Patent Publication Number: US-6665129-B2

Title: Lens barrel and a zoom lens barrel

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a lens barrel, and also relates to a zoom lens barrel. 
     2. Description of the Related Art 
     As an example of a conventional lens barrel in which two or more lens groups move in an optical axis direction, a lens barrel in which front and rear lens groups are guided linearly in the optical axis direction without rotating about the optical axis and in which the lens support frame of each of the front and rear lens groups is provided with a set of follower pins which are respectively engaged in a corresponding set of cam grooves (cam slots) formed on a cam ring is known in the art. In this type of lens barrel, one cam groove is necessary for each follower pin formed on the lens support frame of each of the front and rear lens groups. 
     Accordingly, a large number of cam grooves are necessary if the lens barrel is provided with a plurality of movable lens groups. It is difficult to form a large number of cam grooves on a cam ring due to a limited circumferential area thereon. This is more serious if the diameter of the cam ring is small. 
     One possible solution would be to increase the pitch of each cam groove (i.e., to increase the angle of inclination of each cam groove relative to a circumferential direction of the cam ring), however, this impairs the movement of the follower pins, or may lock the follower pins. If the number of the follower pins is large, the number of the cam grooves formed on the cam ring is also large, which inevitably deteriorates the strength of the cam ring. 
     As an example of a conventional extendable lens barrel such as an extendable zoom lens barrel in which two or more lens groups move in an optical axis direction, an extendable lens barrel in which front and rear lens groups are guided linearly in the optical axis direction without rotating about the optical axis, and in which the lens support frame of each of the front and rear lens groups is provided with a set of follower pins which are respectively engaged in a corresponding set of cam grooves (cam slots) formed on a cam ring, is known in the art. Note that the term ‘extendable lens barrel’ refers to a lens barrel which extends and retracts so that the length thereof is shortest at an accommodation position, and is longer at a photographing position. 
     In such an extendable lens barrel, if the lens barrel is retracted to move each of the front and rear lens groups to an accommodation position thereof, each follower pin of the lens support frame of each lens group is accommodated and positioned at an end (accommodation position) of the associated cam groove with no gap between the follower pin and the end of the cam groove, and accordingly, each follower pin cannot move from the accommodation position thereof unless the extendable lens barrel is extended to move each of the front and rear lens groups out of the accommodation position thereof. 
     However, if the respective accommodation positions of the front and rear lens groups are precisely determined by the profiles of the cam grooves, the front and rear lens groups at their respective accommodation positions need to have a certain gap therebetween so as not to collide with each other, which makes it difficult to reduce the length of the extendable lens barrel in an accommodation state thereof. 
     In addition, since it is necessary to adjust the fixing position of at least one lens group relative to a support frame thereof in the optical axis direction when the lens group is fixed to the lens support frame during assembly, a marginal space for the fixing position adjustment needs to be secured between the respective accommodation positions of adjacent lens groups, which makes it more difficult to reduce the length of the extendable lens barrel in an accommodation state thereof. 
     In an extendable lens barrel which extends and retracts between an extended position (ready-to-photograph position) and an accommodation position, if it is desired to reduce the length of the extendable lens barrel in an accommodation state thereof, all of the movable lens groups positioned in the lens barrel need to be retracted in order to be as compact as possible in an accommodation position of the extendable lens barrel. 
     However, in the case where the extendable lens barrel is provided at the front end thereof with a photographing aperture and at least one lens barrier blade which opens and closes the photographing aperture in a plane perpendicular to the optical axis of the extendable lens barrel, a substantial gap has to be provided between the front surface of the front-end lens group and the lens barrier blade, which increases the length of the extendable lens barrel. 
     SUMMARY OF THE INVENTION 
     The present invention provides a lens barrel wherein a cam ring does not need to have a large diameter even if the lens barrel is provided with a large number of movable lens groups, and wherein the cam ring can be provided with cam grooves which ensure smooth movements of the associated follower pins while keeping any decrease in strength of the cam ring to a minimum. 
     The present invention also provides a lens barrel wherein each of a plurality of lens groups provided in the lens barrel can be accommodated (housed) as compact as possible while preventing adjacent lens groups from contacting with each other even if the lens barrel is of a type wherein at least one lens group is fixed to the lens support frame thereof while the fixing position of the lens group is being adjusted relative to the lens support frame thereof in the optical axis direction, or even if the lens barrel is an extendable lens barrel provided at the front end thereof with a photographing aperture and at least one lens barrier blade which opens and closes the photographing aperture. 
     The present invention also provides a extendable zoom lens barrel having two or more movable lens groups, wherein front and rear lens groups can be accommodated compact in size in the zoom lens barrel when it is in an accommodation state. 
     For example, in an embodiment, a lens barrel is provided, including first and second lens groups guided in an optical axis direction without rotating about the optical axis; a first support frame which supports the first lens group, the first support frame including a first cam follower; a second support frame which supports the second lens group, the second support frame including a second cam follower; and a cam ring which is driven to rotate about the optical axis, the cam ring including a cam groove in which the first cam follower and the second cam follower are engaged. The cam groove is formed as a continuous groove and includes a first groove portion via which the first cam follower is guided to move the first lens group, and a second groove portion via which the second cam follower is guided to move the second lens group. The cam groove is formed so that one of the first cam follower and the second cam follower enters a corresponding one of the first groove portion and the second groove portion after passing through the other of the first groove portion and the second groove portion in a preparation stage of the lens barrel. 
     The first groove portion can include a first zooming section for moving the first lens group to perform a zooming operation; and a first accommodation section in which the first cam follower is positioned when the first lens group is accommodated. The second groove portion can include a second zooming section for moving the second lens group to perform the zooming operation; and a second accommodation section in which the second cam follower is positioned when the first lens group is accommodated. The first zooming section, the second zooming section, the first accommodation section and the second accommodation section are arranged in that order from one end of the cam groove, so that the first cam follower passes through the second zooming section when moving between the first accommodation section and the first zooming section. 
     It is desirable for the first lens group to be positioned in front of the second lens group. 
     Each of the first lens group and the second lens group can constitute a lens element of a zoom lens optical system, the cam ring being driven to move the first lens group and the second lens group in the optical axis direction while changing a distance therebetween to vary a focal length. 
     The lens barrel can further include a third lens group positioned behind the second lens group and guided in the optical axis direction, wherein the third lens group constitutes a focusing lens and is moved in the optical axis direction to perform a focusing operation. 
     In another embodiment, a lens barrel includes a front lens support member which supports a front lens group and is guided in an optical axis direction without rotating about the optical axis; a rear lens support member which supports a rear lens group and is guided in the optical axis direction without rotating about the optical axis; and a support frame movement mechanism for moving the front lens support member and the rear lens support member between respective ready-to-photograph positions and respective accommodation positions located behind the respective ready-to-photograph positions, in the optical axis direction. The support frame movement mechanism brings the front lens support member and the rear lens support member into contact with each other at the respective accommodation positions without causing the front lens group and the second lens group to come into contact with each other. 
     The support frame movement mechanism can include a biasing device; the support frame movement mechanism can bring the front lens support member and the rear lens support member into contact with each other, with a biasing force of the biasing device, at the respective accommodation positions. 
     The support frame movement mechanism can include a first cam follower formed on the front lens support member; a second cam follower formed on the rear lens support member; and a cam ring which is driven to rotate about the optical axis, the cam ring including a cam groove in which the first cam follower and the second cam follower are engaged. The cam groove can include a front lens group moving section for moving the front lens support member to the ready-to-photograph position; a rear lens group moving section for moving the rear lens support member to the ready-to-photograph position; a front lens group accommodation section for allowing the front lens group to move to the accommodation position which is located behind a position of the front lens group in the optical axis direction when the first cam follower is positioned in the front lens group moving section; and a rear lens group accommodation section for allowing the rear lens group to move to the accommodation position which is located behind a position of the rear lens group in the optical axis direction when the second cam follower is positioned in the rear lens group moving section. The front lens group accommodation section defines a clearance between the front lens group accommodation section and the first cam follower so that the first cam follower is movable in the optical axis direction in the front lens group accommodation section. The rear lens group accommodation section defines a clearance between the rear lens group accommodation section and the second cam follower so that the second cam follower is movable in the optical axis direction in the rear lens group accommodation section. 
     The front lens group moving section and the rear lens group moving section constitute a first zooming section and a second zooming section, respectively, for varying a focal length to perform a zooming operation. 
     The biasing device can bias the front lens support member in a direction toward the accommodation position so that the front lens support member moves to come in contact with the rear lens support member. 
     It is desirable for an area of the front lens group accommodation section to be smaller than an area of the rear lens group accommodation section. 
     In another embodiment, lens barrel having an optical system including of a plurality of lens groups, the lens barrel including a lens supporting frame to which a frontmost lens group of the plurality of lens groups is supported; a first moving frame to which the lens supporting frame is supported via male and female screw-threaded portions meshing with each other, one and the other of the male and female screw-threaded portions being formed on the lens supporting frame and the first moving frame, respectively; a second moving frame to which a rear lens group of the plurality of lens groups which is positioned behind the frontmost lens group is supported; and a support frame movement mechanism for moving the first moving frame and the second moving frame between respective ready-to-photograph positions and respective accommodation positions located behind the respective ready-to-photograph positions in a direction of an optical axis. A supporting position of the lens supporting frame, relative to the first moving frame in the optical axis direction, can be adjusted via the male and female screw-threaded portions during assembly. The support frame movement mechanism brings the lens supporting frame and the second moving frame into contact with each other when positioning the first moving frame and the second moving frame at the respective accommodation positions without causing the frontmost lens group and the rear lens group to come into contact with each other regardless of the adjustment of the supporting position of the lens supporting frame. 
     The support frame movement mechanism can include a biasing device; and the support frame movement mechanism can bring the lens supporting frame and the second moving frame into contact with each other, with a biasing force of the biasing device, when the first moving frame and the second moving frame are positioned at the respective accommodation positions. 
     The support frame movement mechanism can include a first cam follower formed on the first moving frame; a second cam follower formed on the second moving frame; and a cam ring which is driven to rotate about the optical axis, the cam ring including a cam groove in which the first cam follower and the second cam follower are engaged. The cam groove can include a frontmost lens group moving section for moving the first moving frame to the ready-to-photograph position; a rear lens group moving section for moving the second lens frame to the ready-to-photograph position; a frontmost lens group accommodation section for allowing the frontmost lens group to move to the accommodation position which is located behind a position of the frontmost lens group in the optical axis direction when the first cam follower is positioned in the frontmost lens group moving section; and a rear lens group accommodation section for allowing the rear lens group to move to the accommodation position which is located behind a position of the rear lens group in the optical axis direction when the second cam follower is positioned in the rear lens group moving section. The frontmost lens group accommodation section defines a clearance between the frontmost lens group accommodation section and the first cam follower so that the first cam follower is movable in the optical axis direction in the frontmost lens group accommodation section, whereby the frontmost lens group and the rear lens group are not in contact with each other in a state where the lens supporting frame and the second moving frame are in contact with each other when the first moving frame and the second moving frame are located at the respective accommodation positions. The clearance includes at least a predetermined width corresponding to a maximum variation amount by the adjustment of the supporting position of the lens supporting frame. 
     The support frame movement mechanism can include a first cam follower formed on the first moving frame; a second cam follower formed on the second moving frame; and a cam ring which is driven to rotate about the optical axis, the cam ring including a cam groove in which the first cam follower and the second cam follower are engaged. A width of the cam groove in the optical axis direction at a position corresponding to the accommodation position, when the first moving frame is positioned at the accommodation position, is formed so that a clearance between the cam groove and the first cam follower is greater than an adjustment range of a position of the lens supporting frame relative to the first moving frame in the optical axis direction. 
     The biasing device can include at least one helical compression spring. 
     In another embodiment, a lens barrel can include at least one lens group which moves in an optical axis direction; a ring member which is provided around the lens group, the ring member having a photographing aperture at a front end of the ring member; and at least one barrier blade positioned at the front end of the ring member, the barrier blade being driven to open and close the photographing aperture. A frontmost surface of the lens group can be formed as a convex surface. The barrier blade can include a concave surface which is formed on a rear surface thereof so that a shape of the concave surface corresponds to a shape of a corresponding portion of the frontmost surface. The barrier blade can be driven to open and close the photographing aperture in a plane perpendicular to the optical axis at a position where collision between the barrier blade and the convex front surface of the lens group is avoided due to the presence of the concave surface. 
     It is desirable for the lens group to include a front lens group and a rear lens group, the extendable lens barrel further including a front lens support member which supports the front lens group and is guided in the optical axis direction without rotating about the optical axis; a rear lens support member which supports the rear lens group and is guided in the optical axis direction without rotating about the optical axis; and a support frame movement mechanism for moving the front lens support member and the rear lens support member between respective ready-to-photograph positions and respective accommodation positions located behind the respective ready-to-photograph positions in the optical axis direction, respectively. The support frame movement mechanism can bring the front lens support member and the rear lens support member into contact with each other at the respective accommodation positions without causing the front lens group and the second lens group to come into contact with each other. 
     The support frame movement mechanism can include a biasing device, and the support frame movement mechanism can bring the front lens support member and the rear lens support member into contact with each other, with a biasing force of the biasing device, at the respective accommodation positions. 
     It is desirable for the front lens group to include a frontmost lens group of the lens group. The front lens support member includes a lens supporting frame to which the frontmost lens group is supported, and a first moving frame to which the lens supporting frame is supported via male and female screw-threaded portions meshing with each other, one and the other of the male and female screw-threaded portions being formed on the lens supporting frame and the first moving frame, respectively. A supporting position of the lens supporting frame, relative to the first moving frame in the optical axis direction, can be adjusted via the male and female screw-threaded portions during assembly. The support frame movement mechanism can bring the lens supporting frame and the rear lens support member into contact with each other when positioning the front lens support member and the rear lens support member at the respective accommodation positions without causing the frontmost lens group and the rear lens group to come into contact with each other regardless of the adjustment of the supporting position of the lens supporting frame. 
     The lens group can constitute a lens element of a zoom lens optical system for varying a focal length. 
     The barrier blade can include a pair of barrier blades; and the concave surface can include a pair of semi-circular concave faces which are formed on the pair of barrier blades, respectively, the pair of semicircular concave faces together forming a circular concave face, the circular concave face corresponding to the shape of a central portion of the convex frontmost surface in a state where the pair of barrier blades are closed. 
     In another embodiment, a lens barrel is provided, including front and rear lens groups guided in a direction of an optical axis without rotating about the optical axis; a first support frame which supports the front lens group, the first support frame including a first cam follower; a second support frame which supports the rear lens group, the second support frame including a second cam follower; and a cam ring which is driven to rotate about the optical axis, the cam ring including a first groove portion and a second groove portion in which the first cam follower and the second cam follower are engaged, respectively. The first groove portion includes a first operating section for moving the front lens group to a position wherein a photographing operation is performed; and a first accommodation section for moving the front lens group to a first accommodation position which is located behind a position of the front lens group in the optical axis direction when the first cam follower is positioned in the first operating section. The second groove portion includes a second operating section for moving the rear lens group to perform the zooming operation; and a second accommodation section for photographing the rear lens group to a second accommodation position which is located behind a position of the rear lens group in the optical axis direction when the second cam follower is positioned in the second photographing section. At least one of the first accommodation section and the second accommodation section is formed to allow corresponding at least one of the first cam follower and the second cam follower to move in the optical axis direction in corresponding the at least one of the first accommodation section and the second accommodation section. 
     The first accommodation section and the second accommodation section can be formed so that the front lens group and the rear lens group are released from constraints of the first accommodation section and the second accommodation section at the first accommodation position and the second accommodation position, respectively. 
     The cam groove can be formed as a continuous groove including the first groove portion and the second groove portion. 
     It is desirable for the first groove portion, the second groove portion, the first accommodation section and the second accommodation section to be arranged in that order from one end of the cam groove, so that the first cam follower passes through the second groove portion when moving between the first accommodation section and the first groove portion. 
     The second operating section of the second cam groove can be formed adjacent to the first operating section of the first cam groove portion, and the first accommodation section of the first cam groove portion can be formed adjacent to the second operating section, and the second accommodation section of the second cam groove can be formed adjacent to the first accommodation section. 
     In another embodiment, a lens barrel is provided, including a plurality of lens groups guided in an optical axis direction without rotating about the optical axis; a plurality of support frames which support the plurality of lens groups, respectively; a plurality of cam followers formed on the plurality of support frames, respectively; and a cam ring which is driven to rotate about the optical axis, the cam ring including a plurality of cam grooves in which the plurality of cam followers are engaged, respectively. Each of the plurality of cam grooves includes an operating section for moving corresponding one of the plurality of lens groups to perform a photographing operation; and an accommodation section for moving the corresponding one lens group to an accommodation position which is located behind a position of the corresponding one lens group in the optical axis direction when corresponding one of the plurality of cam followers is engaged in the operating section. The accommodation section is formed to allow the corresponding one cam follower to move in the optical axis direction in the accommodation section. 
     The present disclosure relates to subject matter contained in Japanese Patent Applications Nos. 2001-83262, 2001-83263, 2001-83683, 2001-83684 and 2001-83691 (all filed on Mar. 22, 2001) which are expressly incorporated herein by reference in their entireties. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described below in detail with reference to the accompanying drawings in which: 
     FIG. 1 is an exploded perspective view of an embodiment of a zoom lens barrel according to the present invention; 
     FIG. 2 is an exploded perspective view of an upper left portion of the zoom lens barrel shown in FIG. 1; 
     FIG. 3 is an exploded perspective view of a middle portion of the zoom lens barrel shown in FIG. 1; 
     FIG. 4 is an exploded perspective view of a lower right portion of the zoom lens barrel in FIG. 1; 
     FIG. 5 is an axial cross sectional view of the zoom lens barrel shown in FIG. 1, above the optical axis, showing the zoom lens barrel in an accommodation state; 
     FIG. 6 is an axial cross sectional view of the zoom lens barrel shown in FIG. 1, above the optical axis, taken along a plane different from that of FIG. 5, showing the zoom lens barrel in an accommodation state; 
     FIG. 7 is an axial cross sectional view of the zoom lens barrel shown in FIG. 1, showing the zoom lens barrel in an accommodation state above the optical axis, and further showing the zoom lens barrel in a ready-to-photograph state below the optical axis; 
     FIG. 8 is a developed view of an outer peripheral surface of a cam ring provided as an element of the zoom lens barrel shown in FIG. 1; 
     FIG. 9 is a developed view of one of three cam grooves formed on an inner peripheral surface of the cam ring, showing the profile of the cam groove; 
     FIG. 10 is a developed view of two of the three cam grooves shown in FIGS. 8 and 9, showing the relationship between the cam grooves, first follower pins formed on a first lens frame, and second follower pins formed on a second lens frame; 
     FIG. 11 is a schematic developed view of a cam-ring-control cam slot formed on a stationary ring and an associated rotation transfer groove formed on a rotatable ring; 
     FIG. 12 is a front elevational view of the zoom lens barrel with a barrier blade support front plate removed therefrom in a state where a pair of lens barrier blades are closed; 
     FIG. 13 is a view similar to that of FIG.  12  and illustrates the barrier drive ring and peripheral elements thereof in a state where the pair of lens barrier blades are open; 
     FIG. 14 is a view similar to that of FIG.  12  and illustrates the pair of barrier blades of a barrier unit, showing the relationship between the pair of barrier blades and an inner ring; 
     FIG. 15 is a graph showing variations of respective axial positions of first and second lens groups (first and second lens frames) in a range of movement including a zooming section and a retracting section; 
     FIG. 16 is a developed view of the cam ring and the barrier drive ring, showing the positional relationship therebetween; and 
     FIG. 17 is an enlarged perspective view of a lens barrier blade shown in FIGS.  1  and  2 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of a zoom lens barrel, an exploded perspective view of which is shown in FIG. 1, is a extendable zoom lens barrel of a digital camera. 
     The zoom lens barrel  1  is provided with a support frame movement mechanism which is constructed from a set of three follower pins (first cam followers)  18   f  fixed to a first lens group moving frame (first support frame)  18 , a set of three follower pins (second cam followers)  19   f  fixed to a second lens group moving frame (second support frame/rear lens support member)  19 , a cam ring  15 , a set of three linear guide holes  18   a  of the first lens group moving frame  18 , a set of three linear guide bosses  17   d  (only one of which appears in FIGS. 1 and 3) of an inner ring (ring member)  17 , a set of three linear guide grooves  18   c  of the first lens group moving frame  18 , and a set of three linear guide keys  19   a  of the second lens group moving frame  19 . 
     As can be clearly seen in FIGS. 5 through 7, the zoom lens barrel  1  is provided with a photographing optical system constructed from three lens groups: a first lens group L1, a second lens group L2, and a third lens group L3, in that order from the object side (the left side as viewed in each of FIGS.  5  through  7 ). The first and second lens groups L1 and L2 are driven to move along an optical axis O relative to the third lens group L3 while varying the distance therebetween to perform a zooming operation. The third lens group L3 serves as a focusing lens group, and is driven to move along the optical axis O to perform a focusing operation. 
     The zoom lens barrel  1  is provided with a housing  11 , a shaft holding member  12  and a stationary ring  13 , which are all stationary members fixed to a camera body (not shown). Accordingly, the housing  11 , the shaft holding member  12  and the stationary ring  13  do not move in the direction of the optical axis O (i.e., in the optical axis direction) or rotates about the optical axis O. The housing  11  is provided at a rear end thereof with a flange  11   a  (see FIG.  4 ), while the stationary ring  13  is provided at a rear end thereof with a flange  13   a  which is fixed to the flange  11   a  of the housing  11 . The housing  11  is provided with an outer cylindrical portion  11   b  and a filter holding portion  11   c  to which a low-pass filter  11   d  is fixed. As shown in FIGS. 5 through 7, the low-pass filter  11   d  is positioned in front of a CCD (solid-state image pick-up device)  10   a  fixed to a stationary base  10  positioned in the camera body. 
     The stationary ring  13  is positioned inside the outer cylindrical portion  11   b  of the housing  11 . The zoom lens barrel  1  is provided, on the stationary ring  13  between the stationary ring  13  and the outer cylindrical portion  11   b , with a rotatable ring  14 . The stationary ring  13  is positioned inside the rotatable ring  14  which supports the cam ring  15  therein. The stationary ring  13  is provided with a set of three cam slots (cam-ring-control cam slots)  13   b  formed on the stationary ring  13  as through-slots at equi-angular intervals in a circumferential direction thereof. The cam ring  15  is provided at the rear end thereof with a thick-wall cylinder portion  15   a . A set of three follower pins  15   b  fixed to the thick-wall cylinder portion  15   a  at equiangular intervals in a circumferential direction of the cam ring  15  pass through the set of three cam slots  13   b  of the stationary ring  13  to be engaged in a set of three rotation transfer grooves  14   a  (only of which appears in FIGS. 1 and 4) which are formed on an inner peripheral surface of the rotatable ring  14 . 
     FIG. 11 shows a developed view of one of the three cam slots  13   b  and the associated one of the three rotation transfer grooves  14   a . Each rotation transfer groove  14   a  includes a linear groove portion  14   a   1 , an inclined groove portion  14   a   2 , and a circumferential groove portion  14   a   3  in that order from the front end to the rear end of the linear groove portion  14   a   1  (from left to right as viewed in FIG.  11 ). The linear groove portion  14   a   1 , which occupies a major portion of the rotation transfer groove  14   a , extends parallel to the optical axis O. The circumferential groove portion  14   a   3  of each rotation transfer groove  14   a  is used only when the zoom lens barrel  1  is assembled/disassembled. Each cam slot  13   b  includes a linear slot portion  13   b   1 , a state-changing slot portion  13   b   2 , a zooming slot portion  13   b   3 , and a terminal slot portion  13   b   4 , in that order from the end (the lower end as viewed in FIG. 11) of the cam slot  13   b  which closest to the rear end of the stationary ring  13 . The linear slot portion  13   b   1  extends parallel to the optical axis O. The state-changing slot portion  13   b   2  extends in a direction inclined with respect to both the optical axis O and a circumferential direction of the stationary ring  13 . The zooming slot portion  13   b   3  extends in a circumferential direction of the stationary ring  13 . The terminal slot portion  13   b   4  is used only when the zoom lens barrel  1  is assembled/disassembled. 
     The rotating barrel  14  rotates about the optical axis O in a rotational range between an accommodation position (accommodation position) and a telephoto extremity via a wide-angle extremity. This rotational range includes a preparation section (preparation stage) which extends between a position close to the accommodation position and the wide-angle extremity, and a zooming section which extends between the wide-angle extremity to the telephoto extremity (see FIG.  11 ). If the rotatable ring  14  rotates relative to the stationary ring  13  in a state where each follower pin  15   b  is engaged in the inclined groove portion  14   a   2  of the associated rotation transfer groove  14   a  and the linear slot portion  13   b   1  of the associated cam slot  13   b  (i.e., in a state where the rotatable ring  14  is in the accommodation position and where the cam ring  15  is fully retracted), each follower pin  15   b  of the cam ring  15  is pressed by a side edge of the linear slot portion  13   b   1  of the associated cam slot  13   b , which causes the cam ring  15  to move in the optical axis direction along the linear slot portion  13   b   1  without rotating about the optical axis O. If the rotatable ring  14  rotates relative to the stationary ring  13  in a state where each follower pin  15   b  is engaged in the linear groove portion  14   a   1  of the associated rotation transfer groove  14   a  and the state-changing slot portion  13   b   2  of the associated cam slot  13   b  (i.e., in a state where the rotatable ring  14  is in the preparation section), each follower pin  15   b  of the cam ring  15  moves along the state-changing slot portion  13   b   2  of the associated cam slot  13   b , which causes the cam ring  15  to rotate about the optical axis O while moving in the optical axis direction due to the engagement of the follower pin  15   b  with the state-changing slot portion  13   b   2 . If the rotatable ring  14  rotates relative to the stationary ring  13  in a state where each follower pin  15   b  is engaged in the linear groove portion  14   a   1  of the associated rotation transfer groove  14   a  and the zooming slot portion  13   b   3  of the associated cam slot  13   b  (i.e., in a state where the rotatable ring  14  is in the zooming section), each follower pin  15   b  of the cam ring  15  moves along the zooming slot portion  13   b   3  of the associated cam slot  13   b , which causes the cam ring  15  to rotate about the optical axis O without moving in the optical axis direction. 
     The rotatable ring  14  is provided on an outer peripheral surface thereof with a circumferential gear  14   b  which is in mesh with a drive pinion (not shown). The drive pinion is driven by a reversible motor (not shown) to rotate forwardly and reversely. Rotation of the drive pinion causes the rotatable ring  14  to rotate to thereby move the cam ring  15  in the optical axis direction while rotating about the optical axis O. Accordingly, if the accommodation position of the cam ring  15  is regarded as a starting position (reference position) of movement of the cam ring  15 , firstly the cam ring  15  moves linearly in the optical axis direction without rotating about the optical axis O (due to the linear slot portions  13   b   1 ), subsequently the cam ring  15  moves in the optical axis direction while rotating about the optical axis O (due to the state-changing slot portions  13   b   2  in the preparation section), and finally the cam ring  15  rotates about the optical axis O without moving in the optical axis direction (due to the zooming slot portion  13   b   3  in the zooming section). 
     In the present embodiment of the zoom lens barrel  1 , the rotatable ring  14 , the cam ring  15  and a barrier drive ring  31  are rotatable elements. The remaining movable elements, except for the second lens group moving frame  19 , linearly move in the optical axis direction without rotating about the optical axis O. The second lens group moving frame  19  can rotate about the optical axis O slightly. Such linearly moving elements and guiding mechanisms thereof will be hereinafter discussed. The zoom lens barrel  1  is provided between the stationary ring  13  and the cam ring  15  with an outer ring  16  and the inner ring  17  which is provided inside the outer ring  16 . The outer ring  16  and the inner ring  17  are positioned in an annular space between the cam ring  15  and the stationary ring  13 , while the thick-wall cylinder portion  15   a  of the cam ring  15  is engaged with an inner peripheral surface of the stationary ring  13  so that the cam ring  15  can rotate about the optical axis O relative to the stationary ring  13  without tilting relative to the optical axis O. 
     As shown in FIG. 2, the outer ring  16 , which is positioned immediately inside of the stationary ring  13 , includes a main ring body  16   r  and a reinforcing ring  16   x  which are made of synthetic resin and metal, respectively. The main ring body  16   r  is provided at a rear end thereof with a thick-wall cylinder portion  16   a , and is further provided, on the thick-wall cylinder portion  16   a  at equi-angular intervals in a circumferential direction of the main ring body  16   r , with a set of three linear guide keys  16   b  (only one of which appears in FIGS. 1 and 2) which extend radially outwards. The stationary ring  13  is provided on an inner peripheral surface thereof with a set of three linear guide grooves  13   c  which extend parallel to the optical axis O, and in which the set of three linear guide keys  16   b  of the main ring body  16   r  are slidably engaged in the set of three linear guide keys  16   b , respectively. The metal reinforcing ring  16   x  is fitted on, and adhered to, an outer peripheral surface of the main ring body  16   r  in front of the thick-wall cylinder portion  16   a  by an adhesive to reinforce the main ring body  16   r  with a minimum increase in wall thickness of the outer ring  16 , which contributes to a reduction in wall thickness of the zoom lens barrel  1 , i.e., contributes to further miniaturization of the zoom lens barrel  1 . 
     Similar to the outer ring  16 , the inner ring frame  17  includes a main ring body  17   r  and a reinforcing ring  17   x  which are made of synthetic resin and metal, respectively. The main ring body  17   r  is provided at a rear end thereof with a thick-wall cylinder portion  17   a . The metal reinforcing ring  17   x  is fitted on and adhered to an outer peripheral surface of the main ring body  17   r  in front of the thick-wall cylinder portion  17   a  by an adhesive to reinforce the main ring body  17   r  with a minimum increase in wall thickness of the inner ring  17 , which contributes to a reduction in wall thickness of the zoom lens barrel  1 , i.e., contributes to further miniaturization of the zoom lens barrel  1 . 
     The outer ring  16  is provided, on an inner peripheral surface of the main ring body  16   r  at equi-angular intervals in a circumferential direction of the outer ring  16 , with a set of three linear guide grooves  16   c  which extend parallel to the optical axis O. The inner ring  17  is provided on the thick-wall cylinder portion  17   a  with a set of three linear guide keys  17   b  which extend radially outwards to be slidably engaged in the set of three linear guide grooves  16   c  of the main ring body  16   r , respectively. The outer ring  16  is provided at the rear end thereof with a set of three bayonet prongs  16   d  (only one of which appears in FIG. 5) which extend radially inwards. The cam ring  15  is provided, in the vicinity of the rear end thereof immediately in front of the thick-wall cylinder portion  15   a , with a circumferential groove  15   c  in which the set of three bayonet prongs  16   d  are engaged to be movable in the circumferential groove  15   c  within a predetermined angle of rotation. When the cam ring  15  is positioned within an operating angle relative to the outer ring  16 , the cam ring  15  and the outer ring  16  are movable together in the optical axis direction without disengaging from each other, and at the same time, the cam ring  15  is rotatable about the optical axis O relative to the outer ring  16  due to the engagement of the set of three bayonet prongs  16   d  with the circumferential groove  15   c.    
     The main ring body  17   r  of the inner ring  17  is provided in the vicinity of the front end thereof with an inner flange  17   c  which extends radially inwards and to which a barrier unit  40  and the barrier drive ring  31  are fixed. The main ring body  17   r  of the inner ring  17  is provided, on an rear face of the inner flange  17   c  at equi-angular intervals in a circumferential direction of the inner ring  17 , with the set of three linear guide bosses  17   d  (only one of which appears in FIGS.  1  and  3 ). The zoom lens barrel  1  is provided with the first lens group moving frame  18  which is provided in the inner ring  17 . The first lens group moving frame  18  is provided at the front end thereof with an inner flange  18   b  which extends radially inwards to form a circular aperture having the center thereof about the optical axis O. A female thread portion  18   d  is formed on an inner peripheral face of the inner flange  18   b . The first lens group moving frame  18  is provided on the inner flange  18   b  with the set of three linear guide holes  18   a  in which the set of three linear guide bosses  17   d  of the inner ring  17  are slidably engaged, respectively. Each linear guide hole  18   a  is formed having an oval cross section which is elongated in a radial direction of the first lens group moving frame  18 . Even if each linear guide boss  17   d  is fitted in the associated linear guide hole  18   a  with a substantial clearance therebetween, the inner ring  17  is guided in the optical axis direction relative to the first lens group moving frame  18  with a sufficient degree of precision since the first lens group moving frame  18  is slidably fitted into the cam ring  15 . The first lens group moving frame  18  is provided, on an inner peripheral surface thereof at equi-angular intervals in a circumferential direction thereof, with the set of three linear guide grooves  18   c  which extend parallel to the optical axis O. 
     The second lens group moving frame  19  is fitted in the first lens group moving frame  18 . The second lens group moving frame  19  is provided, on an outer peripheral surface thereof at the front end of the outer peripheral surface, with the set of three linear guide keys  19   a  which are slidably engaged into the set of three linear guide grooves  18   c  of the first lens group moving frame  18 , respectively. 
     As shown in FIGS. 5,  6  and  7 , the second lens group L2 includes three lens elements: front, middle and rear lens elements. The front lens element is fixed to the second lens group moving frame  19  to be directly supported thereby. The rear lens element is supported by a support ring  19   d  which is fixed to the second lens group moving frame  19  from rear thereof, so that the rear lens element is supported by the second lens group moving frame  19  via the support ring  19   d . The middle lens element is fixed to the rear lens element so that a rear surface of the middle lens element is cemented to a front surface of the rear lens element. Accordingly, the middle lens element of the second lens group L2 is supported by the second lens group moving frame  19  via the rear lens element of the second lens group L2 and the support ring  19   d.    
     As can be understood from the above description, according to the above described guiding mechanisms of the zoom lens barrel  1 , the outer ring  16  is guided linearly in the optical axis direction without rotating about the optical axis O via the stationary ring  13 , the inner ring  17  is guided linearly in the optical axis direction without rotating about the optical axis O via the outer ring  16 , the first lens group moving frame  18  is guided linearly in the optical axis direction without rotating about the optical axis O via the inner ring  17 , and the second lens group moving frame  19  is guided linearly in the optical axis direction without rotating about the optical axis O via the first lens group moving frame  18 , in that order from the outside to the inside of the zoom lens barrel  1 . Furthermore, the linear guiding mechanism provided between the inner ring  17  and the first lens group moving frame  18  includes the set of three linear guide bosses  17   d , which extend in a direction parallel to the optical axis, and the set of three linear guide holes  18   a , and is positioned in the vicinity of the front end of each of the inner ring  17  and the first lens group moving frame  18 . Due to this structure, no other linear guiding elements have to be provided on either the inside or the outside of the cam ring  15 . This contributes to a reduction of the annular space between the inner ring  17  and the first lens group moving frame  18  to thereby minimize the diameter of the zoom lens barrel  1 . 
     As shown in FIGS. 5 through 7, the zoom lens barrel  1  is provided with a first lens frame (lens supporting frame)  20  to which the first lens group L1 is fixed. The first lens frame  20  is fixed to the first lens group moving frame  18 , so that the first lens frame  20  and the first lens group moving frame  18  constitute a front lens support member. Accordingly, the first lens group L1 is supported by the first lens group moving frame  18  via the first lens frame  20 . More specifically, the first lens frame  20  is provided on an outer peripheral surface thereof with a male thread portion which is in mesh with the female thread portion  18   d  of the inner flange  18   b . The first lens frame  20  is cemented to the first lens group moving frame  18  by an adhesive after the thread engagement position of the male thread portion of the first lens frame  20  with respect to the female thread portion  18   d  of the inner flange  18   b  has been adjusted during assembly. The zoom lens barrel  1  is provided in an annular recess  19   b  of the second lens group moving frame  19  with a shutter unit  21  which is fixed to the second lens group moving frame  19  by set screws (not shown). A light shield ring  19   c  is fitted in the second lens group moving frame  19  from front thereof to be fixed thereto to hold the shutter unit  21  between the light shield ring  19   c  and the second lens group moving frame  19 . The shutter unit  21  is provided with shutter blades  21   a . The shutter unit  21  drives the shutter blades  21   a  to open and close in accordance with information on an object brightness. The zoom lens barrel  1  is provided therein with a flexible printed wiring board (flexible PWB)  21   b  one end (front end) of which is fixed to the shutter unit  21  (see FIG.  7 ). A drive signal is given to the shutter unit  21  via the flexible PWB  21   b . As shown in FIG. 7, the flexible PWB  21   b  extends rearward from the shutter unit  21 , and subsequently bends radially outwards to extend forward. Subsequently, the flexible PWB  21   b  penetrates the stationary ring  13  via a through-slot  28   a  (see FIGS. 4 and 7) formed thereon, and bends radially outwards to extend rearward along a guiding portion  28  of the stationary ring  13  which extends parallel to the optical axis O. A portion of the flexible PWB  21   b  which extends along the outer surface of the guiding portion  28  is cemented thereto. Subsequently, the flexible PWB  21   b  extends rearward to be positioned outside the housing  11 . As shown in FIG. 7, a bending portion  21   bx  of the flexible PWB  21   b  in the vicinity of the through-slot  28   a  passes through a rubber band  29  which is hooked over a hook  11   f  formed at the rear end of the housing  11 . In a state where the zoom lens barrel  1  is fully extended as shown below the optical axis O in FIG. 7, the front end of the stretched rubber band  29  is positioned behind the position of the through-slot  28   a  in the optical axis direction to pull the bending portion  21   bx  obliquely rearwards in a direction away from the optical axis O to prevent the flexible PWB  21   b  from bending to interfere with the photographing optical path of the zoom lens barrel  1 . 
     The zoom lens barrel  1  is provided with a third lens frame  22  to which the third lens group L3 is fixed. As shown in FIG. 4, the third lens frame  22  is guided in the optical axis direction via a pair of linear guide rods  22   a  which extend parallel to the optical axis. The front and rear ends of each linear guide rod  22   a  are fixed to the shaft holding member  12  and the housing  11 , respectively. The third lens frame  22  is driven to move in the optical axis direction by rotation of a feed screw  24  which is driven forwardly and reversely by a step motor (not shown) in accordance with information on a photographing distance. 
     A zooming operation is carried out by moving the first and second lens groups L1 and L2 (the first and second lens group moving frames  18  and  19 ) in the optical axis direction relative to the third lens group L3 while varying the distance therebetween. The cam ring  15  is provided, on an inner peripheral surface thereof at equi-intervals in a circumferential direction of the cam ring  15 , with a set of three lens-drive cam grooves C 1  (see FIGS. 1,  3  and  5 ). The first lens group moving frame  18  and the second lens group moving frame  19 , which are guided linearly in the optical axis direction without rotating about the optical axis O, move in the optical axis direction by rotation of the cam ring  15  in accordance with the profiles of the lens-drive cam grooves C 1 . The developed view of the lens-drive cam grooves C 1  is shown in FIGS. 8 through 10. In FIG. 8 each lens-drive cam groove C 1 , which is formed on an inner peripheral surface of the cam ring  15 , is shown by dotted lines and is shown by solid lines in FIGS. 9 and 10 to clearly indicate the profile thereof. A feature of the zoom lens barrel  1  is that each lens-drive cam groove C 1  is formed as a continuous bottomed groove to have respective cam groove portions for the first and second lens groups L1 and L2, and that the first and second lens groups L1 and L2 are released from the constraints of the set of three lens-drive cam grooves C 1  at their respective accommodation positions so that the first and second lens groups L1 and L2 can be accommodated to be positioned close to each other until the first lens frame  20  and the second lens group moving frame  19  come into contact with each other. 
     Namely, the set of three follower pins  18   f  that are projected radially outwards from the first lens group moving frame  18  and the set of three follower pins  19   f  that are projected radially outwards from the second lens group moving frame  19  are slidably engaged in the set of three lens-drive cam grooves C 1 , respectively. Each lens-drive cam groove C 1 , which is formed as a continuous bottomed groove, has a function to move the first and second lens groups L1 and L2 (the first and second lens group moving frames  18  and  19 ) in their respective zoom paths. Unlike the present embodiment of the zoom lens barrel  1 , in a conventional zoom lens barrel having a cam ring for driving a plurality of movable lens groups, a set of cam grooves is necessary for each of the plurality of movable lens groups. 
     Each lens-drive cam groove C 1  is provided at one end thereof with an insertion end C 1   e  via which one of the three follower pins  18   f  of the first lens group moving frame  18  and one of the three follower pins  19   f  of the second lens group moving frame  19  are inserted into the lens-drive cam groove C 1 . Each lens-drive cam groove C 1  is further provided with a first-lens-group zooming section (front lens group moving section) C 1 Z 1 , a second-lens-group zooming section (rear lens group moving section) C 1 Z 2 , a first-lens-group accommodation section C 1 A 1  and a second-lens-group accommodation section C 1 A 2 , in that order from the insertion end C 1   e . The opposite ends (lower and upper ends as viewed in FIG. 9) of the first-lens-group zooming section C 1 Z 1  determines a telephoto extremity Z 1 T and a wide-angle extremity Z 1 W of the first lens group L1, respectively. The opposite ends (lower and upper ends as viewed in FIG. 9) of the second-lens-group zooming section C 1 Z 2  determines a telephoto extremity Z 2 T and a wide-angle extremity Z 2 W of the second lens group L2, respectively. As shown in FIGS. 8 through 10, the width of each of the first-lens-group accommodation section C 1 A 1  and the second-lens-group accommodation section C 1 A 2  in the optical axis direction is greater than the width of each the first-lens-group zooming section C 1 Z 1  and the second-lens-group zooming section C 1 Z 2  so that the associated follower pins  18   f  and  19   f  can move freely in the first-lens-group accommodation section C 1 A 1  and the second-lens-group accommodation section C 1 A 2 , respectively. Namely, the first-lens-group accommodation section C 1 A 1  extends in a circumferential direction of the cam ring  15 , and also widens in the optical axis direction to form a clearance for the associated follower pin  18   f  of the first lens group moving frame  18  to be movable in the optical axis direction by an amount of movement corresponding to the range of adjustment of the thread engagement position of the male thread portion of the first lens frame  20  with respect to the female thread portion  18   d  of the inner flange  18   b . On the other hand, the second-lens-group accommodation section C 1 A 2  extends in both a circumferential direction of the cam ring  15  and the optical axis direction to form a substantially triangular area to form a clearance for the associated follower pin  19   f  of the second lens group moving frame  19  to be movable freely and widely in both the circumferential direction of the cam ring  15  and the optical axis direction within the triangular area. 
     The relative angular positions of the set of three follower pins  18   f  and the set of three follower pins  19   f  about the optical axis O are determined so that each follower pin  18   f  and each follower pin  19   f  are respectively positioned in the first-lens-group accommodation section C 1 A 1  and the second-lens-group accommodation section C 1 A 2  when the cam ring  15  is positioned in an accommodation position thereof. The first-lens-group accommodation section C 1 A 1  and the second-lens-group accommodation section C 1 A 2 , to some extent, do not constrain movement of the associated follower pins  18   f  and  19   f , respectively. Namely, each follower pin  18   f  and each follower pin  19   f  can move in the first-lens-group accommodation section C 1 A 1  and the second-lens-group accommodation section C 1 A 2 , respectively, in the optical axis direction because of the clearance formed between each groove portion and the associated follower pin. This clearance contributes to further miniaturization of the length of the zoom lens barrel  1  in an accommodation state thereof (i.e., the distance between the first lens group moving frame  18  and the second lens group moving frame  19  in the optical axis direction can be minimized since both moving frames  18  and  19  are released from positioning restrictions of the cam grooves and cam followers thereof). The amount of clearance formed between the first-lens-group accommodation section C 1 A 1  and the associated follower pin  18   f  is sufficient to absorb a variation in position of the associated follower pin  18   f  which is caused by an adjustment of the thread engagement position of the male thread portion of the first lens frame  20  with respect to the female thread portion  18   d  of the inner flange  18   b  in an accommodation state of the zoom lens barrel  1 . 
     The inner flange  17   c  of the inner ring  17  is provided with a set of three engaging protrusions  17   g  (only one of which appears in FIGS. 1,  3  and  5 ) arranged at different angular positions in a circumferential direction of the inner ring  17 . The first lens group moving frame  18  is provided with a set of three recesses  18   g  to correspond to the set of three engaging protrusions  17   g . Three helical compression springs  30  serving as a biasing device are inserted to be held between the set of three engaging protrusions  17   g  and the set of three recesses  18   g , respectively, to press the first lens group moving frame  18  rearwards in the optical axis direction. Therefore, the first lens frame  20 , which is supported by the first lens group moving frame  18 , can retract up to a mechanical contacting point P (see FIGS. 5 and 6) where the first lens frame  20  comes in contact with the light shield ring  19   c  of the second lens group moving frame  19  due to the clearance between the first-lens-group accommodation section C 1 A 1  of each lens-drive cam groove C 1  of the cam ring  15  and the associated follower pin  18   f  of the first lens group moving frame  18 . By providing the helical compression springs  30 , which have a small length, in between the inner ring  17  and the first lens group moving frame  18 , the relative movement between the first and second lens group moving frames  18  and  19  can be reduced, however, even if the helicoid compression springs  30  are not provided, the first and second lens group moving frames  18  and  19  can still retract up so that the first lens frame  20  contacts the mechanical contacting point P. Likewise, the second lens group moving frame  19  can retract up to a mechanically contacting point Q (see FIGS. 5 and 6) where the second lens group moving frame  19  comes in contact with the third lens frame  22  due to a clearance between the second-lens-group accommodation section C 1 A 2  of each lens-drive cam groove C 1  of the cam ring  15  and the associated follower pin  19   f  of the second lens group moving frame  19 . Due to such structures of the mechanical contacting points P and Q, the length of the zoom lens barrel  1  in an accommodation state thereof is successfully reduced as compared with a conventional zoom lens barrel in which the respective accommodation positions of first and second lens groups which correspond to the first and second lens groups L1 and L2 of the present embodiment of the zoom lens barrel are precisely determined by associated cam grooves. Furthermore, the third lens frame  22  can retract up to a mechanical contacting point R where it comes in contact with the housing  11  while compressing a helical compression spring  23  (see FIGS.  1  and  4 ), which is positioned between the third lens frame  22  and the housing  11  to press the third lens frame  22 , forward. The axial cross sectional view of the zoom lens barrel  1  above the optical axis O in each of FIGS. 5,  6  and  7  shows an accommodation state of the zoom lens barrel  1  where the first lens frame  20  is in contact with the light shield ring  19   c  of the second lens group moving frame  19 , where the second lens group moving frame  19  is in contact with the third lens frame  22 , and where the third lens frame  22  is in contact with the housing  11 . The amount of rearward movement of the first lens group moving frame  18  relative to the second lens group moving frame  19  depends on the position of the first lens frame  20  relative to the first lens group moving frame  18  because the position of the first lens frame  20  relative to the first lens group moving frame  18  varies by an adjustment of the thread engagement position of the male thread portion of the first lens frame  20  with respect to the female thread portion  18   d  of the inner flange  18   b  during assembly. Such a variation due to the adjustment is absorbed by extension or compression of the helical compression springs  30  so that the zoom lens barrel  1  can be accommodated with the first lens frame  20 , the second lens group moving frame  19  and the third lens frame  22  being in contact with the light shield ring  19   c , the third lens frame  22  and the housing  11  at the mechanically contacting points P, Q and R, respectively. 
     If the cam ring  15  rotates in a direction from the accommodation position toward a ready-to-photograph position in a zooming section between the telephoto extremity Z 1 T and the wide-angle extremity Z 1 W thereof, each follower pin  18   f  of the first lens group moving frame  18  which is engaged in the first-lens-group accommodation section C 1 A 1  moves from the first-lens-group accommodation section C 1 A 1  to the first-lens-group zooming section C 1 Z 1  via the second-lens-group zooming section C 1 Z 2 , while each follower pin  19   f  of the second lens group moving frame  19  which is engaged in the second-lens-group accommodation section C 1 A 2  moves from the second-lens-group accommodation section C 1 A 2  to the second-lens-group zooming section C 1 Z 2  via the first-lens-group accommodation section C 1 A 1 . Accordingly, the second-lens-group zooming sections C 1 Z 2  of the set of three lens-drive cam grooves C 1  that are used for driving the set of three follower pins  19   f  of the second lens group moving frame  19  are used as mere passing sections for the set of three follower pins  18   f  of the first lens group moving frame  18  via which the set of three follower pins  18   f  move from the first-lens-group accommodation position to the ready-to-photograph position. The above-described structure which provides such passing sections is advantageous to reduce the number of cam grooves which are to be formed on the cam ring  15 , which is in turn advantageous to reduce the angle of inclination of each cam groove with respect to a circumferential direction of the cam ring  15 . 
     The inner ring  17  moves in the optical axis direction independent of the first lens group moving frame  18  in a moving path which is substantially identical to the moving path of the first lens group moving frame  18 . Accordingly, the cam ring  15  is provided, on an outer peripheral surface at equi-intervals in a circumferential direction thereof, with a set of three cam grooves C 2 . The inner ring  17  is provided, on an inner peripheral surface at equi-intervals in a circumferential direction thereof, with a set of three follower pins  17   f  (only one of them appears in FIG. 5) which are slidably engaged in the set of three cam grooves C 2  of the cam ring  15 , respectively. As can be seen in FIG. 8, the profiles of the cam grooves C 2  resemble those of the lens-drive cam grooves C 1 . As shown in FIG. 8, each cam groove C 2  is provided at one end thereof with an insertion end C 2   e  via which one of the three follower pins  17   f  of the inner ring  17  is inserted into the cam groove C 2 . Each cam groove C 2  is further provided with a first section C 2 Z 1  which corresponds to the first-lens-group zooming section C 1 Z 1 , a second section C 2 Z 2  which corresponds to the second-lens-group zooming section C 1 Z 2 , and a barrier drive section C 2 B. The barrier drive section C 2 B extends in a circumferential direction of the cam ring  15 , so that the cam ring  15  rotates about the optical axis O without moving in the optical axis direction relative to the inner ring  17  as long as each follower pin  17   f  is engaged in the barrier drive section C 2 B. As can be clearly seen in FIG. 8, the set of three lens-drive cam grooves C 1  and the set of three cam grooves C 2  are formed on the cam ring  15  at slightly different positions in the optical axis direction, while the set of three follower pins  17   f  that are respectively engaged in the set of three cam grooves C 2  and the set of three follower pins  18   f  that are respectively engaged in the set of three lens-drive cam grooves C 1  are respectively aligned side by side in a direction parallel to the optical axis O. 
     By providing the inner ring  17 , which extends forward so that an outer peripheral surface thereof is exposed to the outside of the zoom lens barrel  1 , as an element separate from the first lens group moving frame  18 , and by guiding the inner ring  17  in the optical axis direction via a cam mechanism independent of the first lens group moving frame  18  as described above, external forces applied to the inner ring  17  can be prevented from being transferred to the first lens group L1 via the first lens group moving frame  18 , which in turn prevents deterioration in optical performance of the zoom lens barrel  1  due to eccentricity of the optical axis of the first lens group L1. In addition, the structure of the cam ring  15  wherein the set of three lens-drive cam grooves C 1  and the set of three cam grooves C 2 , whose cam profiles are similar (though differing slightly in shape) to each other, are formed on the cam ring  15  in slightly different positions thereon in the optical axis direction does not increase the wall thickness of the cam ring  15 ; moreover, external forces applied to the inner ring  17  in a direction radially inwards can be received by the first lens group moving frame  18  via the set of three follower pins  18   f  (i.e. the strength of the whole zoom lens barrel  1  can be reinforced). Furthermore, since the set of three follower pins  17   f  and the set of three follower pins  18   f  are respectively aligned side by side in a direction parallel to the optical axis O, the strength of the spring force of the three helical compression springs  30  that are held between the inner ring  17  and the first lens group moving frame  18  to bias the inner ring  17  and the first lens group moving frame  18  in opposite directions away from each other varies little even if the cam ring  15  rotates relative to the inner ring  17  and the first lens group moving frame  18 . Namely, since the direction of the helical compression springs  30  and aligned direction of the cam followers  17   f  and  18   f  are same and are parallel to the optical axis O, backlash with the cam grooves C 1  and the cam followers  17   f  and backlash with the cam grooves C 2  and cam followers  18   f  are absorbed by the helical compression springs  30 , and accordingly, the optical performance of the zoom lens can be reliably maintained wherever the cam followers  17   f  and  18   f  are positioned in the cam-grooves C 1  and C 2  respectively. 
     The barrier unit  40  is fixed to an inner surface of the main ring body  17   r  to be positioned therein. The barrier drive ring  31  is positioned in the inner ring  17  and held between the barrier unit  40  and the inner flange  17   c  of the inner ring  17  to be rotatable freely about the optical axis O. The cam ring  15  is provided at the front end thereof with a set of three recesses  15   k . The barrier drive ring  31  is provided on an outer peripheral surface thereof with a set of three engaging portions  31   a . The cam ring  15  is provided at one end (upper end as viewed in FIG. 8) of each recesses  15   k  with a rotation transfer face  15   d  which extends parallel to the optical axis O and extends through a corresponding opening  17   z  (see FIG. 7) provided on a circumferential portion of the inner flange  17   c . If the cam ring  15  rotates about the optical axis O in a barrier closing direction (clockwise as viewed from the front of the zoom lens barrel  1 ) with respect to the inner ring  17  with the set of three follower pins  17   f  being respectively engaged within the barrier drive sections C 2 B of the set of three cam grooves C 2  of the cam ring  15 , the three rotation transfer faces  15   d  firstly come into contact with the three engaging portions  31   a  of the barrier drive ring  31  and subsequently press the three engaging portions  31   a  to give a rotational force to the barrier drive ring  31  to close a pair of barrier blades  42 , respectively. As shown in FIG. 8, the set of three recesses  15   k  are formed on the cam ring  15  at portions thereon other than the portions where the three lens-drive cam grooves C 1  and the three cam grooves C 2  are formed. 
     As shown in FIGS. 2 and 14, the barrier unit  40  is provided with a barrier blade support front plate  41 , the pair of barrier blades  42 , two torsion springs  43  and a barrier blade support rear plate  44 , and is formed as a single assembly in advance. The barrier blade support front plate  41  is provided at the center thereof with a substantially rectangular photographing aperture  41   a , and is further provided, on an rear surface thereof on opposite sides of the photographing aperture  41   a , with two bosses  41   b , respectively, which extend rearwards. Each barrier blade  42  is provided at one end thereof with a hole in which one of the two bosses  41   b  is engaged so that each barrier blade  42  is rotatable about the associated boss  41   b . The two torsion springs  43  bias the pair of barrier blades  42  to rotate in opposite rotational directions to shut the pair of barrier blades  42 , respectively. The pair of barrier blades  42  are supported between the barrier blade support front plate  41  and the barrier blade support rear plate  44 . The barrier blade support rear plate  44  is provided at the center thereof with a central aperture  44   b  (see FIG. 2) thereof which is aligned with the photographing aperture  41   a  in the optical axis direction, and is further provided on opposite sides of the central aperture with two slots  44   a . As shown in FIGS. 12 and 13, each barrier blade  42  is provided in the vicinity of the associated boss  41   b  with an engaging projection  42   a  which extends rearward, toward the barrier drive ring  31 , to pass through the associated slot  44   a  of the barrier blade support rear plate  44 . The barrier drive ring  31  is provided on left and right sides of a central opening thereof with two drive projections  31   c  which are respectively engaged with the two engaging projections  42   a  of the pair of barrier blades  42 . FIG. 12 shows the pair of barrier blades  42  with chain lines in a closed state thereof, and FIG. 13 shows the pair of barrier blades  42  with chain lines in a fully open state thereof. FIG. 14 shows fundamental elements of the barrier unit  40  with the barrier blade support front plate  41  removed. 
     The barrier drive ring  31  is biased to rotate in a direction to open the pair of barrier blades  42  by a helical extension spring  45  whose opposite ends are hooked on an engaging projection  31   b  formed on the barrier drive ring  31  and an engaging projection  17   h  formed on a front surface of the inner flange  17   c  of the inner ring  17 . The spring force of the helical extension spring  45  is greater than the total spring force of the two torsion springs  43 . The two drive projections  31   c  of the barrier drive ring  31  come into contact with the two engaging projections  42   a  of the pair of barrier blades  42  to open the pair of barrier blades  42 , respectively, when the barrier drive ring  31  is in a fully rotated position thereof by the spring force of the helical extension spring  45  (see FIG.  13 ). If the barrier drive ring  31  is rotated in a direction to close the pair of barrier blades  42  against the spring force of the helical extension spring  45 , the two drive projections  31   c  respectively move away from the two engaging projections  42   a  of the pair of barrier blades  42  so that the pair of barrier blades  42  are closed by the spring force of the two torsion springs  43  (see FIG.  12 ). 
     The three rotation transfer faces  15   d  of the cam ring  15  respectively come into contact with the three engaging portions  31   a  of the barrier drive ring  31  to press the three engaging portions  31   a  against the spring force of the helical extension spring  45  to rotate the barrier drive ring  31 . When the cam ring  15  is in the accommodation position thereof, the three rotation transfer faces  15   d  are respectively in contact with the three engaging portions  31   a  of the barrier drive ring  31  via three through-slots  17   z  formed on the inner flange  17   c  of the inner ring  17 . The barrier drive ring  31  is rotated about the optical axis O against the spring force of the helical extension spring  45  to close the pair of barrier blades  42 . If the cam ring  15  rotates about the optical axis O in a barrier opening direction (counterclockwise as viewed from the front of the zoom lens barrel  1 ) with respect to the inner ring  17  with the set of three follower pins  17   f  being respectively engaged within the barrier drive sections C 2 B of the set of three cam grooves C 2  of the cam ring  15 , the three rotation transfer faces  15   d  are respectively disengaged from the three engaging portions  31   a  of the barrier drive ring  31  so that the barrier drive ring  31  is rotated in a direction to open the pair of barrier blades  42  by the spring force of the helical extension spring  45 . 
     FIG. 16 shows the movement of the three rotation transfer faces  15   d  of the cam ring  15  in the case where the cam ring  15  rotates so that each follower pin  15   b , which is engaged in the associated cam slot  13   b  of the stationary ring  13 , moves from the linear slot portion  13   b   1  to the state-changing slot portion  13   b   2  of the associated cam slot  13   b , i.e., from the accommodation position to the preparation section (see FIG.  11 ). Due to the engagement of the set of three follower pins  15   b  of the cam ring  15  with the set of three cam slots  13   b  and the set of three rotation transfer grooves  14   a , the cam ring  15  firstly rotates about the optical axis O while moving in the optical axis direction (each rotation transfer face  15   d  moves from a position “1—1” to a position “4—4” via positions “2—2” and “3—3” in FIG.  16 ), and subsequently rotates about the optical axis O without moving in the optical axis direction (each rotation transfer face  15   d  moves from the position “4—4” to a position “5—5” in FIG.  16 ). When moving from the position “4—4” to the position “5—5”, the three rotation transfer faces  15   d  of the cam ring  15  are respectively disengaged from the three engaging portions  31   a  of the barrier drive ring  31  to thereby open the pair of barrier blades  42  by the spring force of the helical extension spring  45 . Conversely, if the cam ring  15  rotates so that each follower pin  15   b  moves from the preparation section to the accommodation position, the movement of each rotation transfer face  15   d  from the position “5—5” to the position “4—4” causes the pair of barrier blades  42  to close. 
     Each of the pair of barrier blades  42  is formed as a substantially plane plate, and is provided on a rear face thereof with a semi-circular concave face  42   b  (see FIGS. 5,  6  and  17 ) so that the rear face of each barrier blade  42  does not come in contact with a frontmost surface (convex surface) L 1   r  of the first lens group L1. The two semi-circular concave faces  42   b  together form a circular concave face the shape of which corresponds to the shape of a central portion of the convex frontmost surface L 1   r  of the first lens group L1 in a state where the pair of barrier blades  42  are closed. The curvature of each semi-circular concave face  42   b  is determined to corresponds to the curvature of the frontmost surface L 1   r  of the first lens group L1. The concave faces  42   b  of the pair of barrier blades  42  make it possible to retreat the inner ring  17  to a rearward limit when the inner ring  17  is accommodated. The concave face  42   b  is formed on each barrier blade  42  when the barrier blades  42  are molded of synthetic resin. 
     After the reinforcing ring  17   x  is fitted on and adhered to the main ring body  17   r , the barrier unit  40  having the above described structure is fitted into the reinforcing ring  17   x  from the front thereof. The barrier blade support front plate  41  is provided on an outer peripheral edge thereof with a plurality of engaging portions which are respectively engaged with a corresponding plurality of hooks formed on an inner peripheral surface of the main ring body  17   r  in front of the inner flange  17   c  to prevent the barrier unit  40  from coming off the front of the inner ring  17 . The barrier drive ring  31  is held between the barrier unit  40  and the inner flange  17   c  of the inner ring  17  to be rotatable about the optical axis O. The main ring body  17   r , which is made of synthetic resin, is provided, at the front end thereof on opposite sides of the central circular opening of the main ring body  17   r , with two cutout portions  17   k  (see FIG. 14) in which respective outer edges of the pair of barrier blades  42  enter when the pair of barrier blades  42  are fully opened as shown in FIG.  14 . The radially outer ends of the two cutout portions  17   k  are fully covered by the reinforcing ring  17   x . The main body ring  17   r  can be provided with the two cutout portions  17   k  each formed as a through hole in a radial direction of the inner ring  17  due to the structure wherein the inner ring  17  is constructed from two separate elements: the synthetic-resin-made main body ring  17   r  and the metal reinforcing ring  17   x . Conventionally, if a set of barrier blades such as the pair of barrier blades  42  of the zoom lens barrel  1  is designed to consist of four blades, the total thickness of the four blades in the optical axis direction increases though the radial width of each blade is reduced. Conversely, if the set of barrier blades is designed to consist of one or two barrier blades, though the total thickness of the blade or blades in the optical axis direction is reduced, the radial width of each blade increases. However, in the present embodiment of the zoom lens barrel  1 , the formation of the two cutout portions  17   k  on the main body ring  17   r  that serve as recesses for the pair of barrier blades  42  contributes to further miniaturization of the diameter of the inner ring  17  without increasing the total thickness of the barrier blades  42  in the optical axis direction. 
     As has been described above, the zooming slot portion  13   b   3  of each cam slot  13   b  of the stationary ring  13  extends in a circumferential direction of the stationary ring  13  and does not extend in the optical axis direction. Therefore, the set of three follower pins  15   b  of the cam ring  15  rotate about the optical axis O without moving in the optical axis direction when following the zooming slot portions  13   b   3  of the set of three cam slots  13   b  in the zooming section (see FIG.  11 ). The zoom lens barrel  1  is provided between the housing  11  and the rotatable ring  14  with a biasing ring  32  which is fitted on a front part of the rotatable ring  14  to remove backlash and play between the set of three follower pins  15   b  and the zooming slot portions  13   b   3  of the set of three cam slots  13   b . The biasing ring  32  and the rotatable ring  14  are provided with three hooks  32   a  and corresponding three hooks  14   c , respectively. Opposite ends of three helical extension springs  33  are hooked on the three hooks  32   a  and the three hooks  14   c , respectively, to constantly bias the biasing ring  32  rearwards in the optical axis direction. The biasing ring  32  is provided, on an inner peripheral surface thereof at equi-angular intervals in a circumferential direction of the biasing ring  32 , with a set of three inward projections  32   c  which extend radially inwards, while the rotatable ring  14  is provided in the vicinity of the front end thereof with a corresponding set of three through-slots  14   d  which extend parallel to the optical axis O so that the set of three inward projections  32   c  penetrate the rotatable ring  14  via the set of three through-slots  14   d  in radially inward directions, respectively. The set of three through-slots  14   d  are formed on the rotatable ring  14  so as to be communicatively connected in front portions of the set of three rotation transfer grooves  14   a  to penetrate therethrough, so that the set of three inward projections  32   c  are positioned in front of the set of three follower pins  15   b  that are engaged in the set of three rotation transfer grooves  14   a , respectively. If each follower pin  15   b  of the cam ring  15  moves from the state-changing slot portion  13   b   2  to the zooming slot portion  13   b   3 , respective rear faces of the set of three inward projections  32   c  come into pressing contact with the set of three follower pins  15   b  to press each follower pin  15   b  rearward in the optical axis direction against the rear side edge of the associated zooming slot portion  13   b   3  to thereby remove backlash and play between the set of three follower pins  15   b  and the zooming slot portions  13   b   3  of the set of three cam slots  13   b.    
     In addition to the above described structures wherein the set of three linear guide grooves  18   c  are formed on an inner peripheral surface of the first lens group moving frame  18  while the set of three linear guide keys  19   a , which are respectively engaged in the set of three linear guide grooves  18   c , are formed on an outer peripheral surface of the second lens group moving frame  19 , a set of three circumferential recesses  18   h  are formed on the first lens group moving frame  18  at the front ends of the set of three linear guide grooves  18   c , respectively. Each circumferential recess  18   h  allows the associated linear guide key  19   a  of the second lens group moving frame  19  to move therein in a circumferential direction about the optical axis O, i.e., allows the second lens group moving frame  19  to rotate about the optical axis O relative to the first lens group moving frame  18  in a range corresponding to the circumferential length of the circumferential recess  18   h . The second lens group moving frame  19  can rotate about the optical axis O relative to the first lens group moving frame  18  along the three circumferential recesses  18   h  only when the second lens group moving frame  19  is in the vicinity of the accommodation position thereof. The first lens group moving frame  18  is provided on the inner flange  18   b  thereof with a set of three circumferential slots  18   j  (see FIGS.  3  and  6 ). The second lens group moving frame  19  is provided at the front end thereof with a set of three front projecting portions  19   e  on respective outer surfaces of which the three linear guide keys  19   a  are formed, respectively. When each linear guide key  19   a  is positioned in the associated circumferential recess  18   h , i.e., when the second lens group L2 is in the vicinity of the accommodation position thereof, the set of three front projecting portions  19   e  of the second lens group moving frame  19  penetrates through the inner flange  18   b  of the first lens group moving frame  18  to project forward from the inner flange  18   b  via the set of three circumferential slots  18   j , respectively. Accordingly, by allowing the three linear guide keys  19   a  to project forward from the inner flange  18   b  through the three circumferential slots  18   j , respectively, the length in the optical axis direction of the three linear guide grooves  18   c  and the circumferential recesses  18   h  which reliably carry out the engaging and disengaging of the three linear guide keys  19   a  with the three linear guide grooves  18   c , and the amount of movement of the first and second lens group moving frames  18  and  19  in the optical axis direction can be maintained without increasing the combined length of the first and second lens group moving frames  18  and  19  at the accommodation positions thereof. The reason why the second lens group moving frame  19  is allowed to rotate relative to the first lens group moving frame  18  along the three circumferential recesses  18   h  only when the second lens group moving frame  19  is in the vicinity of the accommodation position thereof will be hereinafter discussed. 
     In a state where the zoom lens barrel  1  is in an accommodation state, i.e., where each of the set of three follower pins  18   f  of the first lens group moving frame  18  is engaged in the first-lens-group accommodation section C 1 A 1  of the associated lens-drive cam groove C 1 , a rotation of the cam ring  15  in a direction to extend the zoom lens barrel  1  (in a direction indicated by an arrow “X” in FIG. 10, i.e., counterclockwise as viewed from the front of the zoom lens barrel  1 ) causes each follower pin  18   f  of the first lens group moving frame  18  to move from the first-lens-group accommodation section C 1 A 1  to the second-lens-group zooming section C 1 Z 2  of the associated lens-drive cam groove C 1 , to thereby cause the first lens group moving frame  18  to move forward in the optical axis direction. Such a movement of each follower pin  18   f  of the first lens group moving frame  18  is indicated stepwise by first, second, third and fourth positions “1a”, “2a”, “3a” and “4a” in FIG.  10 . Likewise, the corresponding movement of each follower pin  19   f  of the second lens group moving frame  19  is indicated stepwise by first, second, third and fourth positions “1b”, “2b”, “3b” and “4b” in FIG. 10, while the corresponding movement of each linear guide key  19   a  of the second lens group moving frame  19  is indicated stepwise by first, second, third and fourth positions “1c”, “2c”, “3c” and “4c” in FIG.  10 . 
     In addition, such a rotation of the cam ring  15  in the direction X shown in FIG. 10 causes each follower pin  19   f  of the second lens group moving frame  19  which is positioned in the second-lens-group accommodation section C 1 A 2  of the associated lens-drive cam groove C 1  to move from the position “1b” to the position “2b” in the second-lens-group accommodation section C 1 A 2  to come into contact with a surface XX of an inclined side edge of the second-lens-group accommodation section C 1 A 2  which is inclined with respect to a circumferential direction of the cam ring  15 . The position “2b” in the second-lens-group accommodation section C 1 A 2  is positioned on the inclined side edge β of the second-lens-group accommodation section C 1 A 2 . 
     A further rotational movement of the cam ring  15  in the same direction X causes each follower pin  19   f  of the second lens group moving frame  19  to slide on the surface XX of the inclined side edge β in a direction inclined to both the optical axis direction and the circumferential direction of the cam ring  15  in a manner such as the following. 
     At this time, each linear guide key  19   a  is in contact with a side surface (the lower surface as viewed in FIG. 10) of the associated circumferential recess  18   h  of the first lens group moving frame  18  (see the position “2c” of the linear guide key  19   a  shown in FIG.  10 ). Therefore, a forward movement of the first lens group moving frame  18  in the optical axis direction causes the first lens group moving frame  18  to push the second lens group moving frame  19  forward in the optical axis direction via the circumferential recesses  18   h  and the set of three linear guide keys  19   a , and at the same time, causes the second lens group moving frame  19  to rotate about the optical axis O relative to the first lens group moving frame  18  due to the sliding movement of each follower pin  19   f  of the second lens group moving frame  19  on the surface XX of the inclined side edge β from the position “2b” to the position “3b”. Namely, each linear guide key  19   a  moves from the associated circumferential recess  18   h  toward the associated linear guide groove  18   c  while sliding on the side surface (the lower surface as viewed in FIG. 10) of the associated circumferential recess  18   h.    
     Accordingly, if the second lens group moving frame  19  is rotated relative to the first lens group moving frame  18 , the first lens group moving frame  18  can move forward smoothly without interfering with the second lens group moving frame  19 . 
     Thereafter, each linear guide key  19   a  comes into contact with a side edge (the right side edge as viewed in FIG. 10) of the associated linear guide groove  18   c  of the first lens group moving frame  18  to thereby stop the rotation of the second lens group moving frame  19  (see the position “3c”). At this time, each linear guide key  19   a  is ready to enter the associated linear guide groove  18   c  of the first lens group moving frame  18 , so that a further forward movement of the first lens group moving frame  18  causes the set of three linear guide key  19   a  to enter the set of three linear guide grooves  18   c , respectively. After the set of three linear guide keys  19   a  have respectively entered the set of three linear guide grooves  18   c , the second lens group moving frame  19  is prevented from rotating about the optical axis O relative to the first lens group moving frame  18  by engagement of each linear guide key  19   a  with the associated linear guide groove  18   c , while each follower pin  19   f  of the second lens group moving frame  19  slides on the surface XX of the inclined side edge β from the position “3b” to “4b”, which causes the second lens group moving frame  19  to move linearly in a direction opposite to the direction of movement of the first lens group moving frame  18  (see the position “4b” in FIG.  10 ). 
     Further rotational movement of the cam ring  15  causes each follower pin  19   f  of the second lens group moving frame  19  to enter the first-lens-group accommodation section C 1 A 1  of the associated lens-drive cam groove C 1 . Thereafter, if the cam ring  15  rotates in the direction X, the first and second lens group moving frames  18  and  19  move linearly in the optical axis direction in accordance with the respective sections of the set of three lens-drive cam grooves C 1  while the second lens group moving frame  19  is guided linearly in the optical axis direction by the first lens group moving frame  18 . As can be understood from the above description, the substantially triangular shaped second-lens-group accommodation section C 1 A 2  of each lens-drive cam groove C 1  not only secures a clearance for the associated follower pin  19   f  to be movable freely in both the circumferential direction of the cam ring  15  and the optical axis direction within the triangular area, but also makes the second lens group moving frame  19  rotate relative to the first lens group moving frame  18  to lead each linear guide key  19   a  to a position so as to be engaged in the associated linear guide groove  18   c . Moreover, the substantially triangular shaped second-lens-group accommodation section C 1 A 2  of each lens-drive cam groove C 1  allows the first and second lens group moving frames  18  and  19  move in opposite directions in the optical axis direction to ensure the proper engagement of the first lens group moving frame  18  with the second lens group moving frame  19 . 
     On the other hand, in a state where the zoom lens barrel  1  is in a ready-to-photograph state, if the cam ring  15  rotates in a direction to retract the zoom lens barrel  1 , i.e., in a direction opposite to the direction X, each follower pin  18   f  and each follower pin  19   f  return to the first-lens-group accommodation section C 1 A 1  and the second-lens-group accommodation section C 1 A 2 , respectively. 
     The movement of each follower pin will be hereinafter discussed in detail. After passing the first-lens-group accommodation section C 1 A 1 , each follower pin  19   f  slides on the surface of a rear side edge a of the second-lens-group accommodation section C 1 A 2  to move rightward with respect to FIG.  10 . Upon reaching a position on the surface of the rear side edge a immediately before an end al (the upper end as viewed in FIG. 9) thereof, each linear guide key  19   a  comes out of the associated linear guide groove  18   c  to enter the associated circumferential recess  18   h , to thereby allow rotation of the second lens group moving frame  19  relative to the first lens group moving frame  18  possible. Thereafter, each follower pin  19   f  reaches the end α1 of the rear side edge a to rotate about the optical axis O together with the cam ring  15 , namely, the second lens group moving frame  19  rotates about the optical axis O relative to the first lens group moving frame  18 . Thereafter, since the cam ring  15  retreats in the optical axis direction (in the rightward direction with respect to FIG. 9) due to the engagement of the set of three follower pins  15   b  with the linear slot portions  13   b   1  of the set of three cam slots  13   b  of the stationary ring  13 , each follower pin  19   f  finally reaches a terminal α2 in the vicinity of the end al of the rear side edge α. In this manner, the first and second lens group moving frames  18  and  19  move to the respective accommodation positions smoothly. 
     Assuming that the second lens group moving frame  19  is moved to the accommodation position thereof with the second lens group moving frame  19  being guided only linearly in the optical axis direction in a manner similar to that of the first lens group moving frame  18 , each of the three lens-drive cam grooves C 1  has to be formed longer in a circumferential direction of the cam ring  15  (i.e., in an upward direction from the end α1 of the rear side edge a as viewed in FIG.  9 ). However, if the set of three lens-drive cam grooves C 1  are simply formed longer, theses grooves interfere with other cam grooves (e.g., the cam grooves C 2 ). To prevent this problem from occurring, the diameter of the cam ring  15  has to be increased. However, according to the present embodiment of the zoom lens barrel  1 , the portion of each of the three lens-drive cam groove C 1  which is used to accommodate the second lens group moving frame  19  can be designed short in a circumferential direction of the cam ring  15  within a range in which none of the three lens-drive cam grooves C 1  interfere with other cam grooves. This contributes to further miniaturization of the diameter of the cam ring  15 . 
     Since the second-lens-group accommodation section C 1 A 2  of each lens-drive cam groove C 1  is formed having a substantially triangular shape, each lens-drive cam groove C 1  is successfully formed as a short cam groove, which would need to be longer if formed as a linear cam groove. In addition, by forming each lens-drive cam groove C 1  as a short groove in such a manner, the set of three lens-drive cam grooves C 1  can be formed on the cam ring  15  with little inclination with respect to the circumferential direction of the cam ring  15 . Additionally, when the first and second lens group moving frames  18  and  19  move forward from the respective accommodation positions in the optical axis direction, each follower pin  19   f  moves in the second-lens-group accommodation section C 1 A 2  from the position “1b” to the position “4b” via the positions “2b” and “3b” in the above described manner while the second lens group moving frame  19  rotates about the optical axis O relative to the first lens group moving frame  18  because each lens-drive cam groove C 1  is provided with the substantially triangular shape second-lens-group accommodation section C 1 A 2 . 
     FIG. 15 shows the variation in the respective axial positions of first and second lens group moving frames  18  and  19  in a range of movement including a zooming section (between telephoto extremity and wide-angle extremity) and a retracting section (between wide-angle extremity and accommodation position). As can be understood from FIG. 15, the axial position of the first lens group moving frame  18  corresponds to the rotational position (angular position) of the cam ring  15  about the optical axis O due to the profile of each lens-drive cam groove C 1 , while the second lens group moving frame  19  rotates about the optical axis O relative to the cam ring  15  in a range R shown in FIG.  15 . 
     Friction produced between the light shield ring  19   c  of the second lens group moving frame  19  and the first lens frame  20  becomes a problem if the second lens group moving frame  19  rotates relative to the first lens group moving frame  18  in the accommodation position because the first lens frame  20 , which is supported by the first lens group moving frame  18 , is in contact with the light shield ring  19   c  at the mechanically contacting point P (see FIGS.  5  and  6 ). Such friction may cause the first lens frame  20  to rotate relative to the first lens group moving frame  18  to thereby deviate in the optical axis direction relative to the first lens group moving frame  18  because the male thread portion of the first lens frame  20  is in mesh with the female thread portion  18   d  of the inner flange  18   b . To prevent such deviation of the axial position of the first lens frame  20  from occurring, the light shield ring  19   c  is provided, on a front surface thereof with which a rear face of the first lens frame  20  comes into contact, with a low-frictional sheet  26  (see FIGS. 5,  6  and  7 ) which can be made of, e.g., a tetrafluoroethylene resin. 
     The overall movement of the zoom lens barrel  1 , having the above described structure, from the accommodation position to a ready-to-photograph position (a position in the zooming section) will be hereinafter discussed. When the zoom lens barrel  1  is in an accommodation state, the first lens frame  20  which is supported by the first lens group moving frame  18 , which is biased rearward by the three helical compression springs  30 , is retracted to the mechanically contacting point P where the first lens frame  20  comes in contact with the light shield ring  19   c  of the second lens group moving frame  19  due to the clearance between the first-lens-group accommodation section C 1 A 1  of each lens-drive cam groove C 1  of the cam ring  15  and the associated follower pin  18   f  of the first lens group moving frame  18 . The second lens group moving frame  19  is also retracted to the mechanically contacting point Q where the second lens group moving frame  19  comes in contact with the third lens frame  22  due to the clearance between the second-lens-group accommodation section C 1 A 2  of each lens-drive cam groove C 1  of the cam ring  15  and the associated follower pin  19   f  of the second lens group moving frame  19 . Furthermore, the third lens frame  22  is retracted to the mechanically contacting point R wherein the third lens frame  22  comes in contact with the housing  11  by the spring force of the helical compression spring  23  which presses the third lens frame  22  forward. With these three mechanical contacts at the mechanically contacting points P, Q and R, the length of the zoom lens barrel  1  in an accommodation state of the zoom lens barrel  1  is successfully reduced. When the zoom lens barrel  1  is in an accommodation state, the pair of barrier blades  42  are closed to shut the photographing aperture  41   a  (see FIG.  12 ), since the three rotation transfer faces  15   d  respectively press the three engaging portions  31   a  of the barrier drive ring  31  against the spring force of the helical extension spring  45  to rotate the barrier drive ring  31  in a direction to move the two drive projections  31   c  away from the two engaging projections  42   a  of the pair of barrier blades  42 , respectively. 
     In the accommodation state of the zoom lens barrel  1 , if the rotatable ring  14  rotates in a direction to extend the zoom lens barrel  1  relative to the stationary ring  13 , the cam ring  15  which is provided with the set of three follower pins  15   b , moves in the optical axis direction without rotating about the optical axis O due to the engagement of the follower pins  15   b  of the cam ring  15  with the inclined groove portions  14   a   2  of the rotatable ring  14  and the linear slot portions  13   b   1  of the stationary ring  13  (see FIG.  11 ). This linear movement of the cam ring  15  causes a side edge of the first-lens-group accommodation section C 1 A 1  of each lens-drive cam groove C 1  to push the associated follower pin  18   f  forward, and at the same time, causes a side edge of the second-lens-group accommodation section C 1 A 2  of each lens-drive cam groove C 1  to push the associated follower pin  19   f  forward. As a result, the first lens frame  20  and the second lens group moving frame  19  (the light shield ring  19   c ) which are in contact with each other at the mechanically contacting point P move linearly forward to release the contact therebetween, while the second lens group moving frame  19  which is in contact with the third lens frame  22  at the mechanically contacting point Q moves forward linearly to release the contact between the second lens group moving frame  19  with the third lens group L3. 
     If the rotatable ring  14  further rotates in the same direction to extend the zoom lens barrel  1  relative to the stationary ring  13 , the cam ring  15  moves in the optical axis direction while rotating about the optical axis O due to the engagement of the follower pins  15  of the cam ring  15  with the linear groove portions  14   a   1  of the rotatable ring  14  and the state-changing slot portions  13   b   2  of the stationary ring  13 , until the rotatable ring  14  reaches the zooming section. In an early state of this rotation of the cam ring  15  by the state-changing slot portions  13   b   2  of the stationary ring  13 , the three rotation transfer faces  15   d  of the cam ring  15  are respectively disengaged from the three engaging portions  31   a  of the barrier drive ring  31  so that the barrier drive ring  31  is rotated in a direction to open the pair of barrier blades  42  by the spring force of the helical extension spring  45  against the spring force of the two torsion springs  43 . Accordingly, the second lens group moving frame  19  rotates about the optical axis O relative to the first lens group moving frame  18  so that the first lens frame  20  slides on the low-frictional sheet  26  before and after the opening operation of the pair of barrier blades  42 . 
     When each follower pin  15   b  of the cam ring  15  reaches the zooming slot portion  13   b   3  of the associated cam slot  13   b  by rotation of the rotatable ring  14  in the same rotational direction, rear faces  32   b  of the set of three inward projections  32   c  of the biasing ring  32  come into contact with the set of three follower pins  15   b  of the cam ring  15 , respectively (see the zoom lens barrel  1  below the optical axis O in FIG.  7 ). Each follower pin  15   b  is pressed against the rear side edge of the zooming slot portion  13   b   3  of the associated cam slot  13   b  by the rear face  32   b  of the associated inward projection  32   c  since the biasing ring  32  is biased rearward by the three helical extension springs  33 . This state is maintained as long as each follower pin  15   b  is engaged in the zooming slot portion  13   b   3  of the associated cam slot  13   b , while backlash and play of the cam ring  15  with respect to the stationary barrel  13  is removed as long as the cam ring  15  rotates within the zooming section shown in FIG. 11 via the rotatable ring  14 . 
     If the cam ring  15  rotates in a direction from the accommodation rotational position to the zooming section via the preparation section (i.e., in the barrier opening direction), each follower pin  18   f  of the first lens group moving frame  18  which is engaged in the first-lens-group accommodation section C 1 A 1  moves from the first-lens-group accommodation section C 1 A 1  to the first-lens-group zooming section C 1 Z 1  via the second-lens-group zooming section C 1 Z 2 , while each follower pin  19   f  of the second lens group moving frame  19  which is engaged in the second-lens-group accommodation section C 1 A 2  moves from the second-lens-group accommodation section C 1 A 2  to the second-lens-group zooming section C 1 Z 2  via the first-lens-group accommodation section C 1 A 1 . If the cam ring  15  rotates in the zooming range (i.e., in the first-lens-group zooming section C 1 Z 1  and the second-lens-group zooming section C 1 Z 2 ), the first and second lens group moving frames  18  and  19  (the first and second lens groups L1 and L2) move in the optical axis direction in respective zoom paths thereof in accordance with the profiles of the first-lens-group zooming section C 1 Z 1  and the second-lens-group zooming section C 1 Z 2 , to thereby vary the focal length of the photographing optical system which includes the first, second and third lens groups L1, L2 and L3, i.e., to perform a zooming operation. This zooming operation is carried out by manually operating a conventional zoom switch (not shown). Immediately after a release button is depressed, the aforementioned step motor (not shown), which drives feed screw  24  to move the third lens frame  22  (the third lens group L3), rotates by an amount of rotation corresponding to information on a photographing distance to move the third lens group L to bring an object into focus. The shutter unit  21  drives the shutter blades  21   a  to open and close in accordance with the information on the object brightness. 
     If the first lens group moving frame  18  moves linearly in the optical axis direction, the inner ring  17  also moves in the optical axis direction without varying the position thereof relative to the first lens group moving frame  18  due to the engagement of the set of three follower pins  17   f  with the set of three cam grooves C 2  of the cam ring  15 , the profiles of which are similar to those of the lens-drive cam grooves C 1 . At the same time, the outer ring  16  and the inner ring  17 , the respective outer peripheral surfaces of which are exposed to the outside of the zoom lens barrel  1 , move together in the optical axis direction since the outer ring  16  moves together with the cam ring  15  in the optical axis direction at all times due to the engagement of the set of three bayonet prongs  16   d  with the circumferential groove  15   c.    
     If the cam ring  15  rotates in a direction from the zooming section via the preparation section (i.e., in the barrier closing direction), the outer and inner rings  16  and  17  retract together in the optical axis direction by operations reverse to the above described operations. Subsequently, the first lens frame  20 , which supports the first lens group L 1 , and the second lens group moving frame  19 , which supports the second lens group L2, come into contact with each other at their respective rear ends via the three helical compression springs  30 , while the second lens group moving frame  19  retreats until coming into contact with the third lens frame  22  to push the third lens frame  22  against the filter holding portion  11   c  against the helical compression spring  23 , which presses the third lens frame  22  forward. At the same time, the three rotation transfer faces  15   d  respectively press the three engaging portions  31   a  of the barrier drive ring  31  against the spring force of the helical extension spring  45  to rotate the barrier drive ring  31  in a direction to close the pair of barrier blades  42  to shut the photographing aperture  41   a.    
     In the present embodiment of the zoom lens barrel  1 , as described above, the second-lens-group zooming sections C 1 Z 2  of the set of three lens-drive cam grooves C 1 , which are used for driving the set of three follower pins  19   f  of the second lens group moving frame  19 , are designed as mere passing sections for the set of three follower pins  18   f  of the first lens group moving frame  18  via which the set of three follower pins  18   f  move from the first-lens-group accommodation position to a ready-to-photograph position in a zooming section between the telephoto extremity Z 1 T and the wide-angle extremity Z 1 W, so that each of the three follower pins  18   f  and the corresponding one of the three follower pins  19   f  are fitted in a single lens-drive cam groove of the three lens-drive cam grooves C 1 . This construction contributes to a reduction of the number of cam grooves to be formed on the cam ring  15 , which in turn contributes to a reduction of the angle of inclination of the cam grooves with respect to a circumferential direction of the cam ring  15 . 
     In addition, since the first lens group moving frame  18  is biased rearward by the three helical compression springs  30 , the first lens frame  20 , which is supported by the first lens group moving frame  18  positioned at the accommodation position thereof, can retract up to a point (the mechanically contacting point P) where the first lens frame  20  comes in contact with the light shield ring  19   c  of the second lens group moving frame  19  due to the clearance between the first-lens-group accommodation section C 1 A 1  of each lens-drive cam groove C 1  of the cam ring  15  and the associated follower pin  18   f  of the first lens group moving frame  18 . Likewise, the second lens group moving frame  19  can retract up to a point (the mechanically contacting point Q) where the second lens group moving frame  19  comes in contact with the third lens frame  22  due to the clearance between the second-lens-group accommodation section C 1 A 2  of each lens-drive cam groove C 1  of the cam ring  15  and the associated follower pin  19   f  of the second lens group moving frame  19 . Due to such structures having the mechanically contacting points P and Q, the length of the zoom lens barrel  1  in an accommodation state thereof is successfully reduced as compared with a conventional zoom lens barrel in which the respective accommodation positions of first and second lens groups which correspond to the first and second lens groups L1 and L2 of the present embodiment of the zoom lens barrel are precisely determined by associated cam grooves. 
     The axial cross sectional view of the zoom lens barrel  1  above the optical axis O in each of FIGS. 5,  6  and  7  shows an accommodation state of the zoom lens barrel  1  where the first lens frame  20  is in contact with the light shield ring  19   c  of the second lens group moving frame  19 , where the second lens group moving frame  19  is in contact with the third lens frame  22  and where the third lens frame  22  is in contact with the housing  11 . The amount of rearward movement of the first lens group moving frame  18  depends on the position of the first lens frame  20  because the position of the first lens frame  20  relative to the first lens group moving frame  18  varies due to an adjustment of the thread engagement position of the male thread portion of the first lens frame  20  with respect to the female thread portion  18   d  of the inner flange  18   b  during assembly. Such a variation due to the adjustment is absorbed by extension or compression of the helical compression springs  30  so that the zoom lens barrel  1  can be accommodated with the first lens frame  20 , the second lens group moving frame  19  and the third lens frame  22  being in contact with the light shield ring  19   c , the third lens frame  22  and the housing  11  at the mechanically contacting points P, Q and R, respectively. 
     Moreover, although the first lens frame  20  and the second lens group moving frame  19  abut against each other, the first lens group L1 and the second lens group L2 do not abut against each other. Therefore, there is no chance of forming any scratch on either the rearmost surface of the first lens group L1 or the frontmost surface of the second lens group L2. 
     Furthermore, since the first lens frame  20  is brought into contact with the second lens group moving frame  19 , it is not necessary to secure a marginal space between the first and second lens groups L1 and L2 for making an adjustment to the fixing position of the first lens frame  20  relative to the first lens group moving frame  18  in the optical axis direction, wherein the marginal space corresponds to the amount of thread engagement of the first lens frame  20  with the first lens group moving frame  18 . This contributes to further miniaturization of the length of the zoom lens barrel  1  in an accommodation state thereof. 
     Although the first and second lens groups L1 and L2 are accommodated with the first lens frame  20  being in contact with the second lens group moving frame  19  in the present embodiment of the zoom lens barrel  1 , the first and second lens groups L1 and L2 can be provided with a contacting surface of the first lens group L1 being in contact with a corresponding contacting surface of the second lens group L2 if such two contacting surfaces are respectively formed on the first and second lens groups L1 and L2. 
     When the first lens frame  20 , the second lens group moving frame  19  and the third lens frame  22  are positioned at their respective accommodation positions, the frontmost surface L 1   r  of the first lens group L1 does not come into contact with the pair of barrier blades  42  because each of the pair of barrier blades  42  is provided on a rear face thereof with the concave face  42   b  even, if the frontmost lens element of the first lens group L1 slightly moves back and forth due to contraction/expansion of the helical compression springs  30 . 
     The above described linear guiding mechanism for guiding the first and second lens group moving frames  18  and  19  in the optical axis direction without rotating about the optical axis O is not limited solely to such a particular mechanism as long as the general concept of the set of lens-drive cam grooves C 1  is applied. 
     Although the set of three lens-drive cam grooves C are formed on the cam ring  15  in the above illustrated embodiment of the zoom lens barrel  1 , a similar effect can in theory be expected with only one lens-drive cam groove C 1 . 
     The present invention can be applied not only to a extendable zoom lens barrel, the axial length of which in an accommodation state being generally difficult to be minimized, but also to a extendable fixed-focal-length lens barrel having a plurality of movable lens groups. 
     As can be understood from the foregoing, according to a lens barrel to which the present invention is applied, since a follower pin for moving a lens group and another follower pin for moving another lens group can be engaged in a single cam groove, the number of cam grooves which are to be formed on a cam ring can be minimized even if the lens barrel is provided with a large number of movable lens groups. Accordingly, the cam ring does not need to have a large diameter, and the cam ring can be provided with cam grooves which ensure smooth movements of the associated follower pins while keeping a decrease in strength of the cam ring to a minimum. 
     Moreover, each of a plurality of lens groups provided in the lens barrel can be accommodated as compact as possible while preventing adjacent lens groups from contacting each other, even if the lens barrel is of a type wherein at least one lens group is fixed to the lens support frame thereof while the fixing position of the lens group is being adjusted relative to the lens support frame thereof in the optical axis direction, or even if the lens barrel is of a extendable lens barrel provided at the front end thereof with a photographing aperture and a lens barrier which opens and closes the photographing aperture. 
     Furthermore, even if the zoom lens barrel having two or more lens groups is fully retracted, these lens groups can be accommodated compact in the retracted zoom lens barrel. 
     Obvious changes may be made in the specific embodiment of the present invention described herein, such modifications being within the spirit and scope of the invention claimed. It is indicated that all matter contained herein is illustrative and does not limit the scope of the present invention.