in FIG.  140 . The shutter unit  76  is provided between the shutter S and the adjustable diaphragm A with a partition plate  125  which prevents the shutter S and the adjustable diaphragm A from interfering with each other. The shutter S, the partition plate  125  and the adjustable diaphragm A are fixed to the back of the base plate  120  in this order from front to rear in the optical axis direction, and thereafter a blade-holding plate  126  is fixed to the back of the base plate  120  to hold the shutter S, the partition plate  125  and the adjustable diaphragm A between the base plate  120  and the blade-holding plate  126 . The partition plate  125  and the blade-holding plate  126  are provided with a circular aperture  125   a  and a circular aperture  126   a , respectively, through which rays of light of an object image which is to be photographed pass to be incident on the CCD image sensor  60  through the third lens group LG 3  and the low-pass filter LG 4 . The circular apertures  125   a  and  126   a  are aligned with the central circular aperture  120   a  of the base plate  120 . 
   The shutter actuator  131  is provided with a rotor  131   a , a rotor magnet (permanent magnet)  131   b , a stator  131   c  made of steel, and a bobbin  131   d . The rotor  131   a  is provided with a radial arm portion, and an eccentric pin  131   e  which projects rearwards from the tip of the radial arm portion to be inserted into cam grooves S 1   a  and S 2   a  of the pair of shutter blades S 1  and S 2 . Strands (not shown) through which electric current is passed via the flexible PWB  77  to control rotation of the rotor  131   a  are wound on the bobbin  131   d . Passing a current through the strands wound on the bobbin  131   d  causes the rotor  131   a  to rotate forward or reverse depending on the magnetic field which varies in accordance with the direction of the passage of the current. Rotations of the rotor  131   a  forward and reverse cause the eccentric pin  131   e  to swing in forward and revere directions, thus causing the pair of shutter blades S 1  and S 2  to open and close, respectively, by engagement of the eccentric pin  131   e  with the cam grooves S 1   a  and S 2   a.    
   The diaphragm actuator  132  is provided with a rotor  132   a  and a rotor magnet (permanent magnet)  132   b . The rotor  132   a  is provided with a radial arm portion having two ninety-degree bends, and an eccentric pin  132   c  which projects rearwards from the tip of the radial arm portion to be inserted into cam grooves A 1   a  and A 2   a  of the pair of diaphragm blades A 1  and A 2 . Strands (not shown) through which electric current is passed via the flexible PWB  77  to control rotation of the rotor  132   a  are wound on the diaphragm-actuator support member  120   c  and the diaphragm-actuator support cover  122 . Passing a current through the strands wound on the diaphragm-actuator support member  120   c  and the diaphragm-actuator support cover  122  causes the rotor  132   a  to rotate forward or reverse depending on the magnetic field which varies in accordance with the direction of the passage of the current. Rotations of the rotor  132   a  forward and reverse cause the eccentric pin  132   c  to swing in forward and revere directions, thus causing the pair of diaphragm blades A 1  and A 2  to open and close, respectively, by engagement of the eccentric pin  132   c  with the cam grooves A 1   a  and A 2   a.    
   The shutter unit  76  is prepared as a subassembly in advance, and fitted into the second lens group moving frame  8  to be fixed thereto. As shown in  FIGS. 108 and 110 , the shutter unit  76  is supported by the second lens group moving frame  8  therein so that the base plate  120  is positioned immediately in front of the central inner flange  8   s . A terminal end  77   e  of the flexible PWB  77  is fixed to a front surface of the holding plate  121  (see  FIGS. 108 ,  110 ,  133  and  135 ). 
   The second lens group moving frame  8  has a cylindrical shape coaxial to other rotatable rings such as the cam ring  11 . The axis of the second lens group moving frame  8  coincides with the lens barrel axis Z 0  of the zoom lens  71 . The photographing optical axis Z 1  is eccentric downward from the lens barrel axis Z 0  to secure some space in the second lens group moving frame  8  into which the second lens group LG 2  is retracted to the radially-retracted position (see FIGS.  110  through  112 ). On the other hand, the first lens frame  1 , which supports the first lens group LG 1 , is in the shape of a cylinder with its center on the photographing optical axis Z 1 , and is guided along the photographing optical axis Z 1 . Due to this structure, the space in the second lens group moving frame  8  which is occupied by the first lens group LG 1  is secured in the second lens group moving frame  8  below the lens barrel axis Z 0 . Accordingly, sufficient space (upper front space) is easily secured in the second lens group moving frame  8  in front of the central inner flange  8   s  on the opposite side of the lens barrel axis Z 0  from the photographing optical axis Z 1  (i.e., above the lens barrel axis Z 0 ) so that the shutter actuator  131  and supporting members therefor (the shutter-actuator support portion  120   b  and the holding plate  121 ) are positioned in the upper front space along an inner peripheral surface of the second lens group moving frame  8 . With this structure, the first lens frame  1  does not interfere with either the shutter actuator  131  or the holding plate  121  even if the first lens frame  1  enters the second lens group moving frame  8  from the front thereof as shown in FIG.  135 . Specifically, in the retracted state of the zoom lens  71 , the holding plate  121  and the shutter actuator  131 , which is positioned behind the holding plate  121 , are positioned in an axial range in which the first lens group LG 1  is positioned in the optical axis direction; namely, the holding plate  121  and the shutter actuator  131  are positioned radially outside the first lens group LG 1 . This maximizes the utilization of the internal space of the second lens group moving frame  8 , thus contributing to a further reduction of the length of the zoom lens  71 . 
   The first lens frame  1  that holds the first lens group LG 1  is positioned in the first external barrel  12  to be supported thereby via the first lens group adjustment ring  2  as shown in  FIG. 138  to be movable together with the first external barrel  12  in the optical axis direction though the first lens group adjustment ring  2  is not shown in  FIGS. 133 and 135  around the first lens frame  1  for the purpose of illustration. The inner flange  12   c  of the first external barrel  12  is provided, above the portion thereof which holds the first lens frame  1  and the first lens group adjustment ring  2 , with a through hole  12   c   1  which has a substantially arm shape as viewed from or rear of the first external barrel  12  and which penetrates the first external barrel  12  in the optical axis direction. The through hole  12   c   1  is shaped so that the holding plate  121  can enter the through hole  12   c   1  from behind. The holding plate  121  enters the through hole  12   c   1  as shown in  FIG. 138  when the zoom lens  71  is in the retracted position. 
   In the rear internal space of the second lens group moving frame  8  behind the central inner flange  8   s , not only the forwardly-projecting lens holder portion  51   c  (the third lens group LG 3 ) of the AF lens frame  51  moves in and out in the optical axis direction above the photographing optical axis Z 1  that is positioned below the lens barrel axis Z 0 , but also the cylindrical lens holder portion  6   a  retracts into the space on the opposite side of the lens barrel axis Z 0  from the photographing optical axis Z 1  when the zoom lens  71  is retracted into the camera body  72 . Accordingly, there is substantially no extra space in the second lens group moving frame  8  behind the central inner flange  8   s  in a direction (vertical direction) of a straight line M 1  orthogonally intersecting both the lens barrel axis Z 0  and the photographing optical axis Z 1  (see FIG.  112 ). Whereas, two side spaces not interfering with either the second lens group LG 2  or the third lens group LG 3  are successfully secured on respective sides (right and left sides) of the line M 1  in the second lens group moving frame  8  until an inner peripheral surface thereof behind the central inner flange  8   s  in a direction (see  FIG. 112 ) of a straight line M 2  which is orthogonal to the straight line M 1  and intersecting the photographing optical axis Z 1 . As can be seen in  FIGS. 111 and 112 , the left side space of the two side spaces which is positioned on the left side as viewed in  FIG. 112  (on the left side of the lens barrel axis Z 0  and the photographing optical axis Z 1  as viewed from the rear of the second lens frame  8 ) is utilized partly as the space for the swing arm portion  6   c  of the swingable second lens frame  6  to swing therein and partly as the space for accommodating the above described first positioning device, with which the positions of the front and rear second lens frame support plates  36  and  37  relative to the second lens group moving frame  8  can be adjusted. The right side space of the aforementioned two side spaces which is positioned on the right side as viewed in  FIG. 112  is utilized as the space for accommodating the diaphragm actuator  132  and supporting members therefor (the diaphragm-actuator support cover  122  and the cover ring  123 ) so that the diaphragm actuator  132  and the supporting members are positioned along an inner peripheral surface of the second lens group moving frame  8 . More specifically, the diaphragm actuator  132  and the supporting members (the diaphragm-actuator support cover  122  and the cover ring  123 ) lie on the straight line M 2 . Accordingly, as can be understood from  FIGS. 111 ,  112  and  137 , the diaphragm actuator  132 , the diaphragm-actuator support cover  122  and the cover ring  123  do not interfere with either the range of movement of the second lens group LG 2  or the range of movement of the third lens group LG 3 . 
   Specifically, in the inside of the second lens group moving frame  8  behind the central inner flange  8   s , the second lens group LG 2  (the cylindrical lens holder portion  6   a ) and the third lens group LG 3  (forwardly-projecting lens holder portion  51   c ) are accommodated on upper and lower sides of the lens barrel axis Z 0 , respectively, while the above described first positioning device and diaphragm actuator  132  are positioned on right and left sides of the lens barrel axis Z 0  when the zoom lens  71  is in the retracted state. This maximizes the utilization of the internal space of the second lens group moving frame  8  in the retracted state of the zoom lens  71 . In this state, the diaphragm-actuator support cover  122 , the cover ring  123  and the diaphragm actuator  132  are positioned in the space radially outside the space in which the second lens group LG 2  and the third lens group LG 3  are accommodated. This contributes to a further reduction of the length of the zoom lens  71 . 
   In the present embodiment of the zoom lens, the base plate  120  of the shutter unit  120  is positioned in front of the central inner flange  8   s , whereas the diaphragm actuator  132 , the diaphragm-actuator support cover  122  and the cover ring  123  are positioned behind the central inner flange  8   s . In order to allow the diaphragm actuator  132 , the diaphragm-actuator support cover  122  and the cover ring  123  extend behind the central inner flange  8   s , the central inner flange  8   s  is provided with a substantially circular through hole  8   s   1  in which the cover ring  123  is fitted (see FIGS.  110  through  112 ). The central inner flange  8   s  is further provided below the through hole  8   s   1  with an accommodation recess  8   s   2  in which the rearward-projecting portion  120   c   1  of the diaphragm-actuator support member  120   c  is accommodated. 
   The forwardly-projecting lens holder portion  51   c  of the AF lens frame  51  is provided, on the side surface  51   c   4  among the four side surfaces  51   c   3 ,  51   c   4 ,  51   c   5  and  51   c   6  around the forwardly-projecting lens holder portion  51   c , with a recess  51   i  which is formed by cutting out a part of the forwardly-projecting lens holder portion  51   c . The recess  51   i  is formed to correspond to the shapes of outer peripheral surfaces of the ring cover  123  and the accommodation recess  8   s   2  of the second lens group moving frame  8  so that the forwardly-projecting lens holder portion  51   c  does not interfere with the ring cover  123  and the accommodation recess  8   s   2  in the retracted state of the zoom lens  71 . Namely, the outer peripheral portions of the ring cover  123  and the accommodation recess  8   s   2  partly enter the recess  51   i  when the zoom lens  71  is fully retracted into the camera body  72  (see  FIGS. 122 ,  130  and  137 ). This further maximizes the utilization of the internal space of the second lens group moving frame  8  to minimize the length of the zoom lens  71 . 
   In the present embodiment of the zoom lens, even the shutter actuator  131  and the diaphragm actuator  132  are structured in consideration of the utilization of the internal space of the zoom lens  71 . 
   The space in front of the base plate  120  is narrow in the optical axis direction since the shutter unit  76  is supported by the second lens group moving frame  8  therein toward the front thereof as can be seen in  FIGS. 9 and 10 . Due to the limitation of the space in front of the base plate  120 , the shutter actuator  131  adopts the structure, in which the rotor magnet  131   b  and the bobbin  131   d  do not adjoin each other in the optical axis direction but are positioned separately from each other in a direction perpendicular to the optical axis direction, so that variations of the magnetic field generated on the side of the bobbin  131   d  are transferred to the side of the rotor magnet  131   d  via the stator  131   c . This structure reduces the thickness of the shutter actuator  131  in the optical axis direction, thus making it possible for the shutter actuator  131  to be positioned in the limited space in front of the base plate  120  without problems. 
   On the other hand, the space behind the base plate  120  is also limited in a direction perpendicular to the optical axis direction because the second lens group LG 2  and other retractable parts are positioned behind the base plate  120 . Due to the limitation of the space behind the base plate  120 , the diaphragm actuator  132  adopts the structure in which strands are wound directly on the diaphragm-actuator support member  120   c  and the diaphragm-actuator support cover  122  which cover the rotor magnet  132   b . This structure reduces the height of the diaphragm actuator  132  in a direction perpendicular to the optical axis direction, thus making it possible for the diaphragm actuator  132  to be positioned in the limited space behind the base plate  120  without problems. 
   The digital camera  70  is provided above the zoom lens  71  with a zoom viewfinder, the focal length of which varies to correspond to the focal length of the zoom lens  71 . As shown in  FIGS. 9 ,  10  and  143 , the zoom viewfinder is provided with a zoom type viewing optical system including an objective window plate  81   a  (not shown in FIG.  143 ), a first movable power-varying lens  81   b , a second movable power-varying lens  81   c , a mirror  81   d , a fixed lens  81   e , a prism (erecting system)  81   f , an eyepiece  81   g  and an eyepiece window plate  81   h , in that order from the object side along a viewfinder optical axis. The objective window plate  81   a  and the eyepiece window plate  81   h  are fixed to the camera body  72 , and the remaining optical elements ( 81   b  through  81   g ) are supported by a viewfinder support frame  82 . Among the optical elements  81   b  through  81   g  supported by the viewfinder support frame  82 , the mirror  81   d , the fixed lens  81   e , the prism  81   f  and the eyepiece  81   g  are fixed to the viewfinder support frame  82  at their respective predetermined positions thereon. The zoom viewfinder is provided with a first movable frame  83  and a second movable frame  84  which hold the first movable power-varying lens  81   b  and the second movable power-varying lens  81   c , respectively. The first movable frame  83  and the second movable frame  84  are guided in the optical axis direction by a first guide shaft  85  and a second guide shaft  86  which extend in a direction parallel to the photographing optical axis Z 1 , respectively. The first movable power-varying lens  81   b  and the second movable power-varying lens  81   c  have a common optical axis Z 3  which remains in parallel to the photographing optical axis Z 1  regardless of variations of the relative position between the first movable power-varying lens  81   b  and the second movable power-varying lens  81   c . The first movable frame  83  and the second movable frame  84  are biased forward, toward the objective side, by a first compression coil spring  87  and a second compression coil spring  88 , respectively. The zoom viewfinder is provided with a cam-incorporated gear  90  having a substantially cylindrical shape. The cam-incorporated gear  90  is fitted on a rotational shaft  89  to be supported thereon. The rotational shaft  89  is fixed to the viewfinder support frame  82  to extend parallel to the optical axis Z 3  (the photographing optical axis Z 1 ). 
   The cam-incorporated gear  90  is provided at the front end thereof with a spur gear portion  90   a . The cam-incorporated gear  90  is provided immediately behind the spur gear portion  90   a  with a first cam surface  90   b , and is provided between the first cam surface  90   b  and the rear end of the cam-incorporated gear  90  with a second cam surface  90   c . The cam-incorporated gear  90  is biased forward by a compression coil spring  90   d  to remove backlash. A first follower pin  83   a  (see  FIG. 148 ) projected from the first movable frame  83  is pressed against the first cam surface  90   b  by the spring force of the first compression coil spring  87 , while a second follower pin  84   a  (see  FIGS. 143 ,  146  and  148 ) projected from the second movable frame  84  is pressed against the second cam surface  90   c  by the spring force of the second compression coil spring  88 . A rotation of the cam-incorporated gear  90  causes the first movable frame  83  and the second movable frame  84  that respectively hold the first movable power-varying lens  81   b  and the second movable power-varying lens  81   c  to move in the optical axis direction in a predetermined moving manner while changing the space therebetween in accordance with the contours of the first cam surface  90   b  and the second cam surface  90   c  to vary the focal length of the zoom viewfinder in synchronization with the focal length of the zoom lens  71 .  FIG. 156  is a developed view of an outer peripheral surface of the cam-incorporated gear  90 , showing the positional relationship between the first follower pin  83   a  and the first cam surface  90   b  and the positional relationship between the second follower pin  84   a  and the second cam surface  90   c  in each of three different states, i.e., at the wide-angle extremity, the telephoto extremity and the retracted position of the zoom lens  71 . All the elements of the zoom viewfinder except for the objective window plate  81   a  and the eyepiece window plate  81   h  are put together to be prepared as a viewfinder unit (subassembly)  80  as shown in FIG.  143 . The viewfinder unit  80  is mounted on top of the stationary barrel  22  via set screws  80   a  as shown in FIG.  5 . 
   The digital camera  70  is provided between the helicoid ring  18  and the cam-incorporated gear  90  with a viewfinder drive gear  30  and a gear train (reduction gear train)  91 . The viewfinder drive gear  30  is provided with a spur gear portion  30   a  which is in mesh with the annular gear  18   c  of the helicoid ring  18 . Rotation of the zoom motor  150  is transferred from the annular gear  18   c  to the cam-incorporated gear  90  via the viewfinder drive gear  30  and the gear train  91  (see FIGS.  146  and  147 ). The viewfinder drive gear  30  is provided behind the spur gear portion  30   a  with a semi-cylindrical portion  30   b , and is further provided with a front rotational pin  30   c  and a rear rotational pin  30   d  which project from the front end of the spur gear portion  30   a  and the rear end of the semi-cylindrical portion  30   b , respectively so that the front rotational pin  30   c  and the rear rotational pin  30   d  are positioned on a common rotational axis of the viewfinder drive gear  30 . The front rotational pin  30   c  is rotatably fitted into a bearing hole  22   p  (see  FIG. 6 ) formed on the stationary barrel  22  while the rear rotational pin  30   d  is rotatably fitted into a bearing hole  21   g  (see  FIG. 8 ) formed on the CCD holder  21 . Due to this structure, the viewfinder drive gear  30  is rotatable about its rotational axis (the rotational pins  30   c  and  30   d ) extending parallel to the lens barrel axis Z 0  (the rotational axis of the helicoid ring  18 ), and is immovable in the optical axis direction. The gear train  91  is composed of a plurality of gears: a first gear  91   a , a second gear  91   b , a third gear  91   c  and a fourth gear  91   d . Each of the first through third gears  91   a ,  91   b  and  91   c  is a double gear consisting of a large gear and a small gear, and the fourth gear  91   d  is a simple spur gear as shown in  FIGS. 5 and 146 . The first through fourth gears  91   a ,  91   b ,  91   c  and  91   d  are respectively rotatably fitted on four rotational pins projecting from the stationary barrel  22  in parallel to the photographing optical axis Z 1 . As shown in  FIGS. 5 through 7 , a gear hold plate  22  is fixed to the stationary barrel  22  by set screws  92   a  to be positioned immediately in front of the first through fourth gears  91   a ,  91   b ,  91   c  and  91   d  to prevent the first through fourth gears  91   a ,  91   b ,  91   c  and  91   d  from coming off their respective rotational pins. With the gear train  91  fixed properly at their respective fixing positions as shown in  FIGS. 146 through 148 , rotation of the viewfinder drive gear  30  is imparted to the cam-incorporated gear  90  via the gear train  91 .  FIG. 6 through 8  show the zoom lens  71  in a state where the viewfinder drive gear  30 , the viewfinder unit  80  and the gear train  91  are all fixed to the stationary barrel  22 . 
   As described above, the helicoid ring  18  continues to be driven to move forward along the lens barrel axis Z 0  (the photographing optical axis Z 1 ) while rotating about the lens barrel axis Z 0  with respect to the stationary barrel  22  and the first linear guide ring  14  until the zoom lens  71  reaches the wide-angle extremity (zooming range) from the retracted position. Thereafter, the helicoid ring  18  rotates about the lens barrel axis Z 0  at a fixed position with respect to the stationary barrel  22  and the first linear guide ring  14 , i.e., without moving along the lens barrel axis Z 0  (the photographing optical axis Z 1 ).  FIGS. 23 through 25 ,  144  and  145  show different operational states of the helicoid ring  18 . Specifically,  FIGS. 23 and 144  show the helicoid ring  18  in the retracted state of the zoom lens  71 ,  FIGS. 24 and 145  show the helicoid ring  18  at the wide-angle extremity of the zoom lens  71 , and  FIG. 25  shows the telephoto extremity of the zoom lens  71 . In  FIGS. 144 and 145 , the stationary barrel  22  is not shown for the purpose of making the relationship between the viewfinder drive gear  30  and the helicoid ring  18  easier to understand. 
   The viewfinder drive gear  30  does not rotate about the lens barrel axis Z 0  during the time the helicoid ring  18  rotates about the lens barrel axis Z 0  while moving in the optical axis direction, i.e., during the time the zoom lens  71  is extended forward from the retracted position to a position immediately behind the wide-angle extremity (i.e., immediately behind the zooming range). The viewfinder drive gear  30  rotates about the lens barrel axis Z 0  at a fixed position only when the zoom lens  71  is in the zoom ranging between the wide-angle extremity and the telephoto extremity. Namely, in the viewfinder drive gear  30 , the spur gear portion  30   a  is formed thereon to occupy only a front small part of the viewfinder drive gear  30 , so that the spur gear portion  30   a  is not in mesh with the annular gear  18   c  of the helicoid ring  18  in the retracted state of the zoom lens  71  because the annular gear  18   c  is positioned behind the front rotational pin  30   c  the retracted state of the zoom lens  71 . The annular gear  18   c  reaches the spur gear portion  30   a  to mesh therewith immediately before the zoom lens  71  reaches the wide-angle extremity. Thereafter, from the wide-angle extremity to the telephoto extremity, the annular gear  18   c  remains in mesh with the spur gear portion  30   a  because the helicoid ring  18  does not move in the optical axis direction (horizontal direction as viewed in  FIGS. 23 through 25 ,  144  and  145 ). 
   As can be understood from  FIGS. 153 through 155 , the semi-cylindrical portion  30   b  of the viewfinder drive gear  30  is provided with an incomplete cylindrical portion  30   b   1  and a flat surface portion  30   b   2  which is formed as a cut-away portion of the incomplete cylindrical portion  30   b   1  so that the flat surface portion  30   b   2  extends along the rotational axis of the viewfinder drive gear  30 . Accordingly, the semi-cylindrical portion  30   b  has a non-circular cross section, i.e., a substantially D-shaped cross section. As can be seen in  FIGS. 153 through 155 , some specific teeth of the spur gear portion  30   a  adjacent to the flat surface portion  30   b   2  project radially outwards beyond the position of the flat surface portion  30   b   2  in a direction of engagement of the some specific teeth of the spur gear portion  30   a  with the annular gear  18   c  (i.e., horizontal direction as viewed in FIG.  153 ). When the zoom lens  71  is in the retracted state, the viewfinder drive gear  30  is in its specific angular position in which the flat surface portion  30   b   2  faces the annular gear  18   c  of the helicoid ring  18  as shown in FIG.  153 . In this state shown in  FIG. 153 , the viewfinder drive gear  30  cannot rotate even if driven to rotate because the flat surface portion  30   b   2  is in close vicinity of the addendum circle of the annular gear  18   c . Namely, even if the viewfinder drive gear  30  tries to rotate in the state shown in  FIG. 153 , the flat surface portion  30   b   2  would hit some teeth of the annular gear  18   c , so that the viewfinder drive gear  30  cannot rotate. 
   If the helicoid ring  18  moves forward until the annular gear  18   c  of the helicoid ring  18  is properly engaged with the spur gear portion  30   a  of the viewfinder drive gear  30  as shown in  FIG. 145 , the portion of the helicoid ring  18  which includes the entire part of the annular gear  18   c  is positioned in front of the semi-cylindrical portion  30   b  in the optical axis direction. In this state, the viewfinder drive gear  30  rotates by rotation of the helicoid ring  18  since the semi-cylindrical portion  30   b  does not overlap the annular gear  18   c  in radial directions of the zoom lens  71 . 
   Although the helicoid ring  18  is provided in front of the annular gear  18   c  with the set of three rotational sliding projections  18   b  each having a radial height greater than the radial height (tooth depth) of the annular gear  18   c , the set of three rotational sliding projections  18   b  do not interfere with the viewfinder drive gear  30  during the time the helicoid ring  18  moves between the position thereof at the wide-angle extremity and the position thereof at the telephoto extremity while rotating about the lens barrel axis Z 0  because the rotation of the helicoid ring  18  for driving the zoom lens  71  from the retracted position to the wide-angle extremity is completed while the viewfinder drive gear  30  is positioned in between two of the three rotational sliding projections  18   b  in a circumferential direction of the helicoid ring  18 . Thereafter, the set of three rotational sliding projections  18   b  and the spur gear portion  30   a  do not interfere with each other since the set of three rotational sliding projections  18   b  are positioned in front of the spur gear portion  30   a  in the optical axis direction in a state where the annular gear  18   c  is engaged with the spur gear portion  30   a.    
   In the above illustrated embodiment, with respect to the helicoid ring  18  which rotates about the lens barrel axis Z 0  while moving in the optical axis direction in one state and which rotates at a fixed position on the lens barrel axis Z 0  in another state, the spur gear portion  30   a  is formed on the specific portion of the viewfinder drive gear  30  which is engageable with the annular gear  18   c  only when the helicoid ring  18  rotates at its predetermined axial fixed position. Moreover, the semi-cylindrical portion  30   b  is formed on the viewfinder drive gear  30  behind the spur gear portion  30   a  thereof, so that the viewfinder drive gear  30  is prohibited from rotating by interference of the semi-cylindrical portion  30   b  with the annular gear  18   c  during the time the helicoid ring  18  rotates about the lens barrel axis Z 0  while moving in the optical axis direction. Due to this structure, although the viewfinder drive gear  30  does not rotate while the zoom lens  71  is extended or retracted between the retracted position and a position immediately behind the wide-angle extremity, the viewfinder drive gear  30  rotates only when the zoom lens  71  is driven to change its focal length between the wide-angle extremity and the telephoto extremity. In short, the viewfinder drive gear  30  is driven only when the viewfinder drive gear  30  needs to be associated with the photographing optical system of the zoom lens  71 . 
   Assuming the viewfinder drive gear  30  rotates whenever the helicoid ring  18  rotates, a drive transfer system extending from the viewfinder drive gear to a movable lens of the zoom viewfinder has to be provided with an idle running section for disengaging the movable lens from the viewfinder drive gear, because the viewfinder drive gear  30  rotates even when it is not necessary to drive the zoom viewfinder, i.e., when the zoom lens  71  is extended forward to the wide-angle extremity from the retracted state.  FIG. 157  is a developed view, similar to that of  FIG. 156 , of an outer peripheral surface of a cam-incorporated gear  90 ′ (which corresponds to the cam-incorporated gear  90  of the zoom lens  71 ) which is provided with such an idle running section. In each of  FIGS. 156 and 157 , the spur gear portion  90   a  is not shown for clarity. 
   A first cam surface  90   b ′ of the cam-incorporated gear  90 ′, which correspond to the first cam surface  90   b  of the cam-incorporated gear  90 , is provided with a long linear surface  90   b   1 ′ for preventing a follower pin  83   a ′ (which corresponds to the follower pin  83   a ) from moving in an optical axis direction Z 3 ′ (which corresponds to the optical axis Z 3 ) even if the cam-incorporated gear  90  rotates. Likewise, a second cam surface  90   c ′ of the cam-incorporated gear  90 ′, which correspond to the second cam surface  90   c  of the cam-incorporated gear  90 , is provided with a long linear surface  90   c   1 ′ for preventing a follower pin  84   a ′ (which corresponds to the follower pin  84 a) from moving in the optical axis direction Z 3 ′ even if the cam-incorporated gear  90  rotates. As can be understood by a comparison between  FIGS. 156 and 157 , the long linear surface  90   b   1 ′ consumes a large circumferential range of the first cam surface  90   b ′ to thereby shorten the remaining circumferential range of the first cam surface  90   b ′ which is used as a cam surface for moving the follower pin  83   a ′ in the optical axis direction; this inevitably increases the degree of inclination of the cam surface. Likewise, the long linear surface  90   c   1 ′ consumes a large circumferential range of the second cam surface  90   c ′ to thereby shorten the remaining circumferential range of the second cam surface  90   c ′ which is used as a cam surface for moving the follower pin  84   a ′ in the optical axis direction; this inevitably increases the degree of inclination of the cam surface. If the degree of inclination of each of the first cam surface  90   b ′ and the second cam surface  90   c ′ is great, the amount of movement of each follower pin  83 ′ and  84 ′ along the rotational axis of the cam-incorporated gear  90 ′ (i.e., along the optical axis Z 3 ) per unit of rotation of the cam-incorporated gear  90 ′ becomes great, which makes it difficult to move each follower pin  83 ′ and  84 ′ with a high degree of positioning accuracy. If the degree of inclination of each of the first cam surface  90   b ′ and the second cam surface  90   c ′ is reduced to prevent this problem from occurring, the diameter of the cam-incorporated gear  90 ′ has to be increased, which is detrimental to miniaturization of the zoom lens. This problem is also true for the case of adopting a cam plate instead of a cylindrical cam member such as the cam-incorporated gear  90 . 
   In contrast, in the present embodiment of the zoom lens, in which the viewfinder drive gear  30  is not driven when not necessary to rotate, the cam-incorporated gear  90  does not have to be provided on each of the first and second cam surfaces  90   b  and  90   c  with an idle running section. Therefore, an effective circumferential range of a cam surface for moving the follower pin  83   a  or  84   a  in the optical axis direction can be secured on each of the first and second cam surfaces  90   b  and  90   c  without increasing either the degree of inclination of the cam surfaces or the diameter of the cam-incorporated gear  90 . In other words, miniaturizing the drive system for the zoom viewfinder and driving the movable lenses of the viewfinder optical system with high accuracy can be both achieved. In the present embodiment of the zoom lens, the first and second cam surfaces  90   b  and  90   c  of the cam-incorporated gear  90  are provided with linear surfaces  90   b   1  and  90   c   1  which look like the aforementioned linear surfaces  90   b   1 ′ and  90   c   1 ′, respectively, due to the fact that the annular gear  18   c  is brought into engagement with the spur gear portion  30   a  intentionally at the moment immediately before the zoom lens  71  reaches the zooming range (the wide-angle extremity) when the zoom lens  71  is extended forward from the retracted position in consideration of backlash and play among gears shown in  FIGS. 146 through 148 . Nevertheless, the circumferential lengths of the linear surfaces  90   b   1  and  90   c   1  are much smaller than those of the linear surfaces  90   b   1 ′ and  90   c   1 ′ of the comparative embodiment. 
   In the present embodiment of the zoom lens, the annular gear  18   c  is formed so that the spur gear portion  30   a  of the viewfinder drive gear  30  can smoothly mesh with the annular gear  18   c . Specifically, one of a plurality of gear teeth of the annular gear  18   c , i.e., a short gear tooth  18   c   1  is formed to have a shorter tooth depth than those of other normal gear teeth  18   b   2  of the annular gear  18   c.    
     FIGS. 149 through 152  show the positional relationship between the annular gear  18   c  of the helicoid ring  18  and the spur gear portion  30   a  of the viewfinder drive gear  30  in different states in time sequence in the course of variation in state of the zoom lens from the state shown in  FIG. 144  in which the zoom lens  71  is in the retracted state to the state as shown in  FIG. 145  in which the zoom lens  71  is set at wide-angle extremity. The positional relationship between the annular gear  18   c  and the spur gear portion  30   a  is obtained in the middle of rotation of the helicoid ring  18  in a direction from the retracted position to the wide-angle extremity. 
   Subsequently, the short gear teeth  18   c   1  approaches the spur gear portion  30   a  and is positioned in the immediate vicinity of the spur gear portion  30   a  as shown in FIG.  150 .  FIG. 153  shows this state shown in  FIG. 150 , viewed from the front of the viewfinder drive gear  30 . It can be seen from  FIG. 153  that the short gear teeth  18   c   1  is not yet engaged with the spur gear portion  30   a . The normal gear teeth  18   c   2  are positioned farther from the spur gear portion  30   a  than the short gear tooth  18   c   1 , and therefore are not yet engaged with the spur gear portion  30   a  either. No gear teeth serving as gear teeth of the annular gear  18   c  is formed on a specific portion of the outer peripheral surface of the helicoid ring  18 ; the specific portion is right next to the short gear tooth  18   c   1  on one of the opposite sides thereof in the circumferential direction of the helicoid ring  18 . Accordingly, at the stage shown in  FIGS. 150 and 153 , the annular gear  18   c  is not yet engaged with the spur gear portion  30   a , so that rotation of the helicoid rig  18  is not yet transferred to the viewfinder drive gear  30 . In this connection, at the stage shown in  FIGS. 150 and 153 , a part of the annular gear  18   c  still faces the flat surface portion  30   b   2  to prohibit the viewfinder drive gear  30  from rotating. 
   A further rotation of the helicoid ring  18  in the lens barrel advancing direction causes to the short gear tooth  18   c   1  to reach its position shown in FIG.  151 . At this stage shown in  FIG. 151 , the short gear tooth  18   c   1  comes into contact with one of the teeth of the spur gear portion  30   a  and subsequently presses the same in the lens barrel advancing direction (upwards as viewed in  FIGS. 151 ) to start rotating the viewfinder drive gear  30 . 
   A further rotation of the helicoid ring  18  in the lens barrel advancing direction causes a gear tooth of the normal tooth gear  18   c   2 , which is adjacent to the short gear tooth  18   c   1  on one of the opposite sides thereof in the circumferential direction of the helicoid ring  18 , to press the subsequent gear teeth of the spur gear portion  30   a  to keep rotating the viewfinder drive gear  30 . Thereafter, the annular gear  18   c  imparts a further rotation of the helicoid ring  18  to the viewfinder drive gear  30  via the engagement of the normal tooth gear  18   c   2  with the gear teeth of the spur gear portion  30   a . At the stage shown in  FIG. 145  at which the helicoid ring  18  reaches the position thereof at the wide-angle extremity, the short gear teeth  18   c   1  is not used for the subsequent rotation of the helicoid ring  18  in the zooming range between the wide-angle extremity and the telephoto extremity since the short gear teeth  18   c   1  has already passed the point of engagement with the spur gear portion  30   a.    
   Accordingly, in the present embodiment of the zoom lens, a portion of the annular gear  18   c , which is firstly engaged with the spur gear portion  30   a  of the viewfinder drive gear  30 , is formed as at least one short gear tooth ( 18   c   1 ), the teeth depth of which is smaller than those of the other gear teeth of the annular gear  18   c . According to this construction, the annular gear  18   c  can be reliably and surely engaged with the spur gear portion  30   a  upon commencement of engagement therewith. Namely, in the case of tall (normal) gear teeth, since the tips of mutually neighboring tall gear teeth having very different relative angles, the engagement thereof is shallow (the initial engagement range is narrow) so that there is a chance of engagement therebetween failing (miss engagement). Whereas, since the short gear teeth  18   c   1  moves until the relative angle between the short gear teeth  18   c   1  and the tall gear teeth (the spur gear portion  30   a  of the viewfinder drive gear  30 ) becomes substantially the same before engaging, a deeper engagement is achieved (the initial engagement range is wide), so that there is no chance of engagement therebetween failing (missing engagement). Furthermore, this structure reduces the shock at the movement of engagement of the annular gear  18   c  with the spur gear portion  30   a , thus making it possible to smoothly start operations of the zoom viewfinder drive system including the viewfinder drive gear  30  and to reduce the noise produced by the zoom viewfinder drive system. 
   Although the above descriptions have been directed mainly to the features found in operations of the zoom lens  71  when the zoom lens  71  advances from the retracted position toward the zooming range, similar features can surely be expected in operations of the zoom lens  71  when the zoom lens  71  retracts to the retracted position. 
   As can be understood from the above descriptions, according to the above described linear guide structure, a movable member (the second lens group moving frame  8 ), the moving range of which in the optical axis direction is comparatively large compared with the size of a cam ring (the cam ring  11 ), can be guided linearly by a linear guide member (the second linear guide ring  10 ) with reliability over the entire range of movement of the movable member. Since the linear guide member is not larger than the cam ring, the linear guide structure can be designed simple and small. 
   The present invention is not limited solely to the particular embodiment described above. For instance, although each set of the set of linear guide keys  10   c  and the set of guide grooves  8   a  is provided as a set of three keys or grooves formed at different circumferential positions, the number of keys or grooves of each set of the set of linear guide keys  10   c  and the set of guide grooves  8   a  is not limited solely to three but can be any other number. 
   Although the plurality of cam followers  8   b  are divided into two groups: a front group of cam followers ( 8   b - 1 ) and a rear group of cam followers ( 8   b - 2 ) in the optical axis direction and each group of these two groups consists of three cam followers, the number of cam followers  8   b  can be any other number. Likewise, the plurality of inner cam grooves  11   a  are divided into two groups: a front group of cam grooves ( 11   a - 1 ) and a rear group of cam grooves ( 11   a - 2 ) and each of these two groups consists of three cam grooves, the number of cam grooves  11   a  can be any other number. 
   Obvious changes may be made in the specific embodiments of the present invention described herein, such modifications being within the spirit and scope of the invention claimed. It is indicated that all matter contained herein is illustrative and does not limit the scope of the present invention.