Abstract:
The invention relates to a view finder with improved structure for providing accurate posisioning and alignment of optical elements of its optical system. The view finder comprises an object lens system at least one lens of which is movable along an optical axis of the object lens system being guided by a first guide shaft and an eye-piece lens system at least one lens of which is movable along an optical axis of the eye-piece lens system being guided by a second guide shaft, wherein the first guide shaft and the second guide shaft are positioned in axial alignment. Preferably the first guide shaft and the second guide shaft are integrally formed and immovable optical elements such as fixed lens and prism of the optical system are also supported by the same guide shaft.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to a viewfinder. More specifically, this invention relates to a view finder capable of varying a magnification in accordance with a movement of taking lens of camera for zooming.  
         BACKGROUND OF THE INVENTION  
         [0002]    A view finder, capable of varying a magnification in accordance with a movement of taking lens of camera for zooming, usually includes a finder optical system capable of varying magnification of a finder image and an interlocking mechanism for shifting a movable lens in the finder optical system along the optical axis. A real-image type lens system is usually used for the finder optical system, which usually includes two movable lenses. Some finder has a diopter moving mechanism where eyepiece side lens is movable so as to match an eyesight of the user, such as a near-sighted or a far-sighted, in order to make the finder image of a subject clearer.  
           [0003]    Aforementioned movable lenses in the finder optical system for zooming or diopter movement require high accuracy positioning and alignment. Also immovable optical elements other than the movable lenses must be fixed in the finder with high accuracy positioning and alignment. However, positioning and alignment of those optical elements are not always accurate because respective elements are held by respective holding members supported by respective local structure of the finder housing, which cause low optical performance. Also a guide shaft for guiding a movable lens for zooming is different one from a guide shaft for guiding a movable lens for diopter movement, which causes increase of the number of parts and manufacturing costs.  
         SUMMARY OF THE INVENTION  
         [0004]    An object of the invention is to provide a view finder with high accuracy and low cost holding means for holding optical elements constituting a view finder.  
           [0005]    To establish the object, a view finder of this invention commprises an object lens system at least one lens of which is movable along an optical axis of the object lens system being guided by a first guide shaft and an eye-piece lens system at least one lens of which is movable along an optical axis of the eye-piece lens system being guided by a second guide shaft, wherein the first guide shaft and the second guide shaft are positioned in axial alignment. It is preferable to make the first guide shaft and the second guide shaft formed integrally. The diameters of the two parts can be the same.  
           [0006]    Also the optical axis of the object lens system and the optical axis of the eyepiece lens system can be different from each other, and the object lens system can be capable of varying a magnification of a finder image by moving at least one lens of the object lens system and the eyepiece lens system may be capable of diopter movement by moving at lease one lens of the eyepiece lens system.  
           [0007]    There are two types of finder in ways of fixing the guide shaft. A first type view finder comprises a lens movable along the optical axis being guided by a guide shaft which is fixed to a finder housing and an immovable optical element mounted to the guide shaft which constitutes a finder optical system of the view finder together with the movable lens. A second type view finder comprises a lens movable along the optical axis being guided by a guide shaft and an immovable optical element which constitutes a finder optical system of the view finder together with the movable lens, wherein the guide shaft is fixed to the immovable optical element 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The above and other objects and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments when read in association with the accompanying drawings, which are given by way of illustration only and thus are not limiting the present invention. In the drawings, like reference numerals designate like or corresponding parts throughout the several views, and wherein:  
         [0009]    [0009]FIG. 1 shows an external perspective view of a camera where a finder of the invention is used;  
         [0010]    [0010]FIG. 2 shows an exploded perspective view of a finder and shifting mechanism of the invention for shifting a movable lens for varying magnification of finder image;  
         [0011]    [0011]FIG. 3 shows an exploded perspective view of the finder optical elements indicating disposition of the elements;  
         [0012]    [0012]FIG. 4 shows a cross sectional view of the finder of the invention;  
         [0013]    [0013]FIG. 5A. shows a schematic illustrating a mechanism for shifting a diopter moving lens where the diopter moving lens is located near to the user;  
         [0014]    [0014]FIG. 5B. shows a schematic illustrating a mechanism for shifting a diopter moving lens where the diopter moving lens is located far from the user;  
         [0015]    [0015]FIG. 6 shows a cross sectional view of the finder of another embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    [0016]FIG. 1 shows an external perspective view of the camera where a finder of the invention is used. A camera  10  is made of a camera body  11  and a variety of photographing mechanism installed therein. The camera body  11  includes abase frame  12  (See FIG. 2), a front cover  13  for covering front side of the base frame  12  and a rear cover  14  for covering rear side.  
         [0017]    The front side of the camera has a zoom lens barrel  16  for holding a taking lens of zooming optical system, an object side finder window  17 , a distance-measuring light emitting window  20   a  and a distance-measuring light receiving window  20   b , a metering light receiving window  21  and a flash unit  22 . Top side of the camera body  11  has a shutter releasing button  23 . The zoom lens barrel  16  is extended out of the body  10  when power is on. The rear side of the camera body  10  includes switches for zooming the lens barrel, selecting a photographing mode, adjusting diopter of the finder, a LCD panel for displaying a variety of information and an eyepiece window.  
         [0018]    As shown in FIG. 2, a fixed barrel into which the zoom lens barrel  16  is installed, a film cartridge loading chamber  25  and a film take-up chamber  26  for taking up the film withdrawn from the film cartridge are integrally formed in the base frame  12 . The fixed barrel  24  is disposed between the film cartridge loading chamber  25  and the film take-up chamber  26 .  
         [0019]    A view finder  27  is mounted above the fixed barrel  24 . The view finder includes a variety of optical elements, disposed along an optical axis of the finder, constituting a finder optical system and a finder housing frame  30  made of plastic block for holding the variety of optical elements.  
         [0020]    As shown in FIG. 2 and FIG. 3, the finder optical system for a real-image type zoom finder is made of an object lens  31 , first varying magnification lens  33 , second varying magnification lens  34 , a porro-prism  35 , a field of view frame  36 , a reflex mirror  37 , a diopter moving lens  40  and eyepiece lens  41 , which are disposed from object side in order. In the front of the finder housing frame is a finder object side window  17 . The object lens  31  is fitted behind the finder object side window  17  and has a holding portion  31   a  to hold a guide shaft  42  (described later).  
         [0021]    The first and second varying magnification lenses are movable along a finder object side optical axis  58 . They have integrally formed bearing portions  33   a ,  34   a  and guiding projections  33   b ,  34   b  on their both side respectively. The guiding projections  33   b ,  34   b , are engaged with a guide rail  43  set inside the finder housing frame  30  and the bearing portions  33   a ,  33   b  receive a guide shaft  42  parallel to the optical axis  58  therethrough. Thus the movement of the first and second varying magnification lenses  33 ,  34  are guided by the guiderail 43  and the guide shaft  42 . Each bottom side of the bearing portions  33   a ,  34   a  has cam pins  33   c ,  34   c  projecting downwardly respectively, which are engaged with a cam plate  44  fixed to the outer surface of the fixed barrel  24 .  
         [0022]    The cam plate  44  is bent along the outer surface of the fixed barrel  24  so as to rotate around the optical axis of taking lens system along the outer surface of the barrel  24 . A cam groove  45  and a pair of cam grooves  46 , 47  are formed in the cam plate  44 . One end portion  51   a  of an interlocking pin  51  which is extended out from a slot  50  formed in the fixed barrel  24  is engaged with the cam groove  45 . The other end of the interlocking pin  51  is fitted in an opening (not shown) formed in the zoom lens barrel  16  which moves back and forth inside the fixed barrel  24  when zooming is made, which causes the interlocking pin  51  to move along the taking lens optical axis. The movement of the interlocking pin  51  makes the cam plate  44  rotate around the optical axis. The rotational displacement of the cam plate  44  directly indicates the position of the zoom lens barrel  16 , namely the magnification of the zooming.  
         [0023]    The cam pins  33   c ,  34   c  formed in the first and second varying magnification lenses  33 ,  34  respectively are engaged with the pair of cam grooves  46 ,  47  respectively. Patterns of the cam grooves  46 ,  47  are predetermined so that the movement of the first and second varying magnification lenses  33 ,  34  can make finder optical system always match the zoom taking lens system in image magnification as zoom lens barrel  16  moves for zooming. A leaf spring  52  is disposed between the outer surface of the fixed barrel  24  and the cam plate  44  to lightly push up a upper portion of the cam plate  44 , which keeps respective engagement between the cam pins  33   c ,  34   c  and cam grooves  46 ,  47  without fail.  
         [0024]    As shown in FIG. 3, the bearing portions  33   a ,  34   a  have projections  33   d ,  34   d  with recess respectively. Each end of a spring  48  is hooked over each of the projections  33   d ,  34   d  by using the recess. Two lenses  33 ,  34  are spring-biased toward each other by that, by which the cam pin  33   c  is biased to keep in contact with the inner wall of eyepiece side of the cam groove  46 , and the cam pin  34   c  is biased to keep in contact with the inner wall of object side of the cam groove  47 . As a result, two lenses  33 ,  34  are positioned without fluctuation in the optical axis  58 , regardless of rotating direction of the cam plate  44 .  
         [0025]    A porro-prism  35  is made by forming two right angle prism  53 ,  54  integrally, which functions as an erect prism for transforming a reverse image both vertically and horizontally into a correct image. The first right angle prism  53  has a positioning boss  53   a  on its one side.  
         [0026]    [0026]FIG. 4 shows a cross sectional view of the finder of the invention. The positioning boss  53   a  is fitted into a positioning hole  49  which is formed inside the finder housing frame  30 . On the other side of the first right angle prism  53  is formed integrally a holding portion  53   b  to hold the guide shaft  42 . One end of the guide shaft in object side is fitted into a hole formed in the holding portion  31   a  of the object lens  31  and the other end in eyepiece side is fitted into a hole formed in the holding portion  53   b . Thus the guide shaft  42  is fixed inside the finder housing by both the object lens  31  and the porro-prism  53 .  
         [0027]    The field of view frame  36  for framing a field of view is disposed in a position where the image of the subject is focussed through the object lens  31 , the first and second varying magnification lenses  33 ,  34 . The field of view frame  36  is constituted by a focus plate lens  55  where target mark and parallax correction mark are engraved and a focus plate lens holder  56  for holding the focus plate lens  55 . A field of view changing mechanism can be applied to the finder of the invention, which is manually or automatically changed according to the change in size or aspect ratio of image to be photographed on the film or the like.  
         [0028]    The diopter moving lens  40  are movable along an eyepiece side optical axis  59 . It has integrally formed mounting portion  40   a  and a cam pin  40   b  on its both side respectively. The mounting portion  40   a  with a hole is mounted on a guide shaft  42  parallel to the optical axis  59  by the hole. The cam pin  40   b  reaches cam groove  60   a , which is formed on the side of a gear wheel  60  disposed outside the finder housing  30 , through a guide slot  57  formed in the finder housing  30  to be engaged with the cam groove  60   a.    
         [0029]    The gear wheel  60  is supported in the center by a pin  61  (See FIG. 4) formed on the finder housing  30  and is rotated around the pin  61  by a motor (not shown). The gear wheel  60  can be rotated manually by exposing its portion outside of the camera. As shown in FIG. 5A or  5 B, the shape of the cam groove  60   a  is not a circle. The distance between the groove and the rotation center changes along the groove in order to move the diopter moving lens  40  back and forth for diopter movement via the engagement between the cam pin  40   b  and the cam groove  60   a  as the gear wheel rotates.  
         [0030]    The eyepiece lens  41  disposed inside the finder eyepiece window  62  has integrally formed mounting portion  41   a  projecting downwardly. The mounting portion  41   a  has a hole by which the eyepiece lens  41  is mounted on the guide shaft  42  to be held.  
         [0031]    According to aforementioned structure of the finder, the first movable lens  33  and the second movable lens  34  are supported by the guide shaft  42 . Those lenses guided by the guide shaft  42  move back and forth along the finder object side optical axis  58  to vary magnification of the finder image during zooming operation. Also the diopter moving lens  40  supported by the guide shaft  42  is moved back and forth by motor or manual operation for diopter movement.  
         [0032]    Thus all the movable lenses of the finder optical system are supported and positioned by the single common shaft, which will minimize possible fluctuation of positioning or misalignment of the lenses and reduce the number of parts to be used which leads to cost reduction.  
         [0033]    In the aforementioned structure, the guide shaft  42  is directly supported by the object lens  31  and the porro-prism  35  which are fixed to the finder housing. As shown in FIG. 6, it is also possible that the guide shaft  42  is fixed directly in both hole  30   a  and hole  30   b  which are formed in a front wall and a rear wall of the finder housing respectively, and the object lens  31  and the porro-prism  35  are supported by the guide shaft  42 . In this embodiment, the immovable optical elements such as the object lens  31  and the porro-prism  35  are also supported by the same single guide shaft in addition to the movable lenses. In other words, all the optical elements are set directly on the same foundation. This will minimize possible fluctuation of positioning or misalignment of not only the movable lenses but also the immovable optical elements, which leads to total improvement of the finder optical system.  
         [0034]    In the embodiment described above, the object side optical axis  58  is different from the eyepiece side optical axis  59 . But it is possible for both sides to have the common optical axis. The diameter of guide shaft in the eyepiece side can be not only the same as the one in the object side but also different from the one in the object side. Also the guide shaft can be divided into two portions one of which is for the eyepiece side and the other for the object side as long as they have common axis.  
         [0035]    Thus, the present invention is not to be limited to the above embodiments, but on the contrary, various modifications are possible to those skilled in the art without departing from the scope of claims appended hereto.