Abstract:
A field-of-view frame switching device has a rectangular fixed field-of-view frame and a plurality of light-shielding members for covering portions of the rectangular fixed field-of-view frame so as to vary the size of the actual field-of-view frame. The light-shielding members are composed of a first pair of oppositely arranged light-shielding members and a second pair of oppositely arranged light-shielding members. The field-of-view frame switching device also has a coupling member for coupling the light-shielding members together in such relation that, when the first pair of light-shielding members are positioned closer to each other to narrow the actual field-of-view frame, the second pair of light-shielding members are positioned wide apart from each other and that, when the first pair of light-shielding members are positioned wide apart from each other, the second pair of light-shielding members are positioned closer to each other to narrow the actual field-of-view frame. The field-of-view frame switching device further has a moving member for moving at least a part of the light-shielding members in a direction substantially parallel to one side of the rectangular fixed field-of-view frame.

Description:
This application is based on applications Nos. H09-331479 and H09-331361 filed in Japan, the content of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a field-of-view frame switching device, and more particularly to a device for switching the field-of-view frame of a viewfinder designed for the Advanced Photo System, which uses three picture formats. 
     2. Description of the Prior Art 
     In recent years, more and more products conforming to the Advanced Photo System (hereafter referred to as the “APS”) have been introduced to the market. The APS uses three picture formats to allow an image to be taken in one of the following three different frame sizes: the wide format (hereafter referred to as the “H format”) that offers an aspect ratio of 9:16, the standard format (hereafter referred to as the “C format”) that has the same vertical dimension as and a shorter horizontal dimension than the H format and offers an aspect ratio of 2:3, and the panorama format (hereafter referred to as the “P format”) that has the same horizontal dimension as and a shorter vertical dimension than the H format and offers an aspect ratio of 1:3. 
     Typically, a user&#39;s selection of a specific frame size causes the switching of the field-of-view frame of a viewfinder. Conventional field-of-view frame switching devices are disclosed, for example, in Japanese Laid-Open Patent Applications Nos. H6-273826 and H8-146493. How these conventional devices achieve the switching of the field-of-view frame is schematically illustrated in FIGS. 1A to  1 C and in FIGS. 2A to  2 C respectively. Of these figures, FIGS. 1A and 2A show the arrangement of the field-of-view frame for the H format, FIGS. 1B and 2B show the same for the C format, and FIGS. 1C and 2C show the same for the P format. 
     Either of these devices has a fixed field-of-view frame  11  (indicated by broken lines) and two L-shaped movable masks  12  and  13 . The vertical dimension of the C and H formats and the horizontal dimension of the H and P formats are determined by the fixed field-of-view frame  11 . In each of the figures, the hatched area represents the frame size of the selected formats (H, C, or P). 
     In the device shown in FIGS. 1A to  1 C, the two L-shaped movable masks  12  and  13  are individually moved horizontally (i.e. translated in the directions indicated by arrows C) to determine the horizontal dimension of the C format, and they are moved vertically (i.e. translated in the directions indicated by arrows P) to determine the vertical dimension of the P format. By contrast, in the device shown in FIGS. 2A to  2 C, the two L-shaped movable masks  12  and  13  are individually moved obliquely (i.e. translated in the directions indicated by arrows C or P) to determine the horizontal dimension of the C format or the vertical dimension of the P format. 
     However, allowing the movable masks to be moved in both horizontal and vertical directions requires not only a complicated driving mechanism but also an ample space for it. On the other hand, allowing the movable masks to be moved in oblique directions requires a simpler driving mechanism, since they need to be driven along only one axis, but requires them to be driven over a comparatively long movement stroke (see FIGS.  2 B and  2 C). Thus, both of these devices have been imposing various restrictions in the design of compact cameras. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a device that makes it possible, in any photographing system including the APS, to select a field-of-view frame of a viewfinder from among a plurality of choices and that nevertheless requires only a minimal movement stroke of movable masks and a minimal space for a driving mechanism. 
     To achieve the above object, according to one aspect of the present invention, a field-of-view frame switching device is provided with a rectangular fixed field-of-view frame and a plurality of light-shielding members for covering portions of the rectangular fixed field-of-view frame so as to vary the size of the actual field-of-view frame. Here, the light-shielding members are composed of a first pair of oppositely arranged light-shielding members and a second pair of oppositely arranged light-shielding members. This field-of-view frame switching device is provided also with a coupling member for coupling the light-shielding members together in such relation that, when the first pair of light-shielding members are positioned closer to each other to narrow the actual field-of-view frame, the second pair of light-shielding members are positioned wide apart from each other and that, when the first pair of light-shielding members are positioned wide apart from each other, the second pair of light-shielding members are positioned closer to each other to narrow the actual field-of-view frame. This field-of-view frame switching device is provided further with a moving member for moving at least a part of the light-shielding members in a direction substantially parallel to one side of the rectangular fixed field-of-view frame. 
     According to another aspect of the present invention, a field-of-view frame switching device is provided with a rectangular fixed field-of-view frame and a plurality of field-of-view frame switching members for covering portions along the top, bottom, left-hand, and right-hand sides of the rectangular fixed field-of-view frame so as to vary the size of the actual field-of-view frame. This field-of-view frame switching device is provided also with a coupling member for coupling the field-of-view frame switching members together into a single unit. 
     According to still another aspect of the present invention, a field-of-view frame switching device is provided with a rectangular fixed field-of-view frame and a field-of-view frame switching member for covering portions of the rectangular fixed field-of-view frame so as to vary the size of the actual field-of-view frame. Here, the field-of-view frame switching member is divided at least at one side thereof. This field-of-view frame switching device is provided also with a moving member for moving the field-of-view frame switching member. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This and other objects and features of the present invention will become clear from the following description, taken in conjunction with the preferred embodiments with reference to the accompanied drawings in which: 
     FIG. 1A is a diagram illustrating the arrangement of the field-of-view frame switching members in a first conventional example of a field-of-view frame switching device, when the H format is selected; 
     FIG. 1B is a diagram illustrating the arrangement of the field-of-view frame switching members in the first conventional example of a field-of-view frame switching device, when the C format is selected; 
     FIG. 1C is a diagram illustrating the arrangement of the field-of-view frame switching members in the first conventional example of a field-of-view frame switching device, when the P format is selected; 
     FIG. 2A is a diagram illustrating the arrangement of the field-of-view frame switching members in a second conventional example of a field-of-view frame switching device, when the H format is selected; 
     FIG. 2B is a diagram illustrating the arrangement of the field-of-view frame switching members in the second conventional example of a field-of-view frame switching device, when the C format is selected; 
     FIG. 2C is a diagram illustrating the arrangement of the field-of-view frame switching members in the second conventional example of a field-of-view frame switching device, when the P format is selected; 
     FIG. 3 is a sectional view of a camera incorporating the field-of-view frame switching device of a first embodiment of the invention; 
     FIG. 4 is a perspective view of the camera incorporating the field-of-view frame switching device of the first embodiment; 
     FIG. 5A is a diagram illustrating the arrangement of the light-shielding wings of the field-of-view frame switching device of the first embodiment, when the H format is selected; 
     FIG. 5B is a diagram illustrating the arrangement of the light-shielding wings of the field-of-view frame switching device of the first embodiment, when the C format is selected; 
     FIG. 5C is a diagram illustrating the arrangement of the light-shielding wings of the field-of-view frame switching device of the first embodiment, when the P format is selected; 
     FIG. 6A is a diagram illustrating the arrangement of the light-shielding wings of the field-of-view frame switching device of a second embodiment of the invention, when the H format is selected; 
     FIG. 6B is a diagram illustrating the arrangement of the light-shielding wings of the field-of-view frame switching device of the second embodiment, when the C format is selected; 
     FIG. 6C is a diagram illustrating the arrangement of the light-shielding wings of the field-of-view frame switching device of the second embodiment, when the P format is selected; 
     FIG. 7 is a diagram illustrating the arrangement of the light-shielding wings of the field-of-view frame switching device of a third embodiment of the invention, when the P format is selected; 
     FIG. 8A is a perspective view of the driving mechanism of the field-of-view frame switching device of the first to third embodiments; 
     FIG. 8B is a front view of the driving mechanism of the field-of-view frame switching device of the first to third embodiments, when the P format is selected; 
     FIG. 8C is a front view of the driving mechanism of the field-of-view frame switching device of the first to third embodiments, when the C format is selected; 
     FIG. 9A is a diagram illustrating the arrangement of the light-shielding wings of the field-of-view frame switching device of a fourth embodiment of the invention, when the H format is selected; 
     FIG. 9B is a diagram illustrating the arrangement of the light-shielding wings of the field-of-view frame switching device of the fourth embodiment, when the C format is selected; 
     FIG. 9C is a diagram illustrating the arrangement of the light-shielding wings of the field-of-view frame switching device of the fourth embodiment, when the P format is selected; 
     FIG. 10A is a diagram illustrating the arrangement of the light-shielding wings of the field-of-view frame switching device of a fifth embodiment of the invention, when the H format is selected; 
     FIG. 10B is a diagram illustrating the arrangement of the light-shielding wings of the field-of-view frame switching device of the fifth embodiment, when the C format is selected; 
     FIG. 10C is a diagram illustrating the arrangement of the light-shielding wings of the field-of-view frame switching device of the fifth embodiment, when the P format is selected; 
     FIG. 11 is a diagram schematically illustrating how the light-shielding wings are coupled together in the field-of-view frame switching device of the fifth embodiment; 
     FIG. 12 is a diagram schematically illustrating how the light-shielding wings are coupled together in the field-of-view frame switching device of a sixth embodiment of the invention; 
     FIG. 13 is a perspective view of the field-of-view frame switching device of a seventh embodiment of the invention; 
     FIG. 14A is a diagram illustrating the arrangement of the light-shielding wings of the field-of-view frame switching device of an eighth embodiment of the invention, when the H format is selected; 
     FIG. 14B is a diagram illustrating the arrangement of the light-shielding wings of the field-of-view frame switching device of the eighth embodiment, when the C format is selected; 
     FIG. 14C is a diagram illustrating the arrangement of the light-shielding wings of the field-of-view frame switching device of the eighth embodiment, when the P format is selected; 
     FIG. 15 is a perspective view illustrating the driving mechanism of the field-of-view frame switching device of the eighth embodiment; 
     FIG. 16A is a diagram illustrating the arrangement of the light-shielding wings of the field-of-view frame switching device of a ninth embodiment of the invention, when the H format is selected; 
     FIG. 16B is a diagram illustrating the arrangement of the light-shielding wings of the field-of-view frame switching device of the ninth embodiment, when the C format is selected; 
     FIG. 16C is a diagram illustrating the arrangement of the light-shielding wings of the field-of-view frame switching device of the ninth embodiment, when the P format is selected; and 
     FIG. 17 is a perspective view of a lens-shutter camera incorporating a field-of-view frame switching device embodying the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The structure of a camera incorporating the field-of-view frame switching device of a first embodiment of the invention is shown in FIGS. 3 and 4. FIG. 3 shows a sectional view and FIG. 4 shows a perspective view of the camera  1 . 
     The light beam coming from an object travels through a taking lens system  2 , and is then reflected by a quick-return mirror  3  so as to be focused on a focusing screen  4  and form an image thereon. Closely behind the focusing screen  4  along the beam path is disposed a field-of-view frame switching unit  5 . The light beam having passed through the field-of-view frame switching unit  5  is then reflected by a pentagonal roof prism  6  and directed to an eyepiece lens  7  so that the user of the camera can observe an image of the object. 
     When the user presses a release button  14 , the quick-return mirror  3  rotates so that the light beam from the object will be directed straight to a film  9 , and a shutter  8  disposed in front of the film  9  is released to achieve exposure. Note that, instead of the pentagonal roof prism  6 , a pentagonal roof mirror may be used. 
     Although not shown, a fixed field-of-view frame is formed on the focusing screen  4 . This fixed field-of-view frame determines the field-of-view frame for the H format. The field-of-view frame switching unit  5  is provided with a movable mask  12  having light-shielding wings  10  for shielding the field-of-view frame from four sides. When a field-of-view frame switching operation unit  15  is operated, appropriate ones of the light-shielding wings  10  are moved in an appropriate manner so that the field-of-view frame will be switched to that for the P or C format. 
     In the APS, when the images recorded on the film  9  are printed on photographic paper, appropriate frame sizes are selected in accordance with the information recorded on the film  9 . Thus, in principle, partial shielding of the film  9  is unnecessary in the APS. However, in other systems such as the  135  system, switching between a standard frame size and a panorama frame size, for example, cannot be achieved unless a special mechanism for switching the frame size is provided in front of or behind the shutter  8 . 
     Now, the movable mask  12  of the field-of-view frame switching device of the first embodiment will be described with reference to the drawings. FIGS. 5A,  5 B, and  5 C illustrate the arrangement of the light-shielding wings for the H, C, and P formats respectively. 
     The movable mask  12  includes four light-shielding wings, of which two will be referred to as the P light-shielding wings  21  and the other two as the C light-shielding wings  22 . These P and C light-shielding wings  21  and  22  each have a boss  19  or  20  formed thereon, and each boss  19  or  20  is guided along a slide guide groove  18  or  27  so that the P and C light-shielding wings  21  and  22  can move vertically and horizontally (as seen in the figures) respectively. 
     The movable mask  12  as a whole is made of a flexible material, and is so shaped that the P and C light-shielding wings  21  and  22  are rigid enough to keep their field-of-view frame determining edges  29  and  30  straight at all times and simultaneously that the arm-like portions  23  connecting between the P and C light-shielding wings  21  and  22  are flexible enough to change their orientation freely. Alternatively, it is also possible to form the P and C light-shielding wings  21  and  22  out of one material and the arm-like portions  23  out of another material so that only the arm-like portions  23  may be made of a flexible material. 
     The P and C light-shielding wings  21  and  22  each have a stopper  24  or  25  formed thereon. The stoppers  24  and  25 , by being struck against fixed members (not shown) provided near the corresponding sides of the fixed field-of-view frame  11 , serve to position the field-of-view frame determining edges  29  and  30  in such positions where they determine the vertical dimension of the field-of-view frame for the P format and the horizontal dimension of the field-of-view frame for the C format respectively. 
     Having the structure as described above, the movable mask  12  appears as shown in FIG. 5A when the H format is selected. At this time, the bosses  19  and  20  are located substantially at the center of the slide guide grooves  18  and  27 , and all of the P and C light-shielding wings  21  and  22  are located outside the fixed field-of-view frame  11 . 
     When the frame size is switched from the H format to the C format, as shown in FIG. 5B, the bosses  20  are, by the action of a driving mechanism that will be described later, moved inward along the slide guide grooves  27 , and thereby the C light-shielding wings  22 , which are disposed opposite each other, are moved horizontally inward until the stoppers  25  strike against the fixed members (not shown) formed near the corresponding sides of the fixed field-of-view frame  11 . Meanwhile, as the arm-like portions  23  become more and more vertically oriented, the bosses  19  are moved outward along the slide guide grooves  18 , and thereby the P light-shielding wings  21  are moved vertically outward. In this way, the field-of-view frame for the C format is determined. 
     When the frame size is switched from the H format to the P format, as shown in FIG. 5C, the bosses  20  are, by the action of a driving mechanism that will be described later, moved outward along the slide guide grooves  27 , and thereby the C light-shielding wings  22 , which are disposed opposite each other, are moved horizontally outward. Meanwhile, as the arm-like portions  23  become more and more horizontally oriented, the bosses  19  are moved inward along the slide guide grooves  18 , and thereby the P light-shielding wings  21  are moved vertically inward until the stoppers  24  strike against the fixed members (not shown) formed near the corresponding sides of the fixed field-of-view frame  11 . In this way, the field-of-view frame for the P format is determined. 
     Next, the field-of-view frame switching device of a second embodiment of the invention will be described with reference to the drawings. FIGS. 6A,  6 B, and  6 C illustrate the arrangement of the light-shielding wings for the H, C, and P formats respectively. Note that, in FIGS. 6A,  6 B, and  6 C, such components as are found also in FIGS. 5A,  5 B, and  5 C are identified with the same reference numbers. 
     In FIGS. 6A,  6 B, and  6 C, the movable mask  12  includes an upper mask portion  12   a  and a lower mask portion  12   b , and these portions  12   a  and  12   b  are coupled together at two coupling portions  28  that are rotatably fitted around two slide shafts  26 . The upper and lower mask portions  12   a  and  12   b  may be formed as two separate units that are later coupled together at the coupling portions  28 , or alternatively may be formed as a single unit. The slide shafts  26  are guided along slide guide grooves  27  so that they can be moved, by a driving mechanism that will be described later, horizontally (as seen in the figures) in directions in which they recede from each other and approach each other. 
     Two P light-shielding wings  21  are disposed opposite each other, and are coupled to C light-shielding wings  22   a  and  22   b  (here, the right-hand and left-hand C light-shielding wings are each divided into an upper and a lower portion) by arm-like portions  23   b . The C light-shielding wings  22   a  and  22   b  are coupled to the coupling portions  28  by arm-like portions  23   a . The P light-shielding wings  21  and the C light-shielding wings  22   a  and  22   b  each have a stopper  24  or  25  formed thereon. The stoppers  24  and  25 , by being struck against fixed members (not shown) formed near the corresponding sides of the fixed field-of-view frame  11 , serve to position the P light-shielding wings  21  and the C light-shielding wings  22   a  and  22   b  respectively, in appropriate positions. 
     Having the structure as described above, the movable mask  12  appears as shown in FIG. 6A when the H format is selected. At this time, the slide shafts  26  are located substantially at the center of the slide guide  27 , and all of the P and C light-shielding wings  21 ,  22   a , and  22   b  are located outside the fixed field-of-view frame  11 . 
     When the frame size is switched from the H format to the C format, as shown in FIG. 6B, the slide shafts  26  move inward along the slide guide grooves  27 ; meanwhile, the arm-like portions  23   a  and  23   b  are, as it were, folded up. As a result, the C light-shielding wings  22   a  and  22   b  move vertically away from each other, and meanwhile they move horizontally inward until the stoppers  25  strike the fixed members (not shown) formed near the corresponding sides of the fixed field-of-view frame  11 . Even if the slide shafts  26  are moved further inward along the slide guide grooves  27 , only the arm-like portions  23   a  change their rotation, and the C light-shielding wings  22   a  and  22   b  are kept in fixed positions by the stoppers  25 . 
     At this time, the field-of-view determining edges  30  of the C light-shielding wings  22   a  and  22   b  form straight lines. On the other hand, the P light-shielding wings  21  are moved vertically away from each other by the C light-shielding wings  22   a  and  22   b  and the arm-like portions  23   b  so as to be retracted out of the fixed field-of-view frame  11 . In this way, the field-of-view frame for the C format is determined. 
     When the frame size is switched from the H format to the P format, as shown in FIG. 6C, the slide shafts  26  move outward along the slide guide grooves  27 ; meanwhile, the arm-like portions  23   a  and  23   b  are, as it were, unfolded. As a result, the C light-shielding wings  22   a  and  22   b  move vertically closer to each other, and meanwhile they move horizontally outward. This causes the P light-shielding wings  21  to move vertically inward until the stoppers  24  strike the fixed members (not shown) formed near the corresponding sides of the fixed field-of-view frame  11 . At this time, the field-of-view determining edges  29  of the P light-shielding wings  21  determine the field-of-view frame for the P format. 
     As shown in FIGS. 5A-5C and  6 A- 6 C, coupling together the P light-shielding wings  21  and the C light-shielding wings by the use of flexible members (i.e. the arm-like portions  23 ) makes it possible to select a field-of-view frame from three or more choices by moving the oppositely arranged C light-shielding wings  22  in opposite directions along an axis substantially perpendicular to one side ( 11   a ) of the fixed field-of-view frame  11 , with the C light-shielding wings  22  each driven at one point (i.e. the boss  20  or slide shaft  26 ) integral therewith. 
     This can be achieved by the use of a simple driving mechanism that can drive the relevant members along only one axis and with a minimum of space secured to allow the movement of the light-shielding wings. This leads to a significant reduction in the size of the field-of-view frame switching device as a whole. Note that the first embodiment permits a modification in which the bosses  19  are moved vertically (as seen in the figures). Note also that the second embodiment permits a modification in which the P light-shielding wings  21  are each divided into a right-hand and a left-hand portion and the slide shafts  26  are disposed beside the outer edge of each P light-shielding wing  21  so as to be moved vertically. Moreover, in the second embodiment, in which the field-of-view determining edges  29  of the P light-shielding wings  21  are always straight, it is also possible to provide an additional stopper mechanism somewhere between the H-format and P-format positions so as to make available another field-of-view frame for a format other than the H, C, and P formats. 
     In FIG. 5C, as the arm-like portions  23  become more and more parallel oriented with respect to the movement direction (indicated by arrows J) of the C light-shielding wings  22 , the joint portions  23   c  between the C light-shielding wings  22  and the arm-like portions  23  shrink. Since the joint portions  23   c  are resilient, their shrinkage destabilizes the stoppers  24  for the P light-shielding wings  21  by predisposing them to move away from the fixed members (not shown), in the directions indicated by arrows K. 
     In the second embodiment, the C light-shielding wings  22   a  and  22   b  are each divided into an upper and a lower portion, and these portions move vertically (as seen in the figures) in opposite directions. As shown in FIG. 6C, when the P format is selected, the arm-like portions  23   a  and  23   b  form large angles with respect to the movement direction (indicated by arrows J) of the C light-shielding wings  22 , and this helps keep the stoppers  24  securely against the fixed members (not shown). As a result, it is possible to obtain stable and accurate field-of-view frames and also realize smooth movement of the P light-shielding wings  21 . For these reasons, it is preferable that the C light-shielding wings  22   a  and  22   b  be each divided into an upper and a lower portion. 
     Alternatively, as shown in FIG. 7, which illustrates the field-of-view frame switching device of a third embodiment of the invention, it is also possible to divide only one of the C light-shielding wings into two portions ( 22   a  and  22   b ). In this case, the arm-like portions  23   a  and  23   b  are made more rigid than the arm-like portion  23  or the joint portions  23 d so as not to predispose the stoppers  24  to move away from the fixed members (not shown). 
     Next, the driving mechanism used to drive the movable mask in the first to third embodiments will be described with reference to FIGS. 8A to  8 C. FIG. 8A is a perspective view illustrating the arrangement of the components of the driving mechanism  40  when the H format is selected, and FIGS. 8B and 8C are front views illustrating the arrangement of those components when the P and C formats, respectively, are selected. Note that, in these figures, such components as are found also in FIGS. 5A and 6A are identified with the same reference numerals. 
     In FIG. 8A, the movable mask  12  has its arm-like portions  23   a  rotatably fitted to the slide shafts  26 , which are guided along the slide guide grooves  27  so as to be movable substantially parallel to the field-of-view determining edges  29  of the P light-shielding wings  21 . The slide shafts  26  are fitted with slide levers  41  and  42 , which have slide grooves  41   a  and  42   a  and operation grooves  41   b  and  42   b  formed therein. 
     Into the slide grooves  41   a  and  42   a , slide guide bosses  43  are fitted so that the slide levers  41  and  42  are positioned appropriately in the direction perpendicular to the movable mask  12 . Into the operation grooves  41   b  and  42   b , bosses  45   a  and  45   b  formed on an operation lever  44  are fitted so that, as the operation lever  44  rotates about its center of rotation  44   a , the slide levers  41  and  42  move in opposite directions as indicated by arrow J. 
     In this structure, when the user rotates the operation lever  44 , which is disposed outside the camera body, to a position marked [P] on an indication plate  46  as shown in FIG. 8B, the slide levers  41  and  42  move in directions in which they recede from each other. As a result, the distance between the right-hand and left-hand C light-shielding wings  22   a  and  22   b  increases, and the distance between the upper and lower P light-shielding wings  21  decreases. In this way, the frame size is switched to the P format. 
     When the user rotates the operation lever  44  to a position marked [C] on the indication plate  46  as shown in FIG. 8C, the slide levers  41  and  42  move in directions in which they approach each other. As a result, the distance between the right-hand and left-hand C light-shielding wings  22   a  and  22   b  decreases, and thus the frame size is switched to the C format. At this time, the P light-shielding wings  21  are retracted out of the fixed field-of-view frame  11 . It is also possible to switch the field-of-view frame by driving the slide levers  41  and  42  electrically by the use of a motor, a cam, and other components instead of the operation lever  44 . 
     In this way, it is possible to select a field-of-view frame from three choices by the use of a simple driving mechanism  40  that drives the relevant members along only one axis and that therefore occupies a minimum of space. This helps make cameras compact. 
     Next, the field-of-view frame switching device of a fourth embodiment of the invention will be described with reference to the drawings. FIGS. 9A,  9 B, and  9 C show the arrangement of the movable mask  12  for the H, C, and P formats respectively. Note that, in these figures, such components as are found also in FIGS. 5A and 6A are identified with the same reference numerals. 
     As shown in FIG. 9A, the movable mask  12  includes four L-shaped light-shielding wings  151  to  154 , of which two  151  and  153 , arranged opposite each other, have pin guide grooves  155   a  to  155   d  formed therein and the other two  152  and  154 , also arranged opposite each other, have coupling pins  156   a  to  156   d  formed thereon. These four light-shielding wings  151  to  154  are, substantially at the center of each side of the fixed field-of-view frame  11 , coupled together by the coupling pins  156   a  to  156   d  fitted into the pin guide grooves  155   a  to  155   d  in such a way that each of the light-shielding wings  151  to  154  is rotatable about their respective center of rotation  151   a  to  154   a.    
     Thus, this movable mask  12  has each of its four sides divided into two portions so that, as the distance between two opposite sides increases or decreases, the distance between the other two opposite sides decreases or increases. That is, when one of the coupling pins  156   a  to  156   d  is moved in a direction indicated by arrow J 1 , J 2 , K 1 , or K 2 , the light-shielding wings  151  to  154  each rotate about their respective center of rotation  151   a  to  154   a  as shown in FIGS. 9B and 9C. The pin guide grooves  155   a  to  155   d  guide the other three of the coupling pins  156   a  to  156   d  to move them along substantially straight axes until the field-of-view determining edges  30   a  and  30   b  or  29   a  and  29   b  form straight lines. In this way, the field-of-view frame is switched from the H format (FIG. 9A) to the C format (FIG. 9B) or to the P format (FIG.  9 C). 
     Instead of coupling together the light-shielding wings  151  and  152  by the coupling pin  156   b  and coupling together the light-shielding wings  153  and  154  by the coupling pin  156   d , it is also possible to use a driving mechanism like the one shown in FIG. 8A to drive the coupling pins  156   a  and  156   c  in opposite directions along one axis. Also this makes it possible to select a field-of-view frame from three choices. However, it is preferable to couple together the light-shielding wings  151  and  152 , and the light-shielding wings  153  and  154 , by the coupling pin  156   b  and  156   d  respectively, because it is then possible to switch the field-of-view frame simply by driving only one of the coupling pins  156   a  and  156   c  . This leads to further simplification of the driving mechanism. 
     Next, the field-of-view frame switching device of a fifth embodiment of the invention will be described with reference to the drawings. FIGS. 10A,  10 B, and  10 C show the arrangement of the movable mask  12  for the H, C, and P formats respectively. Note that, in these figures, such components as are found also in FIGS. 5A and 6A are identified with the same reference numerals. 
     As shown in FIG. 10A, the C light-shielding wings  22  each have a boss  20  formed thereon. The bosses  20  are guided along slide guide grooves  27  so that the C light-shielding wings  22  can move horizontally (as seen in the figures, as indicated by arrows J). The C light-shielding wings  22  are coupled to the P light-shielding wings  21  by wires  52  and  53 . Moreover, the P light-shielding wings  21  are fitted with pull springs  51  so as to be pulled in directions in which they recede from each other. 
     The wires  52  and  53  change their direction by approximately 90° at guide pins  54 . As a result, when the two oppositely arranged C light-shielding wings  22  are moved in directions in which they recede from each other, the P light-shielding wings  21 , pulled by the wires  52  and  53 , move in directions in which they approach each other. By contrast, when the two oppositely arranged C light-shielding wings  22  are moved in directions in which they approach each other, the P light-shielding wings  21 , pulled by the pull springs  51 , move in directions in which they recede from each other. 
     How the wires  52  and  53  are used to achieve desired coupling is schematically shown in FIG.  11 . The wires  52  and  53  change their direction at the guide pins  54 . Thus, the upper (as seen in the figure) P light-shielding wing  21  is kept at a predetermined distance from the C light-shielding wings  22  by the wires  52   a  and  53   a . On the other hand, the lower (as seen in the figure) P light-shielding wing  21  is kept at a predetermined distance from the C light-shielding wings  22  by the wires  52   b  and  53   b.    
     Having the structure as described above, the movable mask  12  appears as shown in FIG. 10A when the H format is selected. At this time, the bosses  20  are located substantially at the center of the slide guide grooves, and all of the P light-shielding wings  21  and the C light-shielding wings  22  are located outside the fixed field-of-view frame  11 . 
     When the frame size is switched from the H format to the C format, as shown in FIG. 10B, the bosses  20  are moved, by a driving mechanism like the one shown in FIG. 8A, inward along the slide guide grooves  27 , and thereby the C light-shielding wings  22  are moved horizontally inward until the stoppers  25  strike the fixed members (not shown) formed near the corresponding sides of the fixed field-of-view frame  11 . At this time, the wires  52  and  53  tend to slacken, but this is prevented by the action of the pull springs  51 , which pull the P light-shielding wings  21  and thereby retract them out of the fixed field-of-view frame  11 . 
     When the frame size is switched from the H format to the P format, as shown in FIG. 10C, the bosses  20  are moved outward along the slide guide grooves  27 , and thereby the C light-shielding wings  22  are moved horizontally outward. As a result, the P light-shielding wings  21 , pulled by the wires  52  and  53 , move vertically inward until the stoppers  24  strike the fixed members (not shown) formed near the corresponding sides of the fixed field-of-view frame  11 . In this way, the field-of-view frame for the P format is determined. 
     In this way, by coupling together the P and C light-shielding wings  21  and  22  by wires  52  and  53 , it is possible to select a field-of-view frame from three or more choices by the use of a driving mechanism that can drive the oppositely arranged C light-shielding wings in opposite directions substantially parallel to one side of the fixed field-of-view frame, with each of the C light-shielding wings driven at one point integral therewith. This can be achieved by the use of a simple driving mechanism and with a minimum of space secured to allow the movement of the light-shielding wings, and thus helps make cameras compact. 
     Alternatively, as shown in FIG. 12, which illustrates the field-of-view frame switching device of a sixth embodiment of the invention, the wires may be so arranged that each of the P light-shielding wings  21  is, at both ends, coupled to the two C light-shielding wings  22   a  and  22   b . As shown in the figure, in this case, the right-hand C light-shielding wing  22   a  is coupled to the upper P light-shielding wing  21  by the wires  52   a  and  52   c , and is coupled to the lower P light-shielding wing  21  by the wires  52   b  and  52   d.    
     On the other hand, the left-hand C light-shielding wing  22   b  is coupled to the upper P light-shielding wing  21  by the wires  53   a  and  53   c , and is coupled to the lower P light-shielding wing  21  by the wires  53   b  and  53   d . The P light-shielding wings  21  and the C light-shielding wings  22   b  are each pulled outward by a spring  101  or  102 . 
     In this structure, when the right-hand C light-shielding wing  22   a  is moved straight rightward (as seen in the figure), the upper and lower P light-shielding wings  21  move in directions in which they are pulled by the wires  52   a  to  52   d . At this time, the wires  53   a  to  53   d  coupling the left-hand C light-shielding wing  22   b  to the P light-shielding wings  21  tend to slacken, but this is prevented by the action of the spring  102 , which pulls the left-hand C light-shielding wing  22   b  outward and thereby causes the C light-shielding wings  22   a  and  22   b  to move in directions in which they recede from each other. In this way, the field-of-view frame for the P format is determined. 
     When the right-hand C light-shielding wing  22   a  is moved straight leftward (as seen in the figure), the P light-shielding wings  21  are moved vertically outward by the action of the spring  101 , just as in the case shown in FIG.  10 B. At this time, as long as the pulling force of the spring  102  is set to be weaker than that of the spring  101 , this movement of the P light-shielding wings  21  causes the left-hand C light-shielding wing  22   b  to move horizontally inward. In this way, the field-of-view frame for the C format is determined. 
     In this way, it is possible to select a field-of-view frame from three or more choices by the use of a driving mechanism that can drive one point on the C light-shielding wing  22   a  along a straight line. This can be achieved with a simpler driving mechanism than in the fifth embodiment, and thus helps make cameras compact. 
     Next, the field-of-view frame switching device of a seventh embodiment of the invention will be described with reference to FIG.  13 . Note that, in this figure, such components as are found also in FIG. 10 are identified with the same reference numerals. Here, the P light-shielding wings  21  and the C light-shielding wings  22  are made of a flexible material, and are so arranged as to be movable in directions perpendicular to the focusing plate  4  while changing their direction as they pass light-shielding wing guides  162  and  164  that are disposed just outside the fixed field-of-view frame  11 . The P and C light-shielding wings  21  and  22  are coupled together by wires  52  and  53  that are arranged in the same manner as in the fifth or sixth embodiment. 
     At one end of each of the two oppositely arranged C light-shielding wings  22 , a rotary lever  161  is fitted by the use of a coupling pin  163  so as to be rotatable about a center shaft  161   a . One of the rotary levers  161  has an operation lever portion  161   b  formed integrally therewith. 
     In this structure, when the user rotates the operation lever portion  161   b  to a position marked [P], [H], or [C] on an indication plate  46 , the P and C light-shielding wings  21  and  22  each move first straight parallel to the focusing plate  4  and then, outside the fixed field-of-view frame  11 , in a direction perpendicular to the focusing plate  4 , and thereby the respective field-of-view frame is determined. This structure is useful in cases where the space for the movement of the light-shielding wings  21  and  22  cannot be secured within substantially the same plane as the focusing plate  4 , because it even then allows the light-shielding wings to be retracted three-dimensionally. This contributes to efficient use of available space. 
     Next, the field-of-view frame switching device of an eighth embodiment of the invention will be described with reference to the drawings. FIGS. 14A,  14 B, and  14 C show the arrangement of the movable mask  12  for the H, C, and P formats respectively. On the other hand, FIG. 15 shows the driving mechanism  40  used in the eighth embodiment. Note that, in these figures, such components as are found also in FIGS. 5A and 8A are identified with the same reference numerals. 
     As shown in FIG. 15, the movable mask  12  includes two C-shaped P light-shielding wings  21   a  and  21   b  that are disposed horizontally opposite each other and two I-shaped C light-shielding wings  22   c  and  22   d  that are also disposed horizontally opposite each other. Of these light-shielding wings, the left-hand P light-shielding wing  21   a  is coupled to the right-hand C light-shielding wing  22   d  by a slide lever  41 , and the right-hand P light-shielding wing  21   b  is coupled to the left-hand C light-shielding wing  22   c  by another slide lever  42 . 
     Just as in the case shown in FIG. 8A, when the user rotates the operation lever  44 , the slide levers  41  and  42  move in opposite directions as indicated by arrows J, and this causes the P and C light-shielding wings  21   a  to  21   d  to move. 
     As shown in FIGS. 14A to  14 C, in this structure, it is possible to select a field-of-view frame from three choices by moving the C light-shielding wings  22   c  and  22   d , on the one hand, and the P light-shielding wings  21   a  and  21   b , on the other hand, along the same axis (in the directions indicated by arrows J) by the use of a driving mechanism that can drive the relevant members along only one axis. Moreover, this structure requires no vertical (as shown in the figures) movement, and therefore it is possible to realize a field-of-view frame switching device that occupies almost no space above and below the field-of-view frame. 
     In this embodiment, it is also possible to form the P light-shielding wings  21   a  and  21   b  as one continuous component. However, retracting such a P light-shielding wing, which would then be horizontally twice as long as otherwise, out of the fixed field-of-view frame  11  requires not only an unduly large retraction space but also an unduly long movement stroke in one of the directions indicated by arrows J. Moreover, then, the C light-shielding wings ( 22   c  and  22   d ) coupled thereto also need to be moved through an accordingly long movement stroke and thus require an unduly large retraction space. By contrast, by the use of the horizontally (as seen in the figure) divided P light-shielding wings  21   a  and  21   b , it is possible to realize a field-of-view frame switching device that requires a minimum of space to the right and to the left of the field-of-view frame. 
     Next, the field-of-view frame switching device of a ninth embodiment of the invention will be described with reference to the drawings. FIGS. 16A,  16 B, and  16 C show the arrangement of the movable mask  12  for the H, C, and P formats respectively. 
     As shown in FIG. 16A, the movable mask  12  includes four light-shielding wings  61  to  64 , which are all designed to be rotatable about and together with their respective rotary shafts  65  to  68 . These four rotary shafts  65  to  68  are coupled to a driving shaft  70  by belts  69 . As a result, when the drive shaft  70  is rotated in the direction indicated by arrow E, the rotary shafts  65  to  68 , together with the light-shielding wings  61  to  64 , rotate in the directions indicated by arrows F 1  to F 4  respectively, and thereby the field-of-view frame is switched to that for the C format as shown in FIG.  16 B. Thereafter, when the drive shaft  70  is rotated further in the direction indicated by arrow E, the field-of-view frame is switched to that for the P format as shown in FIG.  16 C. 
     Also in this structure, dividing the movable mask into four light-shielding wings  61  to  64  and rotating in different directions these light-shielding wings thus divided at the center of each side of the movable mask helps reduce the retraction space for the light-shielding wings  61  to  64 . In addition, this structure requires a driving mechanism that can simply rotate a target member about one point (i.e. the driving shaft  70 ), and thus helps reduce the space occupied by the driving system. 
     In the field-of-view frame switching devices described heretofore, according as the distance between two opposite light-shielding wings is increased or reduced, the distance between the other two opposite light-shielding wings is reduced or increased. Thus, by driving one point on one of these light-shielding wings in a direction substantially perpendicular to one side of the fixed field-of-view frame, it is possible to select one among a plurality of field-of-view frames. This helps simplify the driving mechanism, minimize the space occupied by the driving mechanism, and minimize the retraction space for the light-shielding wings. 
     Moreover, by appropriately coupling together the light-shielding wings that are disposed along the four sides of the movable mask, it is possible to increase or decrease the distance between two opposite light-shielding wings while decreasing or increasing the distance between the other two opposite light-shielding wings. Thus, by driving one point on one of these light-shielding wings in a direction substantially perpendicular to one side of the fixed field-of-view frame, it is possible to select one among a plurality of field-of-view frames. This helps simplify the driving mechanism, minimize the space occupied by the driving mechanism, and minimize the retraction space for the light-shielding wings. 
     Moreover, when at least one of the light-shielding wings that are disposed along the four sides of the movable mask is divided into two portions, it is possible, simply by moving the thus divided light-shielding wing portions in opposite directions, to select one among a plurality of field-of-view frames accurately and smoothly. This is achieved by the use of a driving mechanism that can simply rotate a target member about one point or move it straight along one axis, and thus helps simplify the driving mechanism, minimize the space occupied by the driving mechanism, and minimize the retraction space for the light-shielding wings. 
     The above descriptions deal only with cases where the field-of-view frame switching device is incorporated into a single-lens-reflex camera. However, the field-of-view frame switching devices of the above embodiments can be used also in LS cameras as shown in FIG.  17 . In this case, the field-of-view frame switching unit  5  is disposed in a position within a camera where the light beam coming from the object forms an image. In cameras that employ a relay lens system, the field-of-view frame switching unit  5  may be disposed on the secondary imaging plane. 
     The field-of-view frame switching devices of the embodiments may be used even in apparatuses other than cameras, for example slide projectors, to switch between a plurality of rectangular field-of-view frames. In FIG. 17, reference numeral  72  represents an objective lens, reference numeral  73  represents a Porro prism, and reference numeral  7  represents an eyepiece lens.