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Home | Alpha Telephone | Domain Names | Web Hosting | Get Traffic | xrEvidence | xrSoccer United States Patent
Optical element retracting mechanism for a photographing lens A retracting mechanism includes a linearly movable ring; a swingable holder supported inside the linearly movable ring; a position-controller holding the swingable holder; a pair of support plates attached to opposite ends of the linearly movable ring; a support plate fixing device fixing the support plates to the linearly movable ring; a rotatable shaft having a pair of eccentric pins at opposite ends thereof, and having a common axis eccentric to the rotatable shaft; and a pair of elongated holes on the pair of support plates, facing each other and elongated parallel to each other, the eccentric pins being engaged in the elongated holes and being movable therein. The pair of support plates are movable relative to the linearly movable ring in directions orthogonal to the optical axis via rotation of the rotatable shaft when the support plate fixing device is in a released state.
Primary Examiner: Gray; David M. Attorney, Agent or Firm: What is claimed is: 1. A retracting mechanism for a retractable lens including an optical system having a plurality of optical elements, said optical element retracting mechanism comprising: a linearly movable ring configured to be guided along an optical axis of said optical system without rotating, said ring further configured to retract toward a plane along said optical axis when said retractable lens moves from an operational state to a fully-retracted state; a swingable holder pivoted on a pivot and swingable about said pivot, said swingable holder positioned inside and supported by said linearly movable ring, said swingable holder supporting a retractable optical element as one of said plurality of optical elements; a position-controller holding said swingable holder such that said retractable optical element remains on said optical axis when said retractable lens is in said ready-to-photograph state, said position-controller configured to rotate said swingable holder about said pivot such that said retractable optical element retracts to a position which deviates from said optical axis when said linearly movable ring, together with said swingable holder, retracts toward said plane; a pair of support plates attached to opposite ends of said linearly movable ring in said optical axis direction, and support opposite ends of said pivot, respectively; a support plate fixing device fixing said pair of support plates to said linearly movable ring, wherein said support plate fixing device is configured to allow said pair of support plates to move relative to said linearly movable ring in directions lying in a plane orthogonal to said optical axis when said support plate fixing device is in a released state; at least one rotatable shaft having a shaft axis substantially parallel to said optical axis, supported by said linearly movable ring to be rotatable about said shaft axis, said rotatable shaft having a pair of eccentric pins at opposite ends thereof, said pair of eccentric pins having a common axis eccentric to said shaft axis of said rotatable shaft; and at least one pair of elongated holes respectively on said pair of support plates, facing each other and elongated substantially parallel to each other, said pair of eccentric pins being engaged in said pair of elongated holes and configured to be movable therein; wherein said pair of support plates are configured to be moved relative to said linearly movable ring in said directions lying in said plane orthogonal to said optical axis via a rotation of said rotatable shaft, without substantially changing a relative position between said pair of support plates, when said support plate fixing device is in said released state. 2. The optical element retracting mechanism according to claim 1, wherein said linearly movable ring comprises a pair of substantially parallel flat surfaces which are separate from each other in said optical axis direction, which extend in a direction substantially orthogonal to said optical axis, and which do not overlap said retractable optical element in said optical axis direction, said pair of support plates pressed against a respective said pair of parallel flat surfaces and fixed thereto by said support plate fixing device. 3. The optical element retracting mechanism according to claim 1, further comprising an internal optical element positioned inside said linearly movable ring on one of opposite sides of said retractable optical element in said optical axis direction, wherein said pair of support plates are attached to said opposite ends of said linearly movable ring and positioned on opposite sides of said internal optical element in said optical axis direction, respectively. 4. The optical element retracting mechanism according to claim 3, wherein said internal optical element comprises at least one of a shutter and a diaphragm. 5. The optical element retracting mechanism according to claim 1, wherein said support plate fixing device comprises: a screw hole located on one of said pair of support plates and penetrating therethrough in said optical axis direction; a screw insertion hole located on the other of said pair of support plates and penetrating therethrough in said optical axis direction; and a set screw inserted into said screw insertion hole and screwed through said screw hole. 6. The optical element retracting mechanism according to claim 1, wherein said rotatable shaft comprises a first rotatable shaft and a second rotatable shaft; wherein said pair of elongated holes comprises a first pair of elongated holes and a second pair of elongated holes; wherein said pair of eccentric pins of said first rotatable shaft are engaged in said first pair of elongated holes, respectively; wherein said pair of eccentric pins of said second rotatable shaft are engaged in said second pair of elongated holes, respectively; and wherein a direction of elongation of said first pair of elongated holes and a direction of elongation of said second pair of elongated holes are generally orthogonal to each other on said pair of support plates, respectively. 7. The optical element retracting mechanism according to claim 1, wherein said swingable holder further comprises: a cylindrical lens holder portion holding said retractable optical element; a pivoted cylindrical portion fitted on said pivot to be rotatable thereon; and a swing arm portion extending between said cylindrical lens holder and said pivoted cylindrical portion and connecting said cylindrical lens holder to said pivoted cylindrical portion. 8. The optical element retracting mechanism according to claim 1, wherein said position-control device comprises: a spring configured to bias said swingable holder to rotate in a direction to position said retractable optical element on said optical axis; and a cam configured to rotate said swingable holder to said deviated position from said optical axis, against the biasing force of said spring, when said linearly movable ring, together with said swingable holder, retracts toward said plane. 9. The optical element retracting mechanism according to claim 1, wherein said plurality of optical elements comprise at least one rear optical element positioned behind said retractable optical element when said retractable lens is in said operational state; and wherein said retractable optical element is configured to be positioned in an off-axis space radially outside an on-axis space in which said rear optical element is positioned, such that said retractable optical element and said rear optical element are in substantially a same positional range in the optical axis direction, when said retractable lens is in said fully-retracted state. 10. The optical element retracting mechanism according to claim 1, wherein said pivot extends substantially parallel to said optical axis. 11. The optical element retracting mechanism according to claim 1, wherein said retractable optical element comprises a lens group. 12. The optical element retracting mechanism according to claim 1, wherein said optical system comprises a zoom photographing optical system; and wherein said retractable optical element comprises a lens group as a part of said zoom photographing optical system. 13. The optical element retracting mechanism according to claim 1, wherein said optical element retracting mechanism is incorporated in a digital camera. 14. The optical element retracting mechanism according to claim 1, wherein at least one elongated hole of said pair of elongated holes that are respectively located on said pair of support plates comprises a through hole which penetrates through a corresponding one of said pair of support plates in said optical axis direction, and wherein one of said pair of eccentric pins which is engaged in said through hole includes an operating portion via which said one of said pair of eccentric pins can be rotated. 15. The optical element retracting mechanism according to claim 14, wherein said operating portion of said rotatable shaft is provided on an end of a front eccentric pin of said pair of eccentric pins, wherein said optical element retracting mechanism further comprises: an outer barrel surrounding said linearly movable ring, and provided with a radially inward flange positioned in front of said linearly movable ring, wherein said radially inward flange has at least one through hole which penetrates through said radially inward flange in said optical axis direction, said operating portion accessible from the front of said linearly movable ring via said through hole. 16. The optical element retracting mechanism according to claim 14, wherein said support plate fixing device comprises: a screw hole located on one of said pair of support plates and penetrating therethrough in said optical axis direction; a screw insertion hole located on the other of said pair of support plates and penetrating therethrough in said optical axis direction; and a set screw inserted into said screw insertion hole and screwed through said screw hole, wherein one of opposite ends of said set screw, which is directed toward a side toward which said operating portion is directed, comprises an operating portion via which said set screw can be rotated. 17. The optical element retracting mechanism according to claim 16, wherein said operating portion of said set screw faces toward a frontward direction in the optical axis direction, wherein said optical element retracting mechanism further comprises: an outer barrel surrounding said linearly movable ring, and provided with a radially inward flange positioned in front of said linearly movable ring, wherein said radially inward flange includes at least one through hole which penetrates through said radially inward flange in said optical axis direction, said operating portion of said set screw accessible from the front of said linearly movable ring via said through hole. 18. The optical element retracting mechanism according to claim 15, wherein said retractable lens comprises a lens barrier mechanism detachably attached to a front part of said radially inward flange to cover said through hole. 19. The optical element retracting mechanism according to claim 17, wherein said retractable lens comprises a lens barrier mechanism detachably attached to a front part of said radially inward flange to cover said through hole. 20. The optical element retracting mechanism according to claim 15, wherein said outer barrel supports one of said plurality of optical elements which is positioned in front of said retractable optical element, said outer barrel retracting toward said plane together with said linearly movable ring along said optical axis when said retractable lens moves from said operational state to said fully-retracted state. 21. The optical element retracting mechanism according to claim 17, wherein said outer barrel supports one of said plurality of optical elements positioned in front of said retractable optical element, said outer barrel retracting toward said plane together with said linearly movable ring along said optical axis when said retractable lens moves from said operational state to said fully-retracted state. 22. The optical element retracting mechanism according to claim 14, wherein said operating portion comprises a slot in which an adjusting tool can be engaged. 23. The optical element retracting mechanism according to claim 16, wherein said operating portion of said set screw comprises a slot in which an adjusting tool can be engaged. 24. A digital camera having a body and a lens barrel, the lens barrel housed within the body, the lens barrel comprising a retractable lens including an optical system having a plurality of optical elements, the lens barrel further, comprising a retracting mechanism, the retracting mechanism comprising: a lineraly movable ring configured to be guided along an optical axis of said optical system, said ring further configured to retract toward a plane along said optical axis when said retractable lens moves from an operational state to a retracted state; a swingable holder pivoted on a pivot and swingable about said pivot, sand swingable holder positioned inside and supported by said linearly movable ring, said swingable holder supporting a retractable optical element as one of said plurality of optical elements; a position-controller configured to hold said swingable holder such that said retractable optical element remains on said optical axis when said retractable lens is in said ready-to-photograph state, said position-controller further configured to rotate said swingable holder about said pivot such that said retractable optical element retracts to a position which deviates from said optical axis when said linearly movable ring, together with said swingable holder, retracts toward said plane; a pair of support plates attached to opposite ends of said linearly movable ring generally in said optical axis direction, and support opposite ends of said pivot, respectively; a support plate fixing device configured to fix said pair of support plates to said linearly movable ring, wherein said support plate fixing device is further configured to allow said pair of support plates to move relative to said linearly movable ring in directions lying in a plane generally orthogonal to said optical axis when said support plate fixing device is in a released state; at least one rotatable shaft having a shaft axis generally parallel to said optical axis, supported by said linearly movable ring to be rotatable about said shaft axis, said rotatable shaft having a pair of ecentric pins at opposite ends thereof, said pair of ecentric pins having a common axis ecentric to tsaid shaft axis of said rotatable shaft; and at least one pair of elongated holes repectively on said pair of support plates, facing each other and elongated substantially parallel to each other, said pair of ecentric pins being engaged in said pair of elongated holes and configured to be movable therein; wherein said pair of support plates are configured to be moved relative to said linearly movable ring in said directions lying in said plane generally orthogonal to said optical axis via a rotation of said rotatable shaft, without substantially changing a relative position between said pair of support plates, when said support plate fixing device is in said released state. 25. The camera according to claim 24, wherein said linearly movable ring comprises a pair of substantially parallel flat surfaces which are separate from each other in said optical axis direction, which extend in a direction substantially orthogonal to said optical axis, and which do not overlap said retractable optical element in said optical axis direction, said pair of support plates pressed against a respective said pair of parallel flat surfaces and fixed thereto by said support plate fixing device. 26. The camera according to claim 24, fruther comprising an internal optical element positioned inside said linearly movable ring on one of opposite sides of said retractable optical element in said optical axis direction, wherein said pair of support plates are attached to said opposite ends of said linearly movable ring and positioned on opposite sides of said internal optical element in said optical axis direction, respectively. 27. The camera according to claim 24, wherein said support plate fixing device comprises: a screw hole located on one of said pair of support plates and penetrating therethrough in said optical axis direction; a screw insertion hole located on the other of said pair of support plates and penetrating therethrough in said optical axis direction; and a set screw inserted into said screw insertion hole and screwed through said screw hole. 28. The camera according to claim 24, wherein said rotatable shaft comprises a first rotatable shaft and a second rotatable shaft; wherein said pair of elongated holes comprises a first pair of elongated holes and a second pair of elongated holes; wherein said pair of ecentric pins of said first rotatable shaft are engaged in said first pair of elongated holes, respectively; wherein said pair of ecentric pins of said second rotatable shaft are engaged in said second pair of elongated holes, repectively; and wherein a direction of elongation of said first pair of elongated holes and a direction of elongation of said second pair of elongated holes are generally orthogonal to each other on said pair of support plates, respectively. 29. The camera according to claim 24, wherein said swingable holder further comprises: a cylindrical lens holder portion holding said retractable optical element; a pivoted cylindrical portion fitted on said pivot to be rotatable thereon; and a swing arm portion extending between said cylindrical lens holder and said pivoted cylindrical portion and connecting said cylindrical lens holder to said pivoted cylindrical portion. 30. The camera according to claim 24, wherein said position-control device comprises: a spring configuredto bias said swingable holder to rotate in a direction to position said retractable optical element on said optical axis; and a cam configured to rotate said swingable holder to said deviated position from said optical axis, against the biasing force of said spring, when said linearly movable ring, together with said swingable holder, retracts toward said plane. 31. The camera according to claim 24, wherein said plurality of optical elements comprise at least one rear optical element positioned behind said retractable optical element when said retractable lens is in said operational state; and wherein said retractable optical element is configured to be positioned in an off-axis space radially outside an on-axis space in which said rear optical element is positioned, such that said retractabl optical element and said rear optical element are in substantially a same positional range in the optical axis direction, where said retractable lens is in said retrated state. 32. The camera according to claim 24, wherein said pivot extends generally parallel to said optical axis. 33. The camera according to claim 24, wherein said retractable optical element comprises a lens group. 34. The camera according to claim 24, wherein said optical system comprises a zoom photographing optical system; and wherein said retractable optical element comprises a lens group as a part of said zoom photographing optical system. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mechanism, incorporated in a retractable photographing (imaging) lens (retractable lens barrel), for retracting a part of a plurality of optical elements, constituting a photographing optical system, to a position deviating from the photographing optical axis of the photographing optical system when the photographing lens is fully retracted. The present invention also relates to a mechanism, which can be incorporated in a photographing lens, for positioning a supported element such as an internal element of the photographing lens. 2. Description of the Related Art Miniaturization of lens barrels incorporated in optical devices such as cameras has been in increasing demand. Above all, further miniaturization of retractable photographing lenses, specifically the length thereof in a non-operating state, has been in strong demand. To meet such demands, the inventor of the present invention has proposed a retractable photographing lens disclosed in U.S. patent application Ser. No. 10/368342 in which an optical element of a photographing optical system is retracted to a position deviating from the photographing optical axis of the photographing optical system, and at the same time, the optical element (together with other optical elements of the photographing optical system) is retracted toward a picture plane when the photographing lens is fully retracted. The mechanism achieving such complicated operations of the optical elements is required to operate with a high degree of accuracy. Moreover, it is required that the position of the retractable optical element can be easily adjusted with a high degree of positioning accuracy with a simple structure. Furthermore, it is required to provide the photographing lens with a simple two-dimensional positioning device with which the position of a supported element (e.g., a movable lens frame or holder) can be adjusted two-dimensionally in directions along a flat plane. SUMMARY OF THE INVENTION The present invention provides a mechanism, incorporated in a retractable photographing lens (retractable lens barrel), which is capable of retracting an optical element of a photographing optical system to a position deviating from the photographing optical axis of the photographing optical system, and at the same time, retracting the optical element toward a picture plane with a high degree of accuracy, wherein the mechanism is provided with a positioning structure with which the position of the optical element can be adjusted. The present invention provides a simple mechanism, which can be incorporated in a photographing lens, for positioning a supported element such as an internal element of the photographing lens, wherein the position of the supported element can be easily adjusted with the positioning mechanism. According to an aspect of the present invention, a retracting mechanism for a retractable lens including an optical system having a plurality of optical elements is included, the optical element retracting mechanism including a linearly movable ring configured to be guided along an optical axis of the optical system without rotating, the ring further configured to retract toward a plane along the optical axis when the retractable lens moves from an operational state to a fully-retracted state; a swingable holder pivoted on a pivot and swingable about the pivot, the swingable holder positioned inside and supported by the linearly movable ring, the swingable holder supporting a retractable optical element as one of the plurality of optical elements; a position-controller holding the swingable holder such that the retractable optical element remains on the optical axis when the retractable lens is in the ready-to-photograph state, the position-controller configured to rotate the swingable holder about the pivot such that the retractable optical element retracts to a position which deviates from the optical axis when the linearly movable ring, together with the swingable holder, retracts toward the plane; a pair of support plates attached to opposite ends of the linearly movable ring in the optical axis direction, and support opposite ends of the pivot, respectively; a support plate fixing device fixing the pair of support plates to the linearly movable ring, wherein the support plate fixing device is configured to allow the pair of support plates to move relative to the linearly movable ring in directions lying in a plane orthogonal to the optical axis when the support plate fixing device is in a released state; at least one rotatable shaft having a shaft axis substantially parallel to the optical axis, supported by the linearly movable ring to be rotatable about the shaft axis, the rotatable shaft having a pair of eccentric pins at opposite ends thereof, the pair of eccentric pins having a common axis eccentric to the shaft axis of the rotatable shaft; and at least one pair of elongated holes respectively on the pair of support plates, facing each other and elongated substantially parallel to each other, the pair of eccentric pins being engaged in the pair of elongated holes and configured to be movable therein. The pair of support plates are configured to be moved relative to the linearly movable ring in the directions lying in the plane orthogonal to the optical axis via a rotation of the rotatable shaft, without substantially changing a relative position between the pair of support plates, when the support plate fixing device is in the released state. The linearly movable ring can include a pair of substantially parallel flat surfaces which are separate from each other in the optical axis direction, which extend in a direction substantially orthogonal to the optical axis, and which do not overlap the retractable optical element in the optical axis direction, the pair of support plates pressed against a respective the pair of parallel flat surfaces and fixed thereto by the support plate fixing device. The optical element retracting mechanism can further include an internal optical element positioned inside the linearly movable ring on one of opposite sides of the retractable optical element in the optical axis direction, wherein the pair of support plates are attached to the opposite ends of the linearly movable ring and positioned on opposite sides of the internal optical element in the optical axis direction, respectively. The internal optical element can include at least one of a shutter and a diaphragm. The support plate fixing device can include a screw hole located on one of the pair of support plates and penetrating therethrough in the optical axis direction; a screw insertion hole located on the other of the pair of support plates and penetrating therethrough in the optical axis direction; and a set screw inserted into the screw insertion hole and screwed through the screw hole. The rotatable shaft can include a first rotatable shaft and a second rotatable shaft. The pair of elongated holes can include a first pair of elongated holes and a second pair of elongated holes. The pair of eccentric pins of the first rotatable shaft are engaged in the first pair of elongated holes, respectively. The pair of eccentric pins of the second rotatable shaft are engaged in the second pair of elongated holes, respectively. A direction of elongation of the first pair of elongated holes and a direction of elongation of the second pair of elongated holes are generally orthogonal to each other on the pair of support plates, respectively. The swingable holder can further include a cylindrical lens holder portion holding the retractable optical element; a pivoted cylindrical portion fitted on the pivot to be rotatable thereon; and a swing arm portion extending between the cylindrical lens holder and the pivoted cylindrical portion and connecting the cylindrical lens holder to the pivoted cylindrical portion. The position-control device can include a spring configured to bias the swingable holder to rotate in a direction to position the retractable optical element on the optical axis; and a cam configured to rotate the swingable holder to the deviated position from the optical axis, against the biasing force of the spring, when the linearly movable ring, together with the swingable holder, retracts toward the plane. The plurality of optical elements can include at least one rear optical element positioned behind the retractable optical element when the retractable lens is in the operational state. The retractable optical element is configured to be positioned in an off-axis space radially outside an on-axis space in which the rear optical element is positioned, such that the retractable optical element and the rear optical element are in substantially a same positional range in the optical axis direction, when the retractable lens is in the fully-retracted state. It is desirable for the pivot to extend substantially parallel to the optical axis. The retractable optical element can be a lens group. The optical system can be a zoom photographing optical system, and the retractable optical element can be a lens group as a part of the zoom photographing optical system. It is desirable for the optical element retracting mechanism to be incorporated in a digital camera. It is desirable for at least one elongated hole of the pair of elongated holes that are respectively located on the pair of support plates to be a through hole which penetrates through a corresponding one of the pair of support plates in the optical axis direction, and one of the pair of eccentric pins which is engaged in the through hole to include an operating portion via which the one of the pair of eccentric pins can be rotated. It is desirable for the operating portion of the rotatable shaft to be provided on an end of a front eccentric pin of the pair of eccentric pins. The optical element retracting mechanism further includes an outer barrel surrounding the linearly movable ring, and provided with a radially inward flange positioned in front of the linearly movable ring. The radially inward flange has at least one through hole which penetrates through the radially inward flange in the optical axis direction, the operating portion accessible from the front of the linearly movable ring via the through hole. The support plate fixing device can include a screw hole located on one of the pair of support plates and penetrating therethrough in the optical axis direction; a screw insertion hole located on the other of the pair of support plates and penetrating therethrough in the optical axis direction; and a set screw inserted into the screw insertion hole and screwed through the screw hole. One of opposite ends of the set screw, which is directed toward a side toward which the operating portion is directed, includes an operating portion via which the set screw can be rotated. It is desirable for the operating portion of the set screw to face toward a frontward direction in the optical axis direction. The optical element retracting mechanism can further include an outer barrel surrounding the linearly movable ring, and provided with a radially inward flange positioned in front of the linearly movable ring, wherein the radially inward flange includes at least one through hole which penetrates through the radially inward flange in the optical axis direction, the operating portion of the set screw accessible from the front of the linearly movable ring via the through hole. The retractable lens can include a lens barrier mechanism detachably attached to a front part of the radially inward flange to cover the through hole. The retractable lens can include a lens barrier mechanism detachably attached to a front part of the radially inward flange to cover the through hole. The outer barrel can support one of the plurality of optical elements which is positioned in front of the retractable optical element, the outer barrel retracting toward the plane together with the linearly movable ring along the optical axis when the retractable lens moves from the operational state to the fully-retracted state. The outer barrel can support one of the plurality of optical elements positioned in front of the retractable optical element, the outer barrel retracting toward the plane together with the linearly movable ring along the optical axis when the retractable lens moves from the operational state to the fully-retracted state. It is desirable for the operating portion to include a slot in which an adjusting tool can be engaged. It is desirable for the operating portion of the set screw to include a slot in which an adjusting tool can be engaged. The present disclosure relates to subject matter contained in Japanese Patent Application Nos. 2002-247338 (filed on Aug. 27, 2002), 2003-25413 (filed on Feb. 3, 2003) and 2003-25416 (filed on Feb. 3, 2002) 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 according to the present invention; FIG. 2 is an exploded perspective view of a structure supporting a first lens group of the zoom lens; FIG. 3 is an exploded perspective view of a structure supporting a second lens group of the zoom lens; FIG. 4 is an exploded perspective view of a lens barrel advancing-retracting structure of the zoom lens for advancing and retracting a third external barrel from a stationary barrel; FIG. 5 is a perspective view, partly exploded, of the zoom lens, showing a fixing procedure of a viewfinder unit to the zoom lens and a fixing procedure of a gear train to the zoom lens; FIG. 6 is a perspective view of a zoom lens assembly made from the elements shown in FIG. 5; FIG. 7 is a side elevational view of the zoom lens assembly shown in FIG. 6; FIG. 8 is a perspective view of the zoom lens assembly shown in FIG. 6, viewed obliquely from behind; FIG. 9 is an axial cross sectional view of an embodiment of a digital camera incorporating the zoom lens assembly shown in FIGS. 6 through 8, wherein an upper half above a photographing optical axis and a lower half below the photographing optical axis show a state of the zoom lens at telephoto extremity and a state of the zoom lens at wide-angle extremity, respectively; FIG. 10 is an axial cross sectional view of the digital camera shown in FIG. 9 in the retracted state of the zoom lens; FIG. 11 is a developed view of the stationary barrel shown in FIG. 1; FIG. 12 is a developed view of a helicoid ring shown in FIG. 4; FIG. 13 is a developed view of the helicoid ring shown in FIG. 1, showing a structure of the inner peripheral surface thereof by broken lines; FIG. 14 is a developed view of the third external barrel shown in FIG. 1; FIG. 15 is a developed view of a first linear guide ring shown in FIG. 1; FIG. 16 is a developed view of a cam ring shown in FIG. 1; FIG. 17 is a developed view of the cam ring shown in FIG. 1, showing a structure of the inner peripheral surface thereof by broken lines; FIG. 18 is a developed view of a second linear guide ring shown in FIG. 1; FIG. 19 is a developed view of a second lens group moving frame shown in FIG. 1; FIG. 20 is a developed view of a second external barrel shown in FIG. 1; FIG. 21 is a developed view of a first external barrel shown in FIG. 1; FIG. 22 is a conceptual diagram of elements of the zoom lens, showing the relationship among these elements in relation to the operations thereof; FIG. 23 is a developed view of the helicoid ring, the third external barrel and the stationary barrel, showing the positional relationship thereamong in the retracted state of the zoom lens; FIG. 24 is a developed view of the helicoid ring, the third external barrel and the stationary barrel, showing the positional relationship thereamong at the wide-angle extremity the zoom lens; FIG. 25 is a developed view of the helicoid ring, the third external barrel and the stationary barrel, showing the positional relationship among thereamong at the telephoto extremity the zoom lens; FIG. 26 is a developed view of the helicoid ring, the third external barrel and the stationary barrel, showing a positional relationship thereof; FIG. 27 is a developed view of the stationary barrel, showing the positions of a set of rotational sliding projections of the helicoid ring with respect to the stationary barrel in the retracted state of the zoom lens; FIG. 28 is a view similar to that of FIG. 27, showing the positions of the set of rotational sliding projections of the helicoid ring with respect to the stationary barrel at the wide-angle extremity of the zoom lens; FIG. 29 is a view similar to that of FIG. 27, showing the positions of the set of rotational sliding projections of the helicoid ring with respect to the stationary barrel at the telephoto extremity of the zoom lens; FIG. 30 is a view similar to that of FIG. 27, showing the positions of the set of rotational sliding projections of the helicoid ring with respect to the stationary barrel; FIG. 31 is a cross sectional view taken along M2--M2 line shown in FIG. 27; FIG. 32 is a cross sectional view taken along M1--M1 line shown in FIG. 23; FIG. 33 is an enlarged cross sectional view of a portion of the upper half of the zoom lens shown in FIG. 9; FIG. 34 is an enlarged cross sectional view of a portion of the lower half of the zoom lens shown in FIG. 9; FIG. 35 is an enlarged cross sectional view of a portion of the upper half of the zoom lens shown in FIG. 10; FIG. 36 is an enlarged cross sectional view of a portion of the lower half of the zoom lens shown in FIG. 10; FIG. 37 is an enlarged perspective view of a portion of the connecting portion between the third external barrel and the helicoid ring; FIG. 38 is a view similar to that of FIG. 37, showing a state where a stop member has been removed; FIG. 39 is a view similar to that of FIG. 38, showing a state where the third external barrel and the helicoid ring have been disengaged from each other in the optical axis direction from the state shown in FIG. 38; FIG. 40 is a perspective view of a portion of the stationary barrel, the stop member and a set screw therefor, showing a state where the stop member and the set screw have been removed from the stationary barrel; FIG. 41 is a perspective view similar to that shown in FIG. 40, showing a state where the stop member is properly fixed the stationary barrel via the set screw; FIG. 42 is an enlarged developed view of a portion of helicoid ring in relation to a corresponding portion of the stationary barrel; FIG. 43 is a view similar to that of FIG. 42, showing the positional relationship between the specific rotational sliding projection of the helicoid ring and the circumferential groove of the stationary barrel; FIG. 44 is a developed view of the third external barrel and the first linear guide ring in relation to a set of roller followers fixed to the cam ring, showing the positional relationship between the helicoid ring and the stationary barrel in the retracted state of the zoom lens; FIG. 45 is a view similar to that of FIG. 44, showing the positional relationship between the helicoid ring and the stationary barrel at the wide-angle extremity of the zoom lens; FIG. 46 is a view similar to that of FIG. 44, showing the positional relationship between the helicoid ring and the stationary barrel at the telephoto extremity of the zoom lens; FIG. 47 is a view similar to that of FIG. 44, showing the positional relationship between the helicoid ring and the stationary barrel; FIG. 48 is a developed view of the helicoid ring and the first linear guide ring, showing the positional relationship therebetween in the retracted state of the zoom lens; FIG. 49 is a view similar to that of FIG. 48, showing the positional relationship between the helicoid ring and the first linear guide ring at the wide-angle extremity of the zoom lens; FIG. 50 is a view similar to that of FIG. 48, showing the positional relationship between the helicoid ring and the first linear guide ring at the telephoto extremity of the zoom lens; FIG. 51 is a view similar to that of FIG. 48, showing the positional relationship between the helicoid ring and the first linear guide ring; FIG. 52 is a developed view of the cam ring, the first external barrel, the second external barrel and the second linear guide ring, showing the positional relationship thereamong in the retracted state of the zoom lens; FIG. 53 is a view similar to that of FIG. 52, showing the positional relationship among the cam ring, the first external barrel, the second external barrel and the second linear guide ring at the wide-angle extremity of the zoom lens; FIG. 54 is a view similar to that of FIG. 52, showing the positional relationship among the cam ring, the first external barrel, the second external barrel and the second linear guide ring at the telephoto extremity of the zoom lens; FIG. 55 is a view similar to that of FIG. 52, showing the positional relationship among the cam ring, the first external barrel, the second external barrel and the second linear guide ring; FIG. 56 is an exploded perspective view of elements of the zoom lens, showing a state where the third external barrel has been removed from the first linear guide ring; FIG. 57 is an exploded perspective view of elements of the zoom lens, showing a state where the second external barrel and a follower-biasing ring spring have been removed from the block of the zoom lens shown in FIG. 56; FIG. 58 is an exploded perspective view of elements of the zoom lens, showing a state where the first external barrel has been removed from the block of the zoom lens shown in FIG. 57; FIG. 59 is an exploded perspective view of elements of the zoom lens, showing a state where the second linear guide ring has been removed from the block of the zoom lens shown in FIG. 58 while the set of roller followers have been removed from the cam ring included in the block; FIG. 60 is a developed view of the helicoid ring, the third external barrel, the first linear guide ring and the follower-biasing ring spring in relation to the set of roller followers fixed to the cam ring, showing the positional relationship thereamong in the retracted state of the zoom lens; FIG. 61 is a view similar to that of FIG. 60, showing the positional relationship among the helicoid ring, the third external barrel and the first linear guide ring at the wide-angle extremity of the zoom lens; FIG. 62 is a view similar to that of FIG. 60, showing the positional relationship among the helicoid ring, the third external barrel and the first linear guide ring at the telephoto extremity of the zoom lens; FIG. 63 is a view similar to that of FIG. 60, showing the positional relationship among the helicoid ring, the third external barrel and the first linear guide ring; FIG. 64 is an enlarged developed view of portions of the third external barrel and the helicoid ring in relation to the set of roller followers fixed to the cam ring, viewed from radially inside the third external barrel and the helicoid ring; FIG. 65 is a view similar to that of FIG. 64, showing a state where the helicoid ring is rotated in a lens barrel advancing direction thereof; FIG. 66 is an enlarged developed view of portions of the third external barrel and the helicoid ring shown in FIG. 64; FIG. 67 is an enlarged developed view of portions of a front rind and a rear ring of a comparative example which are to be compared with the third external barrel and the helicoid ring shown in FIGS. 64 through 66; FIG. 68 is a view similar to that of FIG. 67, showing a state where the rear ring has slightly rotated with respect to the front ring from the state shown in FIG. 67; FIG. 69 is a magnified view of a part of the drawing shown in FIG. 60 (FIG. 44); FIG. 70 is a magnified view of a part of the drawing shown in FIG. 61 (FIG. 45); FIG. 71 is a magnified view of a part of the drawing shown in FIG. 62 (FIG. 46); FIG. 72 is a magnified view of a part of the drawing shown in FIG. 63 (FIG. 47); FIG. 73 is an axial cross sectional view of an upper half of elements of a linear guide structure of the zoom lens shown in FIGS. 5 and 10, showing the linear guide structure at the wide-angle extremity of the zoom lens; FIG. 74 is a view similar to that of FIG. 73, showing the linear guide structure at the wide-angle extremity of the zoom lens; FIG. 75 is a view similar to that of FIG. 74, showing the linear guide structure in the retracted state of the zoom lens; FIG. 76 is a perspective view of a subassembly of the zoom lens shown in FIGS. 5 through 10 which includes the first external barrel, the external barrel, the second linear guide ring, the cam ring and other elements, showing the positional relationship between the first external barrel and the second linear guide ring that are positioned radially inside and outside the cam ring, respectively; FIG. 77 is a perspective view of a subassembly of the zoom lens shown in FIGS. 5 through 10 which includes all the elements shown in FIG. 77 and the first linear guide ring, showing a state where the first external barrel has been extended forward to its assembling/disassembling position; FIG. 78 is a perspective view of the subassembly shown in FIG. 77, viewed obliquely from behind the subassembly; FIG. 79 is a developed view of the cam ring, the second lens group moving frame and the second linear guide ring, showing the positional relationship thereamong in the retracted state of the zoom lens; FIG. 80 is a view similar to that of FIG. 79, showing the positional relationship among the cam ring, the second lens group moving frame and the second linear guide ring at the wide-angle extremity of the zoom lens; FIG. 81 is a view similar to that of FIG. 79, showing the positional relationship among the cam ring, the second lens group moving frame and the second linear guide ring at the telephoto extremity of the zoom lens; FIG. 82 is a view similar to that of FIG. 79, showing a positional relationship among the cam ring, the second lens group moving frame and the second linear guide ring; FIG. 83 is developed view of the cam ring, showing a state where a set of front cam followers of the second lens group moving frame pass through the points of intersection between a set of front inner cam grooves and a set of rear inner cam grooves of the cam ring; FIG. 84 is a perspective view of a portion of the zoom lens shown in FIGS. 5 through 10 which includes the second lens group moving frame, the second linear guide ring, a shutter unit and other elements, viewed obliquely from the front thereof; FIG. 85 is a perspective view of the portion of the zoom lens in FIG. 84, viewed obliquely from behind; FIG. 86 is a view similar to that of FIG. 84, showing the positional relationship between the second lens group moving frame and the second linear guide ring when the second lens group moving frame is positioned at its front limit for the axial movement thereof with respect to the second linear guide ring; FIG. 87 is a perspective view of the portion of the zoom lens in FIG. 86, viewed obliquely from behind; FIG. 88 is a front elevational view of the second linear guide ring; FIG. 89 is a rear elevational view of the second lens group moving frame, the second linear guide ring and other elements in an assembled state thereof; FIG. 90 is a developed view of the first external barrel and the cam ring in relation to a set of cam followers of the first external barrel, showing the positional relationship between the first external barrel and the cam ring in the retracted state of the zoom lens; FIG. 91 is a view similar to that of FIG. 90, showing a state where each cam follower of the first external barrel is positioned at the insertion end of the inclined lead section of the associated outer cam groove of a set of outer cam grooves of the cam ring by a rotation of the cam ring in a lens barrel advancing direction thereof; FIG. 92 is a view similar to that of FIG. 90, showing the positional relationship between the first external barrel and the cam ring at the wide-angle extremity of the zoom lens; FIG. 93 is a view similar to that of FIG. 90, showing the positional relationship between the first external barrel and the cam ring at the telephoto extremity of the zoom lens; FIG. 94 is a view similar to that of FIG. 90, showing a positional relationship between the first external barrel and the cam ring; FIG. 95 is a magnified view of a part of the drawing shown in FIG. 90; FIG. 96 is a magnified view of a part of the drawing shown in FIG. 91; FIG. 97 is view similar to those of FIGS. 95 and 96, showing a state where each cam follower of the first external barrel are positioned in the inclined lead section of the associated outer cam groove of the cam ring; FIG. 98 is a magnified view of a part of the drawing shown in FIG. 92; FIG. 99 is a magnified view of a part of the drawing shown in FIG. 93; FIG. 100 is a magnified view of a part of the drawing shown in FIG. 94; FIG. 101 is a view similar to that of FIG. 95, showing another embodiment of the structure of the set of outer cam grooves of the cam ring, showing the positional relationship between the first external barrel and the cam ring in the retracted state of the zoom lens; FIG. 102 is an exploded perspective view of a structure of the zoom lens for supporting a second lens frame which holds the second lens group, for retracting the second lens frame to a radially retracted position thereof, and for adjusting the position of the second lens frame; FIG. 103 is a perspective view of the structure for the second lens frame shown in FIG. 102 in an assembled state and a position-control cam bar of a CCD holder, viewed obliquely from the front; FIG. 104 is a perspective view of the structure for the second lens frame and the position-control cam bar shown in FIG. 103, viewed obliquely from behind; FIG. 105 is a view similar to that of FIG. 104, showing a state where the position-control cam bar is in the process of entering the cam-bar insertable hole of a rear second lens frame support plate fixed to the second lens group moving frame; FIG. 106 is a front elevational view of the second lens group moving frame; FIG. 107 is a perspective view of the second lens group moving frame; FIG. 108 is a perspective view of the second lens group moving frame and the shutter unit fixed thereto, viewed obliquely from front; FIG. 109 is a perspective view of the second lens group moving frame and the shutter unit shown in FIG. 108, viewed obliquely from behind; FIG. 110 is a front elevational view of the second lens group moving frame and the shutter unit shown in FIG. 108; FIG. 111 is a rear elevational view of the second lens group moving frame and the shutter unit shown in FIG. 108; FIG. 112 is a view similar to that of FIG. 111, showing a state where the second lens frame has retracted to the radially retracted position; FIG. 113 is a cross sectional view taken along M3--M3 line shown in FIG. 110; FIG. 114 is a front elevational view of the structure for the second lens frame shown in FIGS. 105 and 108 through 112, showing a state where the second lens frame is held at a photographing position thereof as shown in FIG. 110; FIG. 115 is a front elevational view of a portion of the structure for the second lens frame shown in FIG. 114; FIG. 116 is a view similar to that of FIG. 115 in a different state; FIG. 117 is a front elevational view of a portion of the structure for the second lens frame shown in FIGS. 105 and 108 through 116; FIG. 118 is a front elevational view of a portion of the structure for the second lens frame shown in FIGS. 105 and 108 through 116, showing the positional relationship between the second lens frame and the position-control cam bar of the CCD holder when the second lens frame is held in a photographing position thereof as shown in FIGS. 109 and 111; FIG. 119 is a view similar to that of FIG. 118, showing a positional relationship between the second lens frame and the position-control cam bar of the CCD holder; FIG. 120 is a view similar to that of 118, showing the positional relationship between the second lens frame and the position-control cam bar of the CCD holder when the second lens frame is held in the radially retracted position as shown in FIG. 112; FIG. 121 is a perspective view of an AF lens frame and the CCD holder shown in FIGS. 1 and 4, showing a state where the AF lens frame is fully retracted to contact with and the CCD holder, viewed obliquely from lower front of the CCD holder; FIG. 122 is a front elevational view of the CCD holder, the AF lens frame and the second lens group moving frame; FIG. 123 is a perspective view of the CCD holder, the AF lens frame, the second lens group moving frame, the second lens frame and other elements; FIG. 124 is a view similar to that of FIG. 123, showing a state where the second lens frame has fully moved rearward and fully rotated to the radially retracted position; FIG. 125 is an axial cross sectional view of a portion of the upper half of the zoom lens shown in FIG. 9, showing a structure wiring a flexible PWB for exposure control in the zoom lens; FIG. 126 is a perspective view of the second lens frame, the flexible PWB and other elements, showing a manner of supporting the flexible PWB by the second lens frame; FIG. 127 is a perspective view of the second lens frame and the AF lens frame, showing a state where the second lens frame has retracted closely to the AF lens frame; FIG. 128 is a side elevational view of the second lens frame and the AF lens frame, showing a state immediately before the second lens frame comes into contact with the AF lens frame; FIG. 129 is a view similar to that of FIG. 128, showing a state where the second lens frame is in contact with the AF lens frame; FIG. 130 is a front elevational view of the second lens frame and the AF lens frame, showing a positional relationship therebetween; FIG. 131 is a perspective view of the first external barrel that surrounds the second lens group moving frame, and the first lens frame for the first lens group that is held by the first external barrel; FIG. 132 is a front elevational view of the first external barrel and the first lens frame; FIG. 133 is a perspective view of the first lens frame, the second lens group moving frame, the AF lens frame and the shutter unit, viewed obliquely from front, showing the positional relationship thereamong at a ready-to-photograph state of the zoom lens; FIG. 134 is a perspective view of the first lens frame, the second lens group moving frame, the AF lens frame and the shutter unit which are shown in FIG. 133, viewed obliquely from rear thereof; FIG. 135 is a view similar to that of FIG. 133, showing the positional relationship among the first lens frame, the second lens group moving frame, the AF lens frame and the shutter unit, showing the positional relationship thereamong in the retracted state of the zoom lens; FIG. 136 is a perspective view of the first lens frame, the second lens group moving frame, the AF lens frame and the shutter unit which are shown in FIG. 135, viewed obliquely from rear thereof; FIG. 137 is a rear elevational view of the first lens frame, the second lens group moving frame, the AF lens frame and the shutter unit which are shown in FIG. 135; FIG. 138 is a perspective view, of the first lens frame, the first external barrel, the second lens group moving frame, the AF lens frame and the shutter unit in the retracted state of the zoom lens, showing the positional relationship thereamong in the retracted state of the zoom lens; FIG. 139 is a front elevational view of the first lens frame, the first external barrel, the second lens group moving frame, the AF lens frame and the shutter unit which are shown in FIG. 138; FIG. 140 is an exploded perspective view of the shutter unit of the zoom lens; FIG. 141 is a longitudinal cross sectional view of a portion of the zoom lens in the vicinity of the first lens group in the upper half of the zoom lens shown in FIG. 9, in which the zoom lens is in a ready-to-photograph state; FIG. 142 is a view similar to that of FIG. 141 and shows the same portion in the upper half of the zoom lens shown in FIG. 10, in which the zoom lens is in the retracted state; FIG. 143 is an exploded perspective view of the viewfinder unit shown in FIGS. 5 through 8; FIG. 144 is a developed view, similar to that of FIG. 23, of the helicoid ring and the third external barrel in relation to a zoom gear and a viewfinder drive gear, showing the positional relationship thereamong in the retracted state of the zoom lens; FIG. 145 is a developed view, similar to that of FIG. 24, of the helicoid ring and the stationary barrel in relation to the zoom gear and the viewfinder drive gear, showing the positional relationship thereamong at the wide-angle extremity the zoom lens; FIG. 146 is a perspective view of a power transmission system of the zoom lens for imparting rotation of a zoom motor from the helicoid ring to movable lenses of a viewfinder optical system incorporated in the viewfinder unit; FIG. 147 is a front elevational view of the power transmission system shown in FIG. 148; FIG. 148 is a side elevational view of the power transmission system shown in FIG. 148; FIG. 149 is an enlarged developed view of the helicoid ring and the viewfinder drive gear, showing a positional relationship therebetween in the middle of rotation of the helicoid ring in the lens barrel advancing direction from the retracted position shown in FIG. 144 to the wide-angle extremity shown in FIG. 145. FIG. 150 is a view similar to that of FIG. 149, showing a state subsequent to the state shown in FIG. 149; FIG. 151 is a view similar to that of FIG. 149, showing a state subsequent to the state shown in FIG. 150; FIG. 152 is a view similar to that of FIG. 149, showing a state subsequent to the state shown in FIG. 151; FIG. 153 is a front elevational view of the helicoid ring and the viewfinder drive gear which are shown in FIG. 150; FIG. 154 is a front elevational view of the helicoid ring and the viewfinder drive gear which are shown in FIG. 151; FIG. 155 is a front elevational view of the helicoid ring and the viewfinder drive gear which are shown in FIG. 152; FIG. 156 is a developed view of a cam-incorporated gear of the viewfinder unit; and FIG. 157 is a developed view, similar to that of FIG. 156, of a comparative example of a cam-incorporated gear incorporating an idle running section which is to be compared with the cam-incorporated gear shown in FIG. 156. DESCRIPTION OF THE PREFERRED EMBODIMENT In some of the drawings, lines of different thicknesses and/or different types of lines are used as the outlines of different elements for the purpose of illustration. Additionally, in some cross sectional drawings, several elements are shown on a common plane, though positioned in different circumferential positions, for the purpose of illustration. In FIG. 22, the symbols "(S)", "(L)", "(R)" and "(RL)" which are each appended as a suffix to the reference numeral of some elements of a present embodiment of a zoom lens (zoom lens barrel) 71 (see FIGS. 5 through 10) indicate that the element is stationary, the element is solely movable linearly along a lens barrel axis Z0 (see FIGS. 9 and 10) without rotating about the lens barrel axis Z0, the element is rotatable about the lens barrel axis Z0 without moving along the lens barrel axis Z0, and the element is solely movable along the lens barrel axis Z0 while rotating about the lens barrel axis Z0, respectively. Additionally, in FIG. 22, the symbol "(R, RL)" which is appended as a suffix to the reference numeral of some elements of the zoom lens 71 indicates that the element rotates about the lens barrel axis Z0 without moving along the lens barrel axis Z0 during a zooming operation and that the element moves along the lens barrel axis Z0 while rotating about the lens barrel axis Z0 during the time the zoom lens 71 advances from or retracts into a camera body 72 upon power being turned ON or OFF, while the symbol "(S, L)" which is appended as a suffix to the reference numeral of some elements of the zoom lens 71 indicates that the element is stationary when the zoom lens 71 in a zooming range in which a zooming operation is possible and that the element moves linearly along the lens barrel axis Z0 without rotating about the lens barrel axis Z0 during the time the zoom lens 71 advances from or retracts into the camera body 72 upon power being turned ON or OFF. As shown in FIGS. 9 and 10, the present embodiment of the zoom lens 71 incorporated in a digital camera 70 is provided with a photographing optical system consisting of a first lens group LG1, a shutter S, an adjustable diaphragm A, a second lens group LG2, a third lens group LG3, a low-pass filter (optical filter) LG4, and a CCD image sensor (solid-state image pick-up device) 60. "Z1" shown in FIGS. 9 and 10 designates the optical axis of the photographing optical system. The photographing optical axis Z1 is parallel to a common rotational axis (the lens barrel axis Z0) of external barrels which form an outward appearance of the zoom lens 71. Moreover, the photographing optical axis Z1 is positioned below the lens barrel axis Z0. The first lens group LG1 and the second lens group LG2 are driven along the photographing optical axis Z1 in a predetermined moving manner to perform a zooming operation, while the third lens group L3 is driven along the photographing optical axis Z1 to perform a focusing operation. In the following descriptions, the term "optical axis direction" means a direction parallel to the photographing optical axis Z1 unless there is a different explanatory note on the expression. As shown in FIGS. 9 and 10, the camera 70 is provided in the camera body 72 thereof with a stationary barrel 22 fixed to the camera body 72, and a CCD holder 21 fixed to a rear portion of the stationary barrel 22. The CCD image sensor 60 is mounted to the CCD holder 21 to be held thereby via a CCD base plate 62. The low-pass filter LG4 is held by the CCD holder 21 to be positioned in front of the CCD 60 via a filter holder portion 21b and an annular sealing member 61. The filter holder portion 21b is a portion formed integrally with the CCD holder 21. The camera 70 is provided behind the CCD holder 21 with an LCD panel 20 which indicates a live image so that the user can see how the image about to be taken looks before photographing, captured images so that the user can review pictures which he or she has already taken, and also various photographing information. The zoom lens 71 is provided in the stationary barrel 22 with an AF lens frame (a third lens frame which supports and holds the third lens group LG3) 51 which is guided linearly in the optical axis direction without rotating about the photographing optical axis Z1. Specifically, the zoom lens 71 is provided with a pair of AF guide shafts 52 and 53 which extend parallel to the photographing optical axis Z1 to guide the AF lens frame 51 in the optical axis direction without rotating the AF lens frame 51 about the photographing optical axis Z1. Front and rear ends of each guide shaft of the pair of AF guide shafts 52 and 53 are fixed to the stationary barrel 22 and the CCD holder 21, respectively. The AF lens frame 51 is provided on radially opposite sides thereof with a pair of guide holes 51a and 51b in which the pair of AF guide shafts 52 and 53 are respectively fitted so that the AF lens frame 51 is slidable on the pair of AF guide shafts 52 and 53. In this particular embodiment, the amount of clearance between the AF guide shaft 53 and the guide hole 51b is greater than that between the AF guide shaft 52 and the guide hole 51a. Namely, the AF guide shaft 52 serves as a main guide shaft for achieving a great positioning accuracy, while the AF guide shaft 53 serves as an auxiliary guide shaft. The camera 70 is provided with an AF motor 160 (see FIG. 1) having a rotary drive shaft which is threaded to serve as a feed screw shaft, and this rotary drive shaft is screwed through a screw hole formed on an AF nut 54 (see FIG. 1). The AF nut 54 is provided with a rotation-preventing protrusion 54a. The AF lens frame 51 is provided with a guide groove 51m (see FIG. 127), extending in a direction parallel to the optical axis Z1, in which the rotation-preventing protrusion 54a is slidably fitted. Furthermore, the AF lens frame 51 is provided with a stopper protrusion 51n (see FIG. 127) which is positioned behind the AF nut 54. The AF lens frame 51 is biased forward in the optical axis direction by an extension coil spring 55 serving as a biasing member, and the forward movement limit of the AF lens frame 51 is determined via engagement between the stopper protrusion 51n and the AF nut 54. The AF lens frame 51 can be moved rearward against the biasing force of the extension coil spring 55 when a rearward force is applied by the AF nut 54. Due to this structure, rotating the rotary drive shaft of AF motor 160 forward and rearward causes the AF lens frame 51 to move forward and rearward in the optical axis direction. In addition, the AF lens frame 51 can be moved rearward against the biasing force of the extension coil spring 55 when a rearward force is directly applied to the AF lens frame 51. As shown in FIGS. 5 and 6, the camera 70 is provided above the stationary barrel 22 with a zoom motor 150 and a reduction gear train box 74 which are mounted on the stationary barrel 22. The reduction gear train box 74 contains a reduction gear train for transferring rotation of the zoom motor 150 to a zoom gear 28 (see FIG. 4). The zoom gear 28 is rotatably fitted on a zoom gear shaft 29 extending parallel to the photographing optical axis Z1. Front and rear ends of the zoom gear shaft 29 are fixed to the stationary barrel 22 and the CCD holder 21, respectively. Rotations of the zoom motor 150 and the AF motor 160 are controlled by a control circuit 140 (see FIG. 22) via a flexible PWB (printed wiring board) 75 which is partly positioned on an outer peripheral surface of the stationary barrel 22. The control circuit 140 comprehensively controls the overall operation of the camera 70. As shown in FIG. 4, the stationary barrel 22 is provided on an inner peripheral surface thereof with a female helicoid 22a, a set of three linear guide grooves 22b, a set of three inclined grooves 22c, and a set of three rotational sliding grooves 22d. Threads of the female helicoid 22a extend in a direction inclined with respect to both the optical axis direction and a circumferential direction of the stationary barrel 22. The set of three linear guide grooves 22b extend parallel to the photographing optical axis Z1. The set of three inclined grooves 22c extend parallel to the female helicoid 22a. The set of three rotational sliding grooves 22d are formed in the vicinity of a front end of the inner peripheral surface of the stationary barrel 22 to extend along a circumference of the stationary barrel 22 to communicate the front ends of the set of three inclined grooves 22c, respectively. The female helicoid 22a is not formed on that specific front area (non-helicoid area 22z) of the inner peripheral surface of the stationary barrel 22 which is positioned immediately behind the set of three rotational sliding grooves 22d (see FIGS. 11, 23 through 26). The zoom lens 71 is provided in the stationary barrel 22 with a helicoid ring 18. The helicoid ring 18 is provided on an outer peripheral surface thereof with a male helicoid 18a and a set of three rotational sliding projections 18b. The male helicoid 18a is engaged with the female helicoid 22a, and the set of three rotational sliding projections 18b are engaged in the set of three inclined grooves 22c or the set of three rotational sliding grooves 22d, respectively (see FIGS. 4 and 12). The helicoid ring 18 is provided on threads of the male helicoid 18a with an annular gear 18c which is in mesh with the zoom gear 28. Therefore, when a rotation of the zoom gear 28 is transferred to the annular gear 18c, the helicoid ring 18 moves forward or rearward in the optical axis direction while rotating about the lens barrel axis Z0 within a predetermined range in which the male helicoid 18a remains in mesh with the female helicoid 22a. A forward movement of the helicoid ring 18 beyond a predetermined point with respect to the stationary barrel 22 causes the male helicoid 18a to be disengaged from the female helicoid 22a so that the helicoid ring 18 rotates about the lens barrel axis Z0 without moving in the optical axis direction relative to the stationary barrel 22 by engagement of the set of three rotational sliding projections 18b with the set of three rotational sliding grooves 22d. The set of three inclined grooves 22c are formed on the stationary barrel 22 to prevent the set of three rotational sliding projections 18b and the stationary barrel 22 from interfering with each other when the female helicoid 22a and the male helicoid 18a are engaged with each other. To this end, each inclined groove 22c is formed on an inner peripheral surface of the stationary barrel 22 to be positioned radially outwards (upwards as viewed in FIG. 31) from the bottom of the female helicoid 22a as shown in FIG. 31. A circumferential space between two adjacent threads of the female helicoid 22a between which one of the three inclined grooves 22c is positioned is greater than that between another two adjacent threads of the female helicoid 22a between which none of the three inclined grooves 22c is positioned. The male helicoid 18a includes three wide threads 18a-W and twelve narrow threads. The three wide threads 18a-W are positioned behind the three rotational sliding projections 18b in the optical axis direction, respectively (see FIG. 12). The circumferential width of each of the three wide threads 18a-W is greater than that of each of the twelve narrow threads so that each of the three wide threads 18a-W can be positioned in the associated two adjacent threads of the female helicoid 22a between which one of the three inclined grooves 22c is positioned (see FIGS. 11 and 12). The stationary barrel 22 is provided with a stop-member insertion hole 22e which radially penetrates through the stationary barrel 22. A stop member 26 having a stop projection 26b is fixed to the stationary barrel 22 by a set screw 67 so that the stop projection 26b can be inserted into and removed from the stop-member insertion hole 22e (see FIGS. 40 and 41). As will be appreciated from FIGS. 9 and 10, the zoom lens 71 of the camera 70 is of a telescoping type having three external telescoping barrels: a first external barrel 12, a second external barrel 13 and a third external barrel 15 which are concentrically arranged about the lens barrel axis Z0. The helicoid ring 18 is provided, on an inner peripheral surface thereof at three different circumferential positions on the helicoid ring 18, with three rotation transfer recesses 18d (see FIGS. 4 and 13) front ends of which are open at the front end of the helicoid ring 18, while the third external barrel 15 is provided, at corresponding three different circumferential positions on the third external barrel 15, with three pairs of rotation transfer projections 15a (see FIGS. 4 and 14) which project rearward from the rear end of the third external barrel 15 to be inserted into the three rotation transfer recesses 18d from the front thereof, respectively. The three pairs of rotation transfer projections 15a and the three rotation transfer recesses 18d are movable relative to each other in a direction of the lens barrel axis Z0, and are not rotatable relative to each other about the lens barrel axis Z0. Namely, the helicoid ring 18 and the third external barrel 15 rotate in one piece. Strictly speaking, the three pairs of rotation transfer projections 15a and the three rotation transfer recesses 18d are slightly rotatable relative to each other about the lens barrel axis Z0 by the amount of clearance between the three pairs of rotation transfer projections 15a and the three rotation transfer recesses 18d, respectively. This structure will be discussed in detail later. The helicoid ring 18 is provided, on front faces of the three rotational sliding projections 18b at three different circumferential positions on the helicoid ring 18, with a set of three engaging recesses 18e which are formed on an inner peripheral surface of the helicoid ring 18 to be open at the front end of the helicoid ring 18. The third external barrel 15 is provided, at corresponding three different circumferential positions on the third external barrel 15, with a set of three engaging projections 15b which project rearward from the rear end of the third external barrel 15, and also project radially outwards, to be engaged in the set of three engaging recesses 18e from the front thereof, respectively. The set of three engaging projections 15b, which are respectively engaged in the set of three engaging recesses 18e, are also engaged in the set of three rotational sliding grooves 22d at a time, respectively, when the set of three rotational sliding projections 18b are engaged in the set of three rotational sliding grooves 22d (see FIG. 33). The zoom lens 71 is provided between the third external barrel 15 and the helicoid ring 18 with three compression coil springs 25 which bias the third external barrel 15 and the helicoid ring 18 in opposite directions away from each other in the optical axis direction. The rear ends of the three compression coil springs 25 are respectively inserted into three spring support holes (non-through hole) 18f which are formed on the front end of the helicoid ring 18, while the front ends of the three compression coil springs 25 are respectively in pressing contact with three engaging recesses 15c formed at the rear end of the third external barrel 15. Therefore, the set of three engaging projections 15b of the third external barrel 15 are respectively pressed against front guide surfaces 22d-A (see FIGS. 28 through 30) of the rotational sliding grooves 22d by the spring force of the three compression coil springs 25. At the same time, the set of three rotational sliding projections 18b of the helicoid ring 18 are respectively pressed against rear guide surfaces 22d-B (see FIGS. 28 through 30) of the rotational sliding grooves 22d by the spring force of the three compression coil springs 25. The third external barrel 15 is provided on an inner peripheral surface thereof with a plurality of relative rotation guide projections 15d which are formed at different circumferential positions on the third external barrel 15, a circumferential groove 15e which extends in a circumferential direction about the lens barrel axis Z0, and a set of three rotation transfer grooves 15f which extend parallel to the lens barrel axis Z0 (see FIGS. 4 and 14). The plurality of relative rotation guide projections 15d are elongated in a circumferential direction of the third external barrel to lie in a plane orthogonal to the lens barrel axis Z0. As can be seen in FIG. 14, each rotation transfer groove 15f intersects the circumferential groove 15e at right angles. The circumferential positions of the three rotation transfer grooves 15f are formed to correspond to those of the three pairs of rotation transfer projections 15a, respectively. The rear end of each rotation transfer groove 15f is open at the rear end of the third external barrel 15. The helicoid ring 18 is provided on an inner peripheral surface thereof with a circumferential groove 18g which extends in a circumferential direction about the lens barrel axis Z0 (see FIGS. 4 and 13). The zoom lens 71 is provided inside the third external barrel 15 and the helicoid ring 18 with a first linear guide ring 14. The first linear guide ring 14 is provided on an outer peripheral surface thereof with a set of three linear guide projections 14a, a first plurality of relative rotation guide projections 14b, a second plurality of relative rotation guide projections 14c, and a circumferential groove 14d in this order from rear to front of the first linear guide ring 14 in the optical axis direction (see FIGS. 4 and 15). The set of three linear guide projections 14a project radially outwards in the vicinity of the rear end of the first linear guide ring 14. The first plurality of relative rotation guide projections 14b project radially outwards at different circumferential positions on the first linear guide ring 14, and are each elongated in a circumferential direction of the first linear guide ring 14 to lie in a plane orthogonal to the lens barrel axis Z0. Likewise, the second plurality of relative rotation guide projections 14c project at different circumferential positions on the first linear guide ring 14, and are each elongated in a circumferential direction of the first linear guide ring 14 to lie in a plane orthogonal to the lens barrel axis Z0. The circumferential groove 14d is an annular groove with its center on the lens barrel axis Z0. The first linear guide ring 14 is guided in the optical axis direction with respect to the stationary barrel 22 by engagement of the set of three linear guide projections 14a with the set of three linear guide grooves 22b, respectively. The third external barrel 15 is coupled to the first linear guide ring 14 to be rotatable about the lens barrel axis Z0 relative to the first linear guide ring 14 by both the engagement of the second plurality of relative rotation guide projections 14c with the circumferential groove 15e and the engagement of the plurality of relative rotation guide projections 15d with the circumferential groove 14d. The second plurality of relative rotation guide projections 14c and the circumferential groove 15e are engaged with each other to be slightly movable relative to each other in the optical axis direction. Likewise, the plurality of relative rotation guide projections 15d and the circumferential groove 14d are engaged with each other to be slightly movable relative to each other in the optical axis direction. The helicoid ring 18 is coupled to the first linear guide ring 14 to be rotatable about the lens barrel axis Z0 relative to the first linear guide ring 14 by engagement of the first plurality of relative rotation guide projections 14b with the circumferential groove 18g. The first plurality of relative rotation guide projections 14b and the circumferential groove 18g are engaged with each other to be slightly movable relative to each other in the optical axis direction. The first linear guide ring 14 is provided with a set of three through-slots 14e which radially penetrate the first linear guide ring 14. As shown in FIG. 15, each through-slot 14e includes a front circumferential slot portion 14e-1, a rear circumferential slot portion 14e-2, and an inclined lead slot portion 14e-3 which connects the front circumferential slot portion 14e-1 with the rear circumferential slot portion 14e-2. The front circumferential slot portion 14e-1 and the rear circumferential slot portion 14e-2 extend parallel to each other in a circumferential direction of the first linear guide ring 14. The zoom lens 71 is provided with a cam ring 11 a front portion of which is positioned inside the first external barrel 12. A set of three roller followers 32 fixed to an outer peripheral surface of the cam ring 11 at different circumferential positions thereon are engaged in the set of three through-slots 14e, respectively (see FIG. 3). Each roller follower 32 is fixed to the cam ring 11 by set screw 32a. The set of three roller followers 32 are further engaged in the set of three rotation transfer grooves 15f through the set of three through-slots 14e, respectively. The zoom lens 71 is provided between the first linear guide ring 14 and the third external barrel 15 with a follower-biasing ring spring 17. A set of three follower pressing protrusions 17a protrude rearward from the follower-biasing ring spring 17 to be engaged in front portions of the set of three rotation transfer grooves 15f, respectively (see FIG. 14). The set of three follower pressing protrusions 17a press the set of three roller followers 32 rearward to remove backlash between the set of three roller followers 32 and the set of three through-slots 14e when the set of three roller followers 32 are engaged in the front circumferential slot portions 14e-1 of the set of three through-slots 14e, respectively. Advancing operations of movable elements of the zoom lens 71 from the stationary barrel 22 to the cam ring 11 will be discussed hereinafter with reference to the above described structure of the digital camera 70. Rotating the zoom gear 28 in a lens barrel advancing direction by the zoom motor 150 causes the helicoid ring 18 to move forward while rotating about the lens barrel axis Z0 due to engagement of the female helicoid 22a with the male helicoid 18a. This rotation of the helicoid ring 18 causes the third external barrel 15 to move forward together with the helicoid ring 18 while rotating about the lens barrel axis Z0 together with the helicoid ring 18, and further causes the first linear guide ring 14 to move forward together with the helicoid ring 18 and the third external barrel 15 because each of the helicoid ring 18 and the third external barrel 15 is coupled to the first linear guide ring 14 to make respective relative rotations between the third external barrel 15 and the first linear guide ring 14 and between the helicoid ring 18 and the first linear guide ring 14 possible and to be movable together along a direction of a common rotational axis (i.e., the lens barrel axis Z0) due to the engagement of the first plurality of relative rotation guide projections 14b with the circumferential groove 18g, the engagement of the second plurality of relative rotation guide projections 14c with the circumferential groove 15e and the engagement of the plurality of relative rotation guide projections 15d with the circumferential groove 14d. Rotation of the third external barrel 15 is transferred to the cam ring 11 via the set of three rotation transfer grooves 15f and the set of three roller followers 32, which are engaged in the set of three rotation transfer grooves 15f, respectively. Since the set of three roller followers 32 are also engaged in the set of three through-slots 14e, respectively, the cam ring 11 moves forward while rotating about the lens barrel axis Z0 relative to the first linear guide ring 14 in accordance with contours of the lead slot portions 14e-3 of the set of three through-slots 14e. Since the first linear guide ring 14 itself moves forward together with the third lens barrel 15 and the helicoid ring 18 as described above, the cam ring 11 moves forward in the optical axis direction by an amount of movement corresponding to the sum of the amount of the forward movement of the first linear guide ring 14 and the amount of the forward movement of the cam ring 11 by engagement of the set of three roller followers 32 with the lead slot portions 14e-3 of the set of three through-slots 14e, respectively. The above described rotating-advancing operations of the cam ring 11, the third external barrel 15 and the helicoid ring 18 are performed while the set of three rotational sliding projections 18b are moving in the set of three inclined grooves 22c, respectively, only when the male helicoid 18a and the female helicoid 22a are engaged with each other. When the helicoid ring 18 moves forward by a predetermined amount of movement, the male helicoid 18a and the female helicoid 22a are disengaged from each other so that the set of three rotational sliding projections 18b move from the set of three inclined grooves 22c to the set of three rotational sliding grooves 22d, respectively. Since the helicoid ring 18 does not move in the optical axis direction relative to the stationary barrel 22 even if rotating upon the disengagement of the male helicoid 18a from the female helicoid 22a, the helicoid ring 18 and the third external barrel 15 rotate at respective axial fixed positions thereof without moving in the optical axis direction due to the engagement of the set of three rotational sliding projections 18b with the set of three rotational sliding grooves 22d. Furthermore, at substantially the same time when the set of three rotational sliding projections 18b slide into the set of three rotational sliding grooves 22d from the set of three inclined grooves 22c, respectively, the set of three roller followers 32 enter the front circumferential slot portions 14e-1 of the set of three through-slots 14e, respectively. In this state, since the first linear guide ring 14 stops while the set of three roller followers 32 have respectively moved into the front circumferential slot portions 14e-1, the cam ring 11 is not given any force to make the cam ring 11 move forward. Consequently, the cam ring 11 only rotates at an axial fixed position in accordance with rotation of the third external barrel 15. Rotating the zoom gear 28 in a lens barrel retracting direction thereof by the zoom motor 150 causes the aforementioned movable elements of the zoom lens 71 from the stationary barrel 22 to the cam ring 11 to operate in the reverse manner to the above described advancing operations. In this reverse operation, the above described movable elements of the zoom lens 71 retract to their respective retracted positions shown in FIG. 10 by rotation of the helicoid ring 18 until the set of three roller followers 32 enter the rear circumferential slot portions 14e-2 of the set of three through-slots 14e, respectively. The first linear guide ring 14 is provided on an inner peripheral surface thereof with a set of three pairs of first linear guide grooves 14f which are formed at different circumferential positions to extend parallel to the photographing optical axis Z1, and a set of six second linear guide grooves 14g which are formed at different circumferential positions to extend parallel to the photographing optical axis Z1. Each pair of first linear guide grooves 14f are positioned on the opposite sides of the associated linear guide groove 14g (every other linear guide groove 14g) in a circumferential direction of the first linear guide ring 14. The zoom lens 71 is provided inside the first linear guide ring 14 with a second linear guide ring 10. The second linear guide ring 10 is provided on an outer edge thereof with a set of three bifurcated projections 10a which project radially outwards from a ring portion 10b of the second linear guide ring 10. Each bifurcated projection 10a is provided at a radially outer end thereof with a pair of radial projections which are respectively engaged in the associated pair of first linear guide grooves 14f (see FIGS. 3 and 18). On the other hand, a set of six radial projections 13a which are formed on an outer peripheral surface of the second external barrel 13 at a rear end thereof to project radially outwards (see FIG. 3) are engaged in the set of six second linear guide grooves 14g, respectively to be slidable therealong. Therefore, each of the second external barrel 13 and the second linear guide ring 10 is guided in the optical axis direction via the first linear guide ring 14. The zoom lens 71 is provided inside the cam ring 11 with a second lens group moving frame 8 which indirectly supports and holds the second lens group LG2 (see FIG. 3). The first external barrel 12 indirectly supports the first lens group LG1, and is positioned inside the second external barrel 13 (see FIG. 2). The second linear guide ring 10 serves as a linear guide member for guiding the second lens group moving frame 8 linearly without rotating the same, while the second external barrel 13 serves as a linear guide member for guiding the first external barrel 12 linearly without rotating the same. The second linear guide ring 10 is provided on the ring portion 10b with a set of three linear guide keys 10c (specifically two narrow linear guide keys 10c and a wide linear guide key 10c-W) which project forward in parallel to one another (see FIGS. 3 and 18) from the ring portion 10b. The second lens group moving frame 8 is provided with a corresponding set of three guide grooves 8a (specifically two narrow guide grooves 8a and a wide guide groove 8a-W) in which the set of three linear guide keys 10c are engaged, respectively. As shown in FIGS. 9 and 10, a discontinuous outer edge of the ring portion 10b is engaged in a discontinuous circumferential groove 11e formed on an inner peripheral surface of the cam ring 11 at the rear end thereof to be rotatable about the lens barrel axis Z0 relative to the cam ring 11 and to be immovable relative to the cam ring 11 in the optical axis direction. The set of three linear guide keys 10c project forward from the ring portion 10b to be positioned inside the cam ring 11. Opposite edges of each linear guide key 10c in a circumferential direction of the second linear guide ring 10 serve as parallel guide edges which are respectively engaged with circumferentially-opposed guide surfaces in the associated guide groove 8a of the second lens group moving frame 8, which is positioned in the cam ring 11 to be supported thereby, to guide the second lens group moving frame 8 linearly in the optical axis direction without rotating the same about the lens barrel axis Z0. The wide linear guide key 10c-W has a circumferential width greater than those of the other two linear guide keys 10c to also serve as a support member for supporting a flexible PWB (printed wiring board) 77 (see FIGS. 84 through 87) used for exposure control. The wide linear guide key 10c-W is provided thereon with a radial through hole 10d through which the flexible PWB 77 passes (see FIG. 18). A portion of the ring portion 10b from which the wide linear guide key 10c-W projects forward is partly cut out so that the rear end of the radial through hole 10d extends through the rear end of the ring portion 10b. As shown in FIGS. 9 and 125, the flexible PWB 77 for exposure control passes through the radial through hole 10d to extend forward along an outer surface of the wide linear guide key 10c-W from the rear of the ring portion 10b, and subsequently bends radially inwards in the vicinity of the front end of the wide linear guide key 10c-W to extend rearward along an inner surface of the wide linear guide key 10c-W. The wide guide groove 8a-W has a circumferential width greater than those of the other two guide grooves 8a so that the wide linear guide key 10c-W can be engaged in the wide guide groove 8a-W to be slidable therealong. As can be clearly seen in FIG. 19, the second lens group mov |