Abstract:
A zoom lens assembly that couples a second lens group to a first lens group such that one cam mechanism is able to transfer both lens groups. The second lens group is further provided with a second cam mechanism that is able to provide the fine distance adjustment of the second lens group required for proper focus correction.

Description:
FIELD OF INVENTION  
         [0001]    The present invention relates generally to a zoom lens apparatus and more particularly to a zoom lens apparatus having a distance adjustment mechanism for a first and a second lens group.  
         BACKGROUND OF INVENTION  
         [0002]    Conventional zoom lens apparatus generally have tow movable lens groups. The first lens group is generally used for magnification proposes and the second lens group is used for focus correction. The distance between the two lens groups from the film surface is adjusted to achieve the zooming effect. The group  1  lens element is used for focal length adjustment. The group  2  lens element is used for focus correction, and is positioned between the group  1  lens element and the film surface. To achieve the zooming effect, the group  1  lens element is moved farther away from the film surface by a distance X. For refocusing the image onto the film, the group  2 , lens element is also moved farther away from the film surface by distance Y. To achieve proper focus, distance Y is larger than distance X, the exact relationship being readily derived from well known optics principles such as Snell&#39;s law. In order to maintain the ratio between distances X and Y, many different cam and gear mechanisms have been designed in the art. Some conventional cameras provide a cam mechanism having a cam barrel with two independent helicoid cam groove with different gradients for the movement of the two lens groups. The first cam is used for moving the first lens group, while the second cam grove is used for moving the second lens group. Since the distance Y is greater than X, the gradient (defined as the angle formed between the traverse-sectional edge of the groove barrel and the groove) for the second groove is larger (steeper) than the first groove. The steeper gradient results in poor accuracy and less reliable mechanical movement for the group  2  lens element. There is therefore a need to provide an improved lens movement mechanism for zoom lenses.  
         SUMMARY OF INVENTION  
         [0003]    Accordingly, the present invention provides a zoom lens assembly that couples the second lens group to the first lens group such that one cam mechanism is able to transfer both lens groups. The second lens group is further provided with a second cam mechanism that is able to provide the fine distance adjustment of the second lens group required for proper focus correction.  
           [0004]    The zoom lens assembly according to the present invention contains a first lens group with a first lens defining an optical axis. A first cam mechanism, adapted for mounting onto a camera, is coupled to the first lens group for moving the first lens group along the optical axis. A second lens is provide within the second lens group and has an optical axis that is aligned with the optical axis of the first lens. The second lens group is coupled to the first cam mechanism such that moving the first lens group along the optical axis by operating the first cam mechanism translates into a movement of the second lens group. In addition, a second cam mechanism is used to further couple the first cam mechanism and the second lens group such that operating the first cam mechanism causes an additional movement of the second lens group along the same optical axis.  
           [0005]    In the preferred embodiment, the first lens group is attached inside a first cam barrel such that the optical axis coincide with the longitudinal axis of the first cam barrel. A rotary barrel, adapted for rotatable attachment onto the mount opening of a camera, is slidably fitted over the first cam barrel and is provided with a cam drive to slidably engage a helicoid cam groove cut on the first cam barrel.  
           [0006]    In the preferred embodiment, the second lens group is fitted within a second cam barrel which contains one or more helicoid channels and one or more longitudinal channels. The second lens group is also attached to the first cam barrel in a manner such that there is no rotational movement of the second lens group relative to the first cam barrel. The cam drive engages to the longitudinal channel such that rotating the rotary barrel caused a corresponding rotation of a second cam barrel. At least one second cam drive is mounted on the second lens group and interacts slidably with the helicoid channel such that the rotational twist of the rotary barrel by the user is translated into additional longitudinal movement of the second lens group.  
           [0007]    An important result of this combination cam mechanism is that the helicoid channel that is cut into the second cam barrel does not need to have a steep gradient, because the purpose of the helicoid channel is only for the fine positioning required for focus correction of the second lens group. The larger travelling distance that is required for focal length adjustment for both lens groups is rendered by the helicoid groove of the first cam barrel. As a result, the helicoid channel has a shallow gradient that gives greater stability and the reliability. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0008]    [0008]FIG. 1 is the exploded perspective view of the zoom mechanism assembly according to our embodiment of the present invention.  
         [0009]    [0009]FIG. 2 is the side view of the present zoom lens assembly fitted with a camera.  
         [0010]    [0010]FIG. 3A is the top perspective view of the assembled zoom assembly of FIG. 1 in the wide-angle position with a section cut-away to illustrate the relative positions of the various parts.  
         [0011]    [0011]FIG. 3B is the bottom perspective view of the assembled zoom assembly of FIG. 1 in the wide-angle position with a section cut-away to illustrate the relative positions of the various parts.  
         [0012]    [0012]FIG. 4A is the top perspective view of the same cut-away assembly as FIG. 1, but with the zoom mechanism in the fully extended position.  
         [0013]    [0013]FIG. 4B is the bottom perspective view of the same cut-away assembly as FIG. 1, but with the zoom mechanism in the fully extended position 
     
    
     DETAILED DESCRIPTION  
       [0014]    In the description and the accompanying claims, the terms “comprising”, “including” and “containing” are meant to be open-ended in their meaning, and should be interpreted to have the meaning “but not limited to . . .”. The word “coupling” refers to a direct mechanical coupling or an indirect mechanical coupling via a mechanical connection or an intermediate device. Also, certain elements or parts understood to be components of the zoom lens assembly have been left out of He drawings in order not to obscure the present invention. For example, each lens group may contain a plurality of lens which may be attached to different positions along he optical axis. Different zoom lens designs may contain different number of lens in each lens group. Thus the exact positioning of the various lenses may vary. It is understood that a description of the movement of a particular lens group refers to the movement of all lenses belonging to the same lens group.  
         [0015]    Referring first to FIG. 1, the preferred embodiment of the present invention contains a first lens element  20  having an optical axis  20   a  and a neck  22  for mounting onto the collar  24  of a lens plate  26 . For ease of explanation, those parts are generally referred to as the first lens group.  
         [0016]    The first cam mechanism includes a cylindrical rotary barrel  28  with two cam pins  30  mounted thereon and extending radially inward through two side holes  32 . In the most preferred embodiment, the outer surface of lie rotary barrel  28  is also provided with circumferential ribs  28   a  for engagement onto the mount hole of a camera. The first cam mechanism also includes a cylindrical fast cam barrel  34  having a pair of opposing helicoid cam grooves  36  cut through the cylindrical structure. Within the first cam barrel is a cross plate  38  provided with screw holes for attachment of the fir lets group via te first attachment plate  40 , Flanges  38   a  extend from the film end of the barrel for interaction with the interior of the camera.  
         [0017]    The second lens group contains a second lens element  42  that contains the focus correction lenses. The second lens element  42  has a non-circular shape and in the prefixed embodiment, contains two opposing flat sides  42   a.  Lens frame  44  has an internal opening having the same non-circular shape as the second lens element with two flat sides  44   a  and is adapted to receive the second lens element therein. The exterior of lens fame  44  also retains two flat sides  44   a.  Lens frame  44  is also provided with smooth holes  44   b  on the periphery and running parallel to the optical axis. Another set of radially extending screw holes  44   c  are provided for mounting of two second cam barrel pins  46 .  
         [0018]    The second cam mechanism includes a cylindrical second cam barrel  48  that is adapted for telescopic fitting over  1 ms frame  44 . In the preferred embodiment, second cam barrel  48  contains two helicoid chapels  48   a  cut through the cylindrical wall, and two longitudinal channels  48   b  providing two recesses on the outer wall of the barrel parallel to the optical axis. In the preferred embodiment, a second attachment plate  50  is provided and contains two circular plates  50   a  and  50   b,  and two hollow lens guard rods  50   c.  The first circular plate is the lens guard rod holder and contain a central opening along the optical axis and screw holes  50   e  along the periphery for molting onto cross plate  38  of the first cam barrel. The second circular plate  50   b  also contains a central opening along the optical axis. Another lens (not shown) is fixed between plates  50   a  and  50   b  and is a constituent part of the first lens group. The hollow lens go rod  50   c  has internal threads at the open end. Positioning frame  52  in the preferred embodiment is disk-shaped, with a circular outer edge and a cavity  54  therein. Cavity  54  contains two opposing flat sides  54   a  and has shape corresponding to that of the second lens element. Two through holes  54   d  are also provided.  
         [0019]    The first lens group is assembled by screwing the threaded neck  22  of the first lens element  20  into collar  24  of the first lens plate  26 . The first lens group couples to the first cam mechanism by a screw attachment of the first lens plate  26  to the cross plate  38  of the first cam barrel  34  with the first lens attachment plate  40  inserted therebetween. The second lens element  42  fits within lens frame  44 , which is in turn fitted within the second lens barrel  48 . The two second cam barrel pins  46  are screwed onto radiating screw holes  44   c  and extend radially outwards to engage helicoid channel  48   a.    
         [0020]    The second attachment plate  50  is screwed onto  38  and is also adapted to also fits within the second lens barrel  48  with the guard rods  5 O c  extending longitudinally therethrough. The two guard rods mate with holes  44   b  and through holes  54   d  to lock lens frame  44  within position frame  52  by a screw  56  thereon. The second attachment plate  50  is mounted onto cross plate  38  in the first cam barrel  34 . Referring now to FIG. 2, the entire assembly  60  fits into the mounting hole  62  of a camera  64  with flange  38   a  of the first cam barrel  34  engaging mounting hole  62 .  
         [0021]    Referring also to FIGS. 3A and 3B and FIGS. 4A and 4B, the zoom lens assembly in the wide-angle position has the two lens groups in the fully retracted position along the optical axis such that distance between the two lenses is shortest. When a user wants to zoom onto a distant object, he can grip and twist the exterior of rotary barrel  28 , which is free to rotate within the camera mount hole. Ribs  28   a  engage the rotary barrel  28  to the camera and prevent any movement along the optical as relative to the camera. The rotating action causes the cam pins  30  to act as the cam drive to transfer the first cam barrel  34  outwards away from and inwards towards the film surface of the camera by sliding along cam groove  36 . Since both the first and second lens groups are coupled to the first cam barrel  34 , both groups of lenses are transferred therealong. The cam pins, however, are also engaged within the longitudinal channels  48   b  of the second cam barrel  48 . Thus, the twisting action of the user on the rotary barrel also causes the rotation of the second cam barrel. Since lens fame  44  is coupled to the non-rotating second attachment plate  50 , via rods  50   c,  the second lens group is prevented from rotating concomitantly. The second lens group, however, is also coupled to the rotating second cam barrel by engagement of the second cam pins to the helicoid channel  48   a.  Thus, the rotation of the second cam barrel causes the second lens group to slide longitudinally within the helicoid channel and along the guard rod, translating the rotational movement into additional longitudinal movement of the second lens group. The second lens group is also free to slide longitudinally through cavity  54 . In the same manner, twisting in the reverse direction causes a retraction of the second lens group for different distances. Thus, rotation of second cam barrel  48  enhances additional movement of leas group  2  initiated by pin  46 . As helicoid channel  48   a  is only used for the additional movement, its gradient is less than that in a conventional design.  
         [0022]    While the present invention has been described particularly with references to the aforementioned figures with emphasis on cam barrels containing grooves cut therein, it should be understood that the figures are for illustration only and should not be taken as limitation on the invention. It is contemplated that many changes and modifications may be made by one of ordinary skilled in the art without departing from the spirit and the scope of the invention described. For example, although drive pins, and helicoid and longitudinal grooves are described in the preferred embodiment, other types of drive mechanisms may also be used to serve the same function such as gears and gear drives.