Abstract:
A zoom lens includes a movable barrel mounting a lens holder barrel having a focusing lens element for rotational and axial movement therein and a cam barrel mounting the movable barrel for rectilinear axial movement therein. The movable barrel is axially moved for zooming through slide engagement between zooming cam slots of the cam barrel and rectilinear axial guide slots of a stationary barrel when the cam barrel rotates and allows the lens holder barrel to axially moves through helicoids coupling. The movable barrel is provided with a moving range restrictive structure having a plurality of screw holes and a single screw pin selectively screwed in one of the screw holes for focus adjustment. The screw pin is exposed to the outside of the zoom lens all at once when the cam barrel is in a telephoto or wide-angle position or one by one during rotation of the can barrel.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a zoom lens equipped with a focus adjusting mechanism and a zooming cam mechanism for varying a focal length of a zoom lens optical system. 
   2. Description of Related Art 
   Various types of zoom lenses are typically equipped with zooming cam mechanisms for moving a lens or lenses through a cam mechanism comprising circumferential cams and rectilinear guide slots. Such a zoom lens equipped with a zooming cam mechanism must have a multiple-barrel structure which comprises a cam barrel having zooming cam slots and a guide barrel having rectilinear guide slots inserted in each other so as to move linearly in an axial direction. A lens element or lens elements forming part of a zoom lens system are mounted in a lens holder barrel provided with cam followers. The lens holder barrel is inserted in the cam barrel or the guide barrel with the cam followers in slide engagement with the zooming cam slots and the rectilinear guide slots so as to move in an axial direction during relative rotation between the lens holder barrel and the cam barrel. 
   Some types of zoom lenses are equipped with focus adjusting mechanisms for adjusting focus of the zoom lens. Such a focus adjustment mechanism is used to make adjustment of an axial position or a movable range of a focusing lens element in which the zoom lens system is best focused based on resolusion of images at various focal lengths. A focus adjusting mechanism for performing the axial position adjustment of the focusing lens for best focus comprises a focus adjusting ring member having a cam face and a biasing spring for urging the cam face against an end of a lens holder barrel. Resolusion of an image formed by the zoom lens system is examined while rotating the focus adjusting ring member about an axis of the zoom lens system to move the lens holder barrel in an axial direction. The lens holder barrel is fixed in a position in which the zoom lens system forms a best focused image. Alternatively, the adjustment of a movable range of the focusing lens for best focus is performed by selectively bringing a screw pin into engagement with recesses formed in the lens holder barrel for different movable ranges. Another type of focus adjusting mechanism known from, for example, Japanese Patent No. 3498530 includes a spring installed in and biasing the lens holder barrel that is changeable in biasing force so as to adjust a lens position for best focus. 
   In the prior art zoom lenses in which a focus adjusting mechanisms is installed in a lens holder barrel that is disposed inside the cam barrel, the screw pin for focus adjustment is hidden by the cam barrel when the zoom lens assembled, so that it is impossible to access the screw pin for selecting axial positions or movable ranges of the focusing lens element for best focus from the outside of the zoom lens. Therefore, in order to perform focus adjustment of a zoom lens, it is essential to disassemble the zoom lens. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the present invention to provide a zoom lens which enables easily access to a screw pin for adjusting an axial position or a movable range of a focusing lens element for best focus of the zoom lens from the outside of the zoom lens after the zoom lens has been assembled. 
   The foregoing object is accomplished by a zoom lens comprising a cam barrel having a plurality of circumferential cam slots for zooming, stationary rectilinear guide means having axial guide slots disposed so that the axial guide slots overlap with the circumferential cam slots, respectively, a movable barrel inserted in the cam barrel so as to be rotatable and linearly movable in an axial direction within the cam barrel, the movable barrel having cam followers in slide engagement with both circumferential cam slots and rectilinear guide slots, respectively, so that the movable barrel is linearly moved in the axial direction with relative rotation between the cam barrel and the movable barrel so as thereby to move at least one lens element forming a part of a zoom lens system for zooming, and focus adjusting means for adjusting focus of the zoom lens system. The focus adjusting means comprises a plurality of focus range selection means formed on the movable barrel for providing different ranges of focus of the zoom lens system, a selection member selectively attachable to the focus range selection means so as to enable either one of the different ranges of focus, and an access slot extending continuously from an extreme end of one or more the circumferential cam slot so as to be brought in line with the focus range selection means for access of the selection member to the focus range selection means from the outside of the cam barrel. 
   Each of the circumferential cam slots may have the extension slot so that the focus range selection means are brought in line with the extension slots all at once when the cam barrel is put in an extreme end position of a zooming range, namely a telephoto end position or a wide-angle end position Otherwise, at least one of the circumferential cam slots may have the extension slot so that the focus range selection means are brought in line with the extension slot one by one during relative rotation between the movable barrel and the cam barrel. The focus range selection means may be brought in line with the rectilinear guide slot, besides the extension slot The focus range selection means may comprise a screw hole and a screw ping or a through hole and a pin. In the latter case, the pin can be adapted to be firmly fitted into and removed from the bore by controlling fit tolerance. In the case where the focus of the zoom lens is adjusted by adjusting a position in which a biasing spring for biasing a lens element in either one of axial directions is attached, it is preferred to form the hole as a slot elongated in the axial direction. Further, in the case where the zoom lens comprises a lens holder frame with a focusing lens element mounted therein that is rectilinearly movable within the movable barrel and a focus adjusting cam ring having an end face cam forced against a front end of the lens holder frame which is rotated relatively to the movable barrel so as to be located in the initial position, it is preferred to form a slot elongated in a circumferential direction in the movable barrel when the focus adjusting cam ring has one hole in an external wall. 
   Furthermore, the zoom lens may further comprise a lens holder barrel mounting the focus lens element that is mounted within the movable barrel through helicoid coupling so as that lens holder barrel moves in an axial direction with respect to the movable barrel during rotation relative to the movable barrel according to leads of the helicoid threads for focusing, besides moving in the axial direction together with the movable barrel during relative rotation between the movable barrel and the cam barrel for zooming. In this zoom lens, the focus range selection means may comprise holding means formed in the movable barrel for holding the selection member and restriction means such as recesses formed in the lens holder barrel for defining moving distances of the lend holder barrel differently for the ranges of focus of the zoom lens system, respectively. In the case where a plurality of focus range selection means are equipped, the focus range selection means are offset in an axial direction and the selection member is adapted to be selectively attached to the movable barrel in different circumferential positions. It is of course that the focus range selection means may be offset in a circumferential direction, and hence the selection member is adapted to be selectively attached to the movable barrel in different axial positions. 
   According to the zoom lens of the present invention having circumferential zooming cam slots with extension slots continuously extending therefrom, respectively, in the cam barrel, the focus range selection means are exposed to the outside of the zoom lens all at once when the cam barrel is put in an extreme end of the zooming range, namely a telephoto end position or a wide-angle end position, or one by one during rotation of the cam barrel, so that focus adjustment of the zoom lens is easily performed even after the zoom lens has been assembled. In consequence, efficiency of the focus adjustment is enhanced. Furthermore, the zoom lens allows an increases in the degree of freedom of assembling procedure and, in consequence, enhanced assembling efficiency. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects and features of the present invention will be clearly understood from the following detailed description when reading with reference to the accompanying drawings, wherein the same reference signs have been used to denote same or similar parts throughout the drawings, and in which: 
       FIG. 1  is an exploded perspective view of a zoom lens barrel according to an embodiment of the present invention; 
       FIG. 2  is a graphical illustration showing an axial position of a movable barrel and a rear lens holding barrel with respect to a rotational position of a cam barrel; 
       FIG. 3  is a graphical illustration showing the relationship between rotational and axial positions of the front lens holding barrel; 
       FIG. 4  is an expansion view of the cam barrel in which a cam follower in a telephoto end position and a wide-angle end position; 
       FIG. 5  is an expansion view of a stationary barrel in which cam followers and screw holes are shown in positions when the cam barrel is put in a telephoto end position; and 
       FIG. 6  is an expansion view of the stationary barrel in which cam followers and screw holes are shown in positions when the cam barrel is put in a wide-angle end position. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to the accompanying drawings in detail, and in particular, to  FIG. 1  showing a zoom lens  10  according to an embodiment of the present invention, the zoom lens  10  comprises a plurality of cylindrical barrels, namely a front lens holding barrel  11 , a movable barrel  12 , a cam barrel, a stationary barrel  14 , and a rear lens holding barrel  15 . The front lens holding barrel  11  has a first group of lenses  16  mounted therein. The front lens holding barrel  11  has a first group of lenses  16  mounted therein, and the rear lens holding barrel  15  has a second group of lenses  28  mounted therein. The movable barrel  12  has internal helicoids threads  20 . The front lens holding barrel  11  rotates with respective to the movable barrel  12  during focusing so as thereby to move axially within the movable barrel  12  according to leads of the helicoids threads  17  and  20 . 
   The front lens holding barrel  11  is provided with external helicoid threads  17  and first and second rotational range recesses or through holes  18  and  19  shaped square. The first and second rotational range recesses  18  and  19  are located in positions circumferentially and axially different from each other. The movable barrel  12  is provided with internal helical threads  20  engageable with the helical threads  17  of the front lens holding barrel  11 , a plurality of, for example three in this embodiment, cam follower pins  24  extending radially outward and disposed with regular circumferential intervals and first and second screw holes  21  and  22  (first screw hole  21  is hidden behind the cam follower pin  24 ; see  FIGS. 5 and 6 ). The first and second screw holes  21  and  22  are located in circumferentially different but axially identical positions. As will be described later, each screw hole  21 ,  22  is located at a specific distance from the cam follower pin  24 . The movable barrel  12  is rotationally coupled to the front lens holding barrel  11  through engagement between their helicoids threads  17  and  20 . A screw pin  23  serving as range selection means is engaged in either one of the screw holes  22  which is chosen according to a result of focus adjustment The range selection screw pin  23  engaged in the first screw hole  21  is received in the first rotational range recess  18  so as thereby to restrict rotation of the front lens holding barrel  11  with respect to the movable barrel  12  to a first rotational range AA (see  FIG. 3 ) according to a circumferential length of the first rotational range recess  18 . Similarly, the range selection screw pin  23  engaged in the second screw hole  22  is received in the second rotational range recess  19  so as thereby to restrain rotation of the front lens holding barrel  11  with respect to the movable barrel  12  to a second rotational range AB (see  FIG. 3 ) according to a circumferential length of the second rotational range recess  19 . The rotational range recesses  18  and  19  may be shaped so as to define the rotational ranges of the front lens holding barrel  11  completely different or partly overlapping and essentially have axial widths covering possible axial movement of the front lens holding barrel  11  according to the first rotational range AA or the second rotational range AB. 
   The cam barrel  13  is provided with a plurality of, for example three in this embodiment, first circumferential cam slots  25  disposed with regular circumferential intervals, a plurality of, for example three in this embodiment, second circumferential cam slots  27  disposed with regular circumferential intervals, and a plurality of, a plurality of, for example three in this embodiment, thrust restraint pins  32  extending radially outward and disposed with regular circumferential intervals. The movable barrel  12  is rotationally coupled to the cam barrel  13  through engagement of the cam follower pins  24  of the movable barrel  12  with the circumferential cam slots, respectively. 
   The stationary barrel  14  is provided with a plurality of, for example three in this embodiment, first rectilinear guide sots  30 , a plurality of, for example three in this embodiment, second rectilinear guide slots  31 , and a plurality of, for example three in this embodiment, thrust restraint slots  33  extending circumferentially. The movable barrel  12  and the cam barrel  13  are rotationally or linearly movably coupled to the stationary barrel  13  through engagement of the cam follower pins  24  of the movable barrel  12  with the circumferential cam slots  25  of the cam barrel  13  and the first rectilinear cam slots  30 , respectively. However, the cam barrel  13  is prevented from moving axially within and with respect to the stationary barrel  14  by means of engagement of the thrust restraint pins  32  of the cam barrel with the thrust restraint slots  33  of the stationary barrel  14 . 
   The front lens holding barrel  11  is provided with a plurality of, for example three in this embodiment, cam followers  26  extending radially outward and disposed with regular circumferential intervals. The front lens holding barrel  11  is rotationally coupled to the cam barrel  13  through engagement of the cam follower pins  26  with the second circumferential cam slots  27  of the cam barrel  13 , and further linearly movably coupled to the stationary barrel  13  through engagement of the cam follower pins  26  passing through the second circumferential cam slots  27  with the rectilinear guide slots  31  of the stationary barrel  14 , respectively. 
     FIG. 2  is a graphical illustration showing an axial position of the movable barrel  12  and the rear lens holding barrel  15  with respect to an angle of rotation of a cam barrel  13 . As shown, the cam barrel  13  is rotative to a desired position within a possible zooming range or between opposite extreme ends of zooming, namely a telephoto end position and a wide-angle end position During rotation of the cam barrel  13 , the movable barrel  12  is forced in an axial direction due to axial displacement of the cam follower pins  24  in the cam slots  25  according to an angle of rotation of the cam barrel  13 . Simultaneously, the rear lens holding barrel  15  is forced in an axial direction due to axial displacement of the cam follower pins  26  in the cam slots  27  according to an angle of rotation of the cam barrel  13 . 
     FIG. 3  is a graphical illustration showing the relationship between rotational and axial positions of the front lens holding barrel  11 . As shown, the front lens holding barrel  11  has a maximum rotational range A between extreme rotational positions A 0  and A 5  with respect to the movable barrel  12  and a maximum axial movable range B between extreme movable positions L 0  (on the image side) and L 5  (on the object side). The first rotational range AA is chosen through engagement of the range selection screw pin  23  with either one of the rotational range recesses  18  and  19 , and the second rotational range AB is chosen through engagement of the range selection screw pin  23  with the other of the rotational range recesses  18  and  19 . These first and second rotational ranges AA and AB are established so as to partly overlap in this embodiment. When the first rotational range AA is chosen, the front lens holding barrel  11  is allowed to move in axial direction within a first focus range C between axial positions L 1  and L 3  within the maximum axial movable range B. On the other hand, when the second rotational range AB is chosen, the front lens holding barrel  11  is allowed to move in axial direction within a second focus range D between axial positions L 2  and L 4  that partly overlap with the first focus range C within the maximum axial movable range B. In  FIG. 3 , the axial position L 1  is an extreme movable position on the image side, and the axial position L 4  is an extreme position on the object side. 
   Referring to  FIG. 4  showing the cam barrel in detail, each of the first circumferential cam slots  25  has an extension slot  40  continuously extending from one end thereof as an access path. The extension slots  40  are used to perform focus position adjustment after having assembled all constituent barrels to the zoom lens  10 . Specifically, the extension slots  40  are formed so as to be accessible through the thrust restraint slots  33  of the stationary barrel  14  when the constituent barrels  11  to  15  have been assembled to the zoom lens  10 . Further, as was described previously, the screw hole  21 ,  22  is located at specific same distances from adjacent cam follower pin  24 , respectively, so that the first and second screw holes  21  and  22  are accessible through the extension slots  40  of two adjacent cam slots  25 , respectively, when the cam barrel is put in, for example, the telephoto end position as shown in  FIG. 5 . Therefore, the range selection screw pin  23  can be screwed in or unscrewed from either one of the first and second screw holes  21  and  22  with a screw driver from the outside of the zoom lens  10  until reaching the rotational range recess  18  or  19  so as thereby to choose either one of the first and second rotational ranges AA and AB. In an assembling process, the range selection screw pin  23  is screwed into one screw hole designated in the design specification. 
   In this instance, the first and second screw holes  21  and  22  are exteriorly exposed when the cam barrel  13  is put in the telephoto end position as shown in  FIG. 5  and, however, hidden inside by the stationary barrel  14  when the cam barrel  13  is put in the wide-angle end position as shown in  FIG. 6 . 
   Assembling and focus position adjustment of the zoom lens  10  thus structured is performed as described below. First of all, the zoom lens  10  is assembled by inserting the movable barrel  12  into the cam barrel  13  from the front and then the rear lens holding barrel  15  into the same from the front or vice versa. Subsequently, after inserting the cam barrel  13  into the stationary barrel  14 , the thrust restraint pins  32  are fixedly attached to the cam barrel  13  through the thrust restraint slots  33 , respectively. In this state, the cam barrel  13  is held in the stationary barrel  14  so as to be rotatable but prevented from axially moving. Then, the cam follower pins  24  are fixedly attached to the movable barrel  12  through the first rectilinear guide slots  30  and the first cam slots  25 , respectively, and the cam follower pins  26  are fixedly attached to the rear lens holding barrel  15  through the second rectilinear guide slots  31  and the second circumferential cam slots  27 , respectively. In this state, the movable barrel  12  and the rear lens holding barrel  15  are held in the cam barrel  13  so as to be axially movable in opposite directions according to rotation of the cam barrel  13  with respect to the stationary barrel  14 . Finally, after screwing the front lens holding barrel  11  into the movable barrel  12  through helicoid thread coupling between the helical threads  17  and  20 , the range selection screw pin  23  is screwed in one of the first and second screw holes  21  and  22  until reaching the rotational range recess  18  or  19  as designated in the design specification In this manner, assembling of the zoom lens  10  is completed. 
   The completed zoom lens  10  is sent to an adjusting station for focus adjustment. Focus adjustment is performed through, for example, the following steps. That is, the front lens holding barrel  11  is rotated with respect to the movable barrel  12  until the range selection screw pin  23  abuts against one of opposite extreme edges of the rotational range recess  18  that is on a close-up side. Then, a distance of an object on which the zoom lens  10  is focused is measured. When the object distance falls within a predetermined extent of object distance, the zoom lens  10  is freed from focus adjustment. If the object distance is out of the predetermined extent of object distance, either one of the first and second screw holes  21  and  22  into which the range selection screw pin  23  should be screwed is chosen according to how far or how close the object is. Because, regarding an infinite distance, focusing accuracy can be warranted only by shifting the front lens holding barrel  11 , it is typically suffice to carry out focus adjustment for extremely close ranges. The shift distance can be warranted simply by the circumferential length of the rotational range recess  18  at the design phase. 
   After having made a choice of either the first screw hole  21  or the second screw hole  22 , the cam barrel  13  is rotated with respect to the stationary barrel  14  until reaching the telephoto end position shown in  FIG. 5 . The telephoto end position is provided at an extreme end  25   a  of the first circumferential cam slots  25  opposite to the extension slot  40 , the cam barrel  13  is allowed to be simply and easily rotated until striking the extreme end of the first circumferential cam slots  25 . In the telephoto end position, the first and second screw holes  21  and  22  are positioned so as to be accessible through adjacent two of the thrust restraint slots  33  of the stationary barrel  14  and adjacent two of the extension slot s  40  of adjacent two of the first circumferential cam slots  25  of the cam barrel  13 . In consequently, it is easy to screw the range selection screw pin  23  into the screw hole  21  or  22  from the outside of the stationary barrel  14  without removing the movable barrel  12  and then screwing the range selection screw pin  23 . 
   When screwing the range selection screw pin  23  into the first screw hole  21 , the front lens holder barrel  11  is rotated with respect to the movable barrel  12  beforehand so as to bring the first rotational range recess  18  in line with the first screw hole  21 . In this state, the front lens holder barrel  11  is restricted in rotation within the first rotational range AA so as thereby to move within the first focus range C during focusing as shown in  FIG. 3 . On the other hand, when screwing the range selection screw pin  23  into the second screw hole  22 , the front lens holder barrel  11  is rotated with respect to the movable barrel  12  beforehand so as to bring the second rotational range recess  19  in line with the second screw hole  22 . In this state, the front lens holder barrel  11  is restricted in rotation within the second rotational range AB so as thereby to move within the second focus range D during focusing as shown in  FIG. 3 . 
   Although, in the zoom lens of the described embodiment, the extension slots  40  extending continuously extending from the wide-angle end of the first circumferential cam slots  25  of the cam barrel  13  are used as access paths to the screw holes  21  and  22  which the range selection screw pin  23  is attached to or removed from, nevertheless, they are available for positional and operational adjustment of adjusting focus adjusting springs and/or their associated parts that are hidden inside by the cam barrel  13  and, in consequence, hardly accessible. Although the access path is provided by the extension slots  40  continuously extending from the first circumferential cam slot  25  of the cam barrel  13  so as to be accessible through the thrust restraint slot  33  of the stationary barrel  14 , it may be provided by the thrust restraint slot  33  itself or an extension slot continuously extending from the thrust restraint slot  33 . In this manner, according to the zoom lens of the present invention, slots earmarked for specific purposes or extension slots of the specific slots formed in multiple overlapping barrels are used for forming an access path to the screw holes  21  and  22 . Further, although the first and second screw holes  21  and  22  are disposed so as to be exteriorly exposed all at once through the extension slots  40  when the cam barrel  13  is put in a specific rotational position, namely the telephoto end position in the described embodiment, it may be possible to exteriorly expose the first and second screw holes  21  and  22  one by one during gradual rotation of the cam barrel  13 . 
   Furthermore, although the zoom lens  10  is exemplified as comprising two lens groups, namely the first and second lens groups  16  and  28 , it may comprise three or more lens groups. The screw pin  23  and the screw holes  21  and  22  forming the range selection means may be replaced with a pin and bores. In this case, the pin can be adapted to be firmly fitted into and removed from the bore by controlling fit tolerance. 
   It is to be understood that although the present invention has been described with regard to a preferred embodiments thereof, various other embodiments and variants may occur to those skilled in the art, which are within the scope and spirit of the invention, and such other embodiments and variants are intended to be covered by the following claims.