Mechanism for rotating a cam barrel of a zoom lens

[0002] 1. Field of the Invention

[0003] The present invention relates to a zoom lens, or a zoom lens barrel, and more specifically to a mechanism for rotating a cam barrel of the zoom lens.

[0004] 2. Description of the Related Art

[0005] In conventional zoom lenses having a plurality of lens groups guided in the direction of the optical axis (i.e., the optical axis direction) of the zoom lens without rotating about the optical axis, a mechanism for moving the plurality of lens groups in the optical axis direction in a predetermined moving manner by rotation of a cam barrel having cam grooves is known in the art. Such a conventional mechanism generally includes a gear formed directly on the outer or inner peripheral surface of the cam barrel, and a motor-driven pinion which is in mesh with the gear of the cam barrel to rotate the cam barrel. Such a conventional 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.

[0006] However, such a 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 lens elements, lens frames and other elements. 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 the 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 the photographing lens system of a digital camera, the influence that an eccentricity or tilt of one or more lens groups has on the optical performance of the photographing optical system is larger than the influence that a deviation of one or more lens groups in the optical axis direction has on the optical performance of the photographing optical system.

[0007] From this point of view, in the conventional cam barrel drive mechanism, the cam barrel can be easily deformed minutely since the pinion driven by motor is engaged with the gear formed directly on the cam barrel. In other words, the cam barrel can be easily deformed minutely to cause the rotational center of the cam barrel to deviate from the optical axis of the zoom lens, since the pinion driven by motor is engaged with the gear formed as an integral part of the cam barrel. Once the rotational center of the cam barrel deviates from the optical axis of the zoom lens, the optical centers of the lens groups, whose cam followers are engaged with the cam grooves formed on the cam barrel, also deviate from the optical axis of the zoom lens. Consequently, the optical performance of the zoom lens deteriorates.

[0008] The present invention has been devised in view of the above-mentioned problem, and accordingly, an object of the present invention is to provide a mechanism for rotating a cam barrel of a zoom lens, in which the lens groups are moved forward and rearward in the optical axis direction by rotation of the cam barrel having cam grooves, wherein the cam barrel cannot be easily deformed.

[0009] To achieve the object mentioned above, according to an aspect of the present invention, a cam barrel rotating mechanism of a zoom lens is provided, including a plurality of lens groups guided in a direction of an optical axis without rotating about the optical axis to change a focal length of the zoom lens; a cam barrel rotatably supported to move the plurality of lens groups in the optical axis direction; a rotational barrel positioned around the cam barrel, the rotational barrel being rotatably driven; a plurality of rotation transmission grooves formed on an outer peripheral surface of the cam barrel, at an equi-angular intervals, extending parallel to the optical axis; and a plurality of inward projections formed on an inner peripheral surface of the rotational barrel to be engaged in the plurality of rotation transmission grooves, respectively.

[0010] Preferably, the zoom lens includes a stationary barrel having a female helicoid formed on an inner peripheral surface of the stationary barrel, wherein the cam barrel includes a male helicoid formed on an outer peripheral surface of the cam barrel. The male helicoid of the cam barrel meshes with the female helicoid of the stationary barrel so that the cam barrel moves in the optical axis direction while rotating about the optical axis relative to the stationary barrel.

[0011] The plurality of rotation transmission grooves are formed so as to cut across the male helicoid of the cam barrel in a direction parallel to the optical axis.

[0012] In an embodiment, the stationary barrel includes a fine male thread, wherein the rotational barrel includes a fine female thread which meshes with the fine male thread so that the rotational barrel moves in the optical axis direction while rotating about the optical axis relative to the stationary barrel. A thread lead of each of the fine male thread and the fine female thread is smaller than a thread lead of each of the male helicoid of the cam barrel and the female helicoid of the stationary barrel.

[0013] Preferably, the plurality of inward projections are formed on the rotational barrel at an equi-angular intervals about an axis of the rotational barrel, and the plurality of rotation transmission grooves are formed on the cam barrel at an equi-angular intervals about an axis of the cam barrel.

[0014] Preferably, the plurality of inward projections include three inward projections formed on the rotational barrel at 120° intervals about the axis of the rotational barrel, and the plurality of rotation transmission grooves include three rotation transmission grooves formed on the cam barrel at 120° intervals about the axis of the cam barrel.

[0015] Preferably, a depth of each of the plurality of rotation transmission grooves is substantially identical to a depth of thread of the male helicoid. The length of each of the plurality of inward projections in the optical axis direction is determined so that at least part of each of the plurality of inward projections is constantly engaged with an end surface of a portion of thread of the male helicoid in a corresponding rotation transmission groove of the plurality of rotation transmission grooves.

[0016] In an embodiment, the zoom lens includes a plurality of lens frames each having a plurality of cam followers which extend radially outwards at an equi-angular intervals about the optical axis; and a plurality of cam grooves which are formed on an inner peripheral surface of the cam barrel, and in which the plurality of cam followers of a corresponding lens frame of the plurality of lens frames are respectively engaged.

[0017] The above-described zoom lens can be incorporated in a digital camera.

[0018] According to another aspect of the present invention, a zoom lens is provided, including a plurality of lens frames guided in a direction of an optical axis without rotating about the optical axis; a plurality of cam followers formed on each of the plurality of lens frames which extend radially outwards at an equi-angular intervals about the optical axis; a cam barrel having a plurality of cam grooves which are formed on an inner peripheral surface of the cam barrel, the plurality of cam followers being respectively engaged in the plurality of cam grooves, the plurality of lens frames being moved in the optical axis direction without rotating about the optical axis to change a focal length of the zoom lens by rotation of the cam barrel; a rotational barrel positioned around the cam barrel and driven to rotate about the optical axis; a plurality of rotation transmission grooves formed on an outer peripheral surface of the cam barrel at an equi-angular intervals about the optical axis to extend parallel to the optical axis; and a plurality of inward projections formed on an inner peripheral surface of the rotational barrel to be engaged in the plurality of rotation transmission grooves, respectively.

[0019] The present disclosure relates to subject matter contained in Japanese Patent Application No. 2000-24041 (filed on Feb. 1, 2000) which is expressly incorporated herein by reference in its entirety.

[0020] The present invention will be described below in detail with reference to the accompanying drawings in which:

[0021] FIG. 1 is an exploded perspective view of an embodiment of a zoom lens according to the present invention, showing the overall structure thereof;

[0022] 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;

[0023] 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;

[0024] 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;

[0025] 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;

[0026] FIG. 6 is an exploded perspective view of the linear guide barrel, a linear guide ring and a retainer ring;

[0027] FIG. 7 is a developed view of the linear guide barrel, the linear guide ring and the retainer ring;

[0028] 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);

[0029] 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);

[0030] FIG. 10 is an axial cross sectional view of the zoom lens show 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;

[0031] FIG. 11 is an axial cross sectional view of the zoom lens show 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;

[0032] 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;

[0033] FIG. 13 is an exploded perspective view of the barrier block, viewed from behind the barrier block;

[0034] 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;

[0035] FIG. 15A is a schematic front view of the barrier block, showing two pairs of barrier blades in a fully open position;

[0036] FIG. 15B is a schematic front view of the barrier block, showing the two pairs of barrier blades in a half-closed position;

[0037] FIG. 15C is a schematic front view of the barrier block, showing the two pairs of barrier blades in a fully closed position;

[0038] 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;

[0039] 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;

[0040] 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; and

[0041] FIG. 19 is a fragmentary cross sectional view of the first cam barrel, taken along a rotation transmission groove formed on the cam barrel.

[0042] 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.

[0043] The photographic optical system of the zoom lens includes three lens groups; namely, a first lens group (front lens group) L1 (L), a second lens group (middle lens group) L2 (L) and a third lens group (rear lens group) L3 (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 L1 and L2 along the optical axis O relative to the sensitive surface of a stationary CCD 12a(see FIG. 1) and at the same time changing the space between the first and second lens groups L1 and L2 in a predetermined manner. The zoom lens performs a focusing operation by moving the third lens group L3 along the optical axis O to bring an object into focus. The third lens group L3 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 L1 and L2. 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.

[0044] 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 11a.The stationary barrel 11 is provided on an inner peripheral surface thereof with a female helicoid (female helicoidal thread) 11band three linear guide grooves 11c(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 11care formed to cut across the female helicoid 11b.The three linear guide grooves 11care formed at 120 intervals (i.e., at an equi-angular distance) about the axis of the stationary barrel 11.

[0045] As shown in FIG. 2, the housing 10 is provided with a CCD insertion opening 10a,a filter fixing portion 10band a focusing lens group guide portion 10c.The CCD 12awhich is fixed to a substrate 12 is positioned in the CCD insertion opening 10a.A filter 10dsuch as a low-pass filter is fixed to the filter fixing portion 10b.The third lens group L3 is guided by the focusing lens group guide portion 10cto be movable in the optical axis direction. The axial position of the third lens group L3 on the optical axis O is determined by the direction of rotation of a feed screw 10eand the angle of rotation (amount of rotation) thereof. The feed screw 10eextends parallel to the optical axis O from the camera body in the focusing lens group guide portion 10c.The feed screw 10eis driven by a pulse motor (not shown) provided in the camera body. The angle of rotation of the feed screw 10eis controlled via an encoder (not shown) of the pulse motor.

[0046] 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 13awhich meshes with the fine male thread 11aof the stationary barrel 11. The rotational barrel 13 is provided on an outer peripheral surface thereof with a circumferential gear 13b(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 13b.When the rotational barrel 13 is driven to rotate about the optical axis O, the rotational barrel 13 moves in the optical axis direction while rotating about the optical axis O in accordance with the engagement of the fine female thread 13awith the fine male thread 11a.The rotational barrel 13 is provided at the front end of an inner peripheral surface thereof with three inward projections 13cat 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 13awith the fine male thread 11a,so as to sense the rotational position of the rotational barrel 13 as digital and/or analogue information. The fine female thread 13a,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. Accordingly, the thread lead of each of the fine male thread 11aand the fine female thread 13ais much smaller than the thread lead of each of the male helicoid 17bof the cam barrel 17 and the female helicoid 11bof the stationary barrel 11.

[0047] 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 16a.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 16aand 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.

[0048] 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 18a(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 18ais provided with a spring holding groove 18a1, and a pair of guide grooves 18a2 positioned on the opposite sides of the spring holding groove 18a1 in a circumferential direction of the second cam ring 18 (see FIGS. 8 and 9). Each of the three linear guide portions 18ais further provided, in each spring holding groove 18a1 at the front end (the left end as viewed in FIG. 8 or 9) of each spring holding groove 18a1, with an engaging projection 18a3. All of the spring holding grooves 18a1 and the pairs of guide grooves 18a2 extend parallel to the optical axis O. The first cam barrel 17 is provided on an outer peripheral surface thereof with three stopper portions 17a(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 17ais provided with a stopper projection 17a1, and a pair of guide projections 17a2 positioned on the opposite sides of the stopper projection 17a1 in a circumferential direction of the first cam barrel 17 (see FIG. 4). Each pair of guide projections 17a2 of the first cam barrel 17 are respectively fitted in the corresponding pair of guide grooves 18a2 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 18a3 and the corresponding stopper projection 17a1. 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 constantly 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 18a2 and the guide projections 17a2. 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.

[0049] The first cam barrel 17 is provided on an outer peripheral surface thereof with a male helicoid (male helicoidal thread) 17bthat is engaged with the female helicoid 11bof the stationary barrel 11, and three rotation transmission grooves 17cthat extend parallel to the optical axis O. The three rotation transmission grooves 17care formed so as to cut across the male helicoid 17b.The three rotation transmission grooves 17care formed at 120° intervals about the axis of the first cam barrel 17. The three inward projections 13cof the rotational barrel 13 are respectively engaged with the three rotation transmission grooves 17cto be relatively slidable to each other. The linear guide barrel 16 is provided on the outer flange 16athereof with three linear guide projections 16bat 120° intervals about the axis of the linear guide barrel 16. Each linear guide projection 16bextends radially outwards to be engaged with the corresponding linear guide groove 11cof the stationary barrel 11. The linear guide barrel 16 is further provided with three linear guide slots 16cat 120° intervals about the axis of the linear guide barrel 16 so that the circumferential positions of the three linear guide slots 16ccoincide with those of the three linear guide projections 16b.Each of the three linear guide slots 16cpenetrates the linear guide barrel 16 radially and extends parallel to the optical axis O.

[0050] As can be seen in FIGS. 4, 5 and 6, each of the three linear guide slots 16copens at the rear end of the linear guide barrel 16, and the rear end of each linear guide slot 16cis covered by the corresponding part of the outer flange 16aand the corresponding linear guide projection 16bat the radially outer side of the linear guide barrel 16. The outer flange 16ais provided with three insertion grooves 16hwhich respectively extend along a portion of each three linear guide slots 16cfrom the front end of the outer flange 16ato each respective rear end of the three linear guide slots 16c(i.e., the rear end of the outer flange 16a), so that a follower pin (cam follower) 22dand a follower pin (cam follower) 23dcan be inserted into each linear guide slot 16cfrom the corresponding insertion groove 16h.

[0051] 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 16bof the linear guide barrel 16 is inserted into the corresponding linear guide groove 11cof the stationary barrel 11 via a corresponding introducing groove 11dformed on an inner peripheral surface of the stationary barrel 11, and each of the three inward projections 13cof the rotational barrel 13 is inserted into the corresponding rotation transmission groove 17cof the first cam barrel 17 via a corresponding introducing groove 17dformed on an outer peripheral surface of the first cam barrel 17. After each linear guide projection 16band each inward projection 13care inserted into the corresponding linear guide groove 11cand the corresponding rotation transmission groove 17c,respectively, the female helicoid 11bof the stationary barrel 11 and the male helicoid 17bof the first cam barrel 17 mesh with each other.

[0052] 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 13bcauses 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 13awith the fine male thread 11a.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 13cwith the rotation transmission grooves 17c,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 17bwith the female helicoid 11b.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 16bwith the linear guide grooves 11c,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.

[0053] The first cam barrel 17 is provided on an inner peripheral surface thereof with three first cam grooves 17C1 for driving the first lens group L1, and three second cam grooves 17C2 for driving the second lens group L2. 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 17C1 and 17C2. The three first cam grooves 17C1 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 17C2 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 17C1 and 17C2 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 17C1 and 17C2 determines the telephoto extremity of the corresponding lens groups L1 and L2, respectively; the wide-angle position of each cam groove 17C1 and 17C2 determines the wide-angle extremity of the corresponding lens groups L1 and L2, respectively; and the accommodation position of each cam groove 17C1 and 17C2 determines the position of the corresponding lens groups L1 and L2, 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.

[0054] 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 L1 and the second lens group L2, respectively. The first lens frame 22 is guided by the first cam grooves 17C1 and the linear guide slots 16cto 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 17C2 and the linear guide slots 16cto 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 22bwhich extend rearward from a cylindrical portion 22aof the first lens frame 22. The three resilient extending pieces 22bare formed on the first lens frame 22 at 120° intervals about the axis of the first lens frame 22. Each resilient extending piece 22bis provided on a radially outer surface thereof with a square projection 22cwhich extends radially outwards to be fitted in the corresponding linear guide slot 16cin a slidable manner in the optical axis direction. Each resilient extending piece 22bis further provided on top of each square projection 22cwith the follower pin 22d,which is fixed to the resilient extending piece 22bto extend radially outwards. Each square projection 22cis formed so that the opposite faces thereof, which are respectively in sliding contact with the side faces of the corresponding linear guide slot 16c,extend parallel to each other. The zoom lens is provided with a first lens holder 22ewhich encloses the first lens group L1 to hold the same. The first lens holder 22eis fixed to the cylindrical portion 22aof the first lens frame 22 via male and female threads 22fwhich are formed on an outer peripheral surface of the first lens holder 22eand an inner peripheral surface of the cylindrical portion 22a,respectively. The position of the first lens group L1 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 22f.A wave washer 22his held between the holder 22eand an inner flange 22gof the first lens frame 22 to remove the play between the first lens holder 22e(or the first lens group L1) and the first lens frame 22 (see FIG. 2).

[0055] The second lens frame 23 is provided with three resilient extending pieces 23bwhich extend forward from an annular plate portion 23aof the second lens frame 23. The three resilient extending pieces 23bare formed on the second lens frame 23 at 120° intervals about the axis of the second lens frame 23. Each resilient extending piece 23bis provided on a radially outer surface thereof with a square projection 23cwhich extends radially outwards to be fitted in the corresponding linear guide slot 16cin a slidable manner in the optical axis direction. Each resilient extending piece 23bis further provided on top of each square projection 23cwith the aforementioned follower pin 23d,which is fixed to the resilient extending piece 23bto extend radially outwards. The square projections 23cand the follower pins 23dof the second lens frame 23 are identical to the square projections 22cand the follower pins 22dof the first lens frame 22 except that the resilient extending pieces 23bof the second lens frame 23 extend in the direction opposite to the resilient extending pieces 22bof the first lens frame 22 in the optical axis direction. The zoom lens is provided with a second lens holder 23ewhich encloses the second lens group L2 to hold the same. The second lens holder 23eis fixed to the annular plate portion 23aof the second lens frame 23 via set screws 23f.A shutter block 24 is provided around the second lens group L2. The shutter block 24 is fixed to the annular plate portion 23aof the second lens frame 23 via the set screws 23fthat 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 12aat a shutter release operation.

[0056] 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 22cand corresponding each of the three square projections 23cwith each common corresponding linear guide slot of the three linear guide slots 16c.Each follower pin 22dpenetrates the corresponding linear guide slot 16cof the linear guide barrel 16 to be engaged with the corresponding first cam groove 17C1 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 23dpenetrates the corresponding linear guide slot 16cof the linear guide barrel 16 to be engaged with the corresponding second cam groove 17C2 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 22cand corresponding one of the three square projections 23care inserted into a corresponding linear guide slot of the three linear guide slots 16cfrom the rear end face of the linear guide barrel 16. At the same time, each of the three follower pins 22dand corresponding one of the three follower pins 23dare inserted into corresponding one of the three insertion grooves 16hto be fitted in the corresponding first and second cam grooves 17C1 and 17C2, respectively. It should be noted that the hatched areas of the first and second cam grooves 17C1 and 17C2 in FIG. 3 are used solely for the purpose of inserting each follower pin 22dor 23dinto the corresponding cam groove 17C1 or 17C2 during assembly, and thus are not used when the zoom lens is in operation.

[0057] 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 L1) and the second lens frame 23 (the second lens group L2) linearly move in the optical axis direction while changing the space therebetween in accordance with the contours of the first and second cam grooves 17C1 and 17C2 to thereby carry out a zooming operation.

[0058] 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 16deach of which extends radially outwards. A receiving area 16eis formed between any two adjacent engaging lugs 16dof the linear guide barrel 16 in order to receive one of three radially inward projections 19aof the linear guide ring 19. The linear guide barrel 16 is provided immediately behind the three engaging lugs 16dwith three grooves 16f,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 16fis identical to the radius from the axis of the linear guide barrel 16 to the surface of each receiving area 16e.The linear guide barrel 16 is provided behind the three engaging lugs 16dwith three recesses 16g,respectively, each of which is connected with the corresponding groove 16f.Each recess 16gis 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.

[0059] On the other hand, the linear guide ring 19 is provided with the aforementioned three inward projections 19aat 120° intervals about the axis of the linear guide ring 19. The three inward projections 19acan be inserted into the three receiving areas 16e,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 19abeing properly inserted into the three receiving areas 16e,respectively, each inward projection 19aslides into the corresponding groove 16f.The linear guide ring 19 is provided with three radially outward projections 19bat 120° intervals about the axis of the linear guide ring 19. The circumferential positions of the three outward projections 19bare precisely determined with reference to the circumferential positions of the three inward projections 19a.

[0060] The retainer ring 20 is provided with radially inward blades 20aat 120° intervals about the axis of the retainer ring 20. The three inward blades 20acan be inserted into the three receiving areas 16eof 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 20abeing properly inserted into the three receiving areas 16e,respectively, each inward blade 20aslides into the corresponding groove 16f.The retainer ring 20 is provided on the front end face thereof with a plurality of grooves 20bwhich are recessed rearward, toward the linear guide barrel 16, so that a pin face wrench (not shown) can be engaged with the recessed portions 20bto rotate the retainer ring 20 relative to the linear guide barrel 16.

[0061] When the linear guide ring 19 is fixed to the front end of the linear guide barrel 16, firstly the three inward projections 19aare respectively inserted into the three receiving areas 16e,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 19aslides into the corresponding groove 16f.Subsequently, each inward projection 19ais made to be fitted in the corresponding recess 16g.This engagement of each inward projection 19awith the corresponding recess 16gdetermines the fixed circumferential position of the linear guide ring 19 relative to the linear guide barrel 16. Subsequently, the inward blades 20aof the retainer ring 20 are respectively inserted into the three receiving areas 16e,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 20aslides into the corresponding groove 16fand presses the corresponding inward projection 19ainto the corresponding recess 16g.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 20aof the retainer ring 20 is held in one of the three grooves 16fbetween the corresponding engaging lug 16dand the corresponding inward projection 19a,the inward blades 20aand the engaging lugs 16dfunction 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 20a1 which are formed on the retainer ring 20 and corresponding three detent 16jwhich are formed on the linear guide barrel 16 to be respectively engaged with the three indentations 20a1 constitute the elements of the click-stop device (see FIGS. 6 and 7).

[0062] Accordingly, the outward projections 19bof 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 16b.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 25awhich extend parallel to the optical axis O. The three outward projections 19bof the linear guide ring 19 are respectively engaged with the three linear guide grooves 25ato 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 25bwhich are respectively engaged with three guide grooves 18bformed on outer peripheral surface of the second cam barrel 18 at 120° intervals about the axis thereof.

[0063] As shown in FIG. 8, each of the three guide grooves 18bof the second cam barrel 18 defines an assembling position (or a disassembling position) X at which the three inward pins 25bof the external barrel 25 are respectively inserted into or taken out of the three guide grooves 18bof the second cam barrel 18. Each of the three guide grooves 18bfurther 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 18bare 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 18bare 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 L1) 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.

[0064] 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 L1 and L2 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 L1 and L2 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.

[0065] 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 close two pairs of barrier blades 27cand 27d(i.e. the front pair of barrier blades 27cand the rear pair of barrier blades 27d) by rotating about the optical axis O. The two pairs of barrier blades 27cand 27dtogether function as a lens protection cover for protecting the front surface of the first lens group L1 from getting scratched, etc., when the digital camera is not in use. The barrier block 27 is provided with a panel 27bhaving a photographic aperture 27a,the aforementioned two pairs of barrier blades 27cand 27dsupported by the panel 27btherebehind to open and close the photographic aperture 27a,and two torsion springs 27ewhich constantly bias the two pairs of barrier blades 27cand 27din a direction to close the photographic aperture 27a.The barrier block 27 is further provided with an annular pressure plate 27fwhich holds the two pairs of barrier blades 27cand 27dand the torsion springs 27ebetween the panel 27band the pressure plate 27f.The barrier block 27 having such elements is assembled in advance as a unit. The panel 27bis provided on a rear face thereof with two pivots 27g(see FIGS. 13 and 14) and two engaging pins 27n.The upper front barrier blade 27c1 of the front pair of barrier blades 27cand the upper rear barrier blade 27d1 of the rear pair of barrier blades 27dare pivoted at corresponding one of the two pivots 27g(the right pivot 27gas viewed in FIG. 13), while the lower front barrier blade 27c2 of the front pair of barrier blades 27cand the lower rear barrier blade 27d2 of the rear pair of barrier blades 27dare pivoted at the other pivot 27g(the left pivot 27gas viewed in FIG. 13). Each of the rear pair of barrier blades 27dis constantly biased to rotate in a direction to close the photographic aperture 27aof the panel 27bby the corresponding torsion spring 27ewhose coil portion is fitted on the corresponding engaging pin 27n.Each of the rear pair of barrier blades 27dis provided in the vicinity of the pivoted portion thereof with a driven pin 27hthat is driven to open the corresponding rear barrier blade 27dagainst the spring force of the corresponding torsion spring 27e.Each of the front pair of barrier blades 27cis provided on an outer edge thereof with an engaging projection 27iwhich extends rearward to be engaged with the outer edge of the corresponding rear barrier blade 27dso that the engaging projection 27iof each of the front pair of barrier blades 27ccomes into engagement with the outer edge of the corresponding rear barrier blade 27dto rotate the corresponding front barrier blade 27cin the direction to open the photographic aperture 27atogether with the corresponding rear barrier blade 27dwhen the corresponding rear barrier blade 27dis driven to rotate in the direction to open the photographic aperture 27a.The upper front barrier blade 27c1 is provided on a rear surface thereof with an engaging projection 27j,while the upper rear barrier blade 27d1 is provided on a front surface thereof with an engaging projection 27k(see FIGS. 15A, 15B and 15C). When the upper rear barrier blade 27d1 is driven to rotate in the direction to close the photographic aperture 27a,the engaging projection 27kof the upper rear barrier blade 27d1 is engaged with the engaging projection 27jof the upper front barrier blade 27c1 to drive the upper front barrier blade 27c1 to rotate in the direction to close the photographic aperture 27atogether with the upper rear barrier blade 27d1. Likewise, the lower front barrier blade 27c2 is provided on a rear surface thereof with an engaging projection 27j,while the lower rear barrier blade 27d2 is provided on a front surface thereof with an engaging projection 27k(see FIGS. 15A, 15B and 15C). When the lower rear barrier blade 27d2 is driven to rotate in the direction to close the photographic aperture 27a,the engaging projection 27kof the lower rear barrier blade 27d2 is engaged with the engaging projection 27jof the lower front barrier blade 27c2 to drive the lower front barrier blade 27c2 to rotate in the direction to close the photographic aperture 27atogether with the lower rear barrier blade 27d2.

[0066] The pressure plate 27fis provided with two slots 27mthrough which the two drive pins 27hof the rear pair of barrier blades 27dpenetrate toward the barrier drive ring 26, respectively.

[0067] The barrier drive ring 26 is provided on the front thereof with two protrusions 26b,while the external barrel 25 is provided in the vicinity of the front end thereof with corresponding two protrusions 25c(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 26band corresponding one of the two protrusions 25c,respectively, and one and the other ends of the other helical extension spring 28 are hooked on the other protrusion 26band the other protrusion 25c,respectively. The spring force of each helical extension spring 28 is stronger than the spring force of each torsion spring 27e.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 27cand 27d.The barrier drive ring 26 is provided on the front thereof with two protrusions 26cwhich can be respectively engaged with the two drive pins 27hof the rear pair of barrier blades 27dto open the two pairs of barrier blades 27cand 27d.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 26cis engaged with the corresponding driven pin 27hto push the same in the direction to open the corresponding rear barrier blade 27dagainst the spring force of the corresponding torsion spring 27e,so that the corresponding front barrier blade 27calso opens via the engaging projection 27ithereof (see FIGS. 15A, 15B and 15C).

[0068] On the other hand, the barrier drive ring 26 is provided with a driven lever 26awhich 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 18cformed 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 25bof the external barrel 25 with the guide grooves 18bof the second cam barrel 18 as can be seen in FIGS. 8 and 9. The driven lever 26aand the rotation transfer recess 18care 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 26aapproaches the rotation transfer recess 18cand is then engaged with the rotation transfer recess 18cto apply a force to the barrier drive ring 26 to rotate the same in the direction to close the two pairs of barrier blades 27cand 27d.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 26cof the barrier drive ring 26 disengages from the drive pins 27hof the corresponding rear barrier blade 27d.As a result, each of the rear pair of barrier blades 27dcloses by the spring force of the corresponding torsion spring 27e,so that each of the front pair of barrier blades 27calso closes via the corresponding engaging projections 27jand 27kto thereby close the photographic aperture 27a(see FIG. 14). Conversely, when the zoom barrel advances from the accommodation position thereof to the telephoto extremity thereof, the driven lever 26amoves forwards and then disengages from the rotation transfer recess 18cto thereby allow the barrier drive ring 26 to rotate in the direction to open the two pairs of barrier blades 27cand 27dby the spring force of the helical extension springs 28. As a result, each of the protrusions 26cof the barrier drive ring 26 is engaged with the drive pin 27hof the corresponding rear barrier blade 27dto push the same in the direction to open the corresponding front barrier blade 27cvia the corresponding engaging projection 27ito thereby open the two pairs of barrier blades 27cand 27d.Accordingly, as can be understood by the above description, the two pairs of barrier blades 27cand 27dare 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 26a,whereas the second cam barrel 18 has three rotation transfer recesses 18cformed at 120° intervals about the axis of the second cam barrel 18. One rotation transfer recess 18cwhich is actually used is freely selected from the three rotation transfer recesses 18cduring assembly.

[0069] 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 L1 and L2 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 12aon which subject images are formed through the photographic optical system, and the variations in the axial positions of the first lens group L1 (the principal point of the first lens group L1), the second lens group L2 (the principal point of the first lens group L2), and the barrier block 27 fixed to the front end of the external barrel 25 (more specifically, the photographic aperture 27aformed on the panel 27bof 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 17C1 and 17C2 of the first cam barrel 17 and the guide grooves 18bof the second cam barrel 18 are determined so that the first lens group L1, the second lens group L2 and the barrier block 27 move in the optical axis direction to have the moving paths shown in FIG. 12. The photographic aperture 27ahas 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 27ais greater than the angle of view in the lateral (horizontal) direction of the photographic aperture 27a,while the angle of view in the lateral direction of the photographic aperture 27ais greater than the angle of view in the longitudinal (vertical) direction of the photographic aperture 27a.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 27a,an incident light ray M on the zoom lens along the angle of view in the lateral direction of the photographic aperture 27a,and an incident light ray L on the zoom lens along the angle of view in the diagonal direction of the photographic aperture 27aare shown by two-dot chain lines.

[0070] A light shield barrel 26dwhich 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 26dis rotationally symmetrical about the optical axis O, so that the shielding characteristics of the light shield barrel 26ddo not vary even if the light shield barrel 26drotates forwardly and reversely together with the barrier drive ring 26 about the optical axis O.

[0071] Almost all the above mentioned elements of the zoom lens except for each spring, the feed screw 10e,the set screws 23f,the follower pins 22d,the follower pins 23d,the shutter block 24, the radially inward pins 25b,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 L1, L2 and L3 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 L1 from being scratched.

[0072] In the above illustrated embodiment, although the third lens group L3 functions as focusing lens group, the zoom lens can be modified so that the first lens group L1 or the second lens group L2 functions as focusing lens group. In the case where the second lens group L2 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.

[0073] As has been described above, in the present embodiment of the zoom lens, rotating the rotational barrel 13 about the optical axis O via the gear 13bcauses 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 13awith the fine male thread 11a.At the same time, the rotation of the rotational barrel 13 is transmitted to the first cam barrel 17 due to the engagement of the inward projections 13cwith the rotation transmission grooves 17c,so that the first cam barrel 17 rotates about the optical axis O. When the first cam barrel 17 rotates about the optical axis O, the first cam barrel 17 also moves in the optical axis direction O due to the engagement of the male helicoid 17bwith the female helicoid 11b.This movement of the first cam barrel 17 causes the first and second lens frames 22 and 23, which hold the first and second lens groups L1 and L2, to move in the optical axis direction in accordance with the first and second cam grooves 17C1 and 17C2, respectively, to perform a zooming operation.

[0074] In this zooming operation, rotation is transmitted to the first cam barrel 17 from the rotational barrel 13 via the three inward projections 13cof the rotational barrel 13, which are respectively engaged with the three rotation transmission grooves 17cof the first cam barrel 17. There is little possibility of the first cam barrel 17, which is made of a synthetic resin, being deformed to cause the rotational center of the first cam barrel 17 to deviate from the optical axis O since the inward projections 13care formed on the rotational barrel 13 at an equi-angular intervals (120° intervals in the illustrated embodiment) about the axis of the rotational barrel 13 while the three rotation transmission grooves 17care formed on the first cam barrel 17 also at an equi-angular intervals (120° intervals in the illustrated embodiment) about the axis of the first cam barrel 17. In a conventional zoom lens, for instance, a gear which is in mesh with a drive pinion is formed on a cam barrel which corresponds to the first cam barrel 17, or cam grooves are formed on a rotational barrel which corresponds to the rotational barrel 13. In the present embodiment of the zoom lens, there is a possibility of the rotational barrel 13 being slightly deformed since the drive pinion (not shown) meshes with the circumferential gear 13bof the rotational barrel 13 at a position away from the rotational axis of the first cam barrel 17. However, even if the rotational barrel 13 should be deformed, the deformation of the rotational barrel 13 is not transmitted to the first cam barrel 17. Therefore, there is no possibility of each of the first and second lens groups L1 and L2 being eccentric or having a tilt with respect to the optical axis due to a deformation of the first cam barrel 17.

[0075] FIG. 19 shows a preferred embodiment of construction between the inward projections 13cof the rotational barrel 13 and the rotation transmission grooves 17cof the first cam barrel 17. The depth of each rotation transmission groove 17cis formed to be substantially the same as the depth of thread of the male helicoid 17b,while the length of each inward projection 13cin the optical axis direction is determined so that at least part of each inward projection 13cis constantly engaged with the end surface of a portion of thread of the male helicoid 17bin the corresponding rotation transmission groove 17c.With such structure wherein each rotation transmission groove 17chas a relatively shallow depth while rotation of the rotational barrel 13 is transmitted to the first cam barrel 17 via the end surface of a portion of thread of the male helicoid 17bin each rotation transmission groove 17c,the wall thickness of the first cam barrel 17 can be made small. Consequently, a small and a thin-walled zoom lens can be achieved.

[0076] The present invention can also be applied to a zoom lens including a cam barrel which does not have a male helicoid formed on an outer peripheral surface thereof, but is only provided with rotation transmission grooves corresponding to the rotation transmission grooves 17cof the above illustrated embodiment. In this case, the effect of preventing the cam barrel from being deformed is obtained, similar to the above illustrated embodiment.

[0077] Although the present invention is applied to the zoom lens of a digital camera, the present invention can also be applied to the zoom lens of a lens shutter type camera.

[0078] As can be understood from the foregoing, according to the present invention, in a zoom lens in which lens groups are moved forward and rearward in the optical axis direction by rotation of a cam barrel having cam grooves, the cam barrel is prevented from being deformed, and consequently, the optical performance of the zoom lens is prevented from deteriorating due to a deformation of the cam barrel.

[0079] 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.