Abstract:
An optical apparatus including an optical system for bending an optical path in a first direction into a second direction substantially perpendicular to the first direction; and a prism for bending the optical path in the second direction with a plurality of reflecting surfaces so that the optical path spirals and leads into a third direction substantially perpendicular to the second direction.

Description:
[0001]     This application claims priority from Japanese Patent Application No. 2003-365464 filed Oct. 27, 2003, which is hereby incorporated by reference herein.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an optical apparatus that changes the optical path of incident light and allows the light to exit.  
         [0004]     2. Description of the Related Art  
         [0005]     An optical apparatus such as an analog camera (silver salt film camera) and a digital camera is difficult to reduce in the size, especially in the thickness. One of the reasons is the optical viewfinder. Hitherto, various viewfinders have been proposed in order to reduce the overall size or the thickness of cameras.  
         [0006]     For example, Japanese Patent Laid-Open No. 2000-338403 discloses a real-image-type viewfinder in which the axis of light entering an objective lens is not parallel to the axis of light exiting an eyepiece lens. According to this related art, the axis of light exiting an eyepiece lens system is tilted with respect to the axis of light entering an objective lens system; an image inverting system is composed of a single roof prism only; and the optical axis is bent in the same plane. Therefore, the mechanism of the viewfinder is simplified and miniaturized.  
         [0007]     In addition, Japanese Patent Laid-Open No. 2000-155357 discloses a viewfinder optical system in which a reflecting optical subsystem includes a plurality of free-form reflecting surfaces having power; and at least one of the reflecting surfaces satisfies the following condition: 
 
5°&lt;|θ|&lt;25°
 
 where θ is an angle of reflection of an axial chief ray with respect to the normal. In this related art, attention is directed to a reflecting surface (hereinafter referred to as independent reflecting surface) that does not function as a transmitting surface and has an unlimited angle of reflection. If the reflecting optical subsystem includes at least one independent reflecting surface, it is possible to reduce the angle of reflection of the independent reflecting surface and consequently reduce the decentration aberration even if the independent reflecting surface has strong power. The reflecting optical subsystem bends the optical path effectively, so reduction in the size of the viewfinder is achieved. 
 
         [0008]     Moreover, Japanese Patent Laid-Open No. 2002-350930 discloses a viewfinder system and an optical apparatus using the same. The viewfinder system includes a first reflecting surface for bending a light path disposed between lenses in an objective lens subsystem; and an image-inverting optical subsystem having a roof reflecting-surface. This viewfinder system achieves a reduction in thickness by bending the optical path at an obtuse angle with a mirror having the first reflecting surface.  
         [0009]     The above related arts achieve miniaturization by folding an optical axis of a viewfinder in the same plane. Unlike these, for example, Japanese Patent Laid-Open No. 2002-040361 discloses a display in which an optical axis of a viewfinder is bent in two parallel planes so that an image from a single display element can be seen with both eyes. According to this related art, a three-dimensional optical-path splitting section splits an image from a single display element into light rays for the left eye and light rays for the right eye, and then allows the light rays to enter eyepiece prisms for the left eye and the right eye, respectively. Therefore, an image from a single display element can be led to both eyes without reduction of intensity of the image.  
         [0010]     However, the above related arts have the following problems. In the first related art, since the objective lens system and the eyepiece lens system of the viewfinder are arranged in the thickness direction of the camera, reduction in the thickness is difficult. In the second related art, instead of providing an objective lens subsystem having power, the reflecting optical system including reflecting surfaces having power is provided. Therefore, in the case of an optical zoom viewfinder, a lens system for zooming is necessary. The lens system for zooming prevents reduction in the thickness of the optical apparatus such as a camera. In the third related art, in the case of a camera having the viewfinder system, the objective system of the viewfinder extends in the width direction of the camera. Therefore, reduction in the width of the camera is difficult. In the fourth related art, in the case of an optical zoom viewfinder, reduction in the thickness of the optical apparatus such as a camera is prevented, as in the second related art.  
         [0011]     As described above, it is difficult to reduce the thickness or the overall size of the optical apparatus, such as a camera, having a viewfinder.  
       SUMMARY OF THE INVENTION  
       [0012]     It is an object of the present invention to miniaturize optical apparatus.  
         [0013]     To attain this object, the present invention is an optical apparatus including an optical system for bending an optical path in a first direction into a second direction substantially perpendicular to the first direction; and a prism for bending the optical path in the second direction with a plurality of reflecting surfaces so that the optical path spirals and leads into a third direction substantially perpendicular to the second direction.  
         [0014]     The present invention reduces the height and the thickness of a prism to provide a thin and compact optical apparatus. In addition, the user can easily look through an eyepiece lens, and the parallax in the optical apparatus is small. The present invention is applicable to a viewfinder for a camera and an optical apparatus having the viewfinder.  
         [0015]     Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments (with reference to the attached drawings). 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a perspective view showing a viewfinder according to embodiment 1 of the present invention, the viewfinder being in wide-angle mode.  
         [0017]      FIG. 2  is a top view showing the viewfinder according to embodiment 1 of the present invention.  
         [0018]      FIG. 3  is a front view showing the viewfinder according to embodiment 1 of the present invention.  
         [0019]      FIG. 4  is a right side view showing the viewfinder according to embodiment 1 of the present invention.  
         [0020]      FIG. 5  is a perspective view showing the viewfinder according to embodiment 1 of the present invention, the viewfinder being in telephoto mode.  
         [0021]      FIGS. 6A and 6B  illustrate the relationship between main optical axis entering an objective optical system and planes including main optical axis in an eyepiece prism in embodiment 1 of the present invention.  
         [0022]      FIG. 7  is an overall perspective view showing an optical apparatus having the viewfinder according to embodiment 1 of the present invention.  
         [0023]      FIG. 8  is a perspective view showing main parts of the optical apparatus in  FIG. 7  with the cover removed.  
         [0024]      FIG. 9  illustrates a viewfinder according to embodiment 2 of the present invention.  
         [0025]      FIG. 10  is a top view showing the viewfinder according to embodiment 2 of the present invention.  
         [0026]      FIG. 11  is a front view showing the viewfinder according to embodiment 2 of the present invention.  
         [0027]      FIG. 12  is a right side view showing the viewfinder according to embodiment 2 of the present invention.  
         [0028]      FIGS. 13A and 13B  illustrate the relationship between main optical axis entering an objective optical system and planes including main optical axis in an eyepiece prism in embodiment 2 of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     Embodiment 1  
       [0029]     FIGS.  1  to  5  illustrates a viewfinder  100  according to embodiment 1 of the present invention. More specifically,  FIG. 1  is a perspective view of the viewfinder in wide-angle mode;  FIG. 2  is a top view of the viewfinder;  FIG. 3  is a front view of the viewfinder;  FIG. 4  is a right side view of the viewfinder; and  FIG. 5  is a perspective view of the viewfinder in telephoto mode. The viewfinder of embodiment 1 is a zoom viewfinder, as is clear from the relationship between  FIGS. 1 and 5 .  
         [0030]     In FIGS.  1  to  5 , reference character G 1  denotes a fixed objective lens; reference characters G 2  and G 3  denote objective lenses capable of moving in the direction of the optical axis for zooming; and reference numeral  4  denotes a mirror for bending the optical path. Reference numeral  5  denotes an objective prism. The objective prism  5  has a tilted surface  5   a.  The tilted surface  5   a  has the same angle as a tilted surface  10   b  of an eyepiece prism  10 , and the air gap between the tilted surfaces  5   a  and  10   b  is very small so that an object image goes approximately straight when the object image enters the eyepiece prism  10  through the tilted surface  10   b.  Reference numeral  6  denotes a bending prism having a roof surface  6   a.  The bending prism  6  converts the object image into an erect image in cooperation with the eyepiece prism  10 .  
         [0031]     The above optical elements (G 1  to G 3  and  4  to  6 ) compose an objective optical system. The roof surface  6   a  and a bending surface  6   c  of the bending prism  6  are metal reflective surfaces evaporated with aluminum.  
         [0032]     Reference numerals  10  and  11  denote optical elements composing an eyepiece optical system. Reference numeral  10  denotes an eyepiece prism for leading the object image from the objective optical system to an eyepiece lens  11 . As shown in  FIG. 4 , the eyepiece lens  11  is tilted. The plane between the objective optical system and the eyepiece optical system, that is to say, the plane between the exit surface  6   b  of the bending prism  6  and the entrance surface  10   a  of the eyepiece prism  10  is the primary image-forming plane, which is the object-image forming plane of the objective optical system. The plane is provided with a field frame (not shown).  
         [0033]     The optical path may be bent by the mirror  4  in the opposite direction. That is to say, the layout of the above components may be a mirror-image layout symmetrical to the layout in  FIG. 1 . Holding members for holding the objective lenses G 1  to G 3  and the mirror  4 , a driving device necessary for zooming the objective lenses G 2  and G 3 , and so on are omitted to simplify the drawing.  
         [0034]     Next, with reference to FIGS.  1  to  5 , embodiment 1 of the present invention will be described in detail.  
         [0035]     The light from an object enters the objective lens G 1  via a protective window (not shown). Reference character O 1  denotes the optical axis at this time. The light from the object exits the objective lens G 1  and is then bent by the mirror  4 . Reference character O 2  denotes the optical axis at this time.  
         [0036]     The light from the object bent by the mirror  4  enters the objective lenses G 2  and G 3 , and then enters the objective prism  5 . The light from the object entering the objective prism  5  exits through the tilted surface  5   a  of the objective prism  5 , and then enters the eyepiece prism  10  through the tilted surface  10   b.  As described above, the tilted surfaces  5   a  and  10   b  have the same angle, and the air gap between them is very small. Therefore, the light from the object goes approximately straight and enters the eyepiece prism  10  through the tilted surface  10   b.    
         [0037]     The light from the object entering the eyepiece prism  10  exits through a surface  10   c  of the eyepiece prism  10 , and then enters the bending prism  6 . The light from the object is bent by the roof surface  6   a  and is laterally inversed, and then goes to the bending surface  6   c.  Reference character O 3  denotes the optical axis at this time. The object image is vertically inverted by the bending surface  6   c,  and then exits the bending prism  6  through the exit surface  6   b.  The object image enters the eyepiece prism  10  again through the entrance surface  10   a  of the eyepiece prism  10  and the field frame (not shown) disposed in the primary image-forming plane. Reference character O 4  denotes the optical axis at this time.  
         [0038]     The light from the object again entering the eyepiece prism  10  goes along the optical axis O 4  toward the tilted surface  10   b.  The angle of the bending prism  6  and the angles of the entrance surface  10   a  and the tilted surface  10   b  of the eyepiece prism  10  are determined optimally so that the light from the object is totally reflected by the tilted surface  10   b.  Therefore, all of the light from the object is bent by the tilted surface  10   b.  Reference character O 5  denotes the optical axis at this time.  
         [0039]     The light from the object totally reflected by the tilted surface  10   b  goes to the tilted surface  10   d  through the eyepiece prism  10 . The angle of the tilted surface  10   d  is determined optimally so that the light from the object is totally reflected by the tilted surface  10   d.  Therefore, all of the light from the object is bent by the tilted surface  10   d.  Reference character O 6  denotes the optical axis extending toward the eyepiece lens  11  at this time. The light from the object totally reflected by the tilted surface  10   d  exits through a tilted surface  10   e,  and then enters the eyepiece lens  11 . Therefore, the user can observe the object image through the eyepiece lens  11 .  
         [0040]     Here, the optical axes O 4 , O 5 , and O 6  in the eyepiece prism  10  will be described. Herein, the light ray that enters through the center of the objective lens G 1  and exits through the center of the eyepiece lens  11  is referred to as the main optical axis. Therefore, the optical axes O 1  to O 6  are parts of the main optical axis.  
         [0041]     In  FIGS. 1 and 2 , the axis L is an imaginary axis that extends through the eyepiece prism  10  and is perpendicular to the optical axes O 1  and O 2 . As shown in  FIGS. 1 and 2 , the optical axes O 4 , O 5 , and O 6  form a spiral that winds around the axis L in the eyepiece prism  10 , and then enter the eyepiece lens  11 . In other words, as shown in  FIGS. 6A and 6B , when the light from the object is reflected by the tilted surfaces of the eyepiece prism  10 , the plane including the main axis incident on a tilted surface and the main axis reflected by the tilted surface, for example, the plane P 1  including the optical axes O 4  and O 5 , is tilted with respect to the optical axes O 1  and O 2 , which enter the objective optical system. That is to say, the plane P 1  is neither parallel nor perpendicular to the optical axes O 1  and O 2 .  
         [0042]     The plane P 2  including the optical axes O 5  and O 6  is also tilted with respect to the optical axes O 1  and O 2 .  
         [0043]     In the case where the length of the optical path from the field frame (not shown) disposed in the primary image-forming plane to the eyepiece lens  11  via the eyepiece prism  10  is a fixed value, spiraling the optical path in the eyepiece prism  10  as described above can reduce the height H and the thickness D (see  FIG. 4 ) of the eyepiece prism  10  compared with folding the optical path in the plane parallel or perpendicular to the optical axes O 1  and O 2  entering the objective optical system.  
         [0044]     Next, the case where an optical apparatus, such as a digital camera, has the above viewfinder will be described.  
         [0045]      FIG. 7  is a perspective view of an apparatus  500  having the viewfinder  100 . In  FIG. 7 , reference numeral  501  denotes an image-taking device for taking an image of an object. The image-taking device  501  is composed of a plurality of optical elements and either a CCD or a silver salt film (not shown). Reference numeral  502  denotes a strobe light; reference numeral  503  denotes a viewfinder window for taking an image of the object; and reference numeral  504  denotes an eyepiece section for observing the image of the object. In addition, reference numeral  510  denotes a cover of the optical apparatus  500 .  
         [0046]      FIG. 8  is a perspective view showing main parts of the optical apparatus  500  with the cover  510  removed. In  FIG. 8 , reference numeral  511  denotes a battery, which is a power supply of the optical apparatus  500 ; reference numerals  512  and  513  denote boards provided with control circuits for controlling the optical apparatus  500 ; reference numeral  514  denotes a main capacitor for flashing the strobe light  502 ; and reference numeral  515  denotes a display such as an LCD for displaying the image of the object being taken by the image-taking device  501 . In addition, reference numeral  300  denotes the schematically depicted viewfinder  100 .  
         [0047]     A case where an optical apparatus, such as a digital camera, has the viewfinder  100  ( 300 ) will be described with reference to  FIGS. 7 and 8 .  
         [0048]     In the case of a compact optical apparatus  500 , normally, the user looks through the eyepiece section  504  with the right eye. Therefore, the eyepiece section  504  is preferably disposed on the right side of the optical device  500  when viewed from the image-taking device  501 . Therefore, as shown in  FIG. 8 , the viewfinder  300  is disposed above the image-taking device  501 . As shown with the dashed line, the objective lens G 1  is disposed above the image-taking device  501 , and a mirror (not shown) bends the light from the object in the opposite direction from the direction shown in FIGS.  1  to  6 . The eyepiece prism  10  composing the eyepiece optical system and the bending prism  6  are disposed on the right side of the optical apparatus  500 . Therefore, the user can easily look through the eyepiece section  504 . In addition, since the distance between the image-taking device  501  and the objective lens G 1  is short, the parallax between the viewfinder  300  and the image-taking device  501  is small. Moreover, since the miniaturization of the viewfinder  300  itself can be achieved as described above, the optical apparatus  500  can be miniaturized (both in the width and in the thickness) compared with the conventional apparatus.  
         [0049]     Disposing the main capacitor  514  and the display  515  under the viewfinder  300  makes it possible to dispose the battery  511  in the space on the left side of the optical apparatus  500  in  FIG. 8 . Since the left side space is provided mainly with the battery  511  and the board  512 , the size of the battery  511  can be increased. Therefore, the operation time of the optical apparatus  500  can be increased. That is to say, the display can operate for a longer time; and many more images can be taken with the optical apparatus  500 . This makes the optical apparatus  500  more user-friendly.  
         [0050]     As described above, the main optical axis O 4  is incident on the surface  10   b;  the main optical axis O 5  is reflected by the surface  10   b  and is incident on the surface  10   d;  and the main optical axis O 6  is reflected by the surface  10   d.  Both the plane P 1  including the main optical axes O 4  and O 5  and the plane P 2  including the main optical axes O 5  and O 6  are not parallel to the main axes O 1  and O 2  entering the objective optical system. In other words, the main axes O 4 , O 5 , and O 6  incident on and reflected by the reflecting surfaces of the eyepiece prism  10  form a spiral that winds around the imaginary axis L perpendicular to the main axes O 1  and O 2  entering the objective optical system. Therefore, the height H and the thickness D of the eyepiece prism  10  can be reduced, and consequently the viewfinder according to embodiment 1 is thin and compact.  
         [0051]     As shown in  FIG. 4 , since the eyepiece lens  11  is tilted toward the eyepiece prism  10 , the eyepiece lens  11  does not project in the thickness direction from the eyepiece prism  10 , as in the conventional apparatus. Therefore, the apparatus, such as a camera, having the viewfinder can be reduced in thickness.  
         [0052]     By optimizing the angles of the tilted surfaces of the eyepiece prism  10  and the angle of the eyepiece lens  11 , the reflecting surfaces of the eyepiece prism  10  can totally reflect the light from the object. In this case, the reflecting surfaces need not be evaporated with aluminum. Therefore, the cost can be reduced compared with the conventional viewfinders.  
         [0053]     When the tilted surface  10   b  of the eyepiece prism  10  functions as the objective optical system, the tilted surface  10   b  transmits light. On the other hand, when the surface  10   b  functions as the eyepiece optical system, the surface  10   b  reflects light. Therefore, part of the eyepiece prism  10  is shared by the objective optical system and the eyepiece optical system. Consequently, there is no need to increase the size of the objective prism  5  nor to provide other optical components. This makes it possible to miniaturize the entire viewfinder  100  and to reduce the cost.  
         [0054]     Since the viewfinder  100  ( 300 ) can be made compact and thin, the optical apparatus, such as a camera, using the viewfinder can also be made compact and thin.  
         [0055]     In addition, as shown in  FIG. 8 , when viewed from the image-taking device  501 , the entrance section (objective lens G 1 ) of the objective optical system is disposed directly above the image-taking device  501 , and the eyepiece optical system is disposed on the right side. Therefore, the user can easily look through the viewfinder, and the parallax in the optical apparatus is small.  
         [0056]     In embodiment 1, although the light from the object is totally reflected twice in the eyepiece prism  10 , the light from the object may be reflected at least once. If the shape of the eyepiece prism  10  and the bending prism  6  and the angle of the tilted surfaces of the prisms  10  and  6  are designed optimally so that an erect image can be seen when the user looks through the eyepiece lens  11 , the light from the object may be reflected any number of times. Therefore, the eyepiece prism  10  may have at least one reflecting surface that produces a main optical axis not parallel to the main optical axes O 1  and O 2  entering the objective optical system.  
       Embodiment 2  
       [0057]     In the viewfinder  100  of embodiment 1, the eyepiece lens  11  is tilted toward the eyepiece prism  10 . In the viewfinder  200  of embodiment 2, the optical axis entering the eyepiece lens  11  is parallel to the optical axis O 1  entering the objective lens G 1 .  
         [0058]     FIGS.  9  to  12  illustrate a viewfinder  200  according to embodiment 2 of the present invention. More specifically,  FIG. 9  is a perspective view of the viewfinder in wide-angle mode;  FIG. 10  is a top view of the viewfinder;  FIG. 11  is a front view of the viewfinder; and  FIG. 12  is a right side view of the viewfinder. Incidentally, in the description of embodiment 2, the same reference numerals and characters will be used to designate the same components as those in embodiment 1, so that the description will be omitted. The viewfinder of embodiment 2 is also a zoom viewfinder, as in embodiment 1.  
         [0059]     In FIGS.  9  to  12 , reference numeral  16  denotes a bending prism having reflecting surfaces  16   a  and  16   c.  The bending prism  16  converts the object image into an erect image in cooperation with an eyepiece prism  20 . The reflecting surfaces  16   a  and  16   c  are metal reflective surfaces evaporated with aluminum. The eyepiece prism  20  composes an eyepiece optical system together with the eyepiece lens  11 . The eyepiece prism  20  has a tilted surface  20   b  that has the same angle as the tilted surface  5   a  of the objective prism  5 . There is a very small air gap between the tilted surfaces  5   a  and  20   b.  The plane between the exit surface  16   b  of the bending prism  16  and the entrance surface  20   a  of the eyepiece prism  20  is the primary image-forming plane of the objective optical system. The plane is provided with a field frame (not shown).  
         [0060]     The optical path may be bent by the mirror  4  in the opposite direction. That is to say, the layout of the above components may be a mirror-image layout symmetrical to the layout in  FIG. 9 .  
         [0061]     Next, with reference to FIGS.  9  to  12 , embodiment 2 of the present invention will be described. The light from an object enters the objective lens G 1  via a protective window (not shown). Reference character O 1  denotes the optical axis at this time. The light from the object exits the objective lens G 1  and is then bent by the mirror  4 . Reference character O 2  denotes the optical axis at this time.  
         [0062]     The light from the object bent by the mirror  4  enters the objective lenses G 2  and G 3 , and then enters the objective prism  5 . The light from the object entering the objective prism  5  exits through the tilted surface  5   a  of the objective prism  5 , and then enters the eyepiece prism  20  through the tilted surface  20   b.  As described above, the tilted surfaces  5   a  and  20   b  have the same angle, and the air gap between them is very small. Therefore, the light from the object goes approximately straight and enters the eyepiece prism  20  through the tilted surface  20   b.    
         [0063]     The light from the object entering the eyepiece prism  20  exits through a surface  20   c  of the eyepiece prism  20 , and then enters the bending prism  16 . The light from the object is bent by the reflecting surface  16   a,  and then goes to the reflecting surface  16   c.  Reference character O 3  denotes the optical axis at this time. The object image is vertically inverted by the reflecting surface  16   c,  and then exits the bending prism  16  through the exit surface  16   b.  The object image enters the eyepiece prism  20  again through the entrance surface  20   a  of the eyepiece prism  20  and the field frame (not shown). Reference character O 4  denotes the optical axis at this time.  
         [0064]     The light from the object again entering the eyepiece prism  20  goes along the optical axis O 4  toward the tilted surface  20   b.  The angle of the bending prism  16  and the angles of the entrance surface  20   a  and the tilted surface  20   b  of the eyepiece prism  20  are determined optimally so that the light from the object is totally reflected by the tilted surface  20   b.  Therefore, all of the light from the object is bent by the tilted surface  20   b.  Reference character O 5  denotes the optical axis at this time.  
         [0065]     The light from the object totally reflected by the tilted surface  20   b  goes to a vertical surface  20   f  through the eyepiece prism  20 . The angle of the tilted surface  20   b  is determined optimally so that the light from the object is totally reflected by the vertical surface  20   f.  Therefore, all of the light from the object is bent by the vertical surface  20   f.  Reference character O 6  denotes the optical axis extending toward the tilted surface  20   d  at this time.  
         [0066]     The angle of the tilted surface  20   d  is determined optimally so that the light from the object is totally reflected by the tilted surface  20   d,  and the main optical axis of the light from the object is incident on the vertical surface  20   f  perpendicularly. Therefore, the light from the object totally reflected by the vertical surface  20   f  is then totally reflected by the tilted surface  20   d  and goes to the vertical surface  20   f.  Reference character O 7  denotes the optical axis at this time.  
         [0067]     The light from the object totally reflected by the tilted surface  20   d  exits through a vertical surface  20   f,  and then enters the eyepiece lens  11 . Therefore, the user can observe the object image through the eyepiece lens  11 .  
         [0068]     Here, the optical axes O 4 , O 5 , and O 6  in the eyepiece prism  20  will be described. As in embodiment 1, the light ray that enters through the center of the objective lens G 1  and exits through the center of the eyepiece lens  11  is referred to as the main optical axis. Therefore, the optical axes O 1  to O 7  are parts of the main optical axis.  
         [0069]     In  FIGS. 9 and 10 , the axis L is an imaginary axis that extends through the eyepiece prism  20  and is perpendicular to the optical axes O 1  and O 2 . As shown in  FIGS. 9 and 10 , the optical axes O 4 , O 5 , and O 6  form a spiral that winds around the axis L in the eyepiece prism  20 , and then enter the eyepiece lens  11 . In other words, as shown in  FIGS. 13A and 13B , when the light from the object is reflected by the tilted surface or the vertical surface of the eyepiece prism  20 , the plane including the main axis incident on the tilted surface or the vertical surface and the main axis reflected by the tilted surface or the vertical surface, for example, the plane P 3  including the optical axes O 4  and O 5 , is tilted with respect to the optical axes O 1  and O 2 , which enter the objective optical system. That is to say, the plane P 3  is not parallel to the optical axes O 1  and O 2 .  
         [0070]     The plane P 4  including the optical axes O 5  and O 6  is also tilted with respect to the optical axes O 1  and O 2 .  
         [0071]     In the case where the length of the optical path from the field frame (not shown) to the eyepiece lens  11  via the eyepiece prism  20  is a fixed value, spiraling the optical path in the eyepiece prism  20  as described above can reduce the height H and the thickness D (see  FIG. 12 ) of the eyepiece prism  20  compared with folding the optical path in the plane parallel or perpendicular to the optical axes O 1  and O 2  entering the objective optical system.  
         [0072]     As described above, the main optical axis O 4  is incident on the surface  20   b;  the main optical axis O 5  is reflected by the surface  20   b  and is incident on the surface  20   f;  and the main optical axis O 6  is reflected by the surface  20   f.  Both the plane P 3  including the main optical axes O 4  and O 5  and the plane P 4  including the main optical axes O 5  and O 6  are not parallel to the main axes O 1  and O 2  entering the objective optical system. In other words, the main axes O 4 , O 5 , and O 6  incident on and reflected by the reflecting surfaces of the eyepiece prism  20  form a spiral that winds around the imaginary axis L perpendicular to the main axes O 1  and O 2  entering the objective optical system. Therefore, the height H and the thickness D of the eyepiece prism  20  can be reduced, and consequently the viewfinder according to embodiment 2 is thin and compact.  
         [0073]     By optimizing the angles of the tilted surfaces of the eyepiece prism  20 , the reflecting surfaces of the eyepiece prism  20  can totally reflect the light from the object. In this case, the reflecting surfaces need not be evaporated with aluminum. Therefore, the cost can be reduced compared with the conventional viewfinders.  
         [0074]     When the tilted surface  20   b  of the eyepiece prism  20  functions as the objective optical system, the tilted surface  20   b  transmits light. On the other hand, when the surface  20   b  functions as the eyepiece optical system, the surface  20   b  reflects light. Therefore, part of the eyepiece prism  20  is shared by the objective optical system and the eyepiece optical system. Consequently, there is no need to increase the size of the objective prism  5  nor to provide other optical components. This makes it possible to miniaturize the entire viewfinder  200  and to reduce the cost.  
         [0075]     Since the viewfinder  200  can be made compact and thin, the optical apparatus, such as a camera, using the viewfinder can also be made compact and thin.  
         [0076]     In addition, an optical apparatus having the viewfinder  200  is as shown in  FIG. 8 . When viewed from the image-taking device  501 , the entrance section (objective lens G 1 ) of the objective optical system is disposed directly above the image-taking device  501 , and the eyepiece optical system is disposed on the right side. Therefore, the user can easily look through the viewfinder, and the parallax in the optical apparatus is small.  
         [0077]     In embodiment 2, although the light from the object is totally reflected three times in the eyepiece prism  20 , the light from the object may be reflected at least once. If the shape of the eyepiece prism  20  and the bending prism  16  and the angle of the tilted surfaces of the prisms  20  and  16  are designed optimally so that an erect image can be seen when the user looks through the eyepiece lens  11 , the light from the object may be reflected any number of times.  
         [0078]     While the present invention has been described with reference to what are presently considered to be the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.