Drive mechanism of a zoom lens

A drive mechanism of a zoom lens includes three movable members which support optical elements, including a front movable member, an intermediate movable member and a rear movable member; a common driving member including a driving device for moving the three movable members independently; and a biasing member including first and second spring members which bias the three movable members to remove backlash. One of the front movable member and the rear movable member serves as a common-engaging spring support member to which one end of each of the first and second spring members are connected, and the other of the front movable member and the rear movable member, and the intermediate movable member serve as two single-engaging spring support members to which the other ends of the first spring member and the second spring member are connected, respectively.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a zoom lens, more particularly to a spring-biasing structure in a drive mechanism for driving optical elements of the zoom lens such as lens groups.

2. Description of the Related Art

In drive mechanisms of zoom lenses for driving optical elements such as lens groups, it is generally the case that a plurality of movable frames which are moved in an optical axis direction relative to one another in different moving manners are biased mutually by biasing springs to remove backlash in the plurality of movable frames. Specifically, there is a high possibility of an external force being exerted on one or more external barrels of the zoom lens, and accordingly, external barrels of the zoom lens need to be biased as appropriate by strong biasing forces to be capable of returning to respective normal positions of the external lens barrels when undergoing displacement by an external force. In telescopic zoom lenses having a plurality of external barrels which are concentrically arranged, it is conventional for the frontmost external barrel (usually the radially-innermost external barrel) to hold a lens group having the largest diameter among all the lens groups of a photographing optical system, so that the frontmost external barrel tends to be heavier in weight than the other movable barrels or frames. Therefore, such an external barrel is often required to be biased by a strong biasing spring force in order to be held with stability.

In the case where three movable members (a frontmost movable member, an intermediate movable member and a rearmost movable member) which are aligned along an optical axis are mutually spring-biased, it is conventional for one end of each two biasing springs to be fixed to the frontmost movable member and the rearmost movable member, respectively, and the other ends of the two biasing springs are fixed to the intermediate movable member.

However, the biasing spring forces of the biasing springs vary due to the difference in amount of movement among the movable frames including the aforementioned one or more external barrels, which makes it difficult to apply a stable biasing spring force to each movable frame. For instance, since the minimum biasing force among variations in the biasing spring force exerted on the movable frames needs to be determined as a reference spring load (minimum return force), the spring load grows excessively upon the spring biasing force becoming maximum, and may become a great burden on operations of the zoom lens. As a result, in the case of a motor-driven zoom lens, a zoom motor thereof needs to be a large type producing a strong torque, which makes it difficult to achieve miniaturization and weight reduction of the zoom lens.

In the above described structure in which the two biasing springs are arranged on the front side and the rear side of the intermediate movable member, respectively, each biasing spring cannot have a greater length than the distance between associated two movable members adjacent to the biasing spring in the optical axis direction, and therefore needs to be a biasing spring having a relatively large spring constant. However, using such a biasing spring makes it difficult to assemble the zoom lens. Additionally, in this structure it is difficult to achieve the balance between the two biasing springs.

SUMMARY OF THE INVENTION

The present invention provides a drive mechanism for driving a plurality of optical elements of a zoom lens, wherein the drive mechanism can exert a stable biasing spring force to each of a plurality of movable frames which respectively support the plurality of optical elements without increasing the spring load on the movable frames excessively. The present invention further provides a drive mechanism for driving a plurality of optical elements of a zoom lens, wherein the installation of biasing springs and the setting (adjustment) of the biasing force thereof are easy to carry out.

According to an aspect of the present invention, a drive mechanism of a zoom lens is provided, for driving at least three optical elements of a photographing optical system in an optical axis direction, the drive mechanism including a front movable member, an intermediate movable member and a rear movable member, in that order from the front of the zoom lens in the optical axis direction, which support the three optical elements, respectively, and are guided linearly in the optical axis direction without rotating, wherein one of the front movable member and the rear movable member serves as a common-engaging spring support member, and the other of the front movable member and the rear movable member, and the intermediate movable member serve as two single-engaging spring support members; a common driving member including a driving device for moving the front, intermediate and rear movable members independently of one another in the optical axis direction; and a first spring member and a second spring member for removing backlash between the front, intermediate and rear movable members and the driving device, both one end of the first spring member and one end of the second spring member being connected to the common-engaging spring support member and the other ends of the first spring member and the second spring member being connected to the two single-engaging spring support members, respectively.

It is desirable for the first spring member and second spring member to bias the common-engaging spring support member in the same biasing direction in the optical axis direction.

It is desirable for the first and second spring members to be arranged so that when one of the biasing forces of the first spring member and the second spring member increases, the other of the biasing forces thereof decreases when the front, intermediate and rear movable members are moved independently of one another in the optical axis direction by the common driving member in accordance with a zooming operation of the photographing optical system.

It is desirable for the common-engaging spring support member to be an external member of the zoom lens.

It is desirable for the three optical elements that are supported by the front, intermediate and rear movable members to include three lens groups, respectively, and for the lens group supported by the common-engaging spring support member to have the largest diameter among all the three lens groups.

It is desirable for the optical element supported by the common-engaging spring support member to include a frontmost lens group in the photographing optical system.

It is desirable for the common driving member to include a cam ring rotatable about the optical axis. The driving device of the common driving member includes a first cam groove, a second cam groove and a third cam groove which are formed on at least one of inner and outer peripheral surfaces of the cam ring to have different cam diagrams. The three movable members include a first cam follower, a second cam follower and a third cam follower which are slidably engaged in the first cam groove, the second cam groove and the third cam groove, respectively.

It is desirable for each of the first spring member and the second spring member to include at least one extension coil spring.

It is desirable for the zoom lens to be a telescopic zoom lens having a plurality of external movable barrels which are concentrically arranged about the optical axis, the common-engaging spring support member serving as a radially-innermost external movable barrel among the plurality of external movable barrels.

It is desirable for the rear movable member to be guided linearly in the optical axis direction by a linear guide member provided independently of the three movable members.

In an embodiment, a drive mechanism of a zoom lens is provided, including a front movable member, an intermediate movable member and a rear movable member, in that order from the front of the zoom lens which are guided linearly in an optical axis direction without rotating; a cam ring including cam grooves, into which cam followers formed on the front, intermediate and rear movable members are slidably fitted, the cam ring being rotated about the optical axis to move the front, intermediate and rear movable members independently of one another in the optical axis direction; and a first spring member and a second spring member which bias the front, intermediate and rear movable members mutually in the optical axis direction to remove backlash between the cam followers and the cam grooves. Both one end of the first spring member and one end of the second spring member are connected to one of the front movable member and the rear movable member, and the other ends of the first spring member and the second spring member are connected to the other of the front movable member and the rear movable member, and the intermediate movable member, respectively.

In an embodiment, a drive mechanism of a zoom lens is provided, including a front movable member, an intermediate movable member and a rear movable member, in that order from the front of the zoom lens in the optical axis direction, wherein one of the front movable member and the rear movable member serves as a common-engaging spring support member, and the other of the front movable member and the rear movable member, and the intermediate movable member serve as two single-engaging spring support members; a common driving member including a driving device for moving the front, intermediate and rear movable members independently of one another in an optical axis direction; and a first spring member and a second spring member for removing backlash between the front, intermediate and rear movable members and the driving device, both one end of the first spring member and one end of the second spring member being connected to the common-engaging spring support member and the other ends of the first spring member and the second spring member being connected to the two single-engaging spring support members, respectively.

According to the present invention, the drive mechanism can exert a stable biasing spring force to each of the plurality of movable frames without increasing the spring load on the movable frames excessively with a simple structure. Moreover, the workability of installing the biasing springs and the workability of balancing the biasing spring force of the biasing springs are improved.

The present disclosure relates to subject matter contained in Japanese Patent Application No. 2004-253386 (filed on Aug. 31, 2004) which is expressly incorporated herein by reference in its entirety.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIGS. 1 through 3show an embodiment of a zoom lens according to the present invention in different states.FIG. 1shows a state of the zoom lens10at the wide-angle extremity,FIG. 2shows a state of the zoom lens10at the telephoto extremity, andFIG. 3shows a state of the zoom lens in a retracted position (fully retracted position). The zoom lens10is incorporated in a digital camera (the camera body thereof is not shown in the drawings). As shown inFIGS. 1 and 2, the photographing optical system of the zoom lens10in a ready-to-photograph state of the zoom lens10consists of a first lens group LG1, a second lens group LG2, a shutter S, a third lens group LG3, a fourth lens group LG4, a low-pass filter (optical filter)11, and a CCD image sensor (solid-state image pick-up device)12. The first lens group LG1, the second lens group LG2and the third lens group LG3are driven along a photographing optical axis Z1in a predetermined moving manner to perform a zooming operation, while the fourth lens group L4is driven along the photographing optical axis Z1to perform a focusing operation. In the following description, the term “optical axis direction” refers to a direction parallel to the photographing optical axis Z1unless otherwise stated.

FIG. 4is an exploded view of elements of the zoom lens10, andFIGS. 5 through 12are enlarged views of these elements. The zoom lens10is incorporated in a camera body (not shown), and is provided with a stationary barrel13fixed to the camera body. A CCD holder (stationary member)14is fixed to a rear portion of the stationary barrel13from behind. The CCD image sensor12is mounted to a central portion of the CCD holder14to be held thereby via a CCD base plate15. The low-pass filter11is held by the CCD holder14to be positioned in front of the CCD image sensor12. An annular sealing member16is installed between the lower-pass filter11and the CCD image sensor12to seal the gap therebetween.

The zoom lens10is provided in the stationary barrel13with an AF lens frame (a fourth lens frame which supports and holds the fourth lens group LG4)17which is guided linearly in the optical axis direction without rotating about the photographing optical axis Z1. Specifically, the zoom lens10is provided with a pair of AF guide shafts18A and18B which extend parallel to the photographing optical axis Z1to guide the AF lens frame17in the optical axis direction without rotating the AF lens frame17about the photographing optical axis Z1. Front and rear ends of each guide shaft of the pair of AF guide shafts18A and18B are fixed to the stationary barrel13and the CCD holder14, respectively. The AF lens frame17is provided on radially opposite sides thereof with a pair of guide holes (guide grooves) in which the pair of AF guide shafts18A and18B are respectively fitted so that the AF lens frame17is slidable on the pair of AF guide shafts18A and18B. Portions of the stationary barrel13and the CCD holder14which support the pair of AF guide shafts18A and18B project radially outwards from the outside diameter of the stationary barrel13, and accordingly, the pair of AF guide shafts18A and18B are positioned radially outside of the stationary barrel13.

The zoom lens10is provided therein with an AF motor19which is fixed to the stationary barrel13. The AF lens frame17can be moved forward and rearward in the optical axis direction by a driving force of the AF motor19. A rotary drive shaft of the AF motor19is threaded to serve as a feed screw shaft (rotatable lead screw), and this rotary drive shaft is screwed through a female screw hole formed on an AF nut20(seeFIG. 5). The AF lens frame17is engaged with the AF nut20to be slidable thereon in the optical axis direction, and is biased forward in the optical axis direction by an extension coil spring (biasing member)21, and the forward movement limit of the AF lens frame17is determined via the engagement between surfaces of the AF nut20and the AF lens frame17which are opposed to each other in the optical axis direction. A rearward movement of the AF nut20in the optical axis direction by a rotation of the rotary drive shaft of the AF motor19causes the AF lens frame17to be pressed rearward by the AF nut20to be moved rearward against the biasing force of the extension coil spring21. Due to this structure, rotating the rotary drive shaft of AF motor19forward and rearward causes the AF lens frame17to move forward and rearward in the optical axis direction.

The zoom lens10is provided with a zoom gear22which is supported by the stationary barrel13to be rotatable on a zoom gear shaft22aextending parallel to the photographing optical axis Z1. Front and rear ends of the zoom gear shaft22aare fixed to the stationary barrel13and the CCD holder14, respectively. The zoom gear22is positioned so that the gear teeth thereof partly project radially inwards from an inner peripheral surface of the stationary barrel13, and can be rotated forward and reverse by a zoom motor23(shown conceptually by a labeled rectangle inFIG. 4).

As shown inFIG. 5, the stationary barrel13is provided on an inner peripheral surface thereof with a female helicoid13a, a set of three linear guide grooves13b, a set of three inclined grooves13c, and a set of three rotational guide grooves13d. Threads of the female helicoid13aextend in a direction inclined with respect to both the optical axis direction and a circumferential direction of the stationary barrel13. The set of three linear guide grooves13bextend parallel to the photographing optical axis Z1. The set of three inclined grooves13cextend parallel to the female helicoid13a. The set of three rotational guide grooves13dare formed in the vicinity of a front end of the inner peripheral surface of the stationary barrel13to extend along a circumference of the stationary barrel13to communicate the front ends of the set of three inclined grooves13c, respectively. The female helicoid13ais not formed on a specific front area of the inner peripheral surface of the stationary barrel13which is positioned immediately behind the set of three rotational guide grooves13d. Regarding each set of the above three sets of grooves (the set of three linear guide grooves13b, the set of three inclined grooves13cand the set of three rotational guide grooves13d), although each set of grooves is composed of three grooves which are arranged at different circumferential positions on the inner peripheral surface of the stationary lens barrel13, only some of the three grooves appear inFIG. 5.

The zoom lens10is provided inside the stationary barrel13with a helicoid ring25. As shown inFIG. 6, the helicoid ring25is provided on an outer peripheral surface thereof with a male helicoid25aand a set of three rotational guide projections25b. The male helicoid25ais engaged with the female helicoid13a, and the set of three rotational guide projections25bare engaged in the set of three inclined grooves13cor the set of three rotational guide grooves13d, respectively. The helicoid ring25is provided on threads of the male helicoid25awith an annular gear25cwhich is in mesh with the zoom gear22. Therefore, when a rotation of the zoom gear22is transferred to the annular gear25c, the helicoid ring25moves forward or rearward in the optical axis direction while rotating about the photographing optical axis Z1within a predetermined range in which the male helicoid25aremains in mesh with the female helicoid13a. A forward movement of the helicoid ring25beyond a predetermined point with respect to the stationary barrel13causes the male helicoid25ato be disengaged from the female helicoid13aso that the helicoid ring25rotates about the photographing optical axis Z1without moving in the optical axis direction relative to the stationary barrel13by engagement of the set of three rotational guide projections25bwith the set of three rotational guide grooves13d. In a state where the female helicoid13ais in mesh with the male helicoid25a, the set of three rotational guide projections25bare positioned in the set of three inclined grooves13c, respectively, and accordingly, the set of three rotational guide projections25band the female helicoid13ado not interfere with each other.

As can be appreciated fromFIGS. 1 through 3, the zoom lens10is a telescopic type having three external telescoping barrels: a first external barrel (front movable member/common-engaging spring support member/external member)37, a second external barrel34and a third external barrel26, which are concentrically arranged about the photographing optical axis Z1. The helicoid ring25moves together with the third external barrel26in the optical axis direction while rotating about the photographing optical axis Z1. The helicoid ring25is provided, on an inner peripheral surface thereof at three different circumferential positions on the helicoid ring25, with three rotation transfer recesses (engaging recesses)25d, the front ends of which are open at the front end of the helicoid ring25. The third external barrel26is provided, at corresponding three different circumferential positions on the third external barrel26, with three pairs of rotation transfer projections (engaging projections)26awhich project rearward from the rear end of the third external barrel26to be engageable in the three rotation transfer recesses25dfrom the front thereof, respectively (seeFIG. 14). The three pairs of rotation transfer projections26aand the three rotation transfer recesses25dare movable relative to each other in the direction of the photographing optical axis Z1, and are not rotatable relative to each other about the photographing optical axis Z1. Namely, the helicoid ring25and the third external barrel26rotate integrally. The helicoid ring25is provided, on front faces of the three rotational guide projections25bat three different circumferential positions on the helicoid ring25, with a set of three engaging recesses25ewhich are formed on an inner peripheral surface of the helicoid ring25to be open at the front end of the helicoid ring25. The third external barrel26is provided, at corresponding three different circumferential positions on the third external barrel26, with a set of three engaging projections26bwhich project rearward from the rear end of the third external barrel26, and which also project radially outwards, to be engaged in the set of three engaging recesses25efrom the front thereof, respectively. The set of three engaging projections26b, which are respectively engaged in the set of three engaging recesses25e, are also engaged in the set of three rotational guide grooves13dat a time, respectively, when the set of three rotational guide projections25bare engaged in the set of three rotational guide grooves13d.

The third external barrel26and the helicoid ring25are biased in opposite directions away from each other in the optical axis direction by compression coil springs (not shown). These compression coil springs are installed between the third external barrel26and the helicoid ring25in a compressed fashion. Therefore, the set of three engaging projections26bof the third external barrel26are respectively pressed against front guide surfaces of the rotational guide grooves13dtherein by the spring force of the compression coil springs. At the same time, the set of three rotational guide projections25bof the helicoid ring25are respectively pressed against rear guide surfaces of the rotational guide grooves13dtherein by the spring force of the compression coil springs.

As shown inFIG. 14, the third external barrel26is provided on an inner peripheral surface thereof with a set of three rotation transfer grooves26cwhich extend parallel to the photographing optical axis Z1. The front end of each rotation transfer groove26cis closed at the front end of the third external barrel26, and the rear end of each rotation transfer groove26cis open at the rear end of the third external barrel26. The circumferential positions of the three rotation transfer grooves26ccorrespond to those of the three pairs of rotation transfer projections26a, respectively. More specifically, as shown inFIGS. 14,19and20, each pair of rotation transfer projections26aconsists of a long projection26a1and a short projection26a2which is smaller than the long projection26a1in the amount of projection rearward in the optical axis direction, and the rear end opening of the associated rotation transfer groove26cis positioned between the long projection26a1and the short projection26a2, and accordingly, surfaces of the long projection26a1and the short projection26a2which are opposed to each other in a circumferential direction of the third external barrel26form a part (the rear end opening) of the associated rotation transfer groove26c.

On the other hand, the helicoid ring25is provided on an inner peripheral surface thereof with a set of three relative rotation allowing grooves25fwhich are communicatively connected with the three rotation transfer recesses25d, respectively. The three relative rotation allowing grooves25fextend circumferentially on a circle about the photographing optical axis Z1, and one end (left end as viewed inFIG. 14) of each relative rotation allowing groove25fis communicatively connected with the associated rotation transfer recess25d, and the other end (right end as viewed inFIG. 14) of each relative rotation allowing groove25fis formed as a closed end. In a state where the helicoid ring25and the third external barrel26are coupled to each other, each relative rotation allowing groove25fis communicatively connected with the rear end opening (the right side surface of the associated long projection26a1as viewed inFIG. 20) of the associated rotation transfer groove26cso that the relative rotation allowing groove25fand the rotation transfer groove26ctogether form an L-shaped groove as shown inFIG. 20.

The zoom lens10is provided inside of the third external barrel26and the helicoid ring25with a first linear guide ring30. The helicoid ring25is provided on an inner peripheral surface thereof with a circumferential groove25gwhich extends in a circumferential direction about the photographing optical axis Z1, and the third external barrel26is provided, on an inner peripheral surface thereof in the vicinity of the rear end and the front end of the third external barrel26, with a rear circumferential groove26dand a front circumferential groove26e, respectively, each of which extends in a circumferential direction about the photographing optical axis Z1(seeFIG. 6). As shown inFIGS. 6 and 13, the first linear guide ring30is provided on an outer peripheral surface thereof with a first plurality of relative rotation guide projections30a, a second plurality of relative rotation guide projections30band a third plurality of relative rotation guide projections30c, in that order from the rear of the first linear guide ring30in the optical axis direction. The first plurality of relative rotation guide projections30a, the second plurality of relative rotation guide projections30band the third plurality of relative rotation guide projections30care engaged in the circumferential groove25g, the rear circumferential groove26dand the front circumferential groove26e, respectively. Due to this engagement, the helicoid ring25and the third external barrel26are supported by the first linear guide ring30to be allowed to rotate relative to the first linear guide ring30and to be prevented from moving in the optical axis direction relative to the first linear guide ring30. In addition, the helicoid ring25and the third external barrel26are prevented from being separated totally from each other in the optical axis direction via the first linear guide ring30. The first linear guide ring30is provided, in the vicinity of the rear end thereof at different circumferential positions, with a set of three linear guide projections30dwhich project radially outwards. The first linear guide ring30is guided linearly in the optical axis direction without rotating by the engagement of the set of three linear guide projections30dwith the set of three linear guide grooves13bof the stationary barrel13.

The first linear guide ring30is provided with a set of three through slots30ewhich radially extend through the first linear guide ring30. As shown inFIG. 13, each through slot30eincludes a circumferential slot portion30e-1which extends in a circumferential direction of the first linear guide ring30, a first lead slot portion30e-2which extends obliquely from one end (right end as viewed inFIG. 13) of the circumferential slot portion30e-1, and a second lead slot portion30e-3which extends obliquely from one end (right end as viewed inFIG. 13) of the first lead slot portion30e-2. The angle of inclination of the first lead slot portion30e-2relative to the circumferential direction of the first linear guide ring30is greater than that of the second lead slot portion30e-3. The zoom lens10is provided with a cam ring (common driving member)31a front part of which is fitted in the first external barrel37. A set of three roller followers32fixed to an outer peripheral surface of the cam ring31at different circumferential positions thereon are engaged in the set of three through slots30e, respectively. The set of three roller followers32are further engaged in the set of three rotation transfer grooves26c(or the set of three relative rotation allowing grooves25f) through the set of three through slots30e, respectively.

Advancing operations of movable elements of the zoom lens10from the stationary barrel13to the cam ring31will be discussed hereinafter. Rotating the zoom gear22in a lens barrel advancing direction by the zoom motor23causes the helicoid ring25to move forward while rotating due to engagement of the female helicoid13awith the male helicoid25a. This rotation of the helicoid ring25causes the third external barrel26to move forward together with the helicoid ring25while rotating together with the helicoid ring25, and further causes the first linear guide ring30to move forward together with the helicoid ring25and the third external barrel26because each of the helicoid ring25and the third external barrel26is coupled to the first linear guide ring30, to allow respective relative rotations between the third external barrel26and the first linear guide ring30and between the helicoid ring25and the first linear guide ring30and to be movable together along a direction of a common rotational axis (i.e., the photographing optical axis Z1), due to the engagement of the first plurality of relative rotation guide projections30awith the circumferential groove25g, the engagement of the second plurality of relative rotation guide projections30bwith the rear circumferential groove26d, and the engagement of the third plurality of relative rotation guide projections30cwith the front circumferential groove26e.

In the retracted state of the zoom lens10, the set of three roller followers32are engaged in the circumferential slot portions30e-1of the set of three through slots30e, respectively, and are further engaged in the three relative rotation allowing grooves25fat closed end portions thereof, respectively, as shown inFIGS. 15 and 17.FIGS. 15 and 17show the same state, although the first linear guide ring30is removed inFIG. 17except the set of three through slots30efor the purpose of making the operation of each roller follower32easier to be seen in the drawing. In addition, inFIGS. 15 and 17, the first linear guide ring30(the set of three through slots30e) are shown by solid lines though actually positioned a hidden position below (radially inside) the helicoid ring25and the third external barrel26.

When the helicoid ring25and the third external barrel26are moved forward while rotating, this rotation of the helicoid ring25and the third external barrel26is not transferred to the cam ring31in an initial stage of the forward movement of the helicoid ring25and the third external barrel26because the set of three roller followers32are engaged in the set of three relative rotation allowing grooves25f, respectively. The set of three roller followers32move together with the helicoid ring25, the third external barrel26and the first linear guide ring30in the optical axis direction due to the engagement of the set of three roller followers32with the circumferential slot portions30e-1of the set of three through slots30e, respectively. Accordingly, in an initial stage of the advancing operation of the zoom lens10from the retracted state of the zoom lens10, the cam ring31is moved forward in the optical axis direction without rotating.

FIGS. 16 and 18show a state of the helicoid ring25and the third external barrel26which have been rotated by an angle of approximately 30 degrees from their respective retracted positions from the retracted state of the zoom lens10shown inFIGS. 15 and 17. In the state shown inFIGS. 16 and 18, each roller follower32is engaged in an intersection of the associated relative rotation allowing groove25fand the associated rotation transfer groove26cso that the rotation of the helicoid ring25and the third external barrel26can be transferred to the roller follower32via a side surface (left surface as viewed inFIG. 20) of the rotation transfer groove26cat the left end of the relative rotation allowing groove25f. A further forward movement of the helicoid ring25and the third external barrel26while rotating causes each roller follower32to be moved rightward as viewed inFIGS. 16 and 18from the circumferential slot portions30e-1to the first lead slot portion30e-2of the associated through slot30e. Since the first lead slot portion30e-2of each through slot30eis inclined to the circumferential direction of the first linear guide ring30in a manner to approach the front end (upper end as viewed inFIG. 16) of the first linear guide ring30in a direction away from the circumferential slot portions30e-1of the associated through slot30e, a forward movement of each roller follower32in the first lead slot portion30e-2of the associated through slot30ecauses the roller follower32to be disengaged from the associated relative rotation allowing groove25fto be engaged in the associated rotation transfer groove26c(i.e., the roller follower32is led from the associated relative rotation allowing groove25fto the associated rotation transfer groove26c). In a state where the set of three roller followers32are engaged in the set of three rotation transfer grooves26c, respectively, the torque (rotating force) of the third external barrel26is transferred to the cam ring31via the engagement of the set of three roller followers32with the set of three rotation transfer grooves26cwhenever the third external barrel26rotates. Thereupon, the cam ring31moves forward while rotating relative to the first linear guide ring30in accordance with contours of the first lead slot portions30e-2of the set of three through slots30e. At this time, each roller follower32moves forward in the optical axis direction in the associated rotation transfer groove26cwhile receiving a torque from the same rotation transfer groove26c. Since the first linear guide ring30itself has linearly moved forward together with the helicoid ring25and the third external barrel26as described above, the cam ring31moves forward in the optical axis direction by a resultant amount of movement corresponding to the sum of the amount of the forward movement of the first linear guide ring30(and the helicoid ring25and the third external barrel26) and the amount of the forward movement of the cam ring31via the engagement of the set of three roller followers32with the first lead slot portions30e-2of the set of three through slots30e, respectively.

The above described rotating-advancing operations of the helicoid ring25and the third external barrel26are performed only when the male helicoid25aand the female helicoid13aare engaged with each other. At this time, the set of three rotational guide projections25bare moving in the set of three inclined grooves13c, respectively. When the helicoid ring25is moved forward by a predetermined amount of movement, the male helicoid25aand the female helicoid13aare disengaged from each other so that the set of three rotational guide projections25bmove from the set of three inclined grooves13cinto the set of three rotational guide grooves13d, respectively. Since the helicoid ring25does not move in the optical axis direction relative to the stationary barrel13even if rotating upon the disengagement of the male helicoid25afrom the female helicoid13a, the helicoid ring25and the third external barrel26rotate at respective axial fixed positions thereof without moving in the optical axis direction due to the slidable engagement of the set of three rotational guide projections25bwith the set of three rotational guide grooves13d.

Furthermore, after a lapse of a predetermined period of time from the moment at which the set of three rotational guide projections25bslide into the set of three rotational guide grooves13dfrom the set of three inclined grooves13c, respectively, the set of three roller followers32enter the second lead slot portions30e-3from the first lead slot portions30e-2of the set of three through slots30e, respectively. Since the second lead slot portion30e-3of each through slot30eis inclined to the first linear guide ring30in a direction away from the associated first lead slot portion30e-2and approaching the front end (upper end as viewed inFIG. 16) of the first linear guide ring30, further rotation of the helicoid ring25and the third external barrel26at respective axial fixed positions thereof in a lens barrel advancing direction causes each roller follower32to move forward in the second lead slot portion30e-3of the associated through slot30e. Namely, the cam ring31is moved forward while rotating relative to the first linear guide ring30in accordance with contours of the second lead slot portions30e-3of the set of three through slots30e. The helicoid ring25and the third external barrel26serve as a rotating drive member which transfers torque to the cam ring31via the engagement of the set of three roller followers32with the set of three through slots30eand the engagement of the set of three roller followers32with the set of three rotation transfer grooves26c.

Rotating the zoom gear22in a lens barrel retracting direction thereof via the zoom motor23causes the aforementioned movable elements of the zoom lens10from the stationary barrel13to the cam ring31to operate in the reverse manner to the above described advancing operations. In this reverse operation, the helicoid ring25and the third external barrel26which rotate at respective axial fixed positions thereof move rearward in the optical axis direction while rotating after the male helicoid25aand the female helicoid13aare engaged with each other. The first linear guide ring30linearly moves in the optical axis direction without rotating at all times while following the rearward linear movement of the helicoid ring25and the third external barrel26. When the set of three roller followers32are engaged in the first lead slot portions30e-2or the second lead slot portions32e-3of the set of three through slots30e, respectively, the cam ring31moves rearward in the optical axis direction relative to the helicoid25, the third external barrel26and the first linear guide ring30by rotation of the helicoid ring25and the third external barrel26in the lens barrel retracting direction thereof. At this time, each roller follower32moves rearward in the optical axis direction in the associated rotation transfer groove26cwhile receiving a torque from the same rotation transfer groove26c. Thereafter, upon moving into the circumferential slot portions30e-1from the first lead slot portion30e-2of the associated through slot30e, each roller follower32is disengaged from the associated rotation transfer groove26cat the rear opening end thereof to be engaged in the associated relative rotation allowing groove25f. At this time, the rotation of the helicoid ring25and the third external barrel26stops being transferred to the set of three roller followers32, and accordingly, the cam ring31is moved rearward in the optical axis direction without rotating together with the helicoid ring25, the third external barrel26and the first linear guide ring30. Each roller follower32moves in the associated relative rotation allowing groove25f, and the zoom lens10falls into the retracted position thereof upon each roller follower32reaching the closed end (right end as viewed inFIG. 14) of the associated relative rotation allowing groove25f.

The structure of the zoom lens10radially inside of the cam ring31will be discussed hereinafter. As shown inFIG. 6, the first linear guide ring30is provided on an inner peripheral surface thereof with a set of three pairs of first linear guide grooves30fwhich are formed at different circumferential positions to extend parallel to the photographing optical axis Z1, and a set of six second linear guide grooves30gwhich are formed at different circumferential positions to extend parallel to the photographing optical axis Z1. Each pair of first linear guide grooves30fare respectively positioned on the opposite sides of the associated second linear guide groove30g(every second linear guide groove30g) in a circumferential direction of the first linear guide ring30. The zoom lens10is provided inside of the first linear guide ring30with a second linear guide ring (linear guide member)33. The second linear guide ring33is provided on an outer edge thereof with a set of three bifurcated projections33a(seeFIGS. 7 through 10and24) which project radially outwards from a ring portion33bof the second linear guide ring33. Each bifurcated projection33ais provided at a radially outer end thereof with a pair of radial projections which are respectively engaged in the associated pair of first linear guide grooves30f. On the other hand, a set of six radial projections34a(seeFIG. 12) which are formed on an outer peripheral surface of the second external barrel34at a rear end thereof and project radially outwards are engaged in the set of six second linear guide grooves30gto be slidable therealong, respectively. Therefore, each of the second linear guide ring33and the set of six radial projections34aof the second external barrel34is guided in the optical axis direction via the first linear guide ring30. The zoom lens10is provided inside of the cam ring31with a second lens group moving frame (intermediate movable member/single-engaging spring support member)35which indirectly supports and holds the second lens group LG2. The first external barrel37indirectly supports the first lens group LG1, and is positioned inside of the second external barrel34. The zoom lens10is provided radially inside of the cam ring31with a third lens group moving frame (rear movable member/single-engaging spring support member)36. The second linear guide ring33serves as a linear guide member for guiding both the second lens group moving frame35that supports the second lens group LG2and the third lens group moving frame36that supports the third lens group LG3linearly without rotating the second lens group moving frame35and the third lens group moving frame36, while the set of six radial projections34aof the second external barrel34serve as linear guide members for guiding the first external barrel37linearly without rotating.

As shown inFIGS. 7 through 10and24, the second linear guide ring33is provided on the ring portion33bwith a set of three linear guide keys33cwhich project forward parallel to one another from the ring portion33b. As shown inFIGS. 1 through 3, a discontinuous outer edge of the ring portion33bis engaged in a discontinuous circumferential groove31aformed on an inner peripheral surface of the cam ring31at the rear end thereof to be rotatable about the photographing optical axis Z1relative to the cam ring31and to be immovable relative to the cam ring31in the optical axis direction. The set of three linear guide keys33cproject forward from the ring portion33binto the cam ring31. The second lens group moving frame35is provided with a corresponding set of three guide grooves35ain which the set of three linear guide keys33care engaged, respectively (seeFIGS. 25 through 28). As shown inFIG. 22, the second lens group moving frame35is further provided with a ring portion35bhaving its center on the photographing optical axis Z1, and a set of three rearward projections35cwhich project rearward in parallel to one another from the ring portion35bin the optical axis direction. The aforementioned set of three linear guide grooves35aare formed on the set of three rearward projections35c, respectively. The set of three rearward projections35care arranged at substantially equi-angular intervals in a circumferential direction of the second lens group moving frame35. The set of three rearward projections35care engaged in a corresponding set of three linear guide grooves36aformed on an outer peripheral surface of the third lens group moving frame36at different circumferential positions to be slidable thereon along the set of three linear guide grooves36a, respectively (seeFIGS. 8,9and25through28). As shown inFIG. 23, the third lens group moving frame36is provided with a ring portion36bhaving its center on the photographing optical axis Z1, and a set of six forward projections36cwhich project both radially outwards from the ring portion36band forward in parallel to one another from the ring portion36bin the optical axis direction. Each of the aforementioned set of three linear guide grooves36ais formed by a combination of an outer peripheral surface of the ring portion36b(bottom surface of the linear guide groove36a) and side surfaces of associated adjacent two forward projections36con opposite sides of the outer peripheral surface of the ring portion36bin a circumferential direction thereof. Accordingly, side surfaces of each forward projection36c, which are positioned on the opposite sides thereof in a circumferential direction of the third lens group moving frame36to extend in the optical axis direction, and side surfaces of each rearward projection35c, which are positioned on the opposite sides thereof in a circumferential direction of the second lens group moving frame35to extend in the optical axis direction, serve as linear guide surfaces for guiding the second lens group moving frame35and the third lens group moving frame36linearly in the optical axis direction to be linearly movable in the optical axis direction relative to each other. The second lens group moving frame35and the third lens group moving frame36are biased toward each other in the optical axis direction. Due to this structure of engagement between the second lens group moving frame35and the third lens group moving frame36, the second lens group moving frame35is guided linearly in the optical axis direction by the second linear guide ring33, and the third lens group moving frame36is guided linearly in the optical axis direction by the second lens group moving frame35.

As shown inFIGS. 7 through 10and21, the cam ring31is provided on an inner peripheral surface thereof with a set of three front inner cam grooves (driving device/third cam grooves) CG3, and a set of three rear inner cam grooves (driving device/second cam grooves) CG2formed behind the set of three front inner cam grooves CG3. The set of three front inner cam grooves CG3and the set of three rear inner cam grooves CG2determine the moving manner of the second lens group LG2and the moving manner of the third lens group LG3, respectively. The second lens group moving frame35is provided on outer peripheral surfaces of the set of three rearward projections35cwith a set of three rear cam followers (second cam followers) CF2which are engaged in the set of three rear inner cam grooves CG2of the cam ring31, respectively. The third lens group moving frame36is provided on outer peripheral surfaces of three of the six forward projections36cwith a set of three front cam followers (third cam followers) CF3which are engaged in the set of three front inner cam grooves CG3of the cam ring31, respectively. Each of the following four sets of grooves or followers, i.e., the set of three front inner cam grooves CG3, the set of three rear inner cam grooves CG2, the set of three front cam followers CF3and the set of three rear cam followers CF2, are formed at substantially equi-angular intervals in a circumferential direction about the photographing optical axis Z1. Since each of the second lens group moving frame35and the third lens group moving frame36is guided linearly in the optical axis direction directly or indirectly by the second linear guide ring33, a rotation of the cam ring31causes the second lens group moving frame35and the third lens group moving frame36to move in the optical axis direction in a predetermined moving manner in accordance with contours of the set of three rear inner cam grooves CG2and the front inner cam grooves CG3. This cam mechanism will be discussed in detail later.

The zoom lens10is provided with a second lens frame40which supports the second lens group LG2. The second lens frame40is supported by the ring portion35bof the second lens group moving frame35(seeFIG. 11). The second lens frame40is fixed to the ring portion35bof the second lens group moving frame35by the engagement of amale screw thread (adjusting screw) formed on an outer peripheral surface of the second lens frame40with a female screw thread (adjusting screw) formed on an inner peripheral surface of the second lens group moving frame40. The male screw thread of the second lens frame40and the female screw thread of the second lens group moving frame35are formed with respective centers thereof on the photographing optical axis Z1. Accordingly, the position of the second lens frame40relative to the second lens group moving frame35in the optical axis direction can be adjusted by rotating the second lens frame40relative to the second lens group moving frame35.

The zoom lens10is provided between the second and third lens groups LG2and LG3with a shutter unit41including the shutter S. The shutter unit41is positioned radially inside of the third lens group moving frame36to be supported thereby. An actuator for driving the shutter S is incorporated in the shutter unit41.

The zoom lens10is provided inside of the third lens group moving frame36with a third lens frame (radially-retractable lens frame)42which supports and holds the third lens group LG3to be positioned behind the shutter unit41. The third lens frame42is pivoted about a pivot shaft44which is fixed to the third lens group moving frame36to project forward. The pivot shaft44is positioned a predetermined distance away from the photographing optical axis Z1, and extends parallel to the photographing optical axis Z1. The third lens frame42is swingable about the pivot shaft44between a photographing position shown inFIGS. 1,2,30and32where the optical axis of the third lens group LG3coincides with the photographing optical axis Z1and a radially-retracted position shown inFIGS. 3,31and33where the optical axis of the third lens group LG3is positioned at a radially retracted optical axis Z2(FIGS. 3 and 33). A rotation limit pin (stop pin)46, which prevents the third lens frame42from rotating clockwise as viewed inFIG. 32beyond a predetermined point to determine the photographing position of the third lens frame42, is fixed to the third lens group moving frame36. The third lens frame42is biased to rotate in a direction (clockwise as viewed inFIG. 32) to come into contact with the rotation limit pin46by a torsion coil spring47. A compression coil spring48is fitted on the pivot shaft44to bias the third lens frame42rearward in the optical axis direction to remove backlash between the third lens frame42and the third lens group moving frame36.

The third lens frame42moves together with the third lens group moving frame36in the optical axis direction. As shown inFIGS. 5 and 29, the CCD holder14is provided on a front surface thereof with a position-control cam bar (retracting member/stationary cam bar)49which projects forward from the CCD holder14to be engageable with the third lens frame42. If the third lens group moving frame36moves rearward in a retracting direction to approach the CCD holder14, a retracting cam surface49a(seeFIG. 29) formed on a front end surface of the position-control cam bar49comes into contact with a specific portion of the third lens frame42to rotate the third lens frame42to the radially-retracted position. The position-control cam bar49is further provided along an inner side edge thereof with a radially-retracted-position holding surface49bwhich extends rearward from the retracting cam surface49ain a direction parallel to the photographing optical axis Z1.

As shown inFIG. 12, the second external barrel34is provided on an inner peripheral surface thereof with a set of three linear guide grooves34bwhich are formed at different circumferential positions to extend parallel the photographing optical axis Z1. The first external barrel37is provided on an outer peripheral surface at the rear end thereof with a set of three engaging protrusions37awhich are slidably engaged in the set of three linear guide grooves34b, respectively. Accordingly, the first external barrel37is guided linearly in the optical axis direction without rotating via the first linear guide ring30and the second external barrel34. The second external barrel34is further provided on an inner peripheral surface thereof in the vicinity of the rear end of the second external barrel34with a discontinuous inner flange34cwhich extends along a circumference of the second external barrel34. The cam ring31is provided on an outer peripheral surface thereof with a discontinuous circumferential groove31bin which the discontinuous inner flange34cis slidably engaged so that the cam ring31is rotatable about the photographing optical axis Z1relative to the second external barrel34and so that the second external barrel34is immovable in the optical axis direction relative to the cam ring31(i.e., the second external barrel34moves together with the cam ring31in the optical axis direction). On the other hand, the first external barrel37is provided on an inner peripheral surface thereof with a set of three cam followers (first cam followers) CF1which project radially inwards, and the cam ring31is provided on an outer peripheral surface thereof with a set of three outer cam grooves (driving device/first cam grooves) CG1in which the set of three cam followers CF1are slidably engaged, respectively.

The zoom lens10is provided inside of the first external barrel37with a first lens frame51which is supported by the first external barrel37via a first lens group adjustment ring50. The first lens group LG1is supported by the first lens frame51to be fixed thereto. The first lens frame51is provided on an outer peripheral surface thereof with a partial male screw thread51a, and the first lens group adjustment ring50is provided on an inner peripheral surface thereof with a partial female screw thread50awhich is engaged with the male screw thread51a(seeFIG. 12). The position of the first lens frame51relative to the first lens group adjustment ring50in the optical axis direction can be adjusted during assembly of the zoom lens10via the partial male screw thread51aand the partial female screw thread50a.

The zoom lens10is provided at the front end of the first external barrel37with a lens barrier mechanism54(seeFIG. 4) which automatically closes a front end aperture of the zoom lens10when the zoom lens10is retracted as shown inFIG. 3to protect the frontmost lens element of the photographing optical system of the zoom lens10, i.e. the first lens group LG1, from getting stains and scratches thereon when the digital camera is not in use. The lens barrier mechanism54is provided with a plurality of barrier blades (a front pair of barrier blades and a rear pair of barrier blades)54a. The lens barrier mechanism54operates so that the plurality of barrier blades54aare fully shut in front of the first lens group LG1in the retracted state of the zoom lens10shown inFIG. 3, and are fully opened in a ready-to-photograph state of the zoom lens10shown inFIGS. 1 and 2.

A lens barrel advancing operation and a lens barrel retracting operation of the zoom lens10having the above described structure will be discussed hereinafter. In the state shown inFIG. 3, in which the zoom lens10is in the retracted state, rotating the zoom gear22in the lens barrel advancing direction by the zoom motor23causes a combination of the helicoid ring25and the third external barrel26to move forward while rotating due to the engagement of the female helicoid13awith the male helicoid25a, and further causes the first linear guide ring30to move forward linearly together with the helicoid ring25and the third external barrel26. At this time, firstly the cam ring31does not rotate but only linearly moves forward together with the helicoid ring25, the third external barrel26and the first linear guide ring30, and subsequently torque is transferred to the cam ring31from the third external barrel26to move forward while rotating relative to the first linear guide ring30by the engagement of the set of roller followers32with the first lead slot portions30e-2of the set of through slots30eafter having been rotated by the aforementioned rotation of the combination of the helicoid ring25and the third external barrel26by an angle of approximately 30 degrees. Immediately after the helicoid ring25and the third external barrel26are extended forward to respective predetermined positions thereof, the male helicoid25aof the helicoid ring25and the female helicoid13aof the stationary barrel13are disengaged from each other, so that the helicoid ring25and the third external barrel26rotate about the photographing optical axis Z1without moving in the optical axis direction due to the slidable engagement of the set of three rotational guide projections25bwith the set of three rotational guide grooves13d. After a lapse of a predetermined period of time from the moment at which the helicoid ring25and the third external barrel26stop moving forward in the optical axis direction (i.e., the moment at which the set of three rotational guide projections25bslide into the set of three rotational guide grooves13dfrom the set of three inclined grooves13c, respectively), the set of three roller followers32enter the second lead slot portion30e-3from the first lead slot portions30e-2of the set of three through slots30e, respectively, so that the cam ring31is further moved forward while rotating relative to the first linear guide ring30.

A rotation of the cam ring31causes each of the second lens group moving frame35and the third lens group moving frame36, which are positioned inside of the cam ring31and guided linearly in the optical axis direction without rotating directly or indirectly by the second linear guide ring33, to move in the optical axis direction with respect to the cam ring31in a predetermined moving manner due to the engagement of the set of front cam followers CF3with the set of front inner cam grooves CG3and the engagement of the set of rear cam followers CF2with the set of rear inner cam grooves CG2, respectively. In the state shown inFIG. 3in which the zoom lens10is in the retracted state, the third lens frame42, which is provided in the third lens group moving frame36, has rotated about the pivot shaft44to be held in the radially-retracted position above the photographing optical axis Z1by the position-control cam bar49, so that the optical axis of the third lens group LG3is moved from the photographing optical axis Z1to the retracted optical axis Z2that is positioned above the photographing optical axis Z1. In the course of movement of the third lens group moving frame36from the retracted position to a position in the zooming range as shown inFIGS. 1,2,30and32, the third lens frame42is disengaged from the position-control cam bar49to rotate about the pivot shaft44from the radially-retracted position to the photographing position shown inFIGS. 1,2,30and32, where the optical axis of the third lens group LG3coincides with the photographing optical axis Z1by the sprig force of the torsion coil spring47. Thereafter, the third lens frame42remains held in the photographing position until the zoom lens10is retracted to the position shown inFIG. 3.

Additionally, the rotation of the cam ring31causes the first external barrel37, which is positioned around the cam ring31and guided linearly in the optical axis direction without rotating, to move in the optical axis direction relative to the cam ring31in a predetermined moving manner due to engagement of the set of three cam followers CF1with the set of three outer cam grooves CG1, respectively.

Therefore, an axial position of the first lens group LG1relative to the picture plane (a light-sensitive surface of the CCD image sensor12) when the first lens group LG1is moved forward from the retracted position is determined by the sum of the amount of forward movement of the cam ring31relative to the stationary barrel13and the amount of movement of the first external barrel37relative to the cam ring31, an axial position of the second lens group LG2relative to the picture plane when the second lens group LG2is moved forward from the retracted position is determined by the sum of the amount of forward movement of the cam ring31relative to the stationary barrel13and the amount of movement of the second lens group moving frame35relative to the cam ring31, and an axial position of the third lens group LG3relative to the picture plane when the third lens group LG3is moved forward from the retracted position is determined by the sum of the amount of forward movement of the cam ring31relative to the stationary barrel13and the amount of movement of the third lens group moving frame36relative to the cam ring31. A zooming operation is carried out by moving the first, second and third lens groups LG1, LG2and LG3on the photographing optical axis Z1while changing the distances therebetween. When the zoom lens10is driven to advance from the retracted position shown inFIG. 3, the zoom lens10firstly moves forward to the position shown inFIG. 1, in which the zoom lens10is set at wide-angle extremity. Subsequently, the zoom lens10moves forward to the position shown inFIG. 2, in which the zoom lens10is set at the telephoto extremity by a further rotation of the zoom motor23in a lens barrel advancing direction thereof. As can be seen from these sectional views of the zoom lens10shown inFIGS. 1 and 2, the distance between the first and second lens groups LG1and LG2is minimum and the distance between the second and third lens groups LG2and LG3is great when the zoom lens10is set at the wide-angle extremity. When the zoom lens10is set at the telephoto extremity, the distance between the first and second lens groups LG1and LG2is great and the distance between the second and third lens groups LG2and LG3is small. This variation of the distances among the first, second and third lens groups LG1, LG2and LG3for zooming operation is achieved by contours of the set of three outer cam grooves CG1, the set of three rear inner cam grooves CG2and the set of three front inner cam grooves CG3. In the zooming range between the wide-angle extremity and the telephoto extremity, the helicoid ring25and the third external barrel26rotate without moving in the optical axis direction. On the other hand, in the same zooming range, the cam ring31moves forward and rearward in the optical axis direction while rotating due to the engagement of the set of three roller followers32with the second lead slot portions30e-3of the set of three through slots30eof the first linear guide ring30.

When the first through third lens groups LG1, LG2and LG3are in the zooming range, a focusing operation is carried out by moving the AF lens frame17, which holds the fourth lens group LG4, along the photographing optical axis Z1by rotation of the AF motor19in accordance with an object distance.

Driving the zoom motor23in a lens barrel retracting direction causes the zoom lens10to operate in the reverse manner to the above described advancing operation to retract the zoom lens10as shown inFIG. 3. In the course of this retracting movement of the zoom lens10, the third lens frame42rotates about the pivot shaft44to the radially-retracted position by the position-control cam bar49while moving rearward together with the third lens group moving frame36. When the zoom lens10is retracted to the retracted position shown inFIG. 3, the third lens group LG3is retracted into the space radially outside of the space in which the fourth lens group LG4, the low-pass filter11and the CCD image sensor12are retracted as shown inFIG. 3, i.e., the third lens group LG3is radially retracted into an axial range substantially identical to an axial range in the optical axis direction in which the fourth lens group LG4, the low-pass filter11and the CCD image sensor12are positioned. This structure of the zoom lens10for retracting the third lens group LG3in this manner reduces the length of the zoom lens10when the zoom lens10is fully retracted, thus making it possible to reduce the thickness of the camera body in the horizontal direction as viewed inFIG. 3, i.e., in the optical axis direction.

The cam mechanism, incorporated in the zoom lens10, for moving the second lens group LG2and the third lens group LG3in the optical axis direction in a predetermined moving manner will be discussed in detail.FIGS. 21 and 25through28show the shapes (contours) of the set of three rear inner cam grooves CG2and the set of three front inner cam grooves CG3, which are elements of the cam mechanism. AlthoughFIG. 21is a developed view of the outer peripheral surface of the cam ring31, the set of three rear inner cam grooves CG2and the set of three front inner cam grooves CG3that are formed on the inner peripheral surface of the cam ring31are shown by solid lines inFIG. 21for the purpose of making the shapes of each cam groove easier to be seen. Additionally, although each ofFIGS. 25 through 28is a developed view of the outer peripheral surface of the cam ring31, the second lens group moving frame35and the third lens group moving frame36are shown by solid lines inFIGS. 25 through 28even though positioned radially inside of the cam ring31. As can be seen inFIGS. 25 through 28, the set of three front inner cam grooves CG3is positioned in front of the set of three rear inner cam grooves CG2in the optical axis direction on the inner peripheral surface of the cam ring31. Accordingly, the positional relationship in the optical axis direction between the second lens group LG2and the third lens group LG3is reverse to the positional relationship in the optical axis direction between the set of three rear inner cam grooves CG2, which is configured to move the second lens group LG2, and the set of three front inner cam grooves CG3, which is configured to move the third lens group LG3.

As shown inFIG. 21, each rear inner cam groove CG2is provided at one end (rear end) thereof with an accommodation section CG2-1which is wider than the remaining part of the rear inner cam groove CG2, and which is open on a rear end surface of the cam ring31. Each rear inner cam groove CG2is further provided with an inclined lead groove section CG2-2which extends linearly obliquely from the accommodation section CG2-1toward the front of the cam ring31, and a return cam groove section CG2-3which extends from the left end (left end as viewed inFIG. 21) of the inclined lead groove section CG2-2. Additionally, each rear inner cam groove CG2is provided with a lens-barrel assembling section CG2-4which projects forward from a front edge of the accommodation section CG2-1in the optical axis direction. On the other hand, each front inner cam groove CG3is provided with an accommodation section CG3-1which extends in a circumferential direction of the cam ring31, an inclined lead groove section CG3-2which extends linearly obliquely from the accommodation section CG3-1toward the rear of the cam ring31, and a return cam groove section CG3-3which extends from the left end (left end as viewed inFIG. 21) of the inclined lead groove section CG3-2. The cam ring31is provided on the front edge of the cam ring31with a set of three cutout portions31veach having the shape of a semicircle in cross section. The set of three cutout portions31vare formed to be communicatively connected to one end (right ends as viewed inFIG. 21) of each of the accommodation sections CG3-1of the set of three front inner cam grooves CG3, respectively, so that these one ends of the accommodation sections CG3-1are open on the front edge of the cam ring31. The set of three front cam followers CF3are inserted into the set of three front inner cam grooves CG3through the set of three cutout portions31v, respectively.

In each rear inner cam groove CG2, a specific portion thereof which is defined by the range between a wide-angle extremity position (W) and a telephoto extremity position (T) inFIG. 21is used as a photographing range for performing a zooming operation. Likewise, in each front inner cam groove CG3, a specific portion thereof which is defined by the range between a wide-angle extremity position (W) and telephoto extremity position (T) inFIG. 21is used as a photographing range for performing a zooming operation. When each rear cam follower CF2and each front cam follower CF3are positioned in the associated rear inner cam groove CG2at the wide-angle extremity position (W) thereof and the associated front inner cam groove CG3at the wide-angle extremity position (W) thereof as shown inFIG. 26, respectively, the zoom lens10is at the wide-angle extremity. When each rear cam follower CF2and each front cam follower CF3are positioned in the associated rear inner cam groove CG2at the telephoto extremity position (T) thereof and the associated front inner cam groove CG3at the telephoto extremity position (T) thereof as shown inFIG. 27, respectively, the zoom lens10is at the telephoto extremity. The distance between the second lens group LG2and the third lens group LG3is great and the amount of engagement (the amount of overlap) between the set of three rearward projections35cof the second lens group moving frame35and the set of three linear guide grooves36aof the third lens group moving frame36in the optical axis direction is small when the zoom lens10is at the wide-angle extremity (seeFIGS. 8 and 26). On the other hand, the distance between the second lens group LG2and the third lens group LG3is small and the amount of engagement (the amount of overlap) between the set of three rearward projections35cof the second lens group moving frame35and the set of three linear guide grooves36aof the third lens group moving frame36in the optical axis direction is great when the zoom lens10is at the telephoto extremity (seeFIGS. 9 and 27). In addition, when the zoom lens10is in the retracted state as shown inFIG. 3, each rear cam follower CF2and each front cam follower CF3are positioned in the accommodation section CG2-1of the associated rear inner cam groove CG2and the accommodation section CG3-1of the associated front inner cam groove CG3, respectively (seeFIG. 25).

The inclined lead groove section CG2-2of each rear inner cam groove CG2is inclined to the circumferential direction of the cam ring31to approach the front of the cam ring31in a direction away from the associated accommodation section CG2-1, while the return cam groove section CG2-3of each rear inner cam groove CG2is inclined to the circumferential direction of the cam ring31to approach the rear of the cam ring31in a direction away from the associated inclined lead groove section CG2-2. Conversely, the inclined lead groove section CG3-2of each front inner cam groove CG3is inclined to the circumferential direction of the cam ring31to approach the rear of the cam ring31in a direction away from the associated accommodation section CG3-1, while the return cam groove section CG3-3of each front inner cam groove CG3is inclined to the circumferential direction of the cam ring31to approach the front of the cam ring31in a direction away from the associated inclined lead groove section CG3-2. In other words, with respect to the development view of the cam ring31shown inFIG. 21, each rear inner cam groove CG2is formed in a substantially inverted V shape having the apex thereof at the substantially center of the rear inner cam groove CG2, while each front inner cam groove CG3is formed in a substantially V shape having the bottom thereof at the substantially center of the front inner cam groove CG3. The set of three rear inner cam grooves CG2are arranged circumferentially at predetermined intervals while the set of three front inner cam groove CG3are arranged circumferentially at predetermined intervals in an interengaged manner in the optical axis direction, with the position of the set of three rear inner cam grooves CG2and the position of the set of three front inner cam grooves CG3deviating from each other in the circumferential direction of the cam ring31(specifically, the position of the set of three rear cam followers CF2and the position of the set of three front cam followers CF3deviate from each other in the circumferential direction of the cam ring31). This pattern of the cam grooves CG2and CG3on the cam ring31makes it possible to reduce the space between the set of three rear inner cam grooves CG2and the set of three front inner cam grooves CG3in the optical axis direction, thus making it possible to reduce the length of the cam ring31in the optical axis direction. In the present embodiment of the cam mechanism, the set of three front inner cam grooves CG3are arranged at equi-angular intervals in the circumferential direction of the cam ring31while the set of three rear cam followers CF2are arranged at equi-angular intervals in the circumferential direction of the cam ring31so that the portion of each rear inner cam groove CG2at the wide-angle extremity position (W) substantially corresponds to the accommodation section CG3-1of the associated front inner cam groove CG3in the circumferential direction of the cam ring31. This positional relationship between the set of three rear inner cam grooves CG2and the set of three front inner cam grooves CG3makes it possible for the set of three rear inner cam grooves CG2and the set of three front inner cam grooves CG3to be formed closely to each other in the optical axis direction without intersecting each other. Consequently, the length of the cam ring31in the optical axis direction is much smaller than the sum of a width W1(seeFIG. 21) of the set of three rear inner cam grooves CG2and a width W2(seeFIG. 21) of the set of three front inner cam grooves CG3in the optical axis direction.

Although each rear inner cam groove CG2and each front inner cam groove CG3have cam diagrams having the above described inverted V shape and the above described V shape, respectively, neither the inverted V-shaped cam diagram nor the V-shaped cam diagram has a symmetrical shape. Therefore, in the case where the set of three rear inner cam grooves CG2and the set of three front inner cam grooves CG3are formed on the cam ring31so that the set of three rear inner cam grooves CG2are positioned in front of the set of three front inner cam grooves CG3in the optical axis direction in the reverse fashion to the above described embodiment of the cam mechanism, such a front set of three cam grooves and such a rear set of three cam grooves would intersect each other on the same peripheral area of the cam ring31(within the same range in the optical axis direction) even if the position of the set of three rear inner cam grooves and the position of the set of three front inner cam grooves are adjusted in the circumferential direction of the cam ring31. As described above, a single cam ring provided on an inner or outer peripheral surface thereof with two sets of cam grooves for moving two linearly guided optical elements needs to prevent the associated two sets of cam followers that are respectively engaged in the two sets of cam grooves from being disengaged therefrom at points of intersection of the two sets of cam grooves; however, taking such measures is liable to complicate the cam mechanism. For instance, one known solution to such a problem is to provide the cam ring with an auxiliary cam grooves having the same cam diagraphs as the main cam grooves to prevent the two sets of cam followers from being disengaged from the associated two sets of cam grooves. However, to form such auxiliary cam grooves in addition to the main cam grooves, it is necessary to secure an additional area on the cam ring in which the auxiliary cam grooves are formed, and accordingly, the cam ring increases in size. Conversely, in the present invention, the positions between the set of three rear inner cam grooves CG2and the set of three front inner cam grooves CG3in the optical axis direction have been changed without regard for the positions of lens groups in the optical axis direction, which are guided linearly in the optical axis direction (the second lens group LG2and the third lens group LG3in the above described embodiment of the zoom lens), which makes it possible for the set of three rear inner cam grooves CG2and the set of three front inner cam grooves CG3to be formed on the inner peripheral surface of the cam ring31while being prevented from intersecting each other with no increase in size of the cam ring31.

Since each rear cam follower CF2for moving the second lens group LG2and each front cam follower CF3for moving the third lens group LG3are engaged in the associated rear inner cam groove CG2and the associated front inner cam groove CG3that are positioned in the reverse fashion as described above, there have been various design ideas applied to this arrangement and the assembling structure of the set of three rear cam followers CF2and the set of three front cam followers CF3. Specifically, the set of three rear cam followers CF2are formed on outer peripheral surfaces of the set of three rearward projections35cthat project rearward from the ring portion35bof the second lens group moving frame35, and the set of three front cam followers CF3are formed on outer peripheral surfaces of three of the six forward projections36cthat project forward from the ring portion36cof the third lens group moving frame36. Moreover, the set of three rear cam followers CF2are formed integral with the set of three rearward projections35c. Furthermore, the set of three front cam followers CF3are elements separate from the third lens group moving frame36, and the third lens group moving frame36is provided, on the associated three of the six forward projections36cin the vicinity of front end thereof, with three follower fixing holes36v(seeFIG. 11) in which the set of three front cam followers CF3are inserted to be fixed thereto.

As for the second lens group moving frame35and the third lens group moving frame36, the positions between the ring portion35b, which supports the second lens group LG2, and the ring portion36b, which supports the third lens group LG3, in the optical axis direction cannot be changed, whereas the positions between the set of three rear cam followers CF2and the set of three front cam followers CF3in the optical axis direction, which are respectively provided on the set of three rearward projections35cand the specific three of the six forward projections36cthat are slidably movable relative to each other in the optical axis direction, can be freely changed without interfering with each other. Therefore, the set of three rear cam followers CF2and the set of three front cam followers CF3can be engaged in the set of three rear inner cam grooves CG2and the set of three front inner cam grooves CG3(the positions therebetween in the optical axis direction have been changed), respectively, without interference. The positional relationship between the positions between the set of three rear cam followers CF2and the set of three front cam followers CF3in the optical axis direction is determined by the cam diagrams of the set of three rear inner cam grooves CG2and the set of three front inner cam grooves CG3, and the set of three front cam followers CF3are not always positioned in front of the set of three rear cam followers CF2in the cam ring31. For instance, the set of three front cam followers CF3are positioned in front of the set of three rear cam followers CF2in the optical axis direction to correspond to the positional relationship between the set of three rear inner cam grooves CG2and the set of three front inner cam grooves CG3in the state shown inFIG. 25in which the zoom lens10is in the retracted state and also in the state shown inFIG. 27in which the zoom lens10is at the telephoto extremity, whereas the set of three rear cam followers CF2are positioned in front of the set of three front cam followers CF3in the optical axis direction in the state shown inFIG. 26in which the zoom lens10is at the wide-angle extremity.

A manner of installing the second lens group moving frame35and the third lens group moving frame36to the cam ring31during assembly of the zoom lens10will be discussed hereinafter. Firstly, the second lens frame40is fixed to the second lens group moving frame35, and the shutter block41and the third lens frame42are fixed to the third lens group moving frame36. Thereafter, the set of three rearward projections35cand the set of six forward projections36care engaged with each other to unitize the second lens group moving frame35and the third lens group moving frame36(seeFIG. 7). In this unitized state, the second lens group moving frame35and the third lens group moving frame36are guided linearly in the optical axis direction relative to each other due to the slidable engagement of the set of three rearward projections35cwith the set of six forward projections36c. Subsequently, this unit of the second lens group moving frame35and the third lens group moving frame36is inserted into the cam ring31from the rear end thereof. At this stage, the set of three front cam followers CF3are not yet installed to the third lens group moving frame36. The set of three rear cam followers CF2are inserted into the set of three rear inner cam grooves CG2, respectively, through the respective openings thereof that are formed on a rear end surface of the cam ring31. Subsequently, the second lens group moving frame35and the third lens group moving frame36are pushed forward as an integral unit into the cam ring31so that the set of three rear cam followers CF2enter the lens-barrel assembling sections CG2-4of the set of three rear inner cam grooves CG2, respectively. Thereupon, the three follower fixing holes36v, which are formed on three of the six forward projections36c, are exposed radially outwards from the cam ring31through the set of three cutout portions31vof the cam ring31, respectively, as shown inFIG. 28. At this stage, the set of three front cam followers CF3are installed into the three follower fixing holes36vthrough the set of three cutout portions31v, respectively. Subsequently, slightly rotating the cam ring31in a lens barrel advancing direction (rightward as viewed inFIG. 28) causes the second lens group moving frame35and the third lens group moving frame36to integrally move slightly rearward due to the relationship between the set of three rear cam followers CF2and the second lens group LG2(an inclined surface which connects the lens-barrel assembling section CG2-4to the accommodation section CG2-1in each rear inner cam groove CG2), and simultaneously causes the set of three front cam followers CF3to move into the openings of the set of three front inner cam grooves CG3from the set of three cutout portions31v, respectively. Thereafter, upon the cam ring31being rotated to the position (retracted position) shown inFIG. 25, the set of three rear cam followers CF2have moved away from the openings of the set of three rear inner cam grooves CG2and reached the accommodation sections CG2-1while the set of three front cam followers CF3have moved away from the openings of the set of three front inner cam grooves CG3and reached the accommodation sections CG3-1, so that the second lens group moving frame35and the third lens group moving frame36are supported by the cam ring31therein.

The second lens group moving frame35and the third lens group moving frame36, which are supported by the cam ring31in the above described manner, are biased forward in the optical axis direction by a set of three extension coil springs (first spring members)38(seeFIGS. 4,12and35) and a set of two extension coil springs (second spring members)39(seeFIGS. 4,11and35), respectively. The set of three extension coil springs38are arranged at different circumferential positions in a circumferential direction about the optical axis Z1, and one end (front ends) of each of the three extension coil springs38are hooked over three hooks37p(seeFIGS. 12 and 34) formed on the first external barrel37, and the other ends (rear ends) of the three extension coil springs38are hooked over three hooks35p(seeFIGS. 7 through 11and22) formed on the second lens group moving frame35. The three hooks35pare formed on an outer peripheral surface of the ring portion35bof the second lens group moving frame35at substantially equi-angular intervals at positions different from the positions of the three rearward projections35cin a circumferential direction of the second lens group moving frame35. The second lens group moving frame35is provided, on an outer peripheral surface of the ring portion35bin front of the three hooks35pin the optical axis direction, with three spring retaining portions35rfor retaining the three extension coil springs38to prevent the three extension coil springs38from tilting during assembly, respectively. The set of two extension coil springs39are arranged at substantially radially opposite sides of the optical axis Z1, and one end (front ends) of each of the two extension coil springs39are hooked over two hooks37q(seeFIGS. 12 and 34) formed on the first external barrel37, and the other end (rear ends) of each of the two extension coil springs39are hooked over two hooks36q(seeFIGS. 7 through 11and23) formed on the third lens group moving frame36. The two hooks36qare formed on the ring portion36bof the third lens group moving frame36. The third lens group moving frame36is provided, in front of the two hooks36qin the optical axis direction, with two spring retaining portions36rfor retaining the two extension coil springs39. The two spring retaining portions36rare formed by cutting out portions of two of the six forward projections36c(seeFIG. 23). Each of the two spring retaining portions36rcan accommodate a part of the associated extension coil spring39. As shown inFIG. 34, the first external barrel37is provided on an inner peripheral surface thereof with an annular flange37s. The annular flange37sis provided thereon between the radially inner edge of the annular flange37sand the inner peripheral surface of the first external barrel37with a plurality of through holes which extend through the annular flange37sin the optical axis direction. The three hooks37pand the two spring hooks37qproject radially outwards from the annular flange37sinto these plurality of through holes.

As can be understood from the above description, in the relationship among the first external barrel37, the second lens group moving frame35and the third lens group moving frame36, all of which are driven by a common driving member, i.e., the cam ring31, the first external barrel37, which supports the first lens group LG1that is positioned at the front end of the photographing optical system in the optical axis direction, serves as a common-engaging spring support member (spring-load receiving member) which is spring-engaged in common with the second lens group moving frame35and the third lens group moving frame36, which are positioned behind the first external barrel37in the optical axis direction, via the set of three extension coil springs38and the set of two extension coil springs39. The set of three extension coil springs38and the set of two extension coil springs39bias the second lens group moving frame35and the third lens group moving frame36forward in the optical axis direction relative to the first external barrel37, respectively, and simultaneously bias the first external barrel37rearward in the optical axis direction relative to the second lens group moving frame35and the third lens group moving frame36to remove backlash in each of the first external barrel37(the first lens group LG1), the second lens group moving frame35and the third lens group moving frame36relative to the cam ring31that serves as the aforementioned common-engaging spring support member (i.e., to remove backlash between the cam grooves CG1, CG2and CG3and the cam followers CF1, CF2and CF3, respectively).FIG. 35schematically shows this spring-biasing relationship. Although the first external barrel37is positioned radially outside of the cam ring31while the set of three cam followers CF1and the set of three outer cam grooves CG1are formed on the outer peripheral surface of the cam ring31in the illustrated embodiment of the zoom lens10, the first external barrel37is positioned radially inside of the cam ring31inFIG. 35while all the cam grooves CG1, CG2and CG3are formed on the inner peripheral surface of the cam ring31inFIG. 35for the purpose of making the concept of the present invention easier to understand.

In a telescopic of zoom lens such as the present embodiment of the zoom lens10, it is often the case that the frontmost lens group, which is positioned at the forward end among all the lens groups of a photographing optical system, is the largest in diameter among all the lens groups. The diameter of the first lens group LG1is larger than the diameters of the second, third and fourth lens groups LG2, LG3and LG4in the present embodiment of the zoom lens10. Therefore, the first external barrel37that supports the first lens group LG1is heavier in weight than other movable barrels or frames such as the second lens group moving frame35. In addition, since the first external barrel37is a movable member which forms a part of the exterior of the zoom lens10, it is required to enhance the sustainability of the first external barrel37so that the first external barrel37can return the proper (normal) position thereof when undergoing displacement by external force. On this account, the first external barrel37needs to be biased rearward by a strong spring biasing force to improve the stability of the first external barrel37. In the case of the placement of springs as shown inFIG. 35, the sum of the spring force of the set of three extension coil springs38and the spring force of the set of two extension coil springs39is exerted on the first external barrel37that supports the first lens group LG1, which makes it possible to improve the stability of the first external barrel37(the sustainability of the first external barrel37due to the spring force). More specifically, in the case of regarding the first external barrel37as a reference, both the extension coil springs38and39apply a biasing force on the first external barrel37to bias the first external barrel37rearward in the optical axis direction, the sum of the spring force of the set of three extension coil springs38and the spring force of the set of two extension coil springs39is exerted on the first external barrel37efficiently without the spring force of the set of three extension coil springs38and the spring force of the set of two extension coil springs39being cancelled out.

In the spring-biasing structure of the above illustrated embodiment of the zoom lens, the set of two extension coil springs39are installed to be stretched between the first external barrel37and the third lens group moving frame36which are positioned away from each other in the optical axis direction by a distance greatest among any two of the following three movable members: the first external barrel37, the second lens group moving frame35and the third lens group moving frame36that are driven by the common cam ring31. This arrangement in which a front movable member (the first external barrel37) and a rear movable member (the third lens group moving frame36) which are positioned on opposite sides of an intermediate movable member (the second lens group moving frame35) in the optical axis direction makes it possible to adopt long extension springs such as the two extension coil springs39. A long extension spring such as each extension coil spring39can provide a sufficient spring biasing force more naturally than a short extension spring, and is advantageous to the spring design, i.e., makes it possible to achieve a reduction in spring constant. In addition, at the time of installation of the set of three extension coil springs38and the set of two extension coil springs39, firstly one end (rear ends) of each of the three extension coil springs38is hooked over the second lens group moving frame35and one end (rear ends) of each of the two extension coil springs39is hooked over the third lens group moving frame36, respectively, and subsequently the other ends (front ends) of all the five extension coil springs38and39are hooked over the first external barrel37, to thereby complete the installation of the set of three extension coil springs38and the set of two extension coil springs39. These factors contribute facilitate the assembly of the zoom lens10(ease of installation of the set of three extension coil springs38and the set of two extension coil springs39in the zoom lens10).

Another feature in the relationship between the spring forces of the set of three extension coil springs38and the spring force of the set of two extension coil springs39will be discussed thereinafter. As can be understood by comparingFIG. 1withFIG. 2, the photographing optical system of the zoom lens10is designed so that the second lens group LG2is positioned close to the first lens group LG1while the third lens group LG3is positioned away from the first lens group LG1at the wide-angle extremity of the zoom lens10as shown inFIG. 1, whereas the second lens group LG2is positioned away from the first lens group LG1while the third lens group LG3is positioned closer to the first lens group LG1than that shown inFIG. 1at the telephoto extremity of the zoom lens10as shown inFIG. 2. Namely, regarding the first external barrel37(which serves as a common-engaging spring support member to which both one end of each extension coil spring38and one end of each extension coil spring39are connected) as a reference, the direction of movement of the second lens group LG2is opposite to the direction of movement of the third lens group LG3in the zooming range between the wide-angle extremity and the telephoto extremity (see the contours of the cam grooves CG2and G3shown inFIG. 21). As a result, when the zoom lens10is in a ready-to-photograph state, the spring biasing force of the set of three extension coil springs38(the length of each extension coil spring38) becomes minimum and maximum at the wide-angle extremity and the telephoto extremity of the zoom lens10, respectively, whereas the spring biasing force of the set of two extension coil springs39(the length of each extension coil spring39) becomes maximum and minimum at the wide-angle extremity and the telephoto extremity of the zoom lens10, respectively. Accordingly, the relationship of intensity of the spring biasing force between the set of three extension coil springs38and the set of two extension coil springs39is reversed between the wide-angle extremity and the telephoto extremity of the zoom lens10, and the spring biasing force of one of the set of three extension coil springs38and the set of two extension coil springs39decreases as the other of the set of three extension coil springs38and the set of two extension coil springs39increases. As can be seen from the contours of the set of three rear inner cam grooves CG2and the set of three front inner cam grooves CG3inFIG. 21, even in a middle range of the zooming range between the wide-angle extremity and the telephoto extremity, the direction of movement of the second lens group moving frame35(the second lens group LG2) and the direction of movement of the third lens group moving frame36(the third lens group LG3) in the optical axis direction are substantially opposite to each other so that the spring biasing force of one of the set of three extension coil springs38and the set of two extension coil springs39decreases as the other of the set of three extension coil springs38and the set of two extension coil springs39increases. Due to this manner of variation in spring biasing force, wherein the rate of increase (rate of change) in spring biasing force of the set of three extension coil springs38and the rate of increase (rate of change) in spring biasing force of the set of two extension coil springs39change between a positive increase rate and a negative increase rate and between a negative increase rate and a positive increase rate, respectively, in accordance with the zooming operation of the zoom lens10, the loads of the springs38and39on the first external barrel37are averaged in the entire photographing range (entire zooming range) of the zoom lens10.

For instance, if the rate of increase in spring biasing force of the set of three extension coil springs38and the rate of increase in spring biasing force of the set of two extension coil springs39change in the same positive/negative direction in accordance with the zooming operation of the zoom lens10in a different manner from the above described manner of the illustrated embodiment of the zoom lens10, the difference between the maximum load and the minimum load of the springs38and39on the first external barrel37becomes large. Since the first external barrel37, which serves as an exterior member of the zoom lens10and supports the first lens group LG1having a large diameter, needs to be biased by a strong spring biasing force as mentioned above, the maximum load of the springs38and39on the first external barrel37will become excessively great if the minimum load of the springs38and39on the first external barrel37is taken as a reference load on the first external barrel37in the case where the difference between the maximum load and the minimum load of the springs38and39on the first external barrel37is great. Such an excessive load on the first external barrel37becomes a burden on the motor which drives the zoom lens (which corresponds to the zoom motor23of the present embodiment of the zoom lens), so that the motor is required to be large. This is disadvantageous to miniaturization of the zoom lens10. To reduce the difference between the maximum load and the minimum load, the spring biasing forces of two sets of extension springs which respectively correspond to the set of three extension coil springs38and the set of two extension coil springs39have to be mutually adjusted with precision, which is troublesome and time-consuming. In contrast, in the present embodiment of the zoom lens, the variation in the sum of the load of the set of three extension coil springs38on the first external barrel37and the load of the set of two extension coil springs39on the first external barrel37is small during the entire zooming range, and accordingly, no excessive burden is exerted on the zoom motor23while the first external barrel37can be held easily with stability.

FIGS. 36 and 37each show a comparative example of spring-biasing structure which is to be compared with the spring-biasing structure of the present embodiment of the zoom lens10. Although identical to the spring-biasing structure of the present embodiment of the zoom lens10in that three lens frames are driven by a common cam ring (131or231), the spring-biasing structures shown inFIGS. 36 and 37that bias the three lens frames with respect to one another are different from the spring-biasing structure of the present embodiment of the zoom lens10.

In the comparative example of spring-biasing structure shown inFIG. 36, an intermediate lens frame135that is positioned between a front lens frame137and a rear lens frame136in the optical axis direction holds both one end (rear end) of a front extension spring138and one end (front end) of a rear extension spring139, while the front lens frame137and the rear lens frame136hold the other end (front end) of the front extension spring138and the other end (rear end) of the rear extension spring139, respectively. Although the biasing forces are applied to the single intermediate lens frame135from the two extension springs138and139in the spring-biasing structure shown inFIG. 36, the front extension spring138exerts the spring biasing force thereof on the intermediate lens frame135in a direction of moving the intermediate lens frame135forward while the rear extension spring139exerts the spring biasing force thereof on the intermediate lens frame135in a direction of moving the intermediate lens frame135rearward; consequently, the biasing force of the front extension spring138and the biasing force of the rear extension spring139cancel each other out. Therefore, a loss of spring biasing force is greater in the spring-biasing structure shown inFIG. 36than that in the preset embodiment of the zoom lens, in which the biasing directions of the set of three extension coil springs38and the set of two extension coil springs39relative to the first external barrel37are the same to thereby exert the sum of the spring force of the set of three extension coil springs38and the spring force of the set of two extension coil springs39on the first external barrel37efficiently.

In the comparative example of spring-biasing structure shown inFIG. 37, one end (rear end) of a front extension coil spring238and one end (front end) of a rear compression coil spring239are engaged with an intermediate lens frame235that is positioned between a front lens frame237and a rear lens frame236in the optical axis direction. In this spring-biasing structure, the above described problem in the comparative example of spring-biasing structure shown inFIG. 36has been resolved because both the front extension coil spring238and the rear compression coil spring239exert the spring biasing forces thereof on the intermediate lens frame235in the same direction of moving the intermediate lens frame235forward in the optical axis direction. However, the following problem arises if the front lens frame237, the intermediate lens frame235and the rear lens frame236are moved in the same moving manner as the first external barrel37, the second lens group moving frame35and the third lens group moving frame36of the above illustrated embodiment of the zoom lens. Firstly, the intermediate lens frame235is positioned at a position thereof closest to the front lens frame237while the rear lens frame236is positioned away from the front lens frame237to thereby reduce both the pulling force of the front extension coil spring238and the compressive force of the rear compression coil spring239when the zoom lens10is at the wide-angle extremity. Conversely, when the zoom lens10is at the telephoto extremity, the intermediate lens frame235is positioned away from the front lens frame237and the rear lens frame236is positioned close to the front lens frame237to thereby increase both the pulling force of the front extension coil spring238and the compressive force of the rear extension coil spring239. Accordingly, the difference between the sum of the spring force of the front extension coil spring238and the spring force of the rear compression coil spring239at the wide-angle extremity of the zoom lens10and the sum of the spring force of the front extension coil spring238and the spring force of the rear compression coil spring239at the telephoto extremity of the zoom lens10becomes excessively large. As described above, this kind of unbalanced spring-biasing state becomes a burden on the motor for driving the lens frames235,236and237, and is therefore undesirable.

In either of the two spring-biasing structures shown inFIGS. 36 and 37, the intermediate lens frame (135or235) serve as a common-engaging spring support member to which both one end of one spring (138or238) and one end of another spring (139or239) are connected, so that one or more long extension springs such as the set of two extension coil springs39of the above described embodiment of the zoom lens which are installed to be stretched between other two optical element support members cannot be adopted. As described above, using a long extension spring such as each extension coil spring39instead of a short extension spring simplifies the spring design, improves stability of the spring-biasing structure, and further improves the workability in installing the biasing springs.

As can be understood from the above description, according to the spring-biasing structure incorporated in the present embodiment of the zoom lens, each of three optical element support members (the first external barrel37, the second lens group moving frame35and the third lens group moving frame36) which are driven by the common cam ring31can be biased mutually by biasing springs efficiently by a simple spring-biasing structure to remove backlash in the three optical element support members. Specifically, since the set of three extension coil springs38and the set of two extension coil springs39are positioned to reverse the rate of increase in biasing force between the set of three extension coil springs38and the set of two extension coil springs39while biasing the first external barrel37in the same direction, a stable biasing force can be applied to the first external barrel37without increasing the load of the extension coil springs38and39on the first external barrel37.

Although the set of three extension coil springs38and the set of two extension coil springs39are used in the above illustrated embodiment of the zoom lens, a set of compression coil springs and another set of compression coil springs can be used instead of the set of three extension coil springs38and the set of two extension coil springs39, respectively, to obtain the same effect. If the set of three extension coil springs38and the set of two extension coil springs39are each replaced by a set of compression coil springs, the spring-biasing direction to each of the three movable barrels or frames (the first external barrel37, the second lens group moving frame35and the third lens group moving frame36) is reversed. Namely, the first external barrel37is biased forward in the optical axis direction with respect to the second lens group moving frame35and the third lens group moving frame36, while each of the second lens group moving frame35and the third lens group moving frame36is biased rearward in the optical axis direction with respect to the first external barrel37.

Although the first external barrel37serves as a common-engaging spring support member with which two types of extension springs (38and39) are commonly engaged in the above illustrated embodiment of the zoom lens, it is possible to modify the spring-biasing structure so that the rear movable barrel or frame (which corresponds to the third lens group moving frame36in the above illustrated embodiment of the zoom lens) serves as the common-engaging spring support member. Although the first external barrel37desirably serves as the common-engaging spring support member because the first external barrel37forms a part of the exterior of the zoom lens10and supports the first lens group LG1having the largest diameter as mentioned above in the above illustrated embodiment of the zoom lens, the present invention does not exclude such a modification wherein the rear movable barrel or frame serves as the common-engaging spring support member if such a spring-biasing structure is desirable in terms of optical design.

Although the spring-biasing relationship among the three movable members: the first external barrel37, the second lens group moving frame35and the third lens group moving frame36, has been discussed above, the present invention can also be applied to another type of spring-biasing structure wherein more than three movable members are moved in an optical axis direction by a common driving member. For instance, supposing the AF lens frame17is a movable member which is moved in the optical axis direction by an associated set of cam grooves formed on the cam ring31, it is possible that the AF lens frame17and the first external barrel37be biased toward each other by connecting the ends of a set of biasing springs directly to the AF lens frame17and the first external barrel37, respectively.