Abstract:
A stereo-image photographing apparatus enabling readily, short time-span realization of lens-barrel optical-axis adjustment in left- and right-eye imaging devices is afforded. The apparatus includes: a beam splitter having an incident surface of rectangular form, on which imaging light from a photographic subject is incident, an optically functional surface that reflects first imaging light, being a portion of incident imaging light, in a direction paralleling the shorter side of the rectangular form and that passes second imaging light being the remaining portion of the incident imaging light, a first light-exit surface through which the first imaging light exits, and a second light-exit surface through which the second imaging light exits; a first lens barrel directly facing the first light-exit surface; a second lens barrel directly facing the second light-exit surface; a first imaging device mounted on the first lens barrel; and a second imaging device mounted on the second lens barrel.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    The disclosure of Japanese Patent Application Nos. 2010-288837, filed on Dec. 24, 2010, and 2011-255160, filed on Nov. 22, 2011, is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to stereo-image photographing apparatuses that, employing dual imaging devices, take stereoscopically visible images, and more particularly relates to a stereo-image photographing apparatus whereby precise adjustment of the optical axes of lens barrels mounted on the dual imaging devices is made. 
         [0004]    2. Description of the Background Art 
         [0005]    In a conventional stereo-image photographing apparatus, imaging light from a photographic subject is separated into imaging light for the left eye and imaging light for the right eye, and a left-eye imaging device that receives the left-eye imaging light and a right-eye imaging device that receives the right-eye imaging light are used to generate a stereoscopic image that causes a viewer to sense a stereoscopic effect. 
         [0006]      FIG. 11  is a diagram illustrating a stereo-image photographing apparatus  900  in conventional art. In  FIG. 11 , the stereo-image photographing apparatus  900  includes a left-eye imaging device  910 , a right-eye imaging device  920 , and a semi-reflective mirror  930 . 
         [0007]    The semi-reflective mirror  930  reflects left-eye imaging light that is one portion of the imaging light from a photographic subject, and passes right-eye imaging light that is the remaining portion of the subject imaging light. The left-eye imaging device  910  generates an image for the left eye on the basis of image-constructing imaging light for the left eye, and the right-eye imaging device  920  generates an image for the right eye on the basis of image-constructing imaging light for the right eye. In this manner, the stereo-image photographing apparatus  900  in the conventional art generates, on the basis of the left-eye image and the right-eye image a stereoscopic image that causes a viewer to sense a stereoscopic effect. 
         [0008]    Therein, in order to generate a stereoscopic image that gives a viewer an optimal stereoscopic impression, adjustment of the optical axis of the lens barrel in the left-eye imaging device and the optical axis of the lens barrel in the right-eye imaging device is crucial. Specifically, it is crucial to precisely adjust the optical axes of the lens barrels in the dual imaging devices such that the optical axes lie in the same horizontal plane. That is, adjusting the axes so that the image taken by the left-eye imaging device and the image taken by the right-eye imaging device have no discrepancies (vertical shift, trapezoidal distortion, or the like) other than the difference in imaging position horizontally makes it possible to obtain an image that does not hinder a viewer from recognizing the dual images as a stereo image. In addition, in order to control the intensity of the stereoscopic effect, adjustment of the interval (stereo base) between the construct-image positions along the optical axes of the dual lens barrels, and adjustment of the angle (angle of convergence) that the two optical axes form is crucial. 
         [0009]    Japanese Patent No. 4293821 discloses technology relating to a stereo-image photographing apparatus, employing a semi-reflective mirror, that allows the stereo base to be freely set. 
         [0010]    However, with the stereo-image photographing apparatus in the conventional art, readily realizing, in a short time-span, optical-axis adjustment whereby the angle between the semi-reflective mirror and the optical axis of the lens barrel in the left-eye imaging device and the angle between the semi-reflective mirror and the optical axis of the lens barrel in the right-eye imaging device are brought to precisely 45 degrees has been challenging. 
       SUMMARY OF THE INVENTION 
       [0011]    Therefore, it is an object of the present invention to make available a stereo-image photographing apparatus that makes it possible readily to realize, in a short time-span, optical-axis adjustment of the lens barrels in a left-eye imaging device and a right-eye imaging device. 
         [0012]    In order to attain the object mentioned above, a stereo-image photographing apparatus of the present invention includes: a beam splitter having an incident surface of rectangular form and on which imaging light from a photographic subject is incident, an optically functional surface reflecting first imaging light, being a portion of imaging light incident on the incident surface, in a direction paralleling the shorter side of the rectangular form and passing second imaging light being the remaining portion of the imaging light, a first light-exit surface through which the first imaging light exits, and a second light-exit surface through which the second imaging light exits; a first lens barrel directly facing the first light-exit surface and through which the first imaging light exiting the first light-exit surface constructs an image; a second lens barrel directly facing the second light-exit surface and through which the second imaging light exiting the second light-exit surface constructs an image; a first imaging device mounted on the first lens barrel, for generating a first image on the basis of the first imaging light constructing an image through the first lens barrel; and a second imaging device mounted on the second lens barrel, for generating a second image on the basis of the second imaging light constructing an image through the second lens barrel. 
         [0013]    According to this configuration, by causing the first lens barrel and the second lens barrel to directly face the first light-exit surface and the second light-exit surface, respectively, of the beam splitter, optical axis adjustment is allowed to readily be realized in a short time-span such that the angle between the optically functional surface within the beam splitter and the optical axis of the first lens barrel and the angle between the optically functional surface within the beam splitter and the optical axis of the second lens barrel are brought to precisely 45 degrees. 
         [0014]    Preferably, the stereo-image photographing apparatus of the present invention further includes: a first lens-barrel holding member placing an end of the first lens barrel into abutment with the first light-exit surface such that the optical axis of the first lens barrel is retained perpendicular to the first light-exit surface; and a second lens-barrel holding member placing an end of the second lens barrel into abutment with the second light-exit surface such that the optical axis of the second lens barrel is retained perpendicular to the second light-exit surface. 
         [0015]    According to this configuration, it is possible to more precisely fix the positional relationship between the beam splitter and the first and second lens barrels. 
         [0016]    Moreover, preferably, the first lens barrel is a lens barrel to and from which the first lens barrel holding member is attachable and detachable; and the second lens barrel is a lens barrel to and from which the second lens barrel holding member is attachable and detachable. 
         [0017]    According to this configuration, it is possible to separate the beam splitter and the first and second lens barrels from each other via the first lens barrel holding member and the second lens barrel holding member. As a result, it is possible to replace the first lens barrel and the second lens barrel according to need, and thus it is possible to select an appropriate lens barrel according to purposes of imaging. 
         [0018]    Further, the preferable stereo-image photographing apparatus of the present invention further includes: a beam-splitter holding member retaining the beam splitter; a first imaging-device holding member retaining the first imaging device; a second imaging-device holding member retaining the second imaging device; and a base member retaining the beam-splitter holding member, the first imaging-device holding member, and the second imaging-device holding member. The first imaging-device holding member maintains a positional relationship, determined by the first lens-barrel holding member, between the first lens barrel and the beam splitter, and the second imaging-device holding member maintains a positional relationship, determined by the second lens-barrel holding member, between the second lens barrel and the beam splitter. 
         [0019]    According to this configuration, the positional relationship among the beam splitter, the first imaging device, and the second imaging device is maintained by the base member. As a result, the positional relationship between the beam splitter and the first and second lens barrels can be maintained. 
         [0020]    Moreover, preferably, the beam splitter holding member and the base member are integrally formed. 
         [0021]    According to this configuration, the positional relationship among the beam splitter, the first imaging device, and the second imaging device can be maintained more stably. 
         [0022]    Moreover, the preferable stereo-image photographing apparatus of the present invention further includes a beam splitter holding member which retains the beam splitter, and the beam splitter holding member, the first lens barrel holding member, and the second lens barrel holding member are secured to retain the beam splitter, the first imaging device, and the second imaging device. 
         [0023]    According to this configuration, the first imaging device and the second imaging device which are small in size and light in weight can be mounted directly on the beam splitter. As a result, it is possible to realize a stereo-image photographing apparatus which is small in size and light in weigh as a whole. 
         [0024]    Moreover, the preferable stereo-image photographing apparatus of the present invention further includes at least one of either a slider mounted on the first light-exit surface of the beam splitter to enable the first lens-barrel holding member to parallel-shift along the longer side of the rectangular form, or a slider mounted on the second light-exit surface of the beam splitter to enable the second lens-barrel holding member to parallel-shift along the longer side of the rectangular form. 
         [0025]    According to this configuration, it is possible to change only a stereo base while the angle between the optically functional surface within the beam splitter and the optical axis of the first lens barrel and the angle between the optically functional surface within the beam splitter and the optical axis of the second lens barrel are maintained constant. 
         [0026]    As described above, according to the stereo-image photographing apparatus of the present invention, the optical axes of the lens barrels in the left eye imaging device and the right eye imaging device are allowed to readily be adjusted in a short time-span. 
         [0027]    These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]      FIG. 1  is a schematic diagram illustrating the configuration of a stereo-image photographing apparatus  100  according to a first embodiment of the present invention; 
           [0029]      FIG. 2  is a schematic top view illustrating the configuration of the stereo-image photographing apparatus  100  shown in  FIG. 1 ; 
           [0030]      FIG. 3A  is a diagram illustrating the positional relationship between photographic subjects and first and second imaging devices  111  and  121  shown in  FIGS. 1 and 2 ; 
           [0031]      FIG. 3B  is a diagram illustrating an image for a left eye and an image for a right eye which are generated by the first and second imaging devices  111  and  121 ; 
           [0032]      FIG. 4  is a diagram schematically illustrating an exemplary configuration of a lens barrel holding member which varies a stereo base in the stereo-image photographing apparatus  100  shown in  FIGS. 1 and 2 ; 
           [0033]      FIG. 5  is a diagram schematically illustrating a connection portion between a first lens barrel holding member  112  and a first lens barrel  110  and a connection portion between a second lens barrel holding member  122  and a second lens barrel  120  in the stereo-image photographing apparatus  100  shown in  FIGS. 1 and 2 ; 
           [0034]      FIG. 6  is a diagram schematically illustrating an exemplary configuration of a first imaging device holding member  113  and a second imaging device holding member  123  in the stereo-image photographing apparatus  100  shown in  FIGS. 1 and 2 ; 
           [0035]      FIG. 7  is a diagram schematically illustrating an exemplary configuration in which a beam splitter holding member  106  and a base member  107  are integrated to each other in the stereo-image photographing apparatus  100  shown in  FIGS. 1 and 2 ; 
           [0036]      FIG. 8  is a schematic diagram illustrating the configuration of a stereo-image photographing apparatus  500  according to a second embodiment of the present invention; 
           [0037]      FIG. 9  is a diagram schematically illustrating an exemplary configuration of a lens barrel holding member which varies a stereo base in the stereo-image photographing apparatus  500  shown in  FIG. 8 ; 
           [0038]      FIG. 10  is a diagram schematically illustrating a connection portion between a first imaging device  501  and a first lens barrel holding member  601  in the stereo-image photographing apparatus  500  shown in  FIG. 9 ; 
           [0039]      FIG. 11  is a schematic diagram illustrating the configuration of a stereo-image photographing apparatus  700  according to a third embodiment of the present invention; and 
           [0040]      FIG. 12  is a diagram illustrating a stereo-image photographing apparatus  900  in the conventional art. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0041]    Hereinafter, each embodiment of the present invention will be described with reference to the drawings. 
       First Embodiment 
       [0042]      FIG. 1  is a schematic diagram illustrating the configuration of a stereo-image photographing apparatus  100  according to a first embodiment of the present invention. In  FIG. 1 , the stereo-image photographing apparatus  100  includes a beam splitter  101 , a first lens barrel  110 , a second lens barrel  120 , a first imaging device  111 , and a second imaging device  121 . 
         [0043]    The first imaging device  111  is a left eye imaging device which generates an image for a left eye, and the second imaging device  121  is a right eye imaging device which generates an image for a right eye. 
         [0044]    The beam splitter  101  has an optically functional surface  103  which separates imaging light from a photographic subject, into first imaging light and second imaging light. The optically functional surface  103  has a property to reflect a portion of incident light and pass the remaining portion of the incident light. In use of the stereo-image photographing apparatus  100 , it is preferred to cause the brightness of an image for a right eye to coincide with the brightness of an image for a left eye, and thus a semi-reflective mirror surface is used as the optically functional surface  103 . The optically functional surface  103  is typically formed by depositing metal on an inclined surface of a triangular prism which constitutes the beam splitter  101 , but may be formed of a film having the function of a semi-reflective mirror. 
         [0045]    The imaging light 
         [0000]    from the photographic subject is incident on an incident surface  102  of the beam splitter  101 . The first imaging light reflected by the optically functional surface  103  is incident on the first lens barrel  110 . The first imaging light incident on the first lens barrel  110  constructs an image on an imaging surface of a solid-state image sensor or the like within the first imaging device  111 , and is photoelectrically converted into an image signal for a left eye. In this manner, the first imaging device  111  generates an image for a left eye. 
         [0046]    The second imaging light having passed through the optically functional surface  103  is incident on the second lens barrel  120 . The second imaging light incident on the second lens barrel  120  constructs an image on an imaging surface of a solid-state image sensor or the like within the second imaging device  121 , and is photoelectrically converted into an image signal for a right eye. In this manner, the second imaging device  121  generates an image for a right eye. 
         [0047]    Here, when shift other than a parallax, such as vertical shift or shift caused by a trapezoidal distortion, occurs between a photographic subject image on the image for a left eye and a photographic subject image on the image for a right eye, a viewer cannot observe the two photographic subject images as a stereoscopic image by means of fusion, or can observe the two photographic subject images as a stereoscopic image but feels eye fatigue. In addition, when the amount of a parallax is excessively great, the viewer cannot obtain a preferable stereoscopic effect when observing a stereoscopic image. Thus, it is crucial to precisely adjust the inclination of the optical axis L 1  of the first lens barrel  110  and the inclination of the optical axis L 2  of the second lens barrel  120 . In other words, an optical axis obtained by bending the optical axis L 1  of the first lens barrel  110  by the optically functional surface  103  and an optical axis obtained by causing the optical axis L 2  of the second lens barrel  120  to pass through the optically functional surface  103  have to coincide with each other to be an optical axis L as shown in  FIG. 1 , when seen from the lateral direction of the stereo-image photographing apparatus  100 . 
         [0048]    In the stereo-image photographing apparatus  100  according to the first embodiment of the present invention, the beam splitter  101  has a rectangular parallelepiped shape, and the cross section of the beam splitter  101  has a square shape as shown in  FIG. 1 , when seen from the lateral direction of the stereo-image photographing apparatus  100 . 
         [0049]    A second light-exit surface  105  of the beam splitter  101  is formed so as to be precisely parallel to the incident surface  102 . In addition, a first light-exit surface  104  of the beam splitter  101  is formed so as to be precisely perpendicular to both the incident surface  102  and the second light-exit surface  105 . Moreover, the optically functional surface  103  provided within the beam splitter  101  is formed so as to precisely make an angle of 45 degrees with each of the incident surface  102 , the first light-exit surface  104 , and the second light-exit surface  105 . The incident surface  102 , the optically functional surface  103 , the first light-exit surface  104 , and the second light-exit surface  105  are formed as flat surfaces with very high accuracy by polishing or the like. 
         [0050]    As described above, the flatness and angle of each surface of the beam splitter  101  are very highly precise. Thus, when the first lens barrel  110  is caused to directly face the first light-exit surface  104  and the second lens barrel  120  is caused to directly face the second light-exit surface  105 , the optical axis obtained by bending the optical axis L 1  of the first lens barrel  110  by the optically functional surface  103  and the optical axis obtained by causing the optical axis L 2  of the second lens barrel  120  to pass through the optically functional surface  103  can be caused to coincide with each other to be the optical axis L as shown in  FIG. 1 , when seen from the lateral direction of the stereo-image photographing apparatus  100 . It should be noted that causing a lens barrel to directly face a light-exit surface means to locate the lens barrel in front of the light-exit surface such that the interval between the outer circumference of a lens included in the lens barrel and the light-exit surface is uniformed. In a state where the lens barrel is caused to directly face the light-exit surface, the optical axis of the lens barrel is perpendicular to the light-exit surface. 
         [0051]    As described above, according to the stereo-image photographing apparatus  100  according to the first embodiment of the present invention, since the beam splitter  101  is precisely formed, by causing the first lens barrel  110  and the second lens barrel  120  to directly face the first light-exit surface  104  and the second light-exit surface  105 , respectively, of the beam splitter  101 , optical axis adjustment is allowed to readily be realized in a short time-span such that the angle between the optically functional surface  103  within the beam splitter  101  and the optical axis L 1  of the first lens barrel  110  and the angle between the optically functional surface  103  within the beam splitter  101  and the optical axis L 2  of the second lens barrel  120  are brought to precisely 45 degrees. 
         [0052]    It should be noted that as shown in  FIG. 1 , the stereo-image photographing apparatus  100  may include a first lens barrel holding member  112  which retains the first lens barrel  110  such that an end of the first lens barrel  110  is in contact with the first light-exit surface  104 . The first lens barrel holding member  112  precisely fixes the beam splitter  101  and the first lens barrel  110  such that the first light-exit surface  104  and the optical axis L 1  of the first lens barrel  110  are perpendicular to each other. 
         [0053]    Similarly, the stereo-image photographing apparatus  100  may include a second lens barrel holding member  122  which retains the second lens barrel  120  such that an end of the second lens barrel  120  is in contact with the second light-exit surface  105 . The second lens barrel holding member  122  precisely fixes the beam splitter  101  and the second lens barrel  120  such that the second light-exit surface  105  and the optical axis L 2  of the second lens barrel  120  are perpendicular to each other. 
         [0054]    Further, as shown in  FIG. 1 , the stereo-image photographing apparatus  100  may include a beam splitter holding member  106 , a first imaging device holding member  113 , a second imaging device holding member  123 , and a base member  107 . The beam splitter holding member  106  retains the beam splitter  101 , the first imaging device holding member  113  retains the first imaging device  111 , and the second imaging device holding member  123  retains the second imaging device  121 . The base member  107  retains the beam splitter holding member  106 , the first imaging device holding member  113 , and the second imaging device holding member  123  to fix a mutual positional relationship thereamong. As a result, the positional relationship between the beam splitter  101  and the first and second lens barrels  110  and  120  can be maintained. It should be noted that a front window  108  is provided in the base member  107  such that imaging light can be incident on the incident surface  102  of the beam splitter  101  therethrough. 
         [0055]    Moreover, in order for the viewer to obtain a preferable stereoscopic effect when observing a stereoscopic image, the interval (stereo base) between the construct-image position along the optical axis L 1  of the first lens barrel  110  and the construct-image position along the optical axis L 2  of the second lens barrel  120  and the angle (angle of convergence) that the optical axis L 1  of the first lens barrel  110  and the optical axis L 2  of the second lens barrel  120  form have to appropriately be set. 
         [0056]      FIG. 2  is a schematic top view illustrating the configuration of the stereo-image photographing apparatus  100 , shown in  FIG. 1 , according to the first embodiment of the present invention. In  FIG. 2 , the optical axis obtained by bending the optical axis L 1  of the first lens barrel  110  by the optically functional surface  103  and the optical axis obtained by causing the optical axis L 2  of the second lens barrel  120  to pass through the optically functional surface  103  have to be adjusted to have an appropriate inter-axis distance and angle of convergence when seen from the top surface of the stereo-image photographing apparatus  100 . 
         [0057]    Here, the stereo base and the angle of convergence will be described in detail. It should be noted that for simplification of explanation, a description will be given on the assumption that in the stereo-image photographing apparatus  100  shown in  FIGS. 1 and 2 , the beam splitter  101  having the optically functional surface  103  is omitted and the first imaging device  111  and the second imaging device  121  are present on the same plane. 
         [0058]      FIG. 3A  is a diagram illustrating the positional relationship among the first imaging device  111 , the first lens barrel  110 , the optical axis L 1 , the second imaging device  121 , the second lens barrel  120 , the optical axis L 2 , and photographic subjects.  FIG. 3A  is a diagram illustrating an image for a left eye and an image for a right eye which are generated by the first imaging device  111  and the second imaging device  121 . 
         [0059]    In  FIG. 3A , images of a photographic subject  34  present at a point at which the optical axis L 1  of the first lens barrel  110  intersects with the optical axis L 2  of the second lens barrel  120  are taken at the same lateral position as shown in  FIG. 3B  as an image for a left eye and an image for a right eye. It should be noted that since the optical axes L 1  and L 2  of the lens barrels in the two imaging devices precisely lie in the same horizontal plane, the image for a left eye taken by the first imaging device  111  and the image for a right eye taken by the second imaging device  121  have no discrepancies (vertical shift, trapezoidal distortion, or the like) other than the difference in imaging position horizontally. 
         [0060]    Meanwhile, images of the photographic subject  32  present on the near side of the point at which the optical axis L 1  of the first lens barrel  110  intersects with the optical axis L 2  of the second lens barrel  120  are taken as photographic subject images projected on a virtual screen  36  which is assumed at the same camera distance as that of the photographic subject  34 . In other words, an image of the photographic subject  32  is taken by the first imaging device  111  corresponding to the viewpoint of a left eye, as a photographic subject image  35  projected on the virtual screen  36 . Similarly, an image of the photographic subject  32  is taken by the second imaging device  121  corresponding to the viewpoint of a right eye, as a photographic subject image  33  projected on the virtual screen  36 . 
         [0061]    Therefore, photographic subject images of the photographic subject  32  are taken at positions distant from each other by a parallax amount  37 , as shown in  FIG. 3B , in the stereo-image photographing apparatus  100  including the first imaging device  111  and the second imaging device  121 . 
         [0062]    Here, as the parallax increases (the interval between the photographic subject image  33  and the photographic subject image  35  increases), the stereoscopically displayed photographic subject image is seen as if projecting toward the near side of the virtual screen  36 . Meanwhile, in  FIG. 3A , when the positional relationship between the photographic subject image  33  and the photographic subject image  35  is reversed such that the photographic subject image  33  is located on the right eye side of the photographic subject image  35  and the photographic subject image  35  is located on the left eye side of the photographic subject image  33 , the stereoscopically displayed photographic subject image is seen as if being recessed toward the far side of the virtual screen  36 . 
         [0063]    When the parallax between the image for a left eye and the image for a right eye is excessively great (the photographic subject image excessively projects), eye fatigue of the viewer is caused. This is because the depth distance between the position of the photographic subject  32  and the virtual screen  36  on which the eyes actually focus is excessively large. In addition, when a range of a parallax of each photographic subject is excessively large in the same screen, namely, when the distance between a projecting photographic subject image and a recessed photographic subject image is excessively large, the viewer feels eye fatigue. Further, when the amount of a parallax rapidly changes with time, the eyes of the viewer cannot follow the change, and thus eye fatigue is caused. 
         [0064]    In order to take an appropriate stereoscopic display image such that the viewer obtains a stereoscopic effect from a displayed image while such causes of eye fatigue of the viewer are taken into consideration, the interval (stereo base)  30  between the construct-image position along the optical axis L 1  of the first lens barrel  110  and the construct-image position along the optical axis L 2  of the second lens barrel  120  and the angle of convergence  31  between the optical axis L 1  of the first lens barrel  110  and the optical axis L 2  of the second lens barrel  120  have to be set to appropriate values by comparing to a position distribution of the photographic subject. 
         [0065]      FIG. 4  is a diagram schematically illustrating an exemplary configuration of a lens barrel holding member which varies the stereo base in the stereo-image photographing apparatus  100 , shown in  FIGS. 1 and 2 , according to the first embodiment of the present invention. In  FIG. 4 , the components described with reference to  FIGS. 1 and 2  are designated by the same reference characters, and thus the detailed description thereof is omitted. 
         [0066]    The stereo-image photographing apparatus shown in  FIG. 4  differs from the stereo-image photographing apparatus  100  shown in  FIGS. 1 and 2  in further including a slider which enables the second lens barrel  120  and the second imaging device  121  to parallel-shift along the second light-exit surface  105  of the beam splitter  101 . The slider refers to a mechanism which rectilinearly shiftably guides a moved member, namely, a lens barrel and/or an imaging device along a specified direction and which restricts the lens barrel and/or the imaging device against shifting along another direction (a direction perpendicular to the specified direction). In the example of  FIG. 4 , the slider is realized by being composed of a second imaging device slider portion  201 , a second imaging device rail portion  202 , second lens barrel holding rail portions  211  to  213 , and second lens barrel holding slider portions  221  to  225 . It should be noted that the beam splitter  101  shown in  FIGS. 1 and 2  is covered with a beam splitter holding member side portion  230 , a beam splitter holding member upper portion  235 , and a beam splitter holding member back portion  236  in  FIG. 4 . 
         [0067]    The beam splitter  101  (not shown in  FIG. 4 ) is installed on the beam splitter holding member  106 , and fixed by being covered with the beam splitter holding member side portion  230 , the beam splitter holding member upper portion  235 , and the beam splitter holding member back portion  236 . The second lens barrel holding rail portions  211  to  213  are mounted on the second light-exit surface  105  (not shown in  FIG. 4 ) of the beam splitter  101 . 
         [0068]    To the second lens barrel holding rail portion  212 , the second lens barrel holding slider portion  221  is provided so as to slide along the second light-exit surface  105  of the beam splitter  101  in the lateral direction (in the direction to the far side of the sheet plane and in the direction to the near side of the sheet plane). To the second lens barrel holding rail portion  213 , the second lens barrel holding slider portion  225  is provided so as to slide along the second light-exit surface  105  of the beam splitter  101  in the lateral direction (in the direction to the far side of the sheet plane and in the direction to the near side of the sheet plane. 
         [0069]    The second lens barrel holding slider portions  221  to  225  slide together on the second lens barrel holding rail portions  212  and  213  in the lateral direction in parallel to the second light-exit surface  105  of the beam splitter  101 . Thus, the second lens barrel  120  mounted on the second lens barrel holding rail portions  212  and  213  moves in the lateral direction while maintaining the angle with the second light-exit surface  105  of the beam splitter  101 . By so doing, it is possible to change only the stereo base while the optical axis direction is maintained constant. 
         [0070]    Further, the second imaging device rail portion  202  is secured to the base member  107 , and the second imaging device slider portion  201  is provided so as to slide on the second imaging device rail portion  202  in the lateral direction (in the direction to the far side of the sheet plane and in the direction to the near side of the sheet plane). When the weight of the second imaging device  121  is high, the second imaging device slider portion  201  and the second imaging device rail portion  202  may be configured such that the second imaging device rail portion  202  is movable in the lateral direction while the second imaging device slider portion  201  and the second imaging device rail portion  202  support the second imaging device  121 . By so doing, a portion where the second lens barrel  120  and the second lens barrel holding slider portion  223  are joined to each other can be prevented from being distorted or broken due to the weight. 
         [0071]      FIG. 5  is a diagram schematically illustrating a connection portion between the first lens barrel holding member  112  and the first lens barrel  110  and a connection portion between the second lens barrel holding member  122  and the second lens barrel  120  in the stereo-image photographing apparatus  100  shown in  FIGS. 1 and 2 . In  FIG. 5 , the first lens barrel  110  is fixed to the beam splitter holding member upper portion  235  by a first lens barrel holding ring  301  and a first lens barrel holding screw portion  302 , and the second lens barrel  120  is fixed to the second lens barrel holding slider portion  223  by a second lens barrel holding ring  311  and a second lens barrel holding screw portion  312 . 
         [0072]    As shown in  FIG. 5(   a ), the beam splitter  101  (not shown in  FIG. 5)  is fixed by being covered with the beam splitter holding member at its periphery except its front surface (the incident surface  102  shown in  FIG. 1 ). The first lens barrel holding ring  301  is attached to the beam splitter holding member upper portion  235  of the beam splitter holding member by means of bonding, integral molding, or the like. The first lens barrel holding ring  301  is positioned in place on the base of the first light-exit surface  104  of the beam splitter  101 . The first lens barrel holding screw portion  302  for connecting to the first lens barrel  110  is provided on the first lens barrel holding ring  301 . The thread of the first lens barrel holding screw portion  302  is engaged with a thread portion provided on the inner surface of the first lens barrel  110 , whereby the first lens barrel  110  can be fixed so as to be precisely perpendicular to the first light-exit surface  104  of the beam splitter  101 . 
         [0073]    Further, as shown in  FIG. 5(   b ), the second lens barrel holding ring  311  is attached to the second lens barrel holding slider portion  223  by means of bonding, integral molding, or the like. The second lens barrel holding ring  311  is also positioned in place on the basis of the second light-exit surface  105  of the beam splitter  101 . The second lens barrel holding screw portion  312  for connecting to the second lens barrel  120  is provided on the second lens barrel holding ring  311 . The thread of the second lens barrel holding screw portion  312  is engaged with a thread portion provided on the inner surface of the second lens barrel  120 , whereby the second lens barrel  120  can be fixed so as to be precisely perpendicular to the second light-exit surface  105  of the beam splitter  101 . 
         [0074]      FIG. 6  is a diagram schematically illustrating an exemplary configuration of the first imaging device holding member  113  and the second imaging device holding member  123  in the stereo-image photographing apparatus  100  shown in  FIGS. 1 and 2 . In  FIG. 6 , the first imaging device holding member  113  and the second imaging device holding member  123  retain the first imaging device  111  and the second imaging device  121  by using a top plate  401 , a bottom plate  402 , windage screws  411  and  412 , a ball  413 , an imaging device fixing hole  421 , spring fixing holes  431  to  436 , and springs  441  to  446 . 
         [0075]    The top plate  401  and the bottom plate  402  are attracted to each other by the springs  441  to  446 . The ball  413  and the windage screws  411  and  412  are located between the top plate  401  and the bottom plate  402  and maintain the interval between the top plate  401  and the bottom plate  402 . By so doing, the top plate  401  and the bottom plate  402  are fixed. 
         [0076]    The inclination of the top plate  401  with respect to the bottom plate  402  is adjustable by the two windage screws  411  and  412 . The first imaging device  111  and the second imaging device  121  are fixed by a screw which is screwed into a threaded hole in the bottom surface of the main body through the imaging device fixing hole  421  of the top plate  401 . 
         [0077]    The positions and inclinations of the image sensors included in the first imaging device  111  and the second imaging device  121  can be adjusted by using the adjustment mechanisms of the first imaging device holding member  113  and the second imaging device holding member  123  shown in  FIG. 6 . However, when the connection portion between the first lens barrel holding member  112  and the first lens barrel  110  and the connection portion between the second lens barrel holding member  122  and the second lens barrel  120  have sufficient strength, for example, the ball  413  and the windage screws  411  and  412  may be omitted, and expansion and compression of the springs  441  to  446  may be adjusted only by strengths from the first lens barrel holding member  112  and the second lens barrel holding member  122  to the first imaging device  111  and the second imaging device  121 , whereby it is configured to support the weights of the first imaging device  111  and the second imaging device  121 . Further, when higher accuracy is required, the number of axes to be adjusted may be increased by using a rack-and-pinion mechanism or the like. 
         [0078]      FIG. 7  is a diagram schematically illustrating an exemplary configuration in which the beam splitter holding member  106  and the base member  107  are integrated to each other in the stereo-image photographing apparatus  100  shown in  FIGS. 1 and 2 . As shown in  FIG. 7 , an additional base member  1071  is further provided on the base member  107 , and the beam splitter holding member  106  and the second imaging device rail portion  202  on which the second imaging device holding member  123  is put are produced by integral molding. By so doing, the positional relationship between the beam splitter  101  and the second imaging device  121  can be maintained with higher accuracy. 
       Second Embodiment 
       [0079]      FIG. 8  is a schematic diagram illustrating the configuration of a stereo-image photographing apparatus  500  according to a second embodiment of the present invention.  FIG. 8(   a ) is a side view of the stereo-image photographing apparatus  500 , and  FIG. 8(   b ) is a top view of the stereo-image photographing apparatus  500 . In  FIG. 8 , the components described with reference to  FIGS. 1 and 2  are designated by the same reference characters, and thus the detailed description thereof is omitted. 
         [0080]    As imaging devices, a first imaging device  501  and second imaging device  511  which are small in size and light in weight are used. It should be noted that the first imaging device  501  and the second imaging device  511  includes a first lens barrel  502  and a second lens barrel  512 , respectively. 
         [0081]    The first lens barrel  502  and the second lens barrel  512  are held by the first lens barrel holding member  112  and the second lens barrel holding member  122 , whereby the first imaging device  501  and the second imaging device  511  are supported. The configuration of the present embodiment is effective when the first imaging device  501  and the second imaging device  511  are light in weight to such a degree that the first imaging device  501  and the second imaging device  511  can be held only by the first lens barrel holding member  112  and the second lens barrel holding member  122 . 
         [0082]    As described above, according to the stereo-image photographing apparatus  500  according to the second embodiment of the present invention, when the first imaging device  501  and the second imaging device  511  are small in size and light in weight, each holding member needed in the stereo-image photographing apparatus  100  according to the first embodiment of the present invention is not needed, and thus the stereo-image photographing apparatus  500  can be decreased in size and weight as a whole. 
         [0083]      FIG. 9  is a diagram schematically illustrating an exemplary configuration of a lens barrel holding member which varies the stereo base in the stereo-image photographing apparatus  500 , shown in  FIG. 8 , according to the second embodiment of the present invention. The stereo-image photographing apparatus shown in  FIG. 9  differs from the stereo-image photographing apparatus  500  shown in  FIG. 8  in further including a slider which enables the second lens barrel  512  and the second imaging device  511  to parallel-shift along the second light-exit surface  105  of the beam splitter  101 . The slider shown in  FIG. 9  is the same as that described with reference to  FIG. 4  in the first embodiment of the present invention, and thus the same reference characters are used therefor and the detailed description thereof is omitted. 
         [0084]    In  FIG. 9 , the first imaging device  501  is held only by a first lens barrel holding member  601 . In addition, the second imaging device  511  is held only by a second lens barrel holding member  602 . The second imaging device  511  is set by the second lens barrel holding rail portions  211  to  213  and the second lens barrel holding slider portions  221  to  225  such that the optical axis thereof is perpendicular to the second light-exit surface  105  of the beam splitter  101 , and is movable in the lateral direction while maintaining the angle with the second light-exit surface  105  of the beam splitter  101 . 
         [0085]      FIG. 10  is a diagram schematically illustrating a connection portion between the first imaging device  501  and the first lens barrel holding member  601  in the stereo-image photographing apparatus  500  shown in  FIG. 8 . Needless to say, a connection portion between the second imaging device  511  and the second lens barrel holding member  602  is the same as the connection portion between the first imaging device  501  and the first lens barrel holding member  601 . 
         [0086]    As shown in  FIG. 10 , a lens barrel holding member fixing base  705  is fixed to the first imaging device  501  by screws  703  and  704  being screwed into threaded holes  701  and  702 . The first imaging device  501  is mounted on the first lens barrel holding member  601  via the lens barrel holding member fixing base  705 . 
         [0087]    Specifically, the lens barrel holding member fixing base  705  has the same configuration as those of the first lens barrel holding ring  301  and the first lens barrel holding screw portion  302  shown in  FIG. 5 . Thus, the first lens barrel holding member  601  can be attached to the first imaging device  501  by screwing a thread portion provided on its inner surface onto a thread portion provided on the outer surface of the lens barrel holding member fixing base  705 . The configuration of the beam splitter side portion of the first lens barrel holding member  601  is the same as the configuration described with reference to  FIG. 5 . The first imaging device  501  can be mounted such that the optical axis thereof is perpendicular to the light-exit surface of the beam splitter. 
         [0088]    It should be noted that the first lens barrel holding member  601  is not formed in a shape of a complete cylinder but formed such that the beam splitter side or the first imaging device  501  side is inclined, whereby the first lens barrel holding member  601  can be fixed while the angle between the optical axis and the light-exit surface of the beam splitter is maintained constant. 
         [0089]    As described above, according to the stereo-image photographing apparatus  500  according to the second embodiment of the present invention, since the beam splitter  101  is precisely formed, by causing the first lens barrel  502  and the second lens barrel  512  to directly face the first light-exit surface  104  and the second light-exit surface  105 , respectively, of the beam splitter  101 , optical axis adjustment is allowed to readily be realized in a short time-span such that the angle between the optically functional surface  103  within the beam splitter  101  and the optical axis L 1  of the first lens barrel  502  and the angle between the optically functional surface  103  within the beam splitter  101  and the optical axis L 2  of the second lens barrel  512  are brought to precisely 45 degrees. 
         [0090]    In the present embodiment, the first imaging device  501  and the second imaging device  511  are illustrated as compact digital cameras in  FIGS. 8 to 10 . However, the first imaging device  501  and the second imaging device  511  may also be configured as smaller and more lightweight imaging devices such as mobile phones equipped with cameras. 
         [0091]    In the first and second embodiments of the present invention, the beam splitter, the first lens barrel, and the second lens barrel are precisely fixed such that the end of the first lens barrel is in contact with the first light-exit surface of the beam splitter, the end of the second lens barrel is in contact with the second light-exit surface of the beam splitter, the optical axis of the first lens barrel is retained perpendicular to the first light-exit surface, and the optical axis of the second lens barrel is retained perpendicular to the second light-exit surface. Here, in particular, as shown in  FIGS. 1 and 2 , each of the first lens barrel and the second lens barrel includes a plurality of lens units. As described above, the positional relationship among the beam splitter, the first lens barrel, and the second lens barrel is precisely set and fixed. Thus, even when the positions of lenses in the plurality of lens units are changed to adjust the focal length or focus, only the view angle or focus position can be changed without changing the precisely set positional relationship among the beam splitter, the first lens barrel, and the second lens barrel. In other words, the optical axis of the first lens barrel is retained perpendicular to the first light-exit surface, and the optical axis of the second lens barrel is retained perpendicular to the second light-exit surface. 
         [0092]    Further, in the first and second embodiments of the present invention, the first imaging device and the second imaging device are individually useable as independent imaging devices, but the present invention is not limited thereto. For example, an exterior part may not be mounted on each imaging device, each imaging device may be mounted as a dedicated mechanism on the stereo-image photographing apparatus, and the entire stereo-image photographing apparatus including each holding member such as a base member may be covered with another exterior part. In such a case, each imaging device does not have to be equipped with a finder and a liquid crystal display. 
       Third Embodiment 
       [0093]      FIG. 11  is a schematic diagram illustrating the configuration of a stereo-image photographing apparatus  800  according to a third embodiment of the present invention. The stereo-image photographing apparatus  800  according to the present embodiment differs from the stereo-image photographing apparatuses according to the first and second embodiments in a method for arranging the second lens barrel  120  and the second imaging device  121  which are used for taking an image for a right eye. Hereinafter, the difference between the present embodiment and each embodiment described above will mainly be described. 
         [0094]    The second lens barrel  120  mounted on the second imaging device  121  is located so as to directly face the second light-exit surface  105  of the beam splitter  101  similarly to each embodiment described above. However, the second lens barrel  120  is not in contact with the second light-exit surface  105  and is spaced apart from the second light-exit surface  105  at a predetermined interval. Similarly to the configuration in  FIG. 4 , the second imaging device  121  is mounted to the second imaging device holding member  123  by means of a threaded hole for fitting a tripod, or the like. 
         [0095]    The second imaging device holding member  123  is supported by a slider mechanism so as to be horizontally movable. Specifically, second imaging device slider portions  803  and  804  are fixed to the lower surface of the second imaging device holding member  123 . The second imaging device slider portions  803  and  804  are slidably engaged with second imaging device rail portions  801  and  802  fixed on the base member  107 . The second imaging device rail portions  801  and  802  are positioned in place on the basis of the second light-exit surface  105  of the beam splitter  101 , and are parallel to both the plane including the first light-exit surface  104  and the plane including the second light-exit surface  105 . Further, in order to move the second imaging device holding member  123  along the second imaging device rail portions  801  and  802 , a feed mechanism  805  is provided. The feed mechanism  805  is composed of, for example, a combination of a feed screw or a ball screw and a motor. Moreover, in the present embodiment, instead of providing a light-blocking member on the second light-exit surface  105 , a light-blocking cover  807  is provided so as to cover at least the second light-exit surface  105 , the second lens barrel  120 , the second imaging device  121 , and the second imaging device holding member  123 . 
         [0096]    According to the stereo-image photographing apparatus  800  according to the present embodiment, since the slide direction of the slider mechanism is set on the basis of the second light-exit surface  105  of the beam splitter  101 , the second lens barrel  120  and the second imaging device  121  can be moved in parallel with high accuracy while the inclination of the optical axis of the second lens barrel  120  with respect to the second light-exit surface  105  is maintained. Further, in the present embodiment, any other member does not have to be in contact with the second light-exit surface  105 , and thus damage of the second light-exit surface  105  of the beam splitter  101  is prevented. Moreover, the optical path can be changed on the basis of the interval between the second light-exit surface  105  and the second lens barrel  120 , and thus the configuration of the present embodiment is effective particularly when it is desired to insert an optical element such as a prism between the beam splitter  101  and the first lens barrel  110 . 
         [0097]    As the slider mechanism described in each embodiment described above, the following various variations are considered. 
         [0098]    For example, in the configuration shown in  FIG. 4 , the slider mechanism including the rail and the slider is provided on both the second light-exit surface  105  of the beam splitter  101  and the second imaging device holding member  123 , but the slider mechanism may not be provided on the second light-exit surface  105 , and the second lens barrel  120  may merely be slid in contact with the second light-exit surface  105 . In such a case, a light-blocking cover is provided so as to cover at least the second light-exit surface, the second lens barrel  120 , and the second imaging device  121 . 
         [0099]    Further, in the configurations shown in  FIGS. 4 and 9 , the slider mechanism which slides along the second light-exit surface  105  of the beam splitter  101  is provided. However, the slider mechanism can also be configured to slide along the first light-exit surface  104  in the longitudinal direction of the beam splitter  101 . In addition, the lower surface of the beam splitter  101  can also similarly be used as a reference surface for the slider mechanism. 
         [0100]    Moreover, in the example shown in  FIG. 11 , the slider mechanism is composed of two pairs of the rails and the sliders and the feed mechanism. However, the slider mechanism may be composed of only a rail and a slider, or may be composed of only a feed mechanism. 
         [0101]    Moreover, when the second lens barrel  120  is not in contact with the second light-exit surface  105  of the beam splitter  101  as in the configuration shown in  FIG. 11 , a portion other than the second lens barrel  120 , such as a portion of the second imaging device holding member  123 , may be slid in contact with a surface of the beam splitter  101 . 
         [0102]    Moreover, the slider mechanism does not have to include the rail and the slider engaged with the rail, and various components such as a combination of a groove and a member engaged with the groove can be used as long as they allow shifting along the longer side of the incident surface of the beam splitter  101  and prevent shifting along a line orthogonal to the longer side of the incident surface. 
         [0103]    Moreover, in each embodiment described above, it is configured such that the second imaging device is horizontally movable. However, it may be configured such that the first imaging device is horizontally movable. Further, it can be configured such that both the first imaging device and the second imaging device are horizontally movable, and the present invention is also similarly applicable to such a case. 
         [0104]    The present invention is useable as a stereo 3D imaging apparatus which takes an three-dimensional image, and is useful particularly for a camera and a video camera which require precise optical axis adjustment. 
         [0105]    While the invention has been described in detail, the foregoing description is in all aspects illustrative and not restrictive. It will be understood that numerous other modifications and variations can be devised without departing from the scope of the invention.