Abstract:
A lens frame guiding mechanism of a zoom lens includes lens frames having projections which extend radially outwards, wherein a cam follower is formed on each projection; a linear guide barrel; linear guide slots formed on the linear guide barrel parallel to the optical axis, wherein the projections of each lens frame are respectively engaged in the linear guide slots to be slidable therealong; a cam barrel fitted outside the linear guide barrel to be relatively rotatable about the optical axis thereto; and bottomed cam grooves formed on an inner peripheral surface of the cam barrel, in which the cam followers of a corresponding lens frame are respectively engaged. The lens frames are moved in the direction of the optical axis, without rotating about the optical axis, to change a focal length of said zoom lens via rotation of said cam barrel.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a zoom lens (a zoom lens barrel), and more specifically to a mechanism for guiding more than one lens frame in the direction of the optical axis of the zoom lens. 
     2. Description of the Related Art 
     It is known in the art for a zoom lens, or a zoom lens barrel, having a mechanism which guides a plurality of lens frames (lens groups) in the optical axis direction of the zoom lens without rotating about the optical axis, so that the plurality of lens frames are driven in the optical axis direction in a predetermined moving manner by rotation of a cam barrel having cam grooves. In such a conventional lens guiding mechanism, the plurality of lens frames are individually guided along the optical axis without rotating about the optical axis, while the plurality of lens frames are driven in the optical axis direction in a predetermined moving manner in accordance with the contours of the cam grooves of the cam barrel. Such a conventional lens frame guiding mechanism has no substantial adverse effects if it is used for the zoom lens of a conventional camera using light-sensitive film such as 35 mm or APS compact zoom camera. 
     However, such a conventional lens frame guiding mechanism does have substantial adverse effects if it is used for the zoom lens of a digital camera because object images are formed on the sensitive surface of a small CCD (CCD image sensor) which is much smaller than the picture plane of conventional cameras using light sensitive film. Namely, upon assembly, every lens element of a digital camera must be optically centered, correctly spaced, and held firmly with a relatively high precision, e.g., tens times greater than that required in conventional cameras using light-sensitive film. For instance, if the angle of view is constant, the focal length of a photographing lens becomes shorter as the size of the picture plane reduces, which in turn reduces the sizes of all the elements of the photographing lens such as, e.g., lens elements, lens frames. Therefore, the influence that a tolerance (e.g., 10 μm) has on a photographing lens system of a digital camera is much larger than the influence that the same tolerance would have on a photographing lens system of a conventional camera using light-sensitive film. Accordingly, manufacturing or assembling error which falls within tolerance of optical performance in the photographing optical system of a conventional camera using light-sensitive film can be outside the tolerance of optical performance in the photographing optical system of a digital camera. Specifically in a photographing lens system of a digital camera, the influence that eccentricity or tilt of one or more lens frames (one or more lens groups) relative to another lens frame or frames (lens group or groups) has upon the optical performance of the photographing optical system is larger than the influence of deviation of one or more lens frames (one or more lens groups) in the optical axis direction relative to another lens frame or frames (another lens group or groups). 
     From this point of view, in the conventional lens frame guiding mechanism, a manufacturing or assembling error of each element of the zoom lens tends to exert adverse effects upon the optical performance of the photographing optical system since a plurality of lens frames are independently guided in the optical axis direction without rotating about the optical axis. For instance, if there is positional error such as eccentricity in each of the lens frames, the linear guide member, and the cam barrel, there is a possibility of these errors being accumulated to deteriorate the optical performance of the photographing optical system, especially in the case of eccentricity of one or more lens groups relative to another lens group or groups. 
     SUMMARY OF THE INVENTION 
     The present invention has been device in view of the above-mentioned problems; accordingly, an object of the present invention is to provide a lens frame guiding mechanism, used in a zoom lens, which guides a plurality of lens frames in the optical axis direction of the zoom lens without rotating about the optical axis so that the plurality of lens frames are driven in the optical axis direction by rotation of a cam barrel having cam grooves, wherein eccentricity of one or more lens groups relative to another lens group or groups does not occur easily. 
     To achieve the object mentioned above, according to an aspect of the present invention, a lens frame guiding mechanism of a zoom lens is provided, including a plurality of lens frames each having a plurality of projections which extend radially outwards at predetermined intervals in a circumferential direction, wherein a cam follower is formed on each of the projections; a linear guide barrel provided around the plurality of lens frames, wherein the linear guide barrel is guided along the optical axis of the zoom lens without rotating; a plurality of linear guide slots formed on the linear guide barrel to extend parallel to the optical axis, wherein the number of the plurality of linear guide slots corresponds to the number of the plurality of projections of each of the lens frames so that the plurality of projections of each of the lens frames are respectively engaged in the plurality of linear guide slots to be slidable in a direction of the optical axis; a cam barrel fitted outside the linear guide barrel to be rotatable about the optical axis relative to the linear guide barrel; and a plurality of bottomed cam grooves formed on an inner peripheral surface of the cam barrel for each of the plurality of lens frames, in which the plurality of cam followers of a corresponding lens frame of the plurality of lens frames are respectively engaged, wherein the profiles of the cam grooves for one of the plurality of lens frames is different from the profiles of the cam grooves for the other of the plurality of lens frames. The plurality of lens frames are moved in the direction of the optical axis, without rotating about the optical axis, to change a focal length of the zoom lens via rotation of the cam barrel. 
     Preferably, the number of projections of each of the lens frames, the number of the linear guide slots of the linear guide barrel, and the number of the bottomed cam grooves of the cam barrel are three, respectively arranged at 120° equi-angular intervals. 
     Preferably, the cam barrel is fitted on the linear guide barrel so as to be immovable in the direction of the optical axis relative to the linear guide barrel. 
     Preferably, each of the plurality of projections is formed to have opposite faces extending parallel to each other so that the opposite faces are respectively in sliding contact with side faces of a corresponding linear guide slot of the plurality of linear guide slots. 
     In an embodiment, the zoom lens is incorporated in a digital camera. 
     According to another aspect of the present invention, a lens frame guiding mechanism of a zoom lens is provided, including a first lens frame having a first group of three projections extending radially outwards at equi-angular intervals, each of the first group of three projections having a first cam follower; a second lens frame having a second group of three projections extending radially outwards at equi-angular intervals, each of the second group three projections having a second cam follower; a linear guide barrel provided around the first lens frame and the second lens frame, and guided in a direction of the optical axis without rotating about the optical axis; three linear guide slots formed on the linear guide barrel so as to extend parallel to the optical axis, the first group of three projections and the second group of three projections being slidably engaged in the three linear guide slots, respectively; a cam barrel fitted on the linear guide barrel to be rotatable about the optical axis relative to the linear guide barrel; a first group of three cam grooves which are formed on an inner peripheral surface of the cam barrel so that the three first cam followers are respectively engaged in the first three cam grooves; and a second group of three cam grooves which are formed on the inner peripheral surface of the cam barrel so that the three second cam followers are respectively engaged in the second group of three cam grooves. The first and second lens frames are moved in the direction of the optical axis, without rotating about the optical axis, in a predetermined moving manner to change a focal length of the zoom lens by rotation of the cam barrel. 
     The present disclosure relates to subject matter contained in Japanese Patent Application No.2000-24039 (filed on Feb. 1, 2000) which is expressly incorporated herein by reference in its entirety. 
    
    
     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, showing the overall structure thereof; 
     FIG. 2 is an axial cross sectional view of the zoom lens shown in FIG. 1, showing the zoom lens above the optical axis thereof; 
     FIG. 3 is a developed view of the inner peripheral surface of a first cam barrel, showing the contours of first and second cam grooves formed on the inner peripheral surface of the first cam barrel; 
     FIG. 4 is an exploded perspective view of the first cam barrel shown in FIG. 3, a linear guide barrel, a first lens frame and a second lens frame; 
     FIG. 5 is a fragmentary rear view of the linear guide barrel and the first lens frame, showing the periphery of an insertion groove of the linear guide barrel; 
     FIG. 6 is an exploded perspective view of the linear guide barrel, a linear guide ring and a retainer ring; 
     FIG. 7 is a developed view of the linear guide barrel, the linear guide ring and the retainer ring; 
     FIG. 8 is a developed view of a second cam barrel and a barrier drive ring, showing the positional relationship therebetween when the zoom lens is set at the telephoto extremity thereof (when the zoom lens is in a ready-to-photograph state); 
     FIG. 9 is a developed view of the second cam barrel and the barrier drive ring, showing the positional relationship therebetween when the zoom lens is positioned in the accommodation position (when the power of the zoom lens is turned OFF); 
     FIG. 10 is an axial cross sectional view of the zoom lens shown in FIG. 1, showing the zoom lens above the optical axis thereof, showing the positional relationship between an external barrel and the second cam barrel (a first lens group) when the zoom lens is set at the wide-angle extremity thereof; 
     FIG. 11 is an axial cross sectional view of the zoom lens shown in FIG. 1, showing the zoom lens above the optical axis thereof, and showing the positional relationship between the external barrel and the second cam barrel (the first lens group) when the zoom lens is set at the telephoto extremity thereof; 
     FIG. 12 is an explanatory view showing variations in axial position of the sensitive surface (image plane) of a CCD, the first lens group, a second lens group, and a barrier block when the zoom lens is driven from the accommodation position to the telephoto extremity and thereafter to the wide-angle extremity; 
     FIG. 13 is an exploded perspective view of the barrier block, viewed from behind the barrier block; 
     FIG. 14 is a perspective view of the barrier block with an annular pressure plate being removed from the barrier block, viewed from behind the barrier block; 
     FIG. 15A is a schematic front view of the barrier block, showing two pairs of barrier blades in a fully open position; 
     FIG. 15B is a schematic front view of the barrier block, showing the two pairs of barrier blades in a half-closed position; 
     FIG. 15C is a schematic front view of the barrier block, showing the two pairs of barrier blades in a fully closed position; 
     FIG. 16 is a perspective view of the second cam barrel and the barrier drive ring, showing the positional relationship between a driven lever which extends from the barrier drive ring and a rotation transfer recess formed on the second cam barrel; 
     FIG. 17 is a front view of the external barrel that is supported by the external barrel to be freely rotatable about the optical axis, in a state where the barrier drive ring is rotated to one rotational limit thereof to thereby fully close the two pairs of barrier blades; and 
     FIG. 18 is a front view of the external barrel shown in FIG. 17, in a state where the barrier drive ring is rotated to the other rotational limit thereof to thereby fully open the two pairs of barrier blades. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment of a zoom lens (zoom lens barrel) according to the present invention that is incorporated in a digital camera will be hereinafter discussed. Firstly, the overall structure of the zoom lens will be discussed with reference mainly to FIGS. 1 and 2. In the drawings and the following descriptions, symbols “(F)”, “(L)” and “(RL)” which are each appended as a suffix to the reference numeral of some elements of the zoom lens barrel indicate that the element is stationary, the element is movable linearly along an optical axis O of the zoom lens without rotating about the optical axis O, and the element is movable along the optical axis O while rotating about the optical axis O, respectively. 
     The photographic optical system of the zoom lens includes three lens groups; namely, a first lens group (front lens group) L 1  (L), a second lens group (middle lens group) L 2  (L) and a third lens group (rear lens group) L 3  (L), in this order from the object side (the left side as viewed in FIG.  2 ). The zoom lens performs zooming by moving the first and second lens groups L 1  and L 2  along the optical axis O relative to the sensitive surface of a stationary CCD  12   a  (see FIG. 1) and at the same time changing the space between the first and second lens groups L 1  and L 2  in a predetermined manner. The zoom lens performs a focusing operation by moving the third lens group L 3  along the optical axis O to bring an object into focus. The third lens group L 3  functions as a focusing lens group which is driven along the optical axis O independently of the axial position of each of the first and second lens groups L 1  and L 2 . Thus, the zoom lens is an internal-focusing type zoom lens having a lens construction which allows the focus to be altered by moving the rearmost lens group provided as a focusing lens group internally within the lens barrel. 
     The zoom lens is provided with a housing  10 (F) which is fixed to a camera body of a digital camera (not shown). The housing  10  can be integral with the camera body to be provided as an element thereof. The zoom lens is provided in the housing  10  with a stationary barrel  11 (F) that is fixed to the housing  10 . The stationary barrel  11  is provided on an outer peripheral surface thereof with a fine male thread  11   a.  The stationary barrel  11  is provided on an inner peripheral surface thereof with a female helicoid (female helicoidal thread)  11   b  and three linear guide grooves  11   c  (only one is shown in FIG. 1) extending parallel to the optical axis O , i.e., extending in the optical axis direction. The three linear guide grooves  11   c  are formed to cut across the female helicoid  11   b.  The three linear guide grooves lic are formed at 120° intervals (i.e., at an equi-angular distance) about the axis of the stationary barrel  11 . 
     As shown in FIG. 2, the housing  10  is provided with a CCD insertion opening  10   a,  a filter fixing portion lOb and a focusing lens group guide portion  10   c.  The CCD  12   a  which is fixed to a substrate  12  is positioned in the CCD insertion opening  10   a.  A filter  10   d  such as a low-pass filter is fixed to the filter fixing portion  10   b.  The third lens group L 3  is guided by the focusing lens group guide portion  10   c  to be movable in the optical axis direction. The axial position of the third lens group L 3  on the optical axis O is determined by the direction of rotation of a feed screw  10   e  and the angle of rotation (amount of rotation) thereof. The feed screw  10   e  extends parallel to the optical axis O from the camera body in the focusing lens group guide portion  10   c.  The feed screw Oe is driven by a pulse motor (not shown) provided in the camera body. The angle of rotation of the feed screw  10   e  is controlled via an encoder (not shown) of the pulse motor. 
     The zoom lens is provided on the stationary barrel  11  with a rotational barrel  13  (RL). The rotational barrel  13  is provided on an inner peripheral surface thereof with a fine female thread  13   a  which meshes with the fine male thread  11   a  of the stationary barrel  11 . The rotational barrel  13  is provided on an outer peripheral surface thereof with a circumferential gear  13   b  (see FIG.  1 ). The rotational barrel  13  is driven to rotate about the optical axis O by a drive pinion (not shown) which meshes with the circumferential gear  13   b.  When the rotational barrel  13  is driven to rotate about the optical axis O, the rotational barrel  13  moves in the optical axis  10  direction while rotating about the optical axis O in accordance with the engagement of the fine female thread  13   a  with the fine male thread  11   a.  The rotational barrel  13  is provided at the front end of an inner peripheral surface thereof with three inward projections  13   c  at 120° intervals about the axis of the rotational barrel  13 . As shown in FIG. 1, a flexible coding plate  14  (RL) is fixed on an outer peripheral surface of the rotational barrel  13  along a circumference thereof, while a brush  15  (F) that is in contact with the coding plate  14  is fixed to the housing  10 . The brush  15  remains in sliding contact with the coding plate  14  regardless of a movement of the coding plate  14  relative to the brush  15  when the coding plate  14  moves in the optical axis direction in accordance with the engagement of the fine female thread  13   a  with the fine male thread  11   a,  so as to sense the rotational position of the rotational barrel  13  as digital and/or analogue information. The fine female thread  13   a,  which is provided on the rotational barrel  13 , is provided as a device for supporting the rotational barrel  13  on the stationary barrel  11  so that the rotational barrel  13  can rotate freely about the optical axis O on the stationary barrel  11 . However, alternatively, the rotational barrel  13  can be supported on the stationary barrel  11  so as to be able to rotate freely about the optical axis O without moving in the optical axis direction relative to the stationary barrel  11 . 
     The zoom lens is further provided with a linear guide barrel  16  (L), a first cam barrel  17  (RL) and a second cam barrel  18  (RL). The first cam barrel  17  is fitted on the linear guide barrel  16  to be rotatable about the optical axis O relative to the linear guide barrel  16  and to be immovable in the optical axis direction relative to the linear guide barrel  16 . The second cam barrel  18  is fitted on the front end of the first cam barrel  17  to be rotatable together with the first cam barrel  17  about the optical axis O and also to be movable in the optical axis direction relative to the first cam barrel  17 . The linear guide barrel  16 , the first cam barrel  17  and the second cam barrel  18  are assembled in advance as a unit, and the rear of this barrel unit is positioned in the stationary barrel  11 . 
     The linear guide barrel  16  is provided at the rear end thereof with an outer flange  16   a.  A linear guide ring (flange ring)  19 (L) is fixed to the front end of the linear guide barrel  16  via a retainer ring  20 (L). The first cam barrel  17  is held between the outer flange  16   a  and the linear guide ring  19 , and is rotatable about the optical axis O relative to the linear guide barrel  16  and also movable together with the linear guide barrel  16  in the optical axis direction. 
     The second cam ring  18 , which is fitted on the front end of the first cam barrel  17 , is provided at the rear end thereof with three linear guide portions  18   a  (only two are shown in FIG. 1) at 120° intervals about the axis of the second cam ring  18 . Each of the three linear guide portions  18   a  is provided with a spring holding groove  18   a   1 , and a pair of guide grooves  18   a   2  positioned on the opposite sides of the spring holding groove  18   a   1  in a circumferential direction of the second cam ring  18  (see FIGS.  8  and  9 ). Each of the three linear guide portions  18   a  is further provided, in each spring holding groove  18   a   1  at the front end (the left end as viewed in FIG. 8 or  9 ) of each spring holding groove  18   a   1 , with an engaging projection  18   a   3 . All of the spring holding grooves  18   a   1  and the pairs of guide grooves  18   a   2  extend parallel to the optical axis O. The first cam barrel  17  is provided on an outer peripheral surface thereof with three stopper portions  17   a  (only two are shown in FIG. 1) at 120° intervals about the axis of the first cam barrel  17 . Each of the three stopper portions  17   a  is provided with a stopper projection  17   a   1 , and a pair of guide projections  17   a   2  positioned on the opposite sides of the stopper projection  17   a   1  in a circumferential direction of the first cam barrel  17  (see FIG.  4 ). Each pair of guide projections  17   a   2  of the first cam barrel  17  are respectively fitted in the corresponding pair of guide grooves  18   a   2  of the second cam ring  18  to be slidable in the optical axis direction relative to the second cam ring  18 , with a compression spring  21  being held between each engaging projection  18   a   3  and the corresponding stopper projection  17   a   1 . Due to this structure, the second cam barrel  18  can slide on the first cam barrel  17  in the optical axis direction without rotating about the optical axis O relative to the first cam barrel  17 . The compression springs  21  constatntly bias the second cam barrel  18  toward the front of the zoom lens, so that the front end of the second cam barrel  18  is usually in press-contact with the linear guide ring  19 . The second cam barrel  18  can move rearward, toward the rear of the zoom lens, against the spring force of the compression springs  21  by an amount of movement corresponding to a predetermined clearance in the optical axis direction between the guide grooves  18   a   2  and the guide projections  17   a   2 . The second cam barrel  18  can also be slightly inclined with respect to the first cam barrel  17  (i.e., with respect to the optical axis O) by an amount of inclination corresponding to a predetermined clearance in a radial direction between the inner peripheral surface of the second cam barrel  18  and the corresponding outer peripheral surface of the first cam barrel  17 . 
     The first cam barrel  17  is provided on an outer peripheral surface thereof with a male helicoid (male helicoidal thread)  17   b  that is engaged with the female helicoid  11   b  of the stationary barrel  11 , and three rotation transmission grooves  17   c  that extend parallel to the optical axis O. The three rotation transmission grooves  17   c  are formed so as to cut across the male helicoid  17   b.  The three rotation transmission grooves  17   c  are formed at 120° intervals about the axis of the first cam barrel  17 . The three inward projections  13   c  of the rotational barrel  13  are respectively engaged with the three rotation transmission grooves  17   c  to be relatively slidable to each other. The linear guide barrel  16  is provided on the outer flange  16   a  thereof with three linear guide projections  16   b  at 120° intervals about the axis of the linear guide barrel  16 . Each linear guide projection  16   b  extends radially outwards to be engaged with the corresponding linear guide groove  11   c  of the stationary barrel  11 . The linear guide barrel  16  is further provided with three linear guide slots  16   c  at 120° intervals about the axis of the linear guide barrel  16  so that the circumferential positions of the three linear guide slots  16   c  coincide with those of the three linear guide projections  16   b.  Each of the three linear guide slots  16   c  penetrates the linear guide barrel  16  radially and extends parallel to the optical axis O. 
     As can be seen in FIGS. 4,  5  and  6 , each of the three linear guide slots  16   c  opens at the rear end of the linear guide barrel  16 , and the rear end of each linear guide slot  16   c  is covered by the corresponding part of the outer flange  16   a  and the corresponding linear guide projection  16   b  at the radially outer side of the linear guide barrel  16 . The outer flange  16   a  is provided with three insertion grooves  16   h  which respectively extend along a portion of each three linear guide slots  16   c  from the front end of the outer flange  16   a  to each respective rear end of the three linear guide slots  16   c  (i.e., the rear end of the outer flange  16   a ), so that a follower pin (cam follower)  22   d  and a follower pin (cam follower)  23   d  can be inserted into each linear guide slot  16   c  from the corresponding insertion groove  16   h.    
     When the barrel unit which includes the linear guide barrel  16 , the first cam barrel  17  and the second cam barrel  18  is coupled to the stationary barrel  11  and the rotational barrel  13 , each of the three linear guide projections  16   b  of the linear guide barrel  16  is inserted into the corresponding linear guide groove  11   c  of the stationary barrel  11  via a corresponding introducing groove  11   d  formed on an inner peripheral surface of the stationary barrel  11 , and each of the three inward projections  13   c  of the rotational barrel  13  is inserted into the corresponding rotation transmission groove  17   c  of the first cam barrel  17  via a corresponding introducing groove  17   d  formed on an outer peripheral surface of the first cam barrel  17 . After each linear guide projection  16   b  and each inward projection  13   c  are inserted into the corresponding linear guide groove  11   c  and the corresponding rotation transmission groove  17   c, respectively, the female helicoid  11   b  of the stationary barrel  11  and the male helicoid  17   b  of the first cam barrel  17  mesh with each other. 
     FIG. 2 shows a state where the barrel unit, which includes the linear guide barrel  16 , the first cam barrel  17  and the second cam barrel  18 , has been coupled to the stationary barrel  11  and the rotational barrel  13 . In this state, rotating the rotational barrel  13  about the optical axis O via the gear  13   b  causes the rotational barrel  13  to move in the optical axis direction while rotating about the optical axis O due to the engagement of the fine female thread  13   a  with the fine male thread  11   a.  At the same time, the rotation of the rotational barrel  13  is transmitted to the first cam barrel  17  and the second cam barrel  18 , which is fitted on the first cam barrel  17 , due to the engagement of the inward projections  13   c  with the rotation transmission grooves  17   c,  so that the first cam barrel  17  and the second cam barrel  18  rotate about the optical axis O. At this time, the first cam barrel  17  and the second cam barrel  18  also move in the optical axis direction O due to the engagement of the male helicoid  17   b  with the female helicoid  11   b.  Furthermore, the linear guide barrel  16  moves in the optical axis direction without rotating about the optical axis O due to the engagement of the linear guide projections  16   b  with the linear guide grooves  11   c,  and at the same time the first and second cam barrels  17  and  18 , which rotate about the optical axis O relative to the linear guide barrel  16 , move together with the linear guide barrel  16  in the optical axis direction. 
     The first cam barrel  17  is provided on an inner peripheral surface thereof with three first cam grooves  17 C 1  for driving the first lens group L 1 , and three second cam grooves  17 C 2  for driving the second lens group L 2 . FIG. 3 is a developed view of the inner peripheral surface of the first cam barrel  17 , showing the contours of the first and second cam grooves  17 C 1  and  17 C 2 . The three first cam grooves  17 C 1  are formed on the inner peripheral surface of the first cam barrel  17  at 120° intervals about the axis of the first cam barrel  17 . Likewise, the three second cam grooves  17 C 2  are formed on the inner peripheral surface of the first cam barrel  17  at 120° intervals about the axis of the first cam barrel  17 . Each of the first and second cam grooves  17 C 1  and  17 C 2  has three predetermined positions: an accommodation position, a telephoto position and a wide-angle, in this order along the direction of rotation of the first cam barrel  17  (the vertical direction as viewed in FIG.  3 ). The telephoto position shown in FIG. 3 of each cam groove  17 C 1  and  17 C 2  determines the telephoto extremity of the corresponding lens groups L 1  and L 2 , respectively; the wide-angle position of each cam groove  17 C 1  and  17 C 2  determines the wide-angle extremity of the corresponding lens groups Li and L 2 , respectively; and the accommodation position of each cam groove  17 C 1  and  17 C 2  determines the position of the corresponding lens groups L 1  and L 2 , respectively, when the power of the digital camera is turned OFF. The angle of rotation from the accommodation position to the wide-angle extremity position is shown by “A” in FIG.  3 . 
     The zoom lens is provided with a first lens frame  22 (L) and a second lens frame  23 (L) which support the first lens group L 1  and the second lens group L 2 , respectively. The first lens frame  22  is guided by the first cam grooves  17 C 1  and the linear guide slots  16   c  to be movable in the optical axis direction without rotating about the optical axis O. Likewise, the second lens frame  23  is guided by the second cam grooves  17 C 2  and the linear guide slots  16   c  to be movable in the optical axis direction without rotating about the optical axis O. The first lens frame  22  is provided with three resilient extending pieces  22   b  which extend rearward from a cylindrical portion  22   a  of the first lens frame  22 . The three resilient extending pieces  22   b  are formed on the first lens frame  22  at 120° intervals about the axis of the first lens frame  22 . Each resilient extending piece  22   b  is provided on a radially outer surface thereof with a square projection  22   c  which extends radially outwards to be fitted in the corresponding linear guide slot  16   c  in a slidable manner in the optical axis direction. Each resilient extending piece  22   b  is further provided on top of each square projection  22   c  with the follower pin (cam follower)  22   d,  which is fixed to the resilient extending piece  22   b  to extend radially outwards. Each square projection  22   c  is formed so that the opposite faces thereof, which are respectively in sliding contact with the side faces of the corresponding linear guide slot  16   c,  extend parallel to each other. The zoom lens is provided with a first lens holder  22   e  which encloses the first lens group L 1  to hold the same. The first lens holder  22   e  is fixed to the cylindrical portion  22   a  of the first lens frame  22  via male and female threads  22   f  which are formed on an outer peripheral surface of the first lens holder  22   e  and an inner peripheral surface of the cylindrical portion  22   a,  respectively. The position of the first lens group L 1  relative to the first lens frame  22  in the optical axis direction can be adjusted by varying the amount of engagement between the male and female threads  22   f.  A wave washer  22   h  is held between the holder  22   e  and an inner flange  22   g  of the first lens frame  22  to remove the play between the first lens holder  22   e  (or the first lens group L 1 ) and the first lens frame  22  (see FIG.  2 ). 
     The second lens frame  23  is provided with three resilient extending pieces  23   b  which extend forward from an annular plate portion  23   a  of the second lens frame  23 . The three resilient extending pieces  23   b  are formed on the second lens frame  23  at 120° intervals about the axis of the second lens frame  23 . Each resilient extending piece  23   b  is provided on a radially outer surface thereof with a square projection  23   c  which extends radially outwards to be fitted in the corresponding linear guide slot  16   c  in a slidable manner in the optical axis direction. Each resilient extending piece  23   b  is further provided on top of each square projection  23   c  with the aforementioned follower pin  23   d,  which is fixed to the resilient extending piece  23   b  to extend radially outwards. The square projections  23   c  and the follower pins  23   d  of the second lens frame  23  are identical to the square projections  22   c  and the follower pins  22   d  of the first lens frame  22  except that the resilient extending pieces  23   b  of the second lens frame  23  extend in the direction opposite to the resilient extending pieces  22   b  of the first lens frame  22  in the optical axis direction. The zoom lens is provided with a second lens holder  23   e  which encloses the second lens group L 2  to hold the same. The second lens holder  23   e  is fixed to the annular plate portion  23   a  of the second lens frame  23  via set screws  23   f.  A shutter block  24  is provided around the second lens group L 2 . The shutter block  24  is fixed to the annular plate portion  23   a  of the second lens frame  23  via the set screws  23   f  that are screwed into the rear of the shutter block  24 . The shutter block  24  functions to interrupt light bundles which are incident on the CCD  12   a  at a shutter release operation. 
     Each of the first and second lens frames  22  and  23  is guided linearly in the optical axis direction without rotating about the optical axis O by the engagement of each of the three square projections  22   c  and corresponding each of the three square projections  23   c  with each common corresponding linear guide slot of the three linear guide slots  16   c.  Each follower pin  22   d  penetrates the corresponding linear guide slot  16   c  of the linear guide barrel  16  to be engaged with the corresponding first cam groove  17 C 1  of the first cam barrel  17 , which is fitted on the linear guide barrel  16  to be rotatable about the optical axis relative to linear guide barrel  16 . Likewise, each follower pin  23   d  penetrates the corresponding linear guide slot  16   c  of the linear guide barrel  16  to be engaged with the corresponding second cam groove  17 C 2  of the first cam barrel  17 . When the first and second lens frames  22  and  23  are placed in the linear guide barrel  16  and the first cam barrel  17 , firstly each of the three square projections  22   c  and corresponding one of the three square projections  23   c  are inserted into a corresponding linear guide slot of the three linear guide slots  16   c  from the rear end face of the linear guide barrel  16 . At the same time, each of the three follower pins  22   d  and corresponding one of the three follower pins  23   d  are inserted into corresponding one of the three insertion grooves  16   h  to be fitted in the corresponding first and second cam grooves  17 C 1  and  17 C 2 , respectively. It should be noted that the hatched areas of the first and second cam grooves  17 C 1  and  17 C 2  in FIG. 3 are used solely for the purpose of inserting each follower pin  22   d  or  23   d  into the corresponding cam groove  17 C 1  or  17 C 2  during assembly, and thus are not used when the zoom lens is in operation. 
     According to the above described guide structure, rotating the rotational barrel  13  about the optical axis O causes the barrel unit which includes the linear guide barrel  16 , the first cam barrel  17  and the second cam barrel  18  to move in the optical axis direction. During this movement of the barrel unit, the first and second cam barrels  17  and  18  rotate together about the optical axis O, but the linear guide barrel  16  does not rotate about the optical axis O. As a result, the first lens frame  22  (the first lens group L 1 ) and the second lens frame  23  (the second lens group L 2 ) linearly move in the optical axis direction while changing the space therebetween in accordance with the contours of the first and second cam grooves  17 C 1  and  17 C 2  to thereby carry out a zooming operation. 
     The coupling structure of the linear guide ring  19  and the retainer ring  20  to the front end of the linear guide barrel  16  will be hereinafter discussed with reference to FIGS. 6 and 7. The linear guide barrel  16  is provided, at the front end thereof at 120° intervals about the axis of the linear guide barrel  16 , with three engaging lugs  16   d  each of which extends radially outwards. A receiving area  16   e  is formed between any two adjacent engaging lugs  16   d  of the linear guide barrel  16  in order to receive one of three radially inward projections  19   a  of the linear guide ring  19 . The linear guide barrel  16  is provided immediately behind the three engaging lugs  16   d  with three grooves  16   f,  respectively. The radius of the linear guide barrel  16  from the axis of the linear guide barrel  16  to the bottom surface of each groove  16   f  is identical to the radius from the axis of the linear guide barrel  16  to the surface of each receiving area  16   e.  The linear guide barrel  16  is provided behind the three engaging lugs  16   d  with three recesses  16   g,  respectively, each of which is connected with the corresponding groove  16   f.  Each recess  16   g  is recessed rearward (toward the right as viewed in FIG. 7) in the direction parallel to the optical axis O, i.e., in the optical axis direction. 
     On the other hand, the linear guide ring  19  is provided with the aforementioned three inward projections  19   a  at 120° intervals about the axis of the linear guide ring  19 . The three inward projections  19   a  can be inserted into the three receiving areas  16   e,  respectively. If the linear guide ring  19  is rotated about the axis thereof clockwise as viewed in FIG. 6 relative to the linear guide barrel  16  with the three inward projections  19   a  being properly inserted into the three receiving areas  16   e,  respectively, each inward projection  19   a  slides into the corresponding groove  16   f.  The linear guide ring  19  is provided with three radially outward projections  19   b  at 120° intervals about the axis of the linear guide ring  19 . The circumferential positions of the three outward projections  19   b  are precisely determined with reference to the circumferential positions of the three inward projections  19   a.    
     The retainer ring  20  is provided with radially inward blades  20   a  at 120° intervals about the axis of the retainer ring  20 . The three inward blades  20   a  can be inserted into the three receiving areas  16   e  of the linear guide barrel  16 , respectively. If the retainer ring  20  is rotated about the axis thereof clockwise as viewed in FIG. 6 relative to the linear guide barrel  16  with the three inward blades  20   a  being properly inserted into the three receiving areas  16   e,  respectively, each inward blade  20   a  slides into the corresponding groove  16   f.  The retainer ring  20  is provided on the front end face thereof with a plurality of grooves  20   b  which are recessed rearward, toward the linear guide barrel  16 , so that a pin face wrench (not shown) can be engaged with the recessed portions  20   b  to rotate the retainer ring  20  relative to the linear guide barrel  16 . 
     When the linear guide ring  19  is fixed to the front end of the linear guide barrel  16 , firstly the three inward projections  19   a  are respectively inserted into the three receiving areas  16   e,  and then the linear guide ring  19  is rotated about the axis thereof clockwise as viewed in FIG. 6 relative to the linear guide barrel  16  so that each inward projection  19   a  slides into the corresponding groove  16   f.  Subsequently, each inward projection  19   a  is made to be fitted in the corresponding recess  16   g.  This engagement of each inward projection  19   a  with the corresponding recess  16   g  determines the fixed circumferential position of the linear guide ring  19  relative to the linear guide barrel  16 . Subsequently, the inward blades  20   a  of the retainer ring  20  are respectively inserted into the three receiving areas  16   e,  and then the retainer ring  20  is rotated about the axis thereof clockwise as viewed in FIG. 6 relative to the linear guide barrel  16  so that each inward blade  20   a  slides into the corresponding groove  16   f  and presses the corresponding inward projection  19   a  into the corresponding recess  16   g.  This prevents the linear guide ring  19  from moving in the optical axis direction relative to the linear guide barrel  16 . In this state, since each of the three inward blades  20   a  of the retainer ring  20  is held in one of the three grooves  16   f  between the corresponding engaging lug  16   d  and the corresponding inward projection  19   a,  the inward blades  20   a  and the engaging lugs  16   d  function to prevent the linear guide ring  19  from coming off the front end of the linear guide barrel  16 . Between the linear guide barrel  16  and the retainer ring  20  is provided a click-stop device which prevents the retainer ring  20  from rotating counterclockwise as viewed in FIG. 6 so that the retainer ring  20  cannot come off the front end of the linear guide barrel  16  after the retainer ring  20  is properly engaged with the linear guide barrel  16 . Three indentations  20   a   1  which are formed on the retainer ring  20  and corresponding three detent  16   j  which are formed on the linear guide barrel  16  to be respectively engaged with the three indentations  20   a   1  constitute the elements of the click-stop device (see FIGS.  6  and  7 ). 
     Accordingly, the outward projections  19   b  of the linear guide ring  19  that is fixed to the front end of the linear guide barrel  16  in the above described manner are located at predetermined specific positions (angular positions) relative to the linear guide projections  16   b.  The zoom lens is provided at the front thereof with an external barrel (a hood barrel)  25 (L). The external barrel  25  is provided, on an inner peripheral surface thereof at 120° intervals about the axis of the external barrel  25 , with three linear guide grooves  25   a  which extend parallel to the optical axis O. The three outward projections  19   b  of the linear guide ring  19  are respectively engaged with the three linear guide grooves  25   a  to guide the external barrel  25  to move in the optical axis direction without rotating about the optical axis O. The external barrel  25  is provided at the rear end thereof with three radially inward pins  25   b  which are respectively engaged with three guide grooves  18   b  formed on outer peripheral surface of the second cam barrel  18  at 120° intervals about the axis thereof. 
     As shown in FIG. 8, each of the three guide grooves  18   b  of the second cam barrel  18  defines an assembling position (or a disassembling position) X at which the three inward pins  25   b  of the external barrel  25  are respectively inserted into or taken out of the three guide grooves  18   b  of the second cam barrel  18 . Each of the three guide grooves  18   b  further defines an accommodation position, a telephoto position and a wide-angle extremity, which determine the accommodation position, the telephoto extremity and the wide-angle extremity of the first cam barrel  17 , respectively. The three guide grooves  18   b  are formed to move the external barrel  25  in the optical axis direction in accordance with the rotational position of the second cam barrel  18 , which rotates together with the first cam barrel  17 . More specifically, the three guide grooves  18   b  are formed to make the external barrel  25  function as a movable lens hood so that the external barrel  25  advances relative to the second cam barrel  18  (i.e., the first lens group L 1 ) when the zoom lens is set at the telephoto extremity thereof having a narrow angle of view while the external barrel  25  retreats relative to the second cam barrel  18  when the zoom lens is set at the wide-angle extremity thereof having a wide angle of view. The external barrel  25  is positioned in the wide-angle extremity thereof and the telephoto extremity thereof in FIG.  10  and FIG. 11, respectively. 
     If the external barrel  25  is pressed rearward (i.e., toward the camera body) by an external force when the camera is in use, the compression springs  21  function as shock absorbers which can absorb at least part of such an external force since the compression springs  21  are positioned between the first cam barrel  17 , which guides the first and second lens groups L 1  and L 2  in the optical axis direction, and the second cam barrel  18 , which guides the external barrel  25  in the optical axis direction. Such an external force is transmitted partly to the first cam barrel  17  after having been absorbed to some extent by the compression springs  21 , which prevents large external forces from being applied to the first cam barrel  17 . Consequently, the precision of the axial position of each of the first and second lens groups L 1  and L 2  is influenced negligibly by external forces applied to the external barrel  25 . In FIG. 2, the reference numeral  29 (F) designates a stationary external barrel which is integral with the camera body. The external barrel  25  advances and retreats with respect to the stationary external barrel  29 . 
     The external barrel  25  is provided, at the front thereof in the radially inner side of the external barrel  25 , with a barrier drive ring  26 , so that the barrier drive ring  26  can rotate about the optical axis O. The barrier drive ring  26  functions to open and shut two pairs of barrier blades  27   c  and  27   d  (i.e. the front pair of barrier blades  27   c  and the rear pair of barrier blades  27   d ) by rotating about the optical axis O. The two pairs of barrier blades  27   c  and  27   d  together function as a lens protection cover for protecting the front surface of the first lens group L 1  from getting scratched, etc., when the digital camera is not in use. The barrier block  27  is provided with a panel  27   b  having a photographic aperture  27   a,  the aforementioned two pairs of barrier blades  27   c  and  27   d  supported by the panel  27   b  therebehind to open and close the photographic aperture  27   a,  and two torsion springs  27   e  which constantly bias the two pairs of barrier blades  27   c  and  27   d  in a direction to close the photographic aperture  27   a.  The barrier block  27  is further provided with an annular pressure plate  27   f  which holds the two pairs of barrier blades  27   c  and  27   d  and the torsion springs  27   e  between the panel  27   b  and the pressure plate  27   f.  The barrier block  27  having such elements is assembled in advance as a unit. The panel  27   b  is provided on a rear face thereof with two pivots  27   g  (see FIGS. 13 and 14) and two engaging pins  27   n.  The upper front barrier blade  27   c   1  of the front pair of barrier blades  27   c  and the upper rear barrier blade  27   d   1  of the rear pair of barrier blades  27   d  are pivoted at corresponding one of the two pivots  27   g  (the right pivot  27   g  as viewed in FIG.  13 ), while the lower front barrier blade  27   c   2  of the front pair of barrier blades  27   c  and the lower rear barrier blade  27   d   2  of the rear pair of barrier blades  27   d  are pivoted at the other pivot  27   g  (the left pivot  27   g  as viewed in FIG.  13 ). Each of the rear pair of barrier blades  27   d  is constantly biased to rotate in a direction to close the photographic aperture  27   a  of the panel  27   b  by the corresponding torsion spring  27   e  whose coil portion is fitted on the corresponding engaging pin  27   n.  Each of the rear pair of barrier blades  27   d  is provided in the vicinity of the pivoted portion thereof with a driven pin  27   h  that is driven to open the corresponding rear barrier blade  27   d  against the spring force of the corresponding torsion spring  27   e.  Each of the front pair of barrier blades  27   c  is provided on an outer edge thereof with an engaging projection  27   i  which extends rearward to be engaged with the outer edge of the corresponding rear barrier blade  27   d  so that the engaging projection  27   i  of each of the front pair of barrier blades  27   c  comes into engagement with the outer edge of the corresponding rear barrier blade  27   d  to rotate the corresponding front barrier blade  27   c  in the direction to open the photographic aperture  27   a  together with the corresponding rear barrier blade  27   d  when the corresponding rear barrier blade  27   d  is driven to rotate in the direction to open the photographic aperture  27   a.  The upper front barrier blade  27   c   1  is provided on a rear surface thereof with an engaging projection  27   j,  while the upper rear barrier blade  27   d   1  is provided on a front surface thereof with an engaging projection  27   k  (see FIGS. 15A,  15 B and  15 C). When the upper rear barrier blade  27   d   1  is driven to rotate in the direction to close the photographic aperture  27   a,  the engaging projection  27   k  of the upper rear barrier blade  27   d   1  is engaged with the engaging projection  27   j  of the upper front barrier blade  27   c   1  to drive the upper front barrier blade  27   c   1  to rotate in the direction to close the photographic aperture  27   a  together with the upper rear barrier blade  27   d   1 . Likewise, the lower front barrier blade  27   c   2  is provided on a rear surface thereof with an engaging projection  27   j,  while the lower rear barrier blade  27   d   2  is provided on a front surface thereof with an engaging projection  27   k  (see FIGS. 15A,  15 B and  15 C). When the lower rear barrier blade  27   d   2  is driven to rotate in the direction to close the photographic aperture  27   a,  the engaging projection  27   k  of the lower rear barrier blade  27   d   2  is engaged with the engaging projection  27   j  of the lower front barrier blade  27   c   2  to drive the lower front barrier blade  27   c   2  to rotate in the direction to close the photographic aperture  27   a  together with the lower rear barrier blade  27   d   2 . 
     The pressure plate  27   f  is provided with two slots  27   m  through which the two drive pins  27   h  of the rear pair of barrier blades  27   d  penetrate toward the barrier drive ring  26 , respectively. 
     The barrier drive ring  26  is provided on the front thereof with two protrusions  26   b,  while the external barrel  25  is provided in the vicinity of the front end thereof with corresponding two protrusions  25   c  (see FIGS. 16,  17  and  18 ). Two helical extension springs  28  are positioned between the external barrel  25  and the barrier drive ring  26  so that one and the other ends of one helical extension spring  28  are hooked on one of the two protrusions  26   b  and corresponding one of the two protrusions  25   c,  respectively, and one and the other ends of the other helical extension spring  28  are hooked on the other protrusion  26   b  and the other protrusion  25   c,  respectively. The spring force of each helical extension spring  28  is stronger than the spring force of each torsion spring  27   e.  The barrier drive ring  26  is constantly biased by the two helical extension springs  28  to rotate in the direction to open the two pairs of barrier blades  27   c  and  27   d.  The barrier drive ring  26  is provided on the front thereof with two protrusions  26   c  which can be respectively engaged with the two drive pins  27   h  of the rear pair of barrier blades  27   d  to open the two pairs of barrier blades  27   c  and  27   d.  When the barrier drive ring  26  is rotated to the rotational limit thereof by the spring force of the helical extension springs  28 , each of the two protrusions  26   c  is engaged with the corresponding driven pin  27   h  to push the same in the direction to open the corresponding rear barrier blade  27   d  against the spring force of the corresponding torsion spring  27   e,  so that the corresponding front barrier blade  27   c  also opens via the engaging projection  27   i  thereof (see FIGS. 15A,  15 B and  15 C). 
     On the other hand, the barrier drive ring  26  is provided with a driven lever  26   a  which extends from the rim of the barrier drive ring  26  toward the second cam barrel  18  to be engaged with, and disengaged from, a rotation transfer recess  18   c  formed on an outer peripheral surface of the second cam barrel  18  (see FIGS. 8,  9  and  16 ). Since the barrier drive ring  26  is supported by the external barrel  25  to be rotatable about the optical axis O relative to the external barrel  25 , but immovable in the optical axis direction relative to the external barrel  25 , the barrier drive ring  26  moves toward and away from the rotating second cam barrel  18  if the external barrel  25  linearly moves in the optical axis direction due to the engagement of the inward pins  25   b  of the external barrel  25  with the guide grooves  18   b  of the second cam barrel  18  as can be seen in FIGS. 8 and 9. The driven lever  26   a  and the rotation transfer recess  18   c  are apart from each other when positioned within a photographing range (i.e., between the telephoto extremity and the wide-angle extremity) as shown in FIG.  8 . When the zoom barrel retreats from the telephoto extremity thereof to the accommodation position thereof, the driven lever  26   a  approaches the rotation transfer recess  18   c  and is then engaged with the rotation transfer recess  18   c  to apply a force to the barrier drive ring  26  to rotate the same in the direction to close the two pairs of barrier blades  27   c  and  27   d.  When the barrier drive ring  26  rotates to the rotational limit thereof against the spring force of the helical extension springs  28 , each of the protrusions  26   c  of the barrier drive ring  26  disengages from the drive pins  27   h  of the corresponding rear barrier blade  27   d.  As a result, each of the rear pair of barrier blades  27   d  closes by the spring force of the corresponding torsion spring  27   e,  so that each of the front pair of barrier blades  27   c  also closes via the corresponding engaging projections  27   j  and  27   k  to thereby close the photographic aperture  27   a  (see FIG.  14 ). Conversely, when the zoom barrel advances from the accommodation position thereof to the telephoto extremity thereof, the driven lever  26   a  moves forwards and then disengages from the rotation transfer recess  18   c  to thereby allow the barrier drive ring  26  to rotate in the direction to open the two pairs of barrier blades  27   c  and  27   d  by the spring force of the helical extension springs  28 . As a result, each of the protrusions  26   c  of the barrier drive ring  26  is engaged with the drive pin  27   h  of the corresponding rear barrier blade  27   d  to push the same in the direction to open the corresponding front barrier blade  27   c  via the corresponding engaging projection  27   i  to thereby open the two pairs of barrier blades  27   c  and  27   d.  Accordingly, as can be understood by the above description, the two pairs of barrier blades  27   c  and  27   d  are driven to open and close by rotation of the barrier drive ring  26 . It should be noted that the barrier drive ring  26  has only one driven lever  26   a,  whereas the second cam barrel  18  has three rotation transfer recesses  18   c  formed at 120° intervals about the axis of the second cam barrel  18 . One rotation transfer recess  18   c  which is actually used is freely selected from the three rotation transfer recesses  18   c  during assembly. 
     The external barrel  25  that is guided in the optical axis direction moves forward and rearward in the optical axis direction by rotation of the second cam barrel  18  in the above described manner. On the other hand, the first and second lens groups L 1  and L 2  move forward and rearward in the optical axis direction by rotation of the first cam barrel  17 . FIG. 12 shows the axial position of the sensitive surface (image plane) of the CCD  12   a  on which subject images are formed through the photographic optical system, and the variations in the axial positions of the first lens group L 1  (the principal point of the first lens group L 1 ), the second lens group L 2  (the principal point of the first lens group L 2 ), and the barrier block  27  fixed to the front end of the external barrel  25  (more specifically, the photographic aperture  27   a  formed on the panel  27   b  of the barrier block  27 ), when the zoom lens is driven from the accommodation position to the wide-angle extremity via the telephoto extremity. The contours of the first and second cam grooves  17 C 1  and  17 C 2  of the first cam barrel  17  and the guide grooves  18   b  of the second cam barrel  18  are determined so that the first lens group L 1 , the second lens group L 2  and the barrier block  27  move in the optical axis direction to have the moving paths shown in FIG.  12 . The photographic aperture  27   a  has a generally rectangular shape as viewed from the front of the digital camera. The angle of view in the diagonal direction of the photographic aperture  27   a  is greater than the angle of view in the lateral (horizontal) direction of the photographic aperture  27   a,  while the angle of view in the lateral direction of the photographic aperture  27   a  is greater than the angle of view in the longitudinal (vertical) direction of the photographic aperture  27   a. In FIG. 10, an incident light ray S on the zoom lens along the angle of view in the longitudinal direction of the photographic aperture  27   a,  an incident light ray M on the zoom lens along the angle of view in the lateral direction of the photographic aperture  27   a,  and an incident light ray L on the zoom lens along the angle of view in the diagonal direction of the photographic aperture  27   a  are shown by two-dot chain lines. 
     A light shield barrel  26   d  which extends from the inner edge of the barrier drive ring  26  to the front end of the outer peripheral surface of the first lens frame  22  is adhered to the inner edge of the barrier drive ring  26  by an adhesive. The light shield barrel  26   d  is rotationally symmetrical about the optical axis O, so that the shielding characteristics of the light shield barrel  26   d  do not vary even if the light shield barrel  26   d  rotates forwardly and reversely together with the barrier drive ring  26  about the optical axis O. 
     Almost all the above mentioned elements of the zoom lens except for each spring, the feed screw  10   e,  the set screws  23   f,  the follower pins  22   d,  the follower pins  23   d,  the shutter block  24 , the radially inward pins  25   b,  the flexible coding plate  14  and the brush  15  are made of synthetic resin. Although each lens element of the first, second and third lens groups L 1 , L 2  and L 3  can be made of a plastic, at least the frontmost lens element is preferably a glass lens for the purpose of preventing the front surface of the first lens group L 1  from being scratched. 
     In the above illustrated embodiment, although the third lens group L 3  functions as focusing lens group, the zoom lens can be modified so that the first lens group L 1  or the second lens group L 2  functions as focusing lens group. In the case where the second lens group L 2  functions as focusing lens group, the shutter block can be modified to have an auto-focusing function. Such a shutter block is well-known in the art. 
     In the above described embodiment of the zoom lens, each of the first and second lens frames  22  and  23 , which respectively hold the first and second lens groups L 1  and L 2 , is guided linearly in the optical axis direction without rotating about the optical axis O by the engagement of each of the three square projections  22   c  and corresponding each of the three square projections  23   c  with each common corresponding linear guide slot of the three linear guide slots  16   c.  At the same time, each follower pin  22   d  penetrates through the corresponding linear guide slot  16   c  of the linear guide barrel  16  to be engaged with the corresponding first cam groove  17 C 1  of the first cam barrel  17 , which is fitted on the linear guide barrel  16  to be rotatable about the optical axis relative to linear guide barrel  16 , while each follower pin  23   d  penetrates through the corresponding linear guide slot  16   c  of the linear guide barrel  16  to be engaged with the corresponding second cam groove  17 C 2  of the first cam barrel  17 . 
     According to such a lens frame guiding structure, eccentricity between the first and second lens groups L 1  and L 2  does not occur easily. More specifically, if the linear guide barrel  16  is deformed slightly for some reason, eccentricity between the first and second lens groups L 1  and L 2  does not occur in the case that each of the three square projections  22   c  and corresponding each of the three square projections  23   c  are engaged with each common corresponding linear guide slot of the three linear guide slots  16   c,  as compared with the case that each of the three square projections  22   c  and corresponding each of the three square projections  23   c  are engaged with different linear guide grooves. In addition, the first cam barrel  17  is fitted on the linear guide barrel  16 , while the three follower pins  22   d  and the three follower pins  23   d  are respectively engaged with the first cam grooves  17 C 1  and the second cam grooves  17 C 2 . Due to this structure, even if the first cam barrel  17  is deformed slightly for some reason, and if the first and second lens frames  22  and  23  deviate from the optical axis O by being respectively pressed radially by the bottoms of the first and second cam grooves  17 C 1  and  17 C 2  due to the bottom of each of the first and second cam grooves  17 C 1  and  17 C 2  not being positioned precisely on a common imaginary cylinder, the first and second lens frames  22  and  23  deviate in the same radial direction. As a consequence, eccentricity between the first and second lens groups L 1  and L 2  does not occur easily. As has been described above, in the photographing lens system of a digital camera, the influence that eccentricity or tilt of one or more lens groups relative to another lens group or groups has upon the optical performance of the photographing optical system is larger than the influence of a deviation of one or more lens groups in the optical axis direction relative to another lens group or groups. Accordingly, with the lens frame guiding structure which makes eccentricity between lens groups difficult to occur, an excellent optical performance of the photographing optical system can be maintained. 
     More than one square projection  22   c  of the first lens frame  22 , each of which has one follower pin  22   d,  can be provided at predetermined intervals in a circumferential direction, while more than one square projection  23   c  of the second lens frame  23 , each of which has one follower pin  23   d,  can be provided at predetermined intervals in a circumferential direction. Likewise, more than one linear guide slot  16   c  of the linear guide barrel  16  can be provided at predetermined intervals in a circumferential direction, while more than one pair of cam grooves (i.e., one first cam groove  17 C 1  and the corresponding second cam groove  17 C 2 ) can be provided at predetermined intervals in a circumferential direction. However, in the case of three square projections  22   c,  each having one follower pin  22   d,  being provided at 120° intervals; three square projections  23   c,  each having one follower pin  23   d,  being provided at 120° intervals; three linear guide slot  16   c  being provided at 120° intervals; and three pairs of cam grooves  17 C 1  and  17 C 2  being provided at 120° intervals just like the above illustrated embodiment, the first and second lens frames  22  and  23  can be easily coupled to the linear guide barrel  16  and the first cam barrel  17  in a well-balanced manner with minimum eccentricity between the first and second lens frames  22  and  23 . 
     The present invention can be applied to not only the above illustrated embodiment of the zoom lens having particular structure but also any other zoom lens, as long as the zoom lens has a plurality of lens frames, a linear guide barrel and a cam barrel. 
     Although the present invention is applied to the zoom lens of a digital camera, the present invention can be applied not only to a zoom lens of a digital camera, but also a zoom lens of a lens shutter type camera. 
     As can be understood from the foregoing, according to the present invention, in a zoom lens having a plurality of lens frames, a linear guide barrel and a cam barrel, a lens frame guiding mechanism which makes an eccentricity between lens groups hard to occur to thus prevent the optical performance of the zoom lens optical system from deteriorating due to the eccentricity can be obtained. 
     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.