Patent Publication Number: US-6906871-B2

Title: Cam mechanism for lens barrel

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a cam mechanism for a lens barrel which is provided with a cam ring for moving a linearly guided ring member of the lens barrel. 
   2. Description of the Related Art 
   Photographing lens barrels are often provided with a cam mechanism using a cam ring wherein the cam ring is provided on an inner peripheral surface thereof with a set of cam grooves while a set of cam followers which are respectively engaged in the set of cam grooves are formed on a linearly guided ring member which is linearly guided along a photographing optical axis so that the linearly guided ring member moves between a ready-to-photograph position and an accommodation position (retracted position). Therefore, each cam groove of the cam ring has a photographing section for moving the linearly guided ring member to the ready-to-photograph position and an accommodation section for positioning the linearly guided ring member to the accommodation position, in which no pictures are taken. 
   Such cam grooves of the cam mechanism are conventionally formed on the cam ring so that the opposite ends of each cam groove are formed as closed ends, i.e., so that either end of each cam groove is not open at the front end or the rear end of the cam ring. Even in the case where each cam groove is provided at the front end or the rear end of the cam ring with a cam follower insertion opening through which the associated cam follower is inserted into the cam groove, each of the aforementioned photographing section and the aforementioned accommodation section is shaped so that both the opposite ends thereof are closed (i.e., so that each of the front cam surface and the rear cam surface in each cam groove can be in contact with the associated cam follower). 
   However, the inventors of the present invention have found that the above described conventional cam groove design, which forms each cam groove as a closed cam groove, restricts the reduction in axial length of the cam ring, thus preventing the length of the lens barrel from being minimized. 
   SUMMARY OF THE INVENTION 
   The present invention provides a cam mechanism for a lens barrel which allows a further reduction in length of a cam ring of the lens barrel, to thereby allow a further reduction in length of the lens barrel when the lens barrel is in the accommodation position. 
   According to an aspect of the present invention, a cam mechanism for a lens barrel is provided, including an annular member which is linearly guided along an optical axis, the annular member having at least one cam follower on an outer peripheral surface thereof; a cam ring having at least one cam groove on an inner peripheral surface thereof, the cam groove including a photographing section for moving the annular member to a ready-to-photograph position thereof, and an accommodation section for positioning the annular member to an accommodation position thereof at which no photographing operation is performed, the cam follower being engaged in the cam groove; and a biasing device for biasing the annular member forward to normally press the cam follower against a front cam surface in the cam groove. A rear end portion of the cam groove is open at a rear end surface of the cam ring to serve as the accommodation section, and the cam follower is disengageable from the front cam surface in the cam groove against a biasing force of the biasing device when the cam follower is engaged in the accommodation section. 
   According to this structure, the annular member (cam follower) can be made to move rearward against the spring force of the spring device when engaged in the accommodation section of the cam groove since the opened rear end portion of the cam groove is formed to serve as the accommodation section of the cam groove. This structure makes it possible to shorten the axial length of the cam ring, which in turn makes it possible to shorten the length of the lens barrel. 
   It is desirable for the cam mechanism to further include a linear guide ring which is linearly guided along the optical axis and positioned inside the cam ring. The cam ring can include a circumferential groove formed on an inner peripheral surface of the cam ring in the vicinity of a rear end thereof. The linear guide ring can include an outer flange which is formed on an outer peripheral surface of the linear guide ring to be engaged in the circumferential groove in a manner so that a relative rotation between the outer flange and the circumferential groove about the optical axis is possible and so that the outer flange and the circumferential groove are prevented from moving relative to each other along the optical axis. It is desirable for the accommodation section to overlap the circumferential groove. The outer flange can include at least one cut-out portion which allows the cam follower to enter the cut-out portion when the annular member is in the accommodation position. 
   It is desirable for the lens barrel to include a photographing optical system including an optical member supported by the annular member. The optical member can include at least one intermediate lens group of the photographing optical system. It is desirable for a lens frame of a front lens group of the photographing optical system which is positioned in front of the intermediate lens group be in contact with the annular member when the annular member is in the accommodation position. 
   The biasing device can be a coil spring positioned between the annular member and a lens frame of a rear lens group of the photographing optical system which is positioned behind the intermediate lens group. 
   It is desirable for the lens frame of the rear lens group to be in contact with a light shield plate provided in the lens barrel by spring force of the coil spring when the annular member is in the accommodation position. 
   It is desirable for the lens barrel to be a zoom lens barrel, and for the photographing section to be formed so as to move the annular member to a ready-to-photograph position thereof among a plurality of ready-to-photograph positions corresponding to a plurality of different focal lengths. 
   It is desirable for the accommodation section to be elongated in a circumferential direction of the cam ring. 
   The optical member, which is supported by the annular member, can include a shutter unit which is fixed to the annular member. 
   The annular member can include a ring portion with a center thereof on the optical axis; and at least one guide arm which projects rearward from the ring portion to be linearly guided along the optical axis, wherein the cam follower extends radially outwards from the guide arm. 
   The lens barrel can be a telescoping type zoom lens barrel having a plurality of external telescoping barrels, the light shield plate being fixed to a rear end of an outermost external telescoping barrel of the plurality of external telescoping barrels. 
   In another embodiment, a cam mechanism for a lens barrel is provided, including an annular member which holds at least one lens group at a center of the annular member, and is linearly guided along an optical axis, the annular member including at least one cam follower; and a cam ring positioned coaxially with the annular member and having at least one cam groove, wherein rotation of the cam ring causes the annular member to move along the optical axis due to engagement of the cam groove with the cam follower formed on the annular member. It is desirable for the cam groove to include a photographing section for moving the annular member to a ready-to-photograph position thereof, and an accommodation section for positioning the annular member to a retracted position positioned behind the ready-to-photograph position in the optical axis direction. It is desirable for a rear end portion of the cam groove to be open at a rear end surface of the cam ring to be formed as the accommodation section. It is desirable for the cam follower to be normally pressed against a front cam surface in the cam groove by a biasing device. The cam follower is disengageable from the front cam surface in the cam groove against the spring force of the biasing device when engaged in the retracting section. 
   The present disclosure relates to subject matter contained in Japanese Patent Application No.2002-359802 (filed on Dec. 11, 2002) which is expressly incorporated herein by reference in its entirety. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be discussed below in detail with reference to the accompanying drawings, in which: 
       FIG. 1  is a diagram showing lens-group-moving paths of a step-zoom lens system (which includes a switching lens group) of a zoom lens barrel according to the present invention; 
       FIG. 2  is an exploded perspective view of an embodiment of the zoom lens barrel according to the present invention; 
       FIG. 3  is a longitudinal cross sectional view of the zoom lens barrel shown in  FIG. 2  in the retracted state, showing only an upper half of the zoom lens barrel from an optical axis; 
       FIG. 4  is a longitudinal cross sectional view of the zoom lens barrel shown in  FIG. 2  at the wide-angle extremity which is focused on an image at infinity, showing only an upper half of the zoom lens barrel from the optical axis; 
       FIG. 5  is a longitudinal cross sectional view of the zoom lens barrel shown in  FIG. 2  at telephoto extremity which is focused on an image at infinity, showing only an upper half of the zoom lens barrel from the optical axis; 
       FIG. 6  is a developed view of an inner peripheral surface of a cam ring of the zoom lens barrel shown in  FIG. 2 ; 
       FIG. 7  is a developed view of an inner peripheral surface of a switching ring of the zoom lens barrel shown in  FIG. 2 ; 
       FIG. 8  is a longitudinal cross sectional view of a portion of the zoom lens barrel shown in  FIG. 2 , showing a structure of engagement of a first lens group support ring with a fourth lens frame, showing only an upper half of the portion of the zoom lens barrel from the optical axis; 
       FIG. 9  is a developed perspective view of the switching ring, the first lens group support ring and a first linear guide ring of the zoom lens shown in.  FIG. 2 ; 
       FIG. 10  is a perspective view of a second/third lens group support unit of the zoom lens barrel shown in  FIG. 2 ; 
       FIG. 11  is an exploded perspective view of the second/third lens group support unit shown in  FIG. 10 ; 
       FIG. 12  is a longitudinal cross sectional view of a switching mechanism of the zoom lens barrel shown in  FIG. 2  that includes the second/third lens group support unit shown in  FIG. 10 , showing only an upper half of the switching mechanism from the optical axis; 
       FIG. 13  is a perspective view of an overtravel mechanism incorporated in the second/third lens group support unit shown in  FIGS. 10 and 11 ; 
       FIG. 14  is a developed view of the second/third lens group support unit in a wide-angle mode; 
       FIG. 15  is a developed view of the second/third lens group support unit in a telephoto mode; 
       FIG. 16  is a front elevational view of the second/third lens group support unit in a state shown in  FIG. 14 ; 
       FIG. 17  is a front elevational view of the second/third lens group support unit in a state shown in  FIG. 15 ; 
       FIGS. 18A through 18D  are developed views of the switching ring shown in  FIG. 7 , the first linear guide ring shown in  FIG. 9 and a  switching leaf of the second/third lens group support unit shown in  FIG. 11  in different states, showing transitions in relative position among these three elements of the zoom lens barrel from a state at wide-angle extremity shown in  FIG. 18A  to a state at telephoto extremity shown in  FIG. 18D ; and 
       FIG. 19  is a developed view of a cam groove provided on a cam ring of the zoom lens barrel. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows a zoom lens system provided in an embodiment of a zoom lens barrel according to the present invention. The zoom lens system of the zoom lens barrel  10  includes a positive first lens group L 1 , and a negative second lens group L 2 , a positive third lens group L 3  and a negative fourth lens group L 4 , in that order from the object side (left side as viewed in FIG.  3 ). The second lens group L 2  and the third lens group L 3  serve as a distance-varying lens group (L 23 ) which changes the distance therebetween at an intermediate range of focal length (mode switching section) from a wide distance in a wide-angle range (wide-angle mode section) to a narrow distance in a telephoto range (telephoto mode section) and vice versa. The second lens group L 2  and the third lens group L 3  move together without changing the distance therebetween in each of the wide-angle range and the telephoto range. The first lens group L 1  and the fourth lens group L 4  always move together without changing the distance therebetween. Over the entire zooming range from the short focal length extremity (wide-angle extremity (W)) to the long focal length extremity (telephoto extremity (T)), each of the first lens group L 1 , the distance-varying lens group L 23  and the fourth lens group L 4  moves monotonously in a forward direction from the image side (right side as viewed in  FIG. 1 ) to the object side (left side as viewed in  FIG. 1 ) when a zooming operation is carried out from wide-angle extremity to telephoto extremity, or in a retracting direction from the object side to the image side (from left to right as viewed in  FIG. 1 ) when a zooming operation is carried out from telephoto extremity to wide-angle extremity. The present embodiment of the zoom lens barrel  10  is a step-zoom lens barrel which changes the focal length stepwise (specifically, six different focal lengths) when performing a zooming operation, and the distance-varying lens group L 23  serves as a focusing lens group in the step-zoom lens barrel. Accordingly, solid lines shown in  FIG. 1  which are drawn in association with the first lens group L 1 , the distance-varying lens group L 23  and the fourth lens group L 4 , represent associated cam diagrams (which include cam diagrams for a focusing operation). A reference moving path of the distance-varying lens group L 23  to perform a zooming operation for an image at infinity is represented by one-dot chain lines shown in  FIG. 1  which are drawn in association with the distance-varying lens group L 23 . 
   This type of zoom lens system having a distance-varying lens group in which the distance between two lens elements varies at an intermediate focal length has been proposed in U.S. Pat. No. 6,369,955 (Japanese Unexamined Patent Publication No.2000-275518), the assignee of which is the same as that of the present invention. This zoom lens system includes a plurality of movable lens groups which are moved to vary the focal length of the zoom lens system, and at least one lens group of the plurality of movable lens groups includes two sub-lens groups serving as a switching lens group. One of the two sub-lens groups is moveable, along the optical axis of the zoom lens system, to be selectively positioned at one of the movement extremities of the moveable sub-lens group with respect to the other sub-lens group. In a short-focal-length side zooming range covering the short focal length extremity over an intermediate focal length, the moveable sub-lens group is arranged to position at one of the movement extremities thereof. In a long-focal-length side zooming range covering the long focal length extremity over the intermediate focal length, the moveable sub-lens group is arranged to position at the other of the movement extremities thereof. The moving path of the switching lens group having the two sub-lens groups, and the moving paths of the other lens groups of the plurality of movable lens groups are discontinued at the intermediate focal length. The zoom lens system is arranged to form an image on a predetermined image plane in accordance with a position of the moveable sub-lens group. Although the first through fourth lens groups L 1  through L 4  are shown as a single lens elements in the lens-group-moving paths shown in  FIG. 1 , each of the first through fourth lens groups L 1  through L 4  generally consists of more than one lens element. 
     FIGS. 2 through 19  show the overall structure of the present embodiment of the zoom lens barrel  10 . The zoom lens barrel  10  is provided with a stationary barrel  11  which is fixed to a camera body. As shown in  FIGS. 2 through 5 , the stationary barrel  11  is provided on an inner peripheral surface thereof with a female helicoid  11   a  and a plurality of linear guide grooves  11   b  (only one of them appears in  FIG. 2 ) which extend parallel to an optical axis O. As can be understood from  FIGS. 3 through 5 , the zoom lens barrel  10  is a telescoping type zoom lens which is provided with three external telescoping barrels: a first external barrel (helicoid ring)  12 , a second external barrel (cam ring)  15  and a third external barrel (switching ring)  16 , which are concentrically arranged about the optical axis O. The female helicoid  11   a  of the stationary barrel  11  is engaged with a male helicoid  12   a  which is formed on an outer peripheral surface of the helicoid ring  12  in the vicinity of the rear end thereof. The zoom lens barrel  10  is provided with a second linear guide ring  13  which is fitted in the helicoid ring  12  to be movable together with the helicoid ring  12  along the optical axis O and to be freely rotatable relative to the helicoid ring  12 . Namely, the helicoid ring  12  is provided on an inner peripheral surface thereof with two circumferential grooves  12   c  which extend parallel to each other in a circumferential direction of the helicoid ring  12 , while the second linear guide ring  13  is provided on an outer peripheral surface thereof with a pair of guide projections  13   a  which are respectively engaged in the two circumferential grooves  12   c  of the helicoid ring  12  to be freely movable therein. The pair of guide projections  13   a , which are aligned in a direction parallel to the optical axis O as shown in  FIG. 2 , remain respectively engaged with the two circumferential grooves  12   c  when the zoom lens barrel  10  is in use. The second linear guide ring  13  is provided at the rear end thereof with a plurality of radial projections  13   b  (only one of them appear in  FIG. 2 ) which extend radially outwards to be engaged in the plurality of linear guide grooves  11   b  of the stationary barrel  11 , respectively. 
   The helicoid ring  12  is provided on the thread of the male helicoid  12   a  with a spur gear  12   b  which is engaged with a drive pinion  14 . The drive pinion  14  is provided in a recessed portion  11   c  (see  FIG. 2 ) formed on an inner peripheral surface of the stationary barrel  11 . The drive pinion  14  is supported by the stationary barrel  11  to be freely rotatable in the recessed portion  11   c  on an axis of the drive pinion  14 . Accordingly, forward and reverse rotations of the drive pinion  14  cause the helicoid ring  12  to move forward rearward along the optical axis O while rotating about the optical axis O, thus causing the second linear guide ring  13  to move linearly along the optical axis O along with the helicoid ring  12 . 
   The cam ring  15  is fitted inside the second linear guide ring  13 .  FIG. 6  is a developed view of an inner peripheral surface of the cam ring  15 . The cam ring  15  is provided, on an outer peripheral surface thereof in the vicinity of the rear end of the cam ring  15 , with a guide pin  15   b  which extends radially outwards from a portion of the male helicoid  15   a . The male helicoid  15   a  is engaged with a female helicoid  13   c  formed on an inner peripheral surface of the second linear guide ring  13 , while the guide pin  15   b  is engaged in a clearance slot  13   d  which is formed on the second linear guide ring  13  to extend in a direction both in a circumferential direction of the second linear guide ring  13  and in the optical axis direction (the direction of the optical axis O). The guide pin  15   b  passes through the clearance slot  13   d  to be engaged in a linear guide groove  12   d , which is formed on an inner peripheral surface of the helicoid ring  12   d  (shown by broken lines in  FIG. 2 ) and extends parallel to the optical axis O. Therefore, a rotation of the helicoid ring  12  causes the cam ring  15  to move along the optical axis O while rotating about the optical axis O due to the engagement of the female helicoid  13   c  with the male helicoid  15   a . The cam ring  15  is provided on an inner peripheral surface thereof with a female helicoid  15   c  (see  FIGS. 2 and 6 ) and a set of three bottomed cam grooves  15   d  (only one of them is shown in FIG.  19 ). 
   The zoom lens barrel  10  is provided inside the cam ring  15  with three concentric rings: the switching ring  16 , a first lens group support ring  17  and a first linear guide ring  18 , which fit inside each other in that order in a radially inward direction. The first lens group support ring  17  supports the first lens group L 1 .  FIG. 7  is a developed view of the switching ring  16 . The switching ring  16  and the first lens group support ring  17  move together along the optical axis O while the switching ring  16  is allowed to rotate freely about the optical axis O relative to the first lens group support ring  17 . The first lens group support ring  17  is provided, on an outer peripheral surface thereof in the vicinity of the rear end of the first lens group support ring  17 , with a male helicoid  17   a , and is further provided immediately in front of the male helicoid  17   a  with a guide projection  17   b . The guide projection  17   b  is engaged in a circumferential groove  16   a  (see  FIG. 7 ) which is formed on an inner peripheral surface of the switching ring  16  in the vicinity of the rear end thereof to allow a relative rotation between the guide projection  17   b  and the circumferential groove  16   a  about the optical axis O. 
   The male helicoid  17   a  of the first lens group support ring  17  is engaged with the female helicoid  15   c  of the cam ring  15 . The cam ring  15  is provided on an inner peripheral surface thereof with a set of six rotation transfer grooves  15   e  (only three of them appear in  FIG. 2 ) which extend parallel to the optical axis O, while the switching ring  16  is provided, on an outer peripheral surface thereof in the vicinity of the rear end of the switching ring  16 , with a set of six rotation transfer projections  16   b  (only three of them appear in  FIG. 2 ) which project radially outwards to be engaged in the set of six rotation transfer grooves  15   e , respectively. 
   On the other hand, the second linear guide ring  13  is provided on an inner peripheral surface thereof with a plurality of linear guide grooves  13   e  (only one of them appears in  FIG. 2 ) which extend parallel to the optical axis O, while the first linear guide ring  18  is provided, on an outer peripheral surface thereof in the vicinity of the rear end of the first linear guide ring  18 , with a plurality of guide projections  18   a  (only two of them appear in  FIG. 9 ) which project radially outwards to be engaged in the plurality of linear guide grooves  13   e , respectively. The first linear guide ring  18  is provided on an outer peripheral surface thereof with a linear guide groove  18   b  (see  FIG. 9 ) which extend parallel to the optical axis O, while the first lens group support ring  17  is provided, on an inner peripheral surface thereof in the vicinity of the rear end of the first lens group support ring  17 , with a linear guide projection  17   c  which projects radially inwards to be engaged in the linear guide groove  18   b  (see FIG.  9 ). Therefore, each of the second linear guide ring  13 , the first linear guide ring  18  and the first lens group support ring  17  is movable along the optical axis O without relatively rotating about the optical axis O. The first linear guide ring  18  is provided in the immediate vicinity of the rear end thereof with an outer flange  18   f  (see  FIG. 9 ) which projects radially outwards to be engaged in a circumferential groove  15   f  (see  FIG. 6 ) which is formed on an inner peripheral surface of the cam ring  15  in the immediate vicinity of the rear end thereof so that a relative rotation between the outer flange  18   f  and the circumferential groove  15   f  about the optical axis O is possible, and so that the outer flange  18   f  and the circumferential groove  15   f  move together in the optical axis direction. 
   Therefore, if a rotation of the cam ring  15  is transferred to the switching ring  16  via the engagement of the set of six rotation transfer projections  16   b  with the set of six rotation transfer grooves  15   e , the first lens group support ring  17 , which has the male helicoid  17   a  engaged with the male helicoid  15   c  of the cam ring  15  and is prevented from rotating by the first linear guide ring  18 , moves along the optical axis O. 
   The zoom lens barrel  10  is provided in the rear of the first lens group support ring  17  with a fourth lens group support ring  19 . The fourth lens group support ring  19  is supported by the first lens group support ring  17  to be freely movable linearly along the optical axis O without rotating about the optical axis O relative to the first lens group support ring  17 . The fourth lens group support ring  19  supports the fourth lens group L 4 , and is provided on an outer peripheral surface thereof with a set of three axial arms  19   a  which extend parallel to the optical axis O. The first lens group support ring  17  is provided with a set of three linear guide slots  17   d  which extend parallel to the optical axis O. The fourth lens group support ring  19  and the first lens group support ring  17  are engaged with each other with the set of three axial arms  19   a  being slidably engaged in the set of three linear guide slots  17   d , respectively. 
   The zoom lens barrel  10  is provided in association with the first linear guide ring  18  with a second/third lens group support unit (ring member)  20  (see  FIGS. 10 and 11 ) which supports the second lens group L 2  and the third lens group L 3 . The second/third lens group support unit  20  is provided on a second/third lens group moving ring  21  thereof with a set of three guide arms  20   a  which extend parallel to the optical axis O. The first linear guide ring  18  is provided with a set of three linear guide slots  18   c  in which the set of three guide arms  20   a  are slidably engaged. A set of three cam followers  20   b  are fixed to the set of three guide arms  20   a  in the vicinity of the rear ends thereof, respectively. Each cam follower  20   b  projects radially outwards to be engaged in the associated one of the three bottomed cam grooves  15   d  of the cam ring  15 .  FIG. 10  shows the second/third lens group support unit  20  in an assembled state, while  FIG. 11  shows the second/third lens group support unit  20  in a disassembled state. As shown in  FIGS. 6 and 19 , each of the three bottomed cam grooves  15   d  consists of a photographing section  15   d   1  (which includes the wide-angle mode section, the mode switching section and the telephoto mode section which are shown in  FIG. 19 ) for moving the second/third lens group support unit  20  to a ready-to-photograph position among a plurality of ready-to-photograph positions, an accommodation section  15   d   2  for positioning the second/third lens group support unit  20  to an accommodation position thereof (in which no photographing operation is performed), and a transfer section  15   d   3 , which is positioned between the photographing section  15   d   1  and the accommodation section  15   d   2 , for moving the second/third lens group support unit  20  between the photographing section  15   d   1  and the accommodation section  15   d   2 . The entire portion of the photographing section  15   d   1  and the entire portion of the transfer section  15   d   3  except for an end portion (rear end portion) of the transfer section  15   d   3  in the vicinity of the accommodation section  15   d   2  are formed as narrow-width cam portions in which the associated cam follower  20   b  is engaged with a minimum clearance. The accommodation section  15   d   2  and the aforementioned rear end portion of the transfer section  15   d   3  are formed as open cam portions which are open at a rear end surface of the cam ring  15 . Accordingly, a rotation of the cam ring  15  causes the second/third lens group support unit  20  to move linearly along the optical axis O in accordance with the contours of the set of three cam grooves  15   d . The outer flange  18   f  of the first linear guide ring  18 , which is engaged in the circumferential groove  15   f  of the cam ring  15  so that a relative rotation between the outer flange  18   f  and the circumferential groove  15   f  about the optical axis O is possible, is provided with a set of three cut-out portions  18   f ′. The set of three cut-out portions  18   f ′ are positioned behind the accommodation sections  15   d   2  of the set of three cam grooves  15  to allow the set of three cam followers  20   b  to enter the set of three cut-out portions  18   f ′ (see  FIGS. 3 ,  9  and  18 A; only two of them appear in FIG.  9 ), respectively, so that each cam follower  20   b  can move rearward beyond the front end surface of the outer flange  18   f  when the second/third lens group support unit  20  retracts to its retracted position (accommodation position). 
   The zoom lens barrel  10  is provided between the second/third lens group support unit  20  and the fourth lens group support ring  19  with a compression coil spring (biasing device)  31  for biasing the fourth lens group support ring  19  rearward. Each of the set of three axial arms  19   a  is provided with a claw portion  19   b  (see  FIG. 8 ) which is engaged with an associated inward projection  17   e  (see  FIGS. 8 and 9 ) which is formed on the first lens group support ring  17  at the rear end thereof to determine the rear limit for the axial movement of the fourth lens group support ring  19  with respect to the first lens group support ring  17  against the spring force of the compression coil spring  31  to prevent the fourth lens group support ring  19  from coming out of the first lens group support ring  17 . The fourth lens group support ring  19  remains at its rearmost position with respect to the first lens group support ring  17  in a ready-to-photograph state of the zoom lens barrel  10 . 
   Operations of the above described portions of the zoom lens barrel  10  will be hereinafter discussed before the structure of the second/third lens group support unit  20  is discussed in detail. Rotating the helicoid ring  12  by rotation of the drive pinion  14  causes the helicoid ring  12  to move along the optical axis O while rotating about the optical axis O, thus causing the second linear guide ring  13 , which is prevented from rotating, to move along the optical axis O together with the helicoid ring  12 . This rotation of the helicoid ring  12  is transferred to the cam ring  15  to move the cam ring  15  along the optical axis O together with the first linear guide ring  18 , which is linearly guided, while rotating about the optical axis O. At the same time, this rotation of the cam ring  15  causes the switching ring  16  to move together with the first lens group support ring  17 , which is linearly guided, along the optical axis while rotating about the optical axis O with respect to the first lens group support ring  17 . When the first lens group support ring  17  moves forward from its retracted position shown in  FIG. 4 , the compression coil spring  31  resiliently expands gradually to position the fourth lens group support ring  19  at its rearmost position with respect to the first lens group support ring  17 . This rearmost position corresponds to wide-angle extremity in the zooming range. Thereafter the first lens group support ring  17  and the fourth lens group support ring  19  move together. Since the first lens group support ring  17  and the fourth lens group support ring  19  hold the first lens group L 1  and the fourth lens group L 4 , respectively, the first lens group L 1  and the fourth lens group L 4  move together along the optical axis O to be linearly proportional to the angle of rotation of the helicoid ring  12  (without varying the distance between the first lens group L 1  and the fourth lens group L 4 ) as shown in FIG.  1 . 
   As can be clearly seen in  FIG. 3 , a front end surface of the second/third lens group support unit  20  is positioned very closely to or comes in contact with a rear end surface of a first lens frame  29  (by which the first lens group L 1  is fixed to be supported) when the zoom lens barrel  10  is in the retracted position. The first lens frame  29  is fixed to a front end portion of the first lens group support ring  17 . In the retracted state shown in  FIG. 3 , since the rear of the accommodation section  15   d   2  of each cam groove  15   d  is open, each cam follower  20   b  is disengaged from a front cam surface (front cam edge) in the associated cam groove  15   d  to become capable of moving rearward to thereby reduce the length of the zoom lens barrel  10  in the retracted state when the second/third lens group support unit  20  is pressed rearward by the first lens frame  29  against the spring force of the compression coil spring  31 . At the same time, a fourth lens frame  30 , to which the fourth lens group L 4  is fixed to be supported thereby, is moved rearward to the position where the fourth lens frame  30  contacts with a light shield plate  35  (see  FIG. 3 ) by the spring force of the compression coil spring  31 . The fourth lens frame  30  is fixed to the fourth lens group support ring  19 , while the light shield plate  35  is fixed to a rear end surface of the helicoid ring  12 . 
   On the other hand, the axial position of the second/third lens group support unit  20  is determined by the set of three bottomed cam grooves  15   d , which are formed on an inner peripheral surface of the cam ring  15 . The second/third lens group support unit  20  supports the second lens group L 2  and the third lens group L 3 , while a continuous rotation of the cam ring  15  together with the switching ring  16  provides the second lens group L 2  and the third lens group L 3  respective moving paths thereof shown in FIG.  1 . The structure of the second/third lens group support unit  20 , and associated structures of the cam ring  15  and the switching ring  16  will be hereinafter discussed in detail with reference to  FIGS. 9 through 18D . 
   The set of three guide arms  20   a  are formed on the second/third lens group moving ring  21  of the second/third lens group support unit  20 , while the set of three cam followers  20   b  are fixed to the set of three guide arms  20   a , respectively. The second/third lens group support unit  20  is provided at a front end thereof with a front-end pressing ring plate  22 , and is provided between the second/third lens group moving ring  21  and the front-end pressing ring plate  22  with the second lens frame  23 , a third lens frame  24 , a differential linking ring  25 , a differential ring  26  and a differential spring  27  which are accommodated in the space between the second/third lens group moving ring  21  and the front-end pressing ring plate  22 , in that order from the object side. The third lens group L 3  is fixed to the third lens frame  24  to be supported thereby. A pair of guide pins  22   a  are fixed to the front-end pressing ring plate  22  to extend rearward to be parallel to the optical axis O. The second lens frame  23  is provided with a pair of guide bosses  23   a  which are slidably fitted on the pair of guide pins  22   a , respectively. A pair of compression coil springs  22   b  are loosely fitted on the pair of guide pins  22   a  to press the second lens frame  23  rearward. 
   Each of the third lens frame  24 , the differential linking ring  25  and the differential ring  26  is rotatable about the optical axis O. The second lens frame  23  and the third lens frame  24  have cylindrical portions so that the cylindrical portion of the third lens frame  24  is fitted on the cylindrical portion of the second lens frame  23 . The second lens frame  23  is provided on an outer peripheral surface of the cylindrical portion thereof with a set of four inclined cam edges  23   b  (only one of them appears in  FIG. 11 ) while the third lens frame  24  is provided on an inner peripheral surface of the cylindrical portion thereof with a set of four cam followers  24   a  (only two of them appears in  FIG. 11 ) which are engaged with the set of four inclined cam edges  23   b , respectively. Each cam edge  23   b  extends linearly, and is inclined with respect to both a circumferential direction of the second lens frame  23  and the optical axis direction. The third lens frame  24  is provided on an outer peripheral surface thereof with a pair of rotation transfer projections  24   b  while the differential linking ring  25  is provided on an inner peripheral surface thereof with a pair of rotation transfer grooves  25   a  in which the pair of rotation transfer projections  24   b  are engaged, respectively, so that the third lens frame  24  and the differential linking ring  25  rotate together at all times. The third lens frame  24  is always pressed rearward by the spring force of the pair of compression coil springs  22   b  to be in pressing contact with the second/third lens group moving ring  21  to determine the position of the third lens frame  24  in the optical axis direction with respect to the second/third lens group moving ring  21 . The differential ring  26  is provided on an inner peripheral surface thereof with a pair of forced-rotation transfer grooves  26   a  (only one of them appears in  FIG. 11 ) while the differential linking ring  25  is provided on an outer peripheral surface thereof with a pair of forced-rotation transfer projections  25   b  which are engaged in the pair of forced-rotation transfer grooves  26   a , respectively, with a predetermined circumferential clearance between each forced-rotation transfer projection  25   b  and the associated forced-rotation transfer groove  26   a  (see FIGS.  16  and  17 ). 
   The differential spring  27  is a torsion spring  27  consisting of a loop portion  27   a  with its center substantially on the optical axis O and a pair of engaging radial projections  27   b  which project radially outwards from the opposite ends of the loop portion  27   a , respectively. The loop portion  27   a  is fitted in the differential linking ring  25  to be engaged with an inner peripheral surface thereof by friction. The differential linking ring  25  is provided with a pair of radial through holes  25   c  into which the pair of engaging radial projections  27   b  are inserted from the inside of the differential linking ring  25  to project radially outwards from an outer peripheral surface of the differential linking ring  25 . The differential linking ring  25  is provided on an inner peripheral surface thereof with an inward projection  25   d  (see  FIG. 11 ) which is engaged with the loop portion  27   a  of the differential spring  27  to prevent the differential spring  27  from coming off the differential linking ring  25 . The differential ring  26  is provided with a rotation transfer projection  26   b  which projects rearwards, and the pair of engaging radial projections  27   b  of the differential spring  27  are in pressing in contact with opposite surfaces of the rotation transfer projection  26   b  in a circumferential direction of the differential ring  26  in opposite directions towards each other. The differential linking ring  25  normally rotates together with the differential ring  26  via the differential spring  27  when the differential ring  26  rotates. However, if the differential linking ring  25  reaches one end of the range of rotation thereof (i.e., if a resistance which is generated in the differential linking ring  25  to rotate is greater than a predetermined resistance) when the differential ring  26  rotates, the differential ring  26  rotates relative to the differential linking ring  25  while the differential spring  27  is deformed to open the pair of engaging radial projections  27   b  (i.e., to move the pair of engaging radial projections  27   b  in opposite directions away from each other in a circumferential direction of the differential spring  27 ). 
   The second/third lens group support unit  20  is provided with a switching leaf  28  which is provided on an inner peripheral surface thereof with a rotation transfer groove  28   a  which extends parallel to the optical axis O, while the rotation transfer projection  26   b  is provided with a linking pin  26   c  which projects radially outwards to be engaged in the rotation transfer groove  28   a . As shown in  FIG. 9 , the switching leaf  28  is positioned in a guide slot  18   d  (see  FIG. 9 ) formed on the first linear guide ring  18 , and is supported by the first linear guide ring  18  to be movable in a circumferential direction of the first linear guide ring  18  with respect to the first linear guide ring  18  within a predetermined angle of rotation about the optical axis O. The switching ring  16  is provided on an inner peripheral surface thereof with a switching groove  16   c , while the switching leaf  28  is provided, on an outer peripheral surface thereof in the vicinity of the front end of the switching leaf  28 , with a follower projection  28   b  which is engaged in the switching groove  16   c.    
   As shown in  FIGS. 7 and 18A , the switching groove  16   c  consists of a telephoto section  16   c T, a switching section  16   c K and a wide-angle section  16   c W, in that order from rear to front of the switching groove  16   c  (i.e., from bottom to top as viewed in FIG.  7 ). Each of the telephoto section  16   c T and the wide-angle section  16   c W is inclined with respect to both a circumferential direction of the switching ring  16  and the optical axis direction. The lead angle of each of the telephoto section  16   c T and the wide-angle section  16   c W is the same as that of the threads of the female helicoid  15   c  of the cam ring  15 , and the direction of inclination of each of the telephoto section  16   c T and the wide-angle section  16   c W is opposite to that of the threads of the female helicoid  15   c  of the cam ring  15 . The switching section  16   c K extends parallel to the optical axis O. Therefore, when the cam ring  15  and the switching ring  16  rotate together, the switching leaf  28  does not rotate relative to the first linear guide ring  18  as long as the follower projection  28   b  of the switching leaf  28  remains engaged in either the telephoto section  16   c T or the wide-angle section  16   c W. This keeps the distance between the second lens group L 2  and the third lens group L 3  at either a wide distance in the wide-angle range or a narrow distance in the telephoto range (see FIG.  1 ). However, in the case where the follower projection  28   b  of the switching leaf  28  is engaged in the switching section  16   c K, the switching leaf  28  rotates relative to the first linear guide ring  18  when the cam ring  15  and the switching ring  16  rotate together. This rotation of the switching leaf  28  relative to the first linear guide ring  18  varies the distance between the narrow distance and the wide distance. 
   As shown in  FIGS. 14 and 15 , the third lens frame  24  is provided with a rotational range limit groove  24   c  and the second/third lens group moving ring  21  is provided with a stop projection  21   a  which is engaged in the rotational range limit groove  24   c  to limit the range of rotation (rotational angle) of the third lens frame  24  relative to the second/third lens group moving ring  21  to a sufficient range for the third lens frame  24  to be switched between the wide-angle position and the telephoto position. The range of rotation (rotational angle) of a combination of the switching leaf  28  and the differential ring  26  is determined to be greater than that of the third lens frame  24 , and the difference therebetween is absorbed by the differential spring  27 . 
   If the switching leaf  28  is rotated counterclockwise from the position shown in  FIG. 16  to the position shown in  FIG. 17 , via the engagement of the follower projection  28   b  with the switching groove  16   c  in a state shown in  FIG. 14  where the second lens frame  23  (the second lens group L 2 ) and the third lens frame  24  (the third lens group L 3 ) are sufficiently apart from each other in the optical axis direction, the differential ring  26  rotates. This rotation of the differential ring  26  is transferred to the differential linking ring  25  via the engagement of the pair of engaging radial projections  27   b  of the differential spring  27  with the rotation transfer projection  26   b  to rotate the third lens frame  24  in the same rotational direction as the differential ring  26 . This rotation of the third lens frame  24  causes one end of the rotational range limit groove  24   c  (the left end as viewed in  FIGS. 14 and 15 ) to come into contact with the stop projection  21   a  to thereby prevent the differential linking ring  25 , which rotates together with the third lens frame  24 , from further rotating together with the third lens frame  24 . Even after the differential linking ring  25  is prevented from rotating, the differential ring  26  continues to rotate in the same rotational direction. This overtravel of the differential ring  26  is absorbed by a resilient deformation of the differential spring  27 . At the same time, the rotation of the third lens frame  24  causes the second lens frame  23 , which is biased rearward by the pair of compression coil springs  22   b , to move rearward due to the engagement of the set of four cam followers  24   a  with the set of four inclined cam edges  23   b , thus causing the second lens group L 2  and the third lens group L 3  to approach each other (see FIGS.  15  and  17 ). The pair of forced-rotation transfer projections  25   b  are tightly engaged with the pair of forced-rotation transfer grooves  26   a , respectively, to forcefully transfer rotation of the differential ring  26  to the differential linking ring  25  in the event of the pair of engaging radial projections  27   b  of the differential spring  27  being open due to a resistance in the differential linking ring  25  from rotating caused by some reason. 
   If the switching leaf  28  is rotated reversely, i.e., clockwise from the position shown in  FIG. 17  to the position shown in  FIG. 16 , via the engagement of the follower projection  28   b  with the switching groove  16   c  in a state shown in  FIG. 15  where the second lens frame  23  (the second lens group L 2 ) and the third lens frame  24  (the third lens group L 3 ) are positioned closely to each other in the optical axis direction, the second lens frame  23  (the second lens group L 2 ) and the third lens frame  24  (the third lens group L 3 ) move apart from each other in the optical axis direction in the reverse fashion to the above described fashion. The operations of the differential ring  25 , the differential linking ring  26  and the differential spring  27  are the same as those described above when the switching leaf  28  is rotated counterclockwise as viewed in FIG.  16 . Each inclined cam edge  23   b  of the second lens frame  23  is provided on opposite ends thereof with a front recess  23   b   1  and a rear recess  23   b   2  for holding the associated cam follower  24   a  at a telephoto mode position and a wide-angle mode position with stability, respectively. The four inclined cam edges  23   b  each having such structure are arranged at equi-angular intervals in a circumferential direction of the second lens frame  23  (i.e., a circumferential direction of the third lens frame  24 ) to ensure precision in spacing (i.e., the distance) between the second lens group L 2  and the third lens group L 3  and the precision in positioning the second lens group L 2  and the third lens group L 3  concentrically with the optical axis O. 
   The zoom lens barrel  10  is provided immediately behind the second/third lens group moving ring  21  with a shutter unit  32  which is fixed to the second/third lens group moving ring  21  by set screws (see FIG.  2 ). A flexible printed wiring board (flexible PWB)  33  for electrically connecting the shutter unit  32  to a control circuit of the camera body (not shown) extends from the shutter unit  32 . The zoom lens barrel  10  is provided between an inner peripheral surface of the first lens frame  17  in the vicinity of the front end thereof and a front surface of the second/third lens group support unit  20  with a light shield bellows  34 . 
   Operations of the zoom lens barrel  10  to achieve focus will be hereinafter discussed with reference mainly to FIG.  19 . In the present embodiment of the zoom lens barrel, the set of three bottomed cam grooves  15   d  are also used to achieve focus, i.e., a focusing operation is performed with a rotation of the cam ring  15 . The step-zoom lens barrel  10  has a variable focal length of six different focal lengths: four different focal lengths (steps  1 ,  2 ,  3  and  4 ) in the wide-angle mode and two different focal lengths (steps  5  and  6 ) in the telephoto mode. The contours of the set of three bottomed cam grooves  15   d  are determined so as to move the second/third lens group support unit  20  (the second lens group L 2  and the third lens group L 3 ) between a closest photographing position (N) and an infinite photographing position (∞) in the optical axis direction at each of the six different focal lengths. Specifically, each cam groove  15   d  includes a step- 1  position for the infinite photographing position (∞), a step- 1  position for the closest photographing position (N), a step- 2  position for the closest photographing position (N), a step- 2  position for the infinite photographing position (∞), a step- 3  position for the infinite photographing position (∞), a step- 3  position for the closest photographing position (N), a step- 4  position for the closest photographing position (N), a step- 4  position for the infinite photographing position (∞), the mode switching section, a step- 5  position for the infinite photographing position (∞), a step- 5  position for the closest photographing position (N), a step- 6  position for the closest photographing position (N), and a step- 6  position for the infinite photographing position (∞), in that order in a rotating direction of the cam ring  15 . The angle of rotation (the angular position of the cam ring  15 ) of the cam ring  15  is controlled in accordance with information on a set focal length and an object distance. 
   As shown in  FIG. 19 , each cam groove  15   d  is formed so that the closest photographing positions (N) in two adjacent focal-length step positions are adjacent to each other, and the infinite photographing positions (∞) in two adjacent focal-length step positions are adjacent to each other (with the exception of the adjacent steps  4  (∞) and  5  (∞). This structure is advantageous to simplify the contour of each cam groove  15   d  and to shorten the length thereof. 
   The second/third lens group support unit  20  is an annular member which supports optical elements such as the second lens group L 2 , the third lens group L 3  and the shutter unit  32 . As described above, the second/third lens group support unit  20  has the set of three cam followers  20   b  that are fixed to the set of three guide arms  20   a , respectively. The cam ring  15  is positioned around the second/third lens group support unit  20 , and has the set of three bottomed cam grooves  15   d , in which the set of three cam followers  20   b  are engaged, respectively (see FIG.  6 ). The first linear guide ring  18  is provided between the cam ring  15  and the second/third lens group support unit  20 , and is freely rotatable about the optical axis O relative to the cam ring  15  and movable together with the cam ring  15  along the optical axis O. 
   Features of the present embodiment of the zoom lens barrel  10  reside in mechanical relationship among the second/third lens group support unit  20 , the cam ring  15  and the first linear guide ring  18 . Such features will be hereinafter discussed with reference mainly to  FIGS. 6 and 19 . As described above, each of the three bottomed cam grooves  15   d  includes the photographing section  15   d   1  (which includes the wide-angle mode section, the mode switching section and the telephoto mode section which are shown in  FIG. 19 ) for moving the second/third lens group support unit  20  to a ready-to-photograph position among a plurality of ready-to-photograph positions, the accommodation section  15   d   2  for positioning the second/third lens group support unit  20  to an accommodation position thereof (in which no pictures are taken), and the transfer section  15   d   3  for moving the second/third lens group support unit  20  between the photographing section  15   d   1  and the accommodation section  15   d   2 . The entire portion of the photographing section  15   d   1  and the entire portion of the transfer section  15   d   3  except for an end portion (rear end portion) of the transfer section  15   d   3  in the vicinity of the accommodation section  15   d   2  are formed as narrow-width cam portions in which the associated cam follower  20   b  is engaged with a minimum clearance, whereas the accommodation section  15   d   2  and the aforementioned rear end portion of the transfer section  15   d   3  are formed as open cam portions which are open at a rear end surface of the cam ring  15 . Accordingly, these open cam portions are formed to serve as an accommodation portion for the associated cam follower  20   b.    
   The second/third lens group support unit  20  is biased forward by the compression coil spring  31 . Therefore, each cam follower  20   b  is biased forward by the compression coil spring  31  to come in contact with the front cam surface (front cam edge) in the associated cam groove  15   d  at all times. However, even though the accommodation section  15   d   2  is formed as an open cam portion which is open at a rear end surface of the cam ring  15  while each cam follower  20   b  is disengaged from the front cam surface in the associated cam groove  15   d  in the retracted state shown in  FIG. 3 , there is no problem in controlling the position of the second/third lens group support unit  20  because the position of the second/third lens group support unit  20  does not have to be controlled with precision when the set of three cam followers  20   b  are respectively engaged in the accommodation sections  15   d   2  of the set of three cam grooves  15   d . On the other hand, as can be clearly seen in  FIG. 3 , a front end surface of the second/third lens group support unit  20  (specifically, a front end surface of a second lens frame  23  of the second/third lens group support unit  20 ) is positioned very closely to, or comes in contact with, a rear end surface of the first lens frame  29 , to which the first lens group L 1  is fixed to be supported by the first lens frame  29  when the second/third lens group support unit  20  is in the retracted position. Accordingly, if the rear of the accommodation section  15   d   2  of each cam groove  15   d  is made open, each cam follower  20   b  is disengaged from the front cam surface (front cam edge) in the associated cam groove  15   d  to become capable of moving rearward by the second/third lens group support unit  20  being pressed rearward by the first lens frame  29  against the spring force of the compression coil spring  31 . This makes it possible to reduce the axial length of the cam ring  15 , which in turn makes it possible to reduce the length of the zoom lens barrel  10  in the retracted state. The compression coil spring  31  presses the second/third lens group support unit  20  forward, and presses the fourth lens group support ring  19  rearward at the same time. This makes the fourth lens frame  30 , to which the fourth lens group L 4  is fixed, move rearward to a position where the fourth lens frame  30  comes into contact with the light shield plate  35  as shown in  FIG. 3  when the zoom lens barrel  10  is in the retracted state. 
   The accommodation section  15   d   2  of each cam groove  15   d  of the cam ring  15  overlaps the circumferential groove  15   f  as shown in FIG.  6 . The set of three cam followers  20   b  may interfere with the outer flange  18   f  of the first linear guide ring  18 , which is engaged in the circumferential groove  15   f , if moving rearward with each cam follower  20   b  being apart from the front cam surface in the associated cam groove  15   d . To prevent this problem from occurring, the set of three cut-out portions  18   f ′, that are positioned behind the accommodation sections  15   d   2  to allow the set of three cam followers  20   b  to enter the set of three cut-out portions  18   f ′ (see  FIGS. 3 ,  9  and  18 A), are formed on the outer flange  18   f  so that each cam follower  20   b  can move rearward beyond the front end surface of the outer flange  18   f  when the second/third lens group support unit  20  retracts to its retracted position. Providing the outer flange  18   f  with the set of three cut-out portions  18   f ′ is also advantageous to reduce the axial length of the cam ring  15 . 
   As can be understood from the foregoing, according to the present invention, in a cam mechanism for a lens barrel which includes an annular member that is linearly guided along an optical axis, supporting at least one optical member, and having at least one cam follower; and a cam ring having at least one cam groove on an inner peripheral surface of the cam ring, wherein the cam follower is engaged in the cam groove, and wherein the cam groove includes a photographing section for moving the annular member to a ready-to-photograph position thereof and an accommodation section for positioning the annular member to an accommodation position thereof at which no photographing operation is performed, a further reduction in length of the cam ring is possible and also a further reduction in length of the lens barrel in its retracted state is possible. 
   Obvious changes may be made in the specific embodiment 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.