Abstract:
An appartus, such as a binocular telescope, with an optical system which includes a long optical path. The binocular telescope includes a first objective lens, a second objective lens, a first prism, a second prism, and a group of eye piece lenses. A body of the second prism has an outer surface on which a film made of aluminum is deposited. A light which passes through the first objective lens is firstly reflected by an outer surface, or an exposed side, of the deposited film, and then reflected by an inner surface, or a side contacting the body of the second prism, of the film after the light passes through the second objective lens, the first prism and the second prism. The light reflected by the inner surface of the film is projected outside through the second prism and the eye piece lenses. The single deposited film serves as a pair of reflection elements within the optical system, so that the long optical path is secured therein.

Description:
This application is based on an application No. 10-225861 filed in Japan, the contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical apparatus such as a camera, and particularly relates to an optical system thereof which has a lens, a prism, and the like. 
     2. Description of the Related Art 
     Accompanying a miniaturization (or compactness, or thinnerization) of cameras which have been provided in recent years, there has been a tendency that a length in a direction of optical axis in an optical unit also becomes short. Under this tendency, it becomes difficult to arrange a light projecting/receiving optical system of an active AF, or an AE optical system, with a sufficient focal length. If the focal length of each of these optical systems is made relatively shorter, undesirable influences, such as a deterioration of precision in AF focussing, a difficulty in AE spot photometric measuring, and the like, may be brought to a camera performance. 
     Firstly, a description is made below upon the deterioration, or lowering, of precision in the AF focussing. 
     FIG. 1 is a view showing an optical relation among a photographing optical system, a light projecting lens (or a light emitting lens), a light receiving lens, and so on, of a camera which employs an active AF as a focussing device. An AF beam emitted from a light projecting element (or a light emitting element) such as a LED, is reflected on an object (i.e. a subject to be photographed), and then is projected, or incident, upon a light receiving surface of a photo sensor (or a light receiving element) such as a PSD. Therefore, it is possible to know a position of center of gravity of light distributed over the light receiving surface, such as a distance from a left edge of the photo sensor to the position of center thereof. Therefore, if a relation between the position of center thereof and a distance “D” up to the object to be photographed (i.e. a distance “D” between the object and the camera) is tabled, or memorized, beforehand, it is possible to calculate the distance “D” up to the object. Alternatively, if a formula which represents a relation between the position of center thereof and the distance “D” up to the object, it is also possible to calculate the distance “D” up to the object by calculation, or operation, due to the position of center thereof. 
     The aforementioned table or formula is prepared on the assumption that all the projected AF beam returns to the photo sensor. That is, in order to accurately measure the distance with the aforementioned manner, it is necessary that all the projected AF beam returns to the photo sensor. FIG. 2B shows a situation in which the AF beam is all projected on the object. That is, the horizontally elongate and shaded region in FIG. 2B shows a region on the object in which the AF beam is projected. In this case, all the projected AF beam is reflected on the object, and it is possible to measure the distance (i.e. to focus) up to the object with high accuracy. 
     Meanwhile, in FIG. 2A, only a part of the projected, or emitted, AF beam can hit the object. In this case, what reflects on the object is not all of the projected, or emitted, AF beam (hereinafter, this phenomenon is referred to as “partial reflection” or “vignette (or vignetting)”), and the accuracy of the distance measuring is naturally lowered or reduced. The degree of error of the distance measuring becomes larger as the degree of the vignette increases. 
     Therefore, for the purpose of accurate distance measuring, it is preferable that there is no “partial reflection” at all. However, even if the “partial reflection”, or “vignetting”, occurs, and even if the actual location of the object and its focused location (namely, the distance up to the object measured by means of active AF) are not exactly coincide with each other as a result, the focussing error does not become a substantial problem as far as the object exists within a depth of field relative to the location thus focussed. 
     Referring to FIG. 1, it can be understood that if the depth of field is “dL”, the width “dX” of the region on the photo sensor corresponding to the depth of field can be determined geometrically. Even if the center location of the light distributed on the photo sensor is deviated due to the “partial reflection”, the deviation is not a substantial problem as far as it exists within the region “dX”. Therefore, supposing that the resolution of the photo sensor is fixed or constant, the wider the “dX” becomes, the higher the accuracy of the distance measuring becomes substantially. As explained below, the “dX” can be represented by a formula including the focal length “fr” of the receiving lens. 
     Generally, the following “FORMULA 1” is established between a permissible circle of confusion (or a permissible derangement circle) and a depth of field, where the “F no ” is a f-number of the photographing optical system, and the “n” is a permissible coefficient which can be determined optionally in compliance with a specific design (the “n” can be 0.333, for example).                  1   /   L     -     1   /     (     L   +     d                 L       )         =     n   ·     F   no     ·     σ   /     ft   2                 (     FORMULA                 1     )                                
     On the other hand, from the geometrical relationship shown in FIG. 1, the following “FORMULA 2”, “FORMULA 3” and “FORMULA 4” are established.                  (     L   +     d                 L       )          :        B     =     fr        :          X   1               (     FORMULA                 2     )                 L        :        B     =     fr        :          X   2               (     FORMULA                 3     )                 d                 x     =       X   2     -     X   1               (     FORMULA                 4     )                                
     Elimination of X 1  and X 2  from the “FORMULA 4”, using the “FORMULA 2” and “FORMULA 3”, brings a “FORMULA 5”, as follows.                d                 x     =         B   ·     fr   /   L       -     B   ·     fr   /     (     L   +     d                 L       )           =     B   ·   fr   ·     {       1   /   L     -     1   /     (     L   +     d                 L       )         }                 (     FORMULA                 5     )                                
     Finally, substitution of the “FORMULA 1” into the “FORMULA 5” brings a “FORMULA 6” as follows.                d                 x     =       (     n   ·     F   no     ·   σ   ·   B   ·   fr     )     /     ft   2               (     FORMULA                 6     )                                
     By the way, in the case that the photographing optical system is a zoom lens which is constituted by a pair of lens groups, the diameter of entrance pupil φ does not change, even if the magnification is changed by zooming. Therefore, using a relationship F no =ft/φ, the “FORMULA 6” can also be expressed as a “FORMULA 7” as follows.                d                 x     =         (     n   ·   σ   ·   B   ·   fr     )     /   ft     ·   φ             (     FORMULA                 7     )                                
     From the “FORMULA 6” and “FORMULA 7”, it can be understood that the “dx” is proportional to “fr”. That is, it can be understood that the longer the focal length “fr” of the light receiving lens becomes, the higher the accuracy of the distance measuring (i.e. the accuracy of focussing) becomes. 
     As to the focal length “fs” of the light projecting lens, the longer “fs” becomes, the higher the accuracy of the distance measuring becomes. Next, an explanation thereof is made below. 
     That is, the shorter the focal length “fs” of the light projecting lens becomes, the projected, or emitted, AF beam diverges from the projecting lens or element with a relatively wider angle. As a result, the beam projected area on the object also becomes relatively larger, supposing that the distance to the object is fixed. FIGS. 3A and 3B show this situation explanatorily. 
     Namely, FIG. 3B illustrates a situation in which the focal length of the light projecting lens is relatively longer; therefore, the beam projected area is relatively smaller. On the other hand, FIG. 3A illustrates a situation in which the focal length of the light projecting lens is relatively shorter; therefore, the beam projected area is relatively larger. 
     As apparent from FIGS. 3A and 3B, if the beam projected area relative to the object is relatively larger, there increases the possibility that the aforementioned “partial reflection” occurs, so that the accuracy of the distance measuring becomes lower. In other words, the light projecting lens is superior in the accuracy of the distance measuring if the focal length is relatively longer, similar to the light receiving lens which is superior in the accuracy of the distance measuring if the focal length is relatively longer. 
     Next, an explanation is made below upon a spot photometry (or spot photometric measurement). 
     That is, the spot photometric measurement is a photometric measurement in which an attention is paid to a specified narrow field of a photographing region. Therefore, it is necessary that the focal length of the AE photometric optical system is relatively long. Further, under a recent tendency in which the zoom lens has a high zooming rate, it is necessary to secure a longer focal length in the AE optical system in order to perform the spot photometric measurement with a higher magnification than the focal length in the conventional AE optical system. 
     As explained above, it is preferable that each optical system arranged in the optical apparatuses such as a camera has a longer focal length, in view of the accuracy of distance measuring and spot photometric measurement. However, the longer focal length in the optical system is contradictory to the necessity for thinnerization, or compactness, of cameras. Next, an explanation thereof is made below with reference to FIG.  4 . 
     That is, FIG. 4 is a cross section showing a main part of an optical unit in a conventional camera. In the optical unit  1 , a light projecting optical system (or a light emitting optical system)  20  and a light receiving optical system  30 , of an active AF, are arranged on both sides of a unit body  11 , and a finder optical system  40  is arranged therebetween. 
     The light projecting optical system  20  has a light projecting, or emitting, lens  21  and a light projecting, or emitting, element  22 . The light receiving optical system  30  has a light receiving lens  31  and a photo sensor  32 . The finder optical system  40  has a first objective lens  41 , a second objective lens  42 , a first prism  43 , a second prism  44 , and an eye piece (or an eye piece lens)  45 . 
     In the conventional optical unit typically as shown in FIG. 4, the focal length of each of the light projecting optical system  20  and the light receiving optical system  30  of the active AF, can not be beyond the thickness of the unit body  11  at its maximum. Namely, an increase of the focal length thereof brings a large size of the optical unit (which in turn brings a large size of camera having the optical unit). The reason why it is difficult to prevent the conventional optical unit from becoming large-sized, is that the light travelling in each of the optical systems  20 ,  30  passes only in a straight direction without changing its direction therein. 
     On the other hand, paying attention to the finder optical system  40 , it can be understood that the length of the optical path in the finder optical system  40  is much longer than the thickness of the unit body  11 . The longer optical path length is attributed to an arrangement in which the light passing in the optical system  40  reflects on a plurality of reflecting surfaces  43   a ,  44   a ,  44   b  of the first and second prisms  43 ,  44 , and changes its direction. In other words, utilizing the reflection thereby in the optical system brings such a longer optical path without increasing the thickness of the optical unit. However, if there is arranged an additional reflection member, such as a prism, in the light projecting optical system  20  and the light receiving optical system  30 , it leads rather a large-sized apparatus. 
     It is to be noted that each surface for reflecting light of the prism arranged in the conventional finder optical system  40  shown in FIG. 4 has an aluminum deposited film by vacuum evaporation, as a reflection element, which is formed by depositing aluminum on an outside surface of a body of the prism. As shown in the figure, only one side of the aluminum deposited film (i.e. only an inner surface thereof contacting the body of the prism) is employed as a surface for reflecting light. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the present invention to provide an optical apparatus, such as a camera, having an optical system in which a relatively longer optical path is secured without hindering a thinnerization, or compactness, of the optical apparatus overall. 
     In order to accomplish the above object, according to one aspect of the present invention, there is provided an optical apparatus with a prism, the prism comprising: a first surface through which an incident light passes into a body of the prism; a second surface having an inner side by which the incident light is reflected into a reflection light within the body; and a third surface through which the reflection light is projected outside the body, wherein the prism is arranged such that an outer side of the second surface reflects light toward outside. 
     In the mechanism, the second surface may be provided with a reflection component. The reflection component, for example, may be a metal deposited film or layer, such as an aluminum film or layer, which is deposited on the body of the prism. Alternatively, the reflection component, for example, may be integrally formed with a part of the body of the prism. 
     According to the mechanism, each of the inner side and the outer side of a single surface (i.e. the second surface) is employed as a surface for reflecting light. Namely, there exist a pair of surfaces for reflecting light per single surface. Therefore, according to the mechanism, in contrast with the aforementioned conventional mechanism in which only the inner surface (i.e. inner side) of the reflection element is employed as a surface for reflecting light, and in which the outer surface (i.e. outer side) of the reflection element is not employed as a surface for reflecting light, it is possible to secure a longer optical path, relative to the same size of an optical unit. In other words, according to the mechanism of the one aspect of the present invention, if the length of optical path is the same in contrast with the conventional mechanism, the optical unit is miniaturized, or becomes compact, which in turn makes it possible to make the optical apparatus with the optical unit thinner or compact overall. 
     According to another aspect of the present invention, there is provided an optical apparatus with an optical element, the optical element comprising: a first surface having a first inner side and a first outer side; and a second surface having a second inner side and a second outer side, wherein an incident light which passes into a body of the optical element is reflected by the first inner side of the first surface and the second inner side of the second surface, into a reflection light which is projected outside the body, and wherein each of the first outer side of the first surface and the second outer side of the second surface reflects a light outside. 
    
    
     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 thereof with reference to the accompanying drawings, in which: 
     FIG. 1 is an explanatory view showing an optical relation of a camera which is equipped with an active AF; 
     FIGS. 2A and 2B are explanatory views explaining a conception of “partial reflection (or vignette)” of an AF beam projected; 
     FIGS. 3A and 3B are explanatory views explaining a conception of “partial reflection (or vignette)” of an AF beam projected; 
     FIG. 4 is a cross section of an optical unit of a conventional camera; 
     FIG. 5 is a cross section of an optical unit of a camera, as an optical apparatus, according to a first embodiment of the present invention; 
     FIG. 6 is a perspective view showing a unit body of the optical unit and a finder optical system therein shown in FIG. 5; 
     FIG. 7 is a perspective view showing the unit body and an AE optical system therein shown in FIG. 5; 
     FIG. 8 is a cross section of an optical unit of the camera, as the optical apparatus, according to a second embodiment of the present invention; 
     FIG. 9 is a cross section of an optical unit of the camera, as the optical apparatus, according to a third embodiment of the present invention; 
     FIG. 10 is a cross section of an optical unit of the camera, as the optical apparatus, according to a fourth embodiment of the present invention; 
     FIG. 11 is a cross section of an optical unit of the camera, as the optical apparatus, according to a fifth embodiment of the present invention; 
     FIG. 12 is a cross section of an optical unit of the camera, as the optical apparatus, according to a sixth embodiment of the present invention; and 
     FIG. 13 is a cross section of an optical unit of a binocular telescope, as the optical apparatus, according to a seventh embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before a description of the preferred embodiments proceeds, it is to be noted that like or corresponding parts are designated by like reference numerals throughout the accompanying drawings. 
     With reference to FIGS. 5 through 13, the description is made below upon a camera or a binocular telescope, as an optical apparatus, with an optical unit including at least one optical system, according to each of seven embodiments of the present invention. 
     First, with reference to FIGS. 5 through 7, the description is made below upon the camera with the optical unit according to a first embodiment of the present invention. 
     FIG. 5 shows a cross section of a main part of the optical unit  100  of the camera. The optical unit  100  has an AE optical system  110 , a finder optical system  120 , and a passive AF unit  130 , which are integrally arranged in an unit body  101 . 
     For a better understanding of FIG. 5, the unit body  101  and the finder optical system  120  are shown in FIG. 6 which is a perspective view thereof FIG. 6 shows an assembled state in which the finder optical system  120  is assembled to the unit body  101 , and it also shows a taken-out state in which the finder optical system  120  is taken out from the unit body  101 . Meanwhile, FIG. 7, which is a perspective view, shows the unit body  101 , the AE optical system  110 , and the passive AE unit  130 . In FIG. 7, although the finder optical system  120  is not shown, a first prism  123  which forms a part of the finder optical system  120  is shown, because an aluminum deposited film (or an aluminum deposited layer) is provided on a surface  123   a  of the first prism  123 , and the surface  123   a  not only forms the part of the finder optical system  120 , but also forms a part of the AE optical system  110 . 
     In FIG. 7, an optical path in the finder optical system  120  is shown by a dashed line “A”, and an optical path in the AE optical system  110  is shown by a dashed line “B”. It can be understood that the light along the optical path “B” in the AE optical system  110  is totally reflected on an exposed side (i.e. an outer side) of the aluminum film deposited surface  123   a  of the first prism  123  which is a component of the finder optical system  120 . In this arrangement, a back side (i.e. inner side) of the aluminum film deposited surface  123   a  is employed as a total reflecting surface in the finder optical system  120 . 
     The aluminum deposited film itself has been conventionally well known, and it is made, or formed, by depositing aluminum by vacuum evaporation on a surface of a body of a prism. Different from a conventional art, however, both sides (i.e. outer side and inner side) of the aluminum film deposited surface  123   a  are employed as reflecting surfaces. 
     In this specification, the exposed side (i.e. the outer side) of the aluminum film deposited surface  123   a  is a face of the aluminum film which is open, or exposed, to environment, and the back side (i.e. the inner side) of the aluminum film deposited surface  123   a  is a face of the aluminum film which contacts with an outer surface of a body of a prism. In FIG. 7, the surface on which the ray indicated by the arrow “B” falls, is the exposed side of the aluminum film deposited surface  123   a ; on the other hand, the surface on which the ray indicated by the arrow “A” falls, is the back side thereof. Namely, there exist a pair of reflecting surfaces per a surface of the prism, which is the aluminum film deposited surface  123   a  in the first embodiment. 
     As shown in FIGS. 5 and 6, the finder optical system  120  has a first objective lens  121 , a second objective lens  122 , the first prism  123 , a second prism  124 , and an eye piece lens  125 . Further, there is arranged a finder diaphragm  128  which is located between the first objective lens  121  and the second objective lens  122 . Still further, there is arranged a field pointing frame  129  which is located between the first prism  123  and the second prism  124 . 
     Meanwhile, the AE optical system  110  has a light receiving lens  111 , a reflex mirror  112 , and a photo sensor  113 , as shown in FIG.  7 . 
     Next, with reference to FIG. 8, the description is made below upon the camera with the optical unit according to a second embodiment of the present invention. 
     That is, FIG. 8 shows a cross section of a main part of the optical unit  200  of the camera. In the optical unit  200 , a light projecting, or emitting, optical system  220  and a light receiving optical system  230  of an active AF system are arranged on both sides of a unit body  201 . A finder optical system  210  is arranged between the light projecting optical system  220  and the light receiving optical system  230 . 
     The light projecting optical system  220  has a light projecting, or emitting, lens  221  and a light projecting, or emitting, element  222 . The light receiving optical system  230  has a light receiving lens  231  and a photo sensor (or a light receiving element)  232 . The finder optical system  210  has a first objective lens  211 , a second objective lens  212 , a first prism  213 , a second prism  214 , and an eye piece lens  215 . Although each prism shown in FIG. 8 is formed by laminating some prism pieces, a prism integrally made of one single piece can also be employed. 
     In the second embodiment shown in FIG. 8, an aluminum deposited film is provided on a surface  214   a  of the second prism  214  of the finder optical system  210 . On an outer side of the film deposited surface  214   a , the light travelling in the light receiving optical system  230  totally reflects towards the photo sensor  232 . On the other hand, on an inner side of the film deposited surface  214   a , the light travelling in the finder optical system  210  totally reflects towards the eyepiece lens  215 . 
     According to the second embodiment, the focal length in the active AF system is possible to be longer while the optical unit is compact; therefore, an accuracy in focussing is enhanced. 
     Next, with reference to FIG. 9, the description is made below upon the camera with the optical unit according to a third embodiment of the present invention. 
     That is, FIG. 9 shows a cross section of a main part of the optical unit  300  of the camera. In the optical unit  300 , a light projecting optical system  310  and a light receiving optical system  340  of an active AF system are arranged on both sides of a unit body  301 . There is arranged a finder optical system  320  which is located between the light projecting optical system  310  and the light receiving optical system  340  and which is located closer to the light projecting optical system  310  rather than to the light receiving optical system  340 . There is also arranged an AE optical system  330  which is located between the light projecting optical system  310  and the light receiving optical system  340  and which is located closer to the light receiving optical system  340  rather than to the light projecting optical system  310 . 
     The reason why the light projecting optical system  310  and the light receiving optical system  340  are located with a maximum space therebetween, is to make the longest a distance therebetween in a base length direction which is perpendicular to a direction (i.e. a reference length: refer to FIG. 1) of its optical axis. With this arrangement, it is possible to enhance a precision of the AF operation of the camera. 
     The light projecting optical system  310  has a light projecting lens  311  and a light projecting element  312 . The light receiving optical system  340  has a light receiving lens  341 , a reflex mirror  342 , and a photo sensor  343 . The finder optical system  320  has a first objective lens  321 , a second objective lens  322 , a first prism  323 , a second prism  324 , and an eye piece lens  325 . The AE optical system  330  has a light receiving lens  331  and a photo sensor  332 . Although each prism shown in FIG. 9 is formed by laminating some prism pieces, a prism integrally made of one single piece can also be employed. 
     In the third embodiment shown in FIG. 9, a pair of aluminum film deposited surfaces  324   a ,  324   b  are formed on the second prism  324  of the finder optical system  320 . That is, the light travelling in the AE optical system  330  totally reflects on the outer side of one  324   a  of pair of the aluminum film deposited surfaces towards the photo sensor  332 ; the light travelling in the finder optical system  320  totally reflects on the inner side of the other  324   b  of the pair of the aluminum film deposited surfaces and then on the inner side of the one  324   a  of the pair of aluminum film deposited surfaces towards the eye piece lens  325 ; and the light travelling in the light receiving optical system  340  totally reflects on the reflex mirror  342  and then on the outer side of the other  324   b  of the pair of aluminum film deposited surfaces towards the photo sensor  343 . 
     As can be understood from this third embodiment, a pair of additional reflecting surfaces are available by providing the two aluminum film deposited surfaces, different from the conventional arrangement. Therefore, it is preferable that the film deposited surfaces are provided as many as possible. 
     According to the third embodiment, the focal length in the AE optical system is possible to be longer while the optical unit is compact; therefore, it is possible to realize a spot photometric measurement even in a zoom photographing with a higher magnification. 
     Next, with reference to FIG. 10, the description is made below upon the camera with the optical unit according to a fourth embodiment of the present invention. 
     That is, FIG. 10 shows a cross section of a main part of the optical unit  400  of the camera. In the optical unit  400 , a light projecting optical system  410  and a light receiving optical system  420  of an active AF system are arranged on both sides of a unit body  401 . A finder optical system  430  is arranged between the light projecting optical system  410  and the light receiving optical system  420 . 
     The light projecting optical system  410  has a light projecting lens  411 , a reflex mirror  412 , and a light projecting element  413 . The light receiving optical system  420  has a light receiving lens  420  and a photo sensor  422 . The finder optical system  430  has a first objective lens  431 , a second objective lens  432 , a first prism  433 , a second prism  434 , and an eye piece lens  435 . 
     In the fourth embodiment shown in FIG. 10, an aluminum deposited film is provided on a surface  433   a  of the first prism  433  of the finder optical system  430  by vacuum evaporation. On an outer side of the film deposited surface  433   a , the light travelling in the light projecting optical system  410  totally reflects. On an inner side of the film deposited surface  433   a , the light travelling in the finder optical system  430  totally reflects. 
     Next, with reference to FIG. 11, the description is made below upon the camera with the optical unit according to a fifth embodiment of the present invention. 
     That is, FIG. 11 shows a cross section of a main part of the optical unit  500  of the camera. In the optical unit  500 , a unit body  501  carries only a finder optical system  510 . This finder optical system  510  has a first objective lens  511 , a second objective lens  512 , a first prism  513 , a second prism  514 , an eye piece lens  515 , and a reflex mirror  518 . 
     In the fifth embodiment, an aluminum deposited film is formed on a surface  514   a  of the second prism  514  of the finder optical system. Both of an outer side of the aluminum film deposited surface  514   a  and an inner side thereof serve as total reflecting surfaces in the finder optical system  510  as one optical system. That is, the aforementioned embodiments (i.e. the first through fourth embodiments) are different from this fifth embodiment in that the outer side and the inner side of each reflection member in the aforementioned embodiments serve as different reflecting surfaces in different optical systems. However, through all the aforementioned embodiments (i.e. the first through fifth embodiments), the reflection member has the same effect in that the optical path in each optical system can be made longer with it. 
     Next, with reference to FIG. 12, the description is made below upon the camera with the optical unit according to a sixth embodiment of the present invention. 
     That is, FIG. 12 shows a cross section of a main part of the optical unit  600  of the camera. In the optical unit  600 , a unit body  601  carries a finder optical system  610  and an AE optical system  620 . The finder optical system  610  has a first objective lens  611 , a second objective lens  612 , a first prism  613 , a second prism  614 , an eye piece lens  615 , and a reflex mirror  618 . The AE optical system  620  has a light receiving lens  621 , a reflex mirror  622 , and a photo sensor  623 . 
     In the embodiment shown in FIG. 12, a pair of reflection surfaces  614   a ,  614   b  are formed, like in the third embodiment shown in FIG.  9 . The two reflection surfaces  614   a ,  614   b  have aluminum deposited films which are formed on different surfaces of the second prism  614  of the finder optical system  610 . The light travelling in the finder optical system  610  totally reflects on an outer side of one  614   a  of the pair of film deposited surfaces  614   a ,  614   b , then the reflected light is further reflected on an inner side of the other  614   b  of the pair of film deposited surfaces, and then the reflected light is further reflected on an inner side of the one  614   a  of the pair of film deposited surfaces towards the eye piece lens  615 . Meanwhile, the light travelling in the AE optical system  620  totally reflects on an outer side of the other  614   b  of the pair of film deposited surfaces. 
     In each of the above embodiments, the present invention is applied to a camera in which there is provided a finder (or viewfinder), independently of a photographing lens. However, it is needless to say that the present invention may be applied to a single-lens reflex camera. Also, it is needless to say that the present invention may be applied to any optical apparatus other than the camera. Next, with reference to FIG. 13, the description is made below upon a binocular telescope, as the optical apparatus, with the optical unit, as a seventh embodiment, to which the present invention is applied. 
     That is, FIG. 13 shows a schematic cross section of the binocular telescope  700 . A pair of optical systems thereof are arranged on both sides (i.e. a right side and a left side in the figure) in a relation of a mirror image with each other, relative to a center of a body of the binocular telescope. Therefore, an explanation thereof is made upon a right-hand optical system only. 
     The optical system has a first objective lens  711 , a reflex mirror (or a reflection mirror)  712 , a second objective lens  713 , a first prism  714 , a second prism  715 , and a group of eye piece lenses  716 . A holder  717  which holds the group of eye piece lenses  716 , is rotatably mounted on the body of the binocular telescope  700 , and it allows to perform a focussing operation. 
     In the embodiment shown in FIG. 13, an aluminum deposited film is formed on a surface  715   a  of the second prism  715 . The light travelling, or passing, in the optical system totally reflects on the outer side of the film deposited surface  715   a , and on the inner side thereof. Namely, the deposited film is employed to ensure a long optical path through which the light passes. The long optical path forming in the optical system, makes it possible to realize a high magnification. 
     Although the present invention has been fully described in connection with the preferred embodiments thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications are also apparent to those skilled in the art. The essence of the present invention is that the exposed side (i.e. outer side) of one surface of a prism and the back side (i.e. inner side) thereof are made use of as reflection surfaces. For example, in the aforementioned embodiments, the aluminum deposited film is employed. Alternatively, the deposited film may be made of Ag (silver), Cr (chromium), Cu (copper), Au (gold), or the like, instead of employing the aluminum (Al). 
     Alternatively, the deposited film or layer may be made as a dielectric multi-layered deposition film or layer. 
     Further, instead of employing the deposited film or layer, a plate-like reflex mirror, both surfaces of which serve as reflection surfaces, may be employed. 
     Such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims unless they depart therefrom.