Abstract:
A system includes a first prism pair that converts beams emitted from a subject and having two substantially parallel optical axes arranged side-by-side in one direction into beams arranged side-by-side in a direction intersecting the aforementioned side-by-side direction; and a second prism pair that performs conversion to reduce the distance between the optical axes of the two beams converted by the first prism pair and that has exit surfaces arranged side-by-side in a direction perpendicular to the side-by-side arrangement direction before entering the first prism pair. The first prism pair includes a first parallelogram prism that reflects, twice, the beam containing one of the two optical axes in a first plane containing one of the optical axes, and a second parallelogram prism that reflects, twice, the beam containing the other of the two optical axes in a second plane containing the other optical axis and parallel to the first plane.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of International Application PCT/JP/2010/068672, with an international filing date of Oct. 22, 2010, which is hereby incorporated by reference herein in its entirety. This application claims the benefit of Japanese Patent Application No. 2009-244658, the content of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to stereoscopic image-capturing objective optical systems and endoscopes. 
     BACKGROUND ART 
     Conventionally known stereoscopic image-capturing optical systems capture two parallax images of a single subject by dividing a single image-capturing surface into two (for example, see PTLs 1 and 2). In PTLs 1 and 2, two parallax images arranged in a direction perpendicular to the direction in which parallax occurs are captured. Thus, a stereoscopic image can be captured without sacrificing the resolution in the direction in which parallax occurs, which is important in stereoscopic image-capturing. 
     CITATION LIST 
     Patent Literature 
     
         
         {PTL 1} Japanese Unexamined Patent Application, Publication No. Hei 8-234339 
         {PTL 2} Japanese Unexamined Patent Application, Publication No. 2004-4869 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     The present invention provides a stereoscopic image-capturing objective optical system and an endoscope that are reduced in size, suppress tilting of two parallax images with ease, and can capture a bright stereoscopic image. 
     Solution to Problem 
     A first aspect of the present invention is a stereoscopic image-capturing objective optical system including a first prism pair that converts beams emitted from a single subject and having two substantially parallel optical axes arranged side-by-side in one direction with a certain distance therebetween into beams arranged side-by-side in a direction intersecting the aforementioned side-by-side direction with a certain distance therebetween; and a second prism pair that performs conversion to reduce the distance between the optical axes of the two beams converted by the first prism pair and that has exit surfaces arranged side-by-side in a direction perpendicular to the side-by-side arrangement direction before entering the first prism pair. 
     A second aspect of the present invention is an endoscope having the above stereoscopic image-capturing objective optical system at the tip of an insertion portion thereof. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an (XZ) plan view showing the overall configuration of a stereoscopic image-capturing objective optical system according to an embodiment of the present invention. 
         FIG. 2  is a (YZ) side view of the stereoscopic image-capturing objective optical system in  FIG. 1 . 
         FIG. 3  is a front view of the stereoscopic image-capturing objective optical system in  FIG. 1 , viewed from an object side. 
         FIG. 4  is a front view showing an image-capturing surface of an image-capturing device that captures an image of light focused by the stereoscopic image-capturing objective optical system in  FIG. 1 . 
         FIG. 5  is an (XZ) plan view of a modification of the stereoscopic image-capturing objective optical system in  FIG. 1 . 
         FIG. 6  is a (YZ) side view of the stereoscopic image-capturing objective optical system in  FIG. 5 . 
         FIG. 7  is a front view of the stereoscopic image-capturing objective optical system in  FIG. 5 , viewed from the object side. 
         FIG. 8A  is a diagram showing the lens configuration along the XZ plane, showing a first example of this embodiment. 
         FIG. 8B  is a diagram showing the lens configuration along the YZ plane, showing the first example of this embodiment. 
         FIG. 9A  is a spherical aberration diagram in the XZ cross-section of the lens configuration shown in  FIGS. 8A and 8B . 
         FIG. 9B  is a spherical aberration diagram in the YZ cross-section of the lens configuration shown in  FIGS. 8A and 8B . 
         FIG. 9C  is an aberration diagram showing astigmatism with the lens configuration shown in  FIGS. 8A and 8B , the solid line corresponding to the sagittal direction (YZ direction), and the broken line corresponding to the meridional direction (XZ direction). 
         FIG. 9D  is a distortion diagram in the diagonal direction of the lens configuration shown in  FIGS. 8A and 8B . 
         FIG. 9E  is a magnification chromatic aberration diagram in the diagonal direction of the lens configuration shown in  FIGS. 8A and 8B . 
         FIG. 10A  is a diagram showing the lens configuration along the XZ plane, showing a second example of this embodiment. 
         FIG. 10B  is a diagram showing the lens configuration along the YZ plane, showing the second example of this embodiment. 
         FIG. 11A  is a spherical aberration diagram in the XZ cross-section of the lens configuration shown in  FIGS. 10A and 10B . 
         FIG. 11B  is a spherical aberration diagram in the YZ cross-section of the lens configuration shown in  FIGS. 10A and 10B . 
         FIG. 11C  is an aberration diagram showing astigmatism with the lens configuration shown in  FIGS. 10A and 10B , the solid line corresponding to the sagittal direction (YZ direction), and the broken line corresponding to the meridional direction (XZ direction). 
         FIG. 11D  is a distortion diagram in the diagonal direction of the lens configuration shown in  FIGS. 10A and 10B . 
         FIG. 11E  is a magnification chromatic aberration diagram in the diagonal direction of the lens configuration shown in  FIGS. 10A and 10B . 
         FIG. 12A  is a diagram showing the lens configuration along the XZ plane, showing a third example of this embodiment. 
         FIG. 12B  is a diagram showing the lens configuration along the YZ plane, showing the third example of this embodiment. 
         FIG. 13A  is a spherical aberration diagram in the XZ cross-section of the lens configuration shown in  FIGS. 12A and 12B . 
         FIG. 13B  is a spherical aberration diagram in the YZ cross-section of the lens configuration shown in  FIGS. 12A and 12B . 
         FIG. 13C  is an aberration diagram showing astigmatism with the lens configuration shown in  FIGS. 12A and 12B , the solid line corresponding to the sagittal direction (YZ direction), and the broken line corresponding to the meridional direction (XZ direction). 
         FIG. 13D  is a distortion diagram in the diagonal direction of the lens configuration shown in  FIGS. 12A and 12B . 
         FIG. 13E  is a magnification chromatic aberration diagram in the diagonal direction of the lens configuration shown in  FIGS. 12A and 12B . 
         FIG. 14A  is a diagram showing the lens configuration along the XZ plane, showing a fourth example of this embodiment. 
         FIG. 14B  is a diagram showing the lens configuration along the YZ plane, showing the fourth example of this embodiment. 
         FIG. 15A  is a spherical aberration diagram in the XZ cross-section of the lens configuration shown in  FIGS. 14A and 14B . 
         FIG. 15B  is a spherical aberration diagram in the YZ cross-section of the lens configuration shown in  FIGS. 14A and 14B . 
         FIG. 15C  is an aberration diagram showing astigmatism with the lens configuration shown in  FIGS. 14A and 14B , the solid line corresponding to the sagittal direction (YZ direction), and the broken line corresponding to the meridional direction (XZ direction). 
         FIG. 15D  is a distortion diagram in the diagonal direction of the lens configuration shown in  FIGS. 14A and 14B . 
         FIG. 15E  is a magnification chromatic aberration diagram in the diagonal direction of the lens configuration shown in  FIGS. 14A and 14B . 
         FIG. 16A  is a diagram showing the lens configuration along the XZ plane, showing a fifth example of this embodiment. 
         FIG. 16B  is a diagram showing the lens configuration along the YZ plane, showing the fifth example of this embodiment. 
         FIG. 17A  is a spherical aberration diagram in the XZ cross-section of the lens configuration shown in  FIGS. 16A and 16B . 
         FIG. 17B  is a spherical aberration diagram in the YZ cross-section of the lens configuration shown in  FIGS. 16A and 16B . 
         FIG. 17C  is an aberration diagram showing astigmatism with the lens configuration shown in  FIGS. 16A and 16B , the solid line corresponding to the sagittal direction (YZ direction), and the broken line corresponding to the meridional direction (XZ direction). 
         FIG. 17D  is a distortion diagram in the diagonal direction of the lens configuration shown in  FIGS. 16A and 16B . 
         FIG. 17E  is a magnification chromatic aberration diagram in the diagonal direction of the lens configuration shown in  FIGS. 16A and 16B . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A stereoscopic image-capturing objective optical system and an endoscope according to an embodiment of the present invention will be described below with reference to the drawings. 
     A stereoscopic image-capturing objective optical system  1  according to this embodiment is disposed at the tip of an insertion portion of an endoscope and, as shown in  FIGS. 1 to 3 , includes a pair of first lens groups  2  disposed on the object side, a pair of first prisms (a first prism pair)  3  that shift two beams passing through the pair of first lens groups  2 , a pair of second lens groups  4  that allow the two beams passing through the pair of first prisms  3  to pass therethrough, and a pair of second prisms (a second prism pair)  5  that shift the two beams passing through the pair of second lens groups  4  such that their optical axes approach each other. 
     As shown in  FIGS. 1 and 3 , the first lens groups  2  are arranged side-by-side in one direction and have negative refractive power. Thus, the first lens groups  2  focus light emitted from a large area of a subject disposed on the object side and form two substantially collimated beams. The two beams formed by the pair of first lens groups  2  are substantially parallel to each other with a certain distance therebetween. 
     The prisms  3  constituting the first prism pair are each a parallelogram prism composed of a six-sided parallelepiped. Each prism  3  includes an entrance surface  3   a , an exit surface  3   b , which are parallel to each other, and two parallel reflecting surfaces  3   c  disposed between the entrance surface  3   a  and the exit surface  3   b . When the beams formed by the first lens groups  2  enter the prisms  3  from the entrance surfaces  3   a  of the prisms  3  constituting the first prism pair, the beams are reflected twice by the two reflecting surfaces  3   c  in the prisms  3  and exit from the exit surfaces  3   b.    
     As shown in  FIGS. 1 to 3 , the entrance surfaces  3   a  of the prisms  3 , constituting the first prism pair, are disposed such that their center positions are aligned with the optical axes of the pair of first lens groups  2 . Furthermore, the exit surfaces  3   b  of the prisms  3  are disposed such that their center positions are arranged side-by-side in a direction perpendicular to the direction in which the pair of first lens groups  2  are arranged side-by-side. That is, the pair of first prisms  3  convert the beams so as to rotate the side-by-side direction of the optical axes of the pair of first lens groups  2  by 90°. 
     Furthermore, the pair of second lens groups  4  have positive refractive power that focuses the beams that exit from the exit surfaces  3   b  of the prisms  3  of the first prism pair. Furthermore, a plurality of lenses are arranged in each second lens group  4 , and one or more of these lenses has a toric surface. A toric surface gives a beam passing therethrough different magnifications in two directions perpendicular to each other. In this embodiment, a smaller magnification is given in the direction perpendicular to the side-by-side direction of the first lens groups  2 . 
     The prisms  5  constituting the second prism pair are also each a parallelogram prism composed of a six-sided parallelepiped. Each prism  5  includes an entrance surface  5   a,  an exit surface  5   b , which are parallel to each other, and two parallel reflecting surfaces  5   c  disposed between the entrance surface  5   a  and the exit surface  5   b . When the beams formed by the second lens groups  4  enter the prisms  5  from the entrance surfaces  5   a  of the prisms  5  constituting the second prism pair, the beams are reflected twice by the two reflecting surfaces  5   c  in the prisms  5  and exit from the exit surfaces  5   b.    
     Furthermore, as shown in  FIGS. 1 to 3 , the entrance surfaces  5   a  of the prisms  5 , constituting the second prism pair, are disposed such that their center positions are aligned with the optical axes of the pair of second lens groups  4 . Furthermore, the exit surfaces  5   b  of the prisms  5  are disposed so as to reduce the distance between the optical axes of the beams that exit from the pair of second lens groups  4 . That is, by passing through the pair of second prisms  5 , the two beams passing through the pair of second lens groups  4  exit from the exit surfaces  5   b  of the prisms  5  without a change in the side-by-side direction but with a reduced distance between the optical axes. 
     The beams that exit from the second prism pair directly enter an image-capturing surface  6   a  of an image-capturing device  6 . That is, as shown in  FIG. 4 , beams C 1  and C 2 , which are formed to have a cross-sectional shape long in the side-by-side direction of the first lens groups  2  and short in the direction perpendicular thereto by the pair of second lens groups  4 , enter the image-capturing surface  6   a  such that they are next to each other in the direction perpendicular to the side-by-side direction of the first lens groups  2 . 
     An operation of the thus-configured stereoscopic image-capturing objective optical system  1  according to this embodiment will be described below. 
     In the stereoscopic image-capturing objective optical system  1  according to this embodiment, light emitted from a subject and entering the pair of first lens groups  2  having optical axes separated by a certain distance exits as substantially parallel parallax beams. The beams that exit from the first lens groups  2  enter the entrance surfaces  3   a  of the parallelogram prisms  3  constituting the first prism pair, disposed on the downstream side thereof. 
     Because the parallelogram prisms  3  each have the two reflecting surfaces  3   c  fabricated with precise parallellism, the beam that enters from the entrance surface  3   a  is reflected twice by the two reflecting surfaces  3   c  in the prism  3  and exits from the exit surface  3   b . Because the exit surfaces  3   b  are arranged side-by-side in the direction perpendicular to the side-by-side direction of the entrance surfaces  3   a , the two beams that exit from the two exit surfaces  3   b  are rotated by 90° from the side-by-side direction when they enter the two entrance surfaces  3   a.    
     In this case, the distance between the center positions of the two entrance surfaces  3   a  and the distance between the center positions of the two exit surfaces  3   b  are both relatively large. Therefore, the two parallelogram prisms  3  can be disposed without interfering with each other, even if they are disposed obliquely with respect to the side-by-side direction of the entrance surfaces  3   a  such that the side-by-side direction of the optical axes is rotated by 90°. 
     Furthermore, the optical axes of the beams that exit from the exit surfaces  3   b  of the parallelogram prisms  3  are precisely parallel to the optical axes of the beams when entering the entrance surfaces  3   a . In this case, in this embodiment, by using the parallelogram prisms  3 , the parallelism between the two reflecting surfaces  3   c  is not affected by the mounting accuracy of the parallelogram prisms  3 . Accordingly, even if there is an error in mounting the parallelogram prisms  3 , the parallelism between the entrance optical axes and the exit optical axes can be precisely maintained. 
     Then, the two beams that exit from the exit surfaces  3   b  of the parallelogram prisms  3  enter the pair of second lens groups  4 , where the beams are focused by the positive refractive power. Because one or more of the lenses in the second lens groups  4  has a toric surface, the two beams are converted into beams magnified by different magnifications in two directions perpendicular to each other and thus have a flat cross-section before entering the entrance surfaces  5   a  of the prisms  5  constituting the second prism pair. 
     In this case, in this embodiment, because the two parallax beams separated by a certain distance pass through the pair of second lens groups  4 , a relatively large diameter of the beams can be ensured. Accordingly, the F number can be reduced to obtain a bright stereoscopic image. 
     Because the second prism pair is also composed of the two parallelogram prisms  5 , the beams that enter from the entrance surfaces  5   a  are reflected twice by the two reflecting surfaces  5   c  in the prisms  5  and exit from the exit surfaces  5   b . The optical axes of the beams that exit from the exit surfaces  5   b  of the parallelogram prisms  5  are precisely parallel to the optical axes of the beams when entering the entrance surfaces  5   a . Thus, even if there is an error in mounting the parallelogram prisms  5 , the parallelism between the entrance optical axes and the exit optical axes can be precisely maintained. 
     Furthermore, the long sides of the horizontally elongated rectangular exit surfaces  5   b  of the pair of second prisms  5  are disposed adjacent to each other to reduce the distance between the optical axes only in the direction perpendicular to the side-by-side direction of the entrance optical axes at the pair of first lens groups  2 . This configuration enables the parallax beams to enter the adjacent areas of the image-capturing surface  6   a  of the image-capturing device  6  located downstream of the exit surfaces  5   b  and facing thereto, as shown in  FIG. 4 , and enables an image to be captured. 
     Although the two prisms  5  constituting the second prism pair are disposed such that the center positions of the exit surfaces  5   b  are close enough to each other in this case, because the prisms  5  reduce the distance between the optical axes only in one direction, they can be disposed without interfering with each other. Thus, by reducing the distance between the optical axes of the two beams with the second prism pair in this manner, stereoscopic image-capturing can be performed using the small image-capturing device  6 . 
     As has been described above, with the objective optical system  1  for stereoscopic observation according to this embodiment, there is no need to precisely adjust the angle of the mirrors compared with a conventional optical system employing a plurality of mirrors for reflection, and thus, the prisms  3  and  5  can be easily positioned. Accordingly, a precise position-adjusting mechanism is unnecessary, which makes the system compact. This also reduces the diameter of an insertion portion of an endoscope having the objective optical system  1  for stereoscopic observation according to this embodiment at the tip thereof. 
     Furthermore, because the parallelism between the entrance optical axes and the exit optical axes of the parallelogram prisms  3  and  5  is not degraded even with easy positioning, tilting of an image at the image-capturing surface  6   a  can be prevented. 
     Furthermore, even if the diameter of the beams is increased, the beams can be guided to the image-capturing surface  6   a  without interfering with each other. This leads to an advantage in that the F number can be reduced to capture a bright stereoscopic image. 
     In this embodiment, the side-by-side direction of the two beams is rotated by 90° in the first prisms  3  and is not rotated in the second prisms  5 . Instead of this, the first and second prisms  3  and  5  may share the rotation in the side-by-side direction to achieve a total rotation angle of 90°. In this case, it is preferable that the first prisms  3  be rotated by a larger angle than the second prisms  5 . The reason for this is that, because the second prisms  5  have the exit surfaces  5   a  that are close to each other, the prisms  5  interfere with each other when the side-by-side direction is rotated by a larger angle, requiring removal of the interfering portions and making the shape of the prisms complex. 
     Furthermore, in this embodiment, although the first and second prisms  3  and  5  are each formed of one prism pair, they may be each formed of two or more prism pairs, as shown in  FIGS. 5 to 7 . In the example shown in  FIGS. 5 to 7 , the prisms  3  constituting the first prism pair shift the beams in the direction perpendicular to the side-by-side direction of the two beams, thereby rotating the side-by-side direction by an angle smaller than 90°. Furthermore, the pair of second prisms  5  and a pair of third prisms  7  employed herein shift the beams in the direction parallel to and in the direction perpendicular to the side-by-side direction of the two beams before entering the first prisms  3 , respectively. 
     By doing so, similarly to the above-described embodiment, it is possible to employ the third prisms  7  that have exit surfaces  7   b  facing the image-capturing surface  6   a  and have a function only to reduce the distance between the beams in the direction perpendicular to the side-by-side direction of the optical axes that enters the pair of first lens groups  2 , which can simplify the configuration. Furthermore, even if the positions of two reflecting surfaces  7   c  of the parallelogram prism  7  are shifted as a result of them being easily positionable, the parallelism between the entrance optical axes and the exit optical axes of the parallelogram prism  7  is not degraded. Thus, tilting of an image at the image-capturing surface  6   a  can be prevented. 
     Note that the configuration of the prisms is not limited to that in the above-described embodiment, and a modification is possible such that the first prisms  3  shift the beams in the direction perpendicular to the side-by-side direction of the two beams and the second prisms  5  shift the two beams such that the beams are arranged side-by-side in the direction perpendicular to the side-by-side direction thereof before entering the first prisms  3 . 
     EXAMPLES 
     Now, examples of the stereoscopic image-capturing objective optical system  1  according to this embodiment will be described below with reference to the drawings. In each example, among the two pairs of lens groups  2  and  4  and the two pairs of prisms  3  and  5  (or the three pairs of prisms  3 ,  5 , and  7 ), drawings and lens data are shown with respect to one of the lens groups  2  and  4  and prisms  3  and  5  (or one of the prisms  3 ,  5 , and  7 ), and the description of the other of the lens groups  2  and  4  and the prisms  3  and  5  will be omitted. 
     Example 1 
     Lens configuration diagrams of the stereoscopic image-capturing objective optical system  1  according to Example 1 are shown in  FIGS. 8A and 8B , and the lens data thereof are shown below. Furthermore, aberration diagrams of the objective lens of this example are shown in  FIGS. 9A to 9E .  FIG. 8A  is a lens configuration diagram along the XZ plane, and  FIG. 8B  is a lens configuration diagram along the YZ plane. 
       FIG. 9A  is a spherical aberration diagram in the XZ cross-section,  FIG. 9B  is a spherical aberration diagram in the YZ cross-section, and  FIG. 9C  is a diagram of astigmatism, in which the solid line represents aberration in the sagittal direction (YZ direction), and the broken line represents aberration in the meridional direction (XZ direction),  FIG. 9D  is a distortion diagram in the diagonal direction, and  FIG. 9E  is a magnification chromatic aberration diagram in the diagonal direction. Furthermore, in  FIGS. 9A and 9E , the solid line represents aberration at the e-line (546.07 nm), the one-dot chain line represents aberration at the F-line (486.13 nm), and the broken line represents aberration at the C-line (656.27 nm). 
     
       
         
               
             
               
               
               
               
               
               
             
               
             
               
               
               
               
             
           
               
                   
               
             
             
               
                 Surface Data 
               
             
          
           
               
                   
                 surface number 
                 r 
                 d 
                 ne 
                 vd 
               
               
                   
                   
               
               
                   
                 object plane 
                 ∞ 
                 26 
               
               
                   
                  1 
                 6.4 
                 0.388 
                 1.88815 
                 40.76 
               
               
                   
                  2 
                 1.89 
                 0.836 
               
               
                   
                  3$ 
                 5.833 
                 0.47 
                 2.01169 
                 28.27 
               
               
                   
                  4$ 
                 3.016 
                 0.537 
               
               
                   
                  5 
                 ∞ 
                 3.066 
                 1.77621 
                 49.6 
               
               
                   
                  6 
                 ∞ 
                 0.135 
               
               
                   
                  7$ 
                 2.254 
                 0.339 
                 1.77621 
                 49.6 
               
               
                   
                  8$ 
                 1.624 
                 0.321 
               
               
                   
                  9 
                 ∞ 
                 0.4 
                 1.77621 
                 49.6 
               
               
                   
                 10 
                 ∞ 
                 0.2 
               
               
                   
                 11 (stop) 
                 ∞ 
                 0.213 
               
               
                   
                 12 
                 −11.144 
                 0.342 
                 1.93429 
                 18.9 
               
               
                   
                 13 
                 −3.629 
                 0.521 
               
               
                   
                 14 
                 11.974 
                 1.178 
                 1.48915 
                 70.23 
               
               
                   
                 15 
                 −1.646 
                 0.389 
                 1.85504 
                 23.78 
               
               
                   
                 16 
                 −2.247 
                 0.11 
               
               
                   
                 17 
                 ∞ 
                 2.793 
                 1.88815 
                 40.76 
               
               
                   
                 18 
                 ∞ 
                 0.11 
               
               
                   
                 19 
                 10.8766 
                 1.632 
                 1.77621 
                 49.6 
               
               
                   
                 20 
                 −2.109 
                 0.318 
                 1.93429 
                 18.9 
               
               
                   
                 21 
                 −24.357 
                 0.107 
               
               
                   
                 22 
                 ∞ 
                 0.376 
                 1.51564 
                 75 
               
               
                   
                 23 
                 ∞ 
                 0.218 
               
               
                   
                 24 
                 ∞ 
                 2.8 
                 1.51825 
                 64.14 
               
               
                   
                 25 
                 ∞ 
                 0.97 
                 1.50801 
                 60 
               
               
                   
                 image plane 
                 ∞ 
                 0 
               
               
                   
                   
               
             
          
           
               
                 Aspherical Surface Data 
               
             
          
           
               
                   
                   
                 RDX 
                 RDY 
               
               
                   
                   
               
               
                   
                 third surface TOC 
                 5.833 
                 −9.04 
               
               
                   
                 fourth surface TOC 
                 3.016 
                 1.478 
               
               
                   
                 seventh surface TOC 
                 2.254 
                 1.502 
               
               
                   
                 eighth surface TOC 
                 1.624 
                 1.354 
               
               
                   
                   
               
             
          
         
       
     
     Example 2 
     Lens configuration diagrams of the stereoscopic image-capturing objective optical system according to Example 2 are shown in  FIGS. 10A and 10B , and the lens data thereof are shown below. Furthermore, aberration diagrams of the objective lens of this example are shown in  FIGS. 11A to 11E .  FIG. 10A  is a lens configuration diagram along the XZ plane, and  FIG. 10B  is a lens configuration diagram along the YZ plane. 
       FIG. 11A  is a spherical aberration diagram in the XZ cross-section,  FIG. 11B  is a spherical aberration diagram in the YZ cross-section, and  FIG. 11C  is a diagram of astigmatism, in which the solid line represents aberration in the sagittal direction (YZ direction), and the broken line represents aberration in the meridional direction (XZ direction),  FIG. 11D  is a distortion diagram in the diagonal direction, and  FIG. 11E  is a magnification chromatic aberration diagram in the diagonal direction. Furthermore, in  FIGS. 11A and 11E , the solid line represents aberration at the e-line (546.07 nm), the one-dot chain line represents aberration at the F-line (486.13 nm), and the broken line represents aberration at the C-line (656.27 nm). 
     
       
         
               
             
               
               
               
               
               
               
             
               
             
               
               
               
               
             
           
               
                   
               
             
             
               
                 Surface Data 
               
             
          
           
               
                   
                 surface number 
                 r 
                 d 
                 ne 
                 vd 
               
               
                   
                   
               
               
                   
                 object plane 
                 ∞ 
                 26 
               
               
                   
                  1 
                 12.747 
                 0.388 
                 1.88815 
                 40.76 
               
               
                   
                  2 
                 2.57 
                 0.435 
               
               
                   
                  3$ 
                 2.779 
                 0.421 
                 2.01169 
                 28.27 
               
               
                   
                  4$ 
                 1.382 
                 0.411 
               
               
                   
                  5 
                 ∞ 
                 2 
                 1.77621 
                 49.6 
               
               
                   
                  6 
                 ∞ 
                 0.078 
               
               
                   
                  7$ 
                 1.676 
                 0.672 
                 1.77621 
                 49.6 
               
               
                   
                  8$ 
                 1.348 
                 0.287 
               
               
                   
                  9 
                 ∞ 
                 0.3 
                 1.51825 
                 64.14 
               
               
                   
                 10 
                 ∞ 
                 0.2 
               
               
                   
                 11 (stop) 
                 ∞ 
                 0.19 
               
               
                   
                 12 
                 −33.73 
                 0.405 
                 1.93429 
                 18.9 
               
               
                   
                 13 
                 −3.05 
                 0.628 
               
               
                   
                 14 
                 29.794 
                 1.506 
                 1.48915 
                 70.23 
               
               
                   
                 15 
                 −1.292 
                 0.378 
                 1.85504 
                 23.78 
               
               
                   
                 16 
                 −1.895 
                 0.098 
               
               
                   
                 17 
                 19.637 
                 0.924 
                 1.77621 
                 49.6 
               
               
                   
                 18 
                 −2.655 
                 0.267 
                 1.93429 
                 18.9 
               
               
                   
                 19 
                 −29.47 
                 0.418 
               
               
                   
                 20 
                 ∞ 
                 3.56 
                 1.88815 
                 40.76 
               
               
                   
                 21 
                 ∞ 
                 0.565 
               
               
                   
                 22 
                 ∞ 
                 1.1 
                 1.51825 
                 64.14 
               
               
                   
                 23 
                 ∞ 
                 0.7 
                 1.50801 
                 60 
               
               
                   
                 image plane 
                 ∞ 
                 0 
               
               
                   
                   
               
             
          
           
               
                 Aspherical Surface Data 
               
             
          
           
               
                   
                   
                 RDX 
                 RDY 
               
               
                   
                   
               
               
                   
                 third surface TOC 
                 2.779 
                 −12.761 
               
               
                   
                 fourth surface TOC 
                 1.382 
                 1.159 
               
               
                   
                 seventh surface TOC 
                 1.676 
                 12.56 
               
               
                   
                 eighth surface TOC 
                 1.348 
                 6.743 
               
               
                   
                   
               
             
          
         
       
     
     Example 3 
     Lens configuration diagrams of the stereoscopic image-capturing objective optical system according to Example 3 are shown in  FIGS. 12A and 12B , and the lens data thereof are shown below. Furthermore, aberration diagrams of the objective lens of this example are shown in  FIGS. 13A to 13E .  FIG. 12A  is a lens configuration diagram along the XZ plane, and  FIG. 12B  is a lens configuration diagram along the YZ plane. 
       FIG. 13A  is a spherical aberration diagram in the XZ cross-section,  FIG. 13B  is a spherical aberration diagram in the YZ cross-section, and  FIG. 13C  is a diagram of astigmatism, in which the solid line represents aberration in the sagittal direction (YZ direction), and the broken line represents aberration in the meridional direction (XZ direction),  FIG. 13D  is a distortion diagram in the diagonal direction, and  FIG. 13E  is a magnification chromatic aberration diagram in the diagonal direction. Furthermore, in  FIGS. 13A and 13E , the solid line represents aberration at the e-line (546.07 nm), the one-dot chain line represents aberration at the F-line (486.13 nm), and the broken line represents aberration at the C-line (656.27 nm). 
     
       
         
               
             
               
               
               
               
               
               
             
               
             
               
               
               
               
             
           
               
                   
               
             
             
               
                 Surface Data 
               
             
          
           
               
                   
                 surface number 
                 r 
                 d 
                 ne 
                 vd 
               
               
                   
                   
               
               
                   
                 object plane 
                 ∞ 
                 29.73 
               
               
                   
                  1 
                 29.785 
                 0.444 
                 1.88815 
                 40.76 
               
               
                   
                  2 
                 3.304 
                 0.18 
               
               
                   
                  3$ 
                 2.45 
                 0.245 
                 2.01169 
                 28.27 
               
               
                   
                  4$ 
                 1.347 
                 0.405 
               
               
                   
                  5 
                 ∞ 
                 2 
                 1.77621 
                 49.6 
               
               
                   
                  6 
                 ∞ 
                 0.101 
               
               
                   
                  7$ 
                 1.781 
                 0.92 
                 1.77621 
                 49.6 
               
               
                   
                  8$ 
                 1.391 
                 0.66 
               
               
                   
                  9 (stop) 
                 ∞ 
                 0.019 
               
               
                   
                 10 
                 −6.261 
                 0.369 
                 1.93429 
                 18.9 
               
               
                   
                 11 
                 −2.23 
                 0.44 
               
               
                   
                 12 
                 ∞ 
                 0.343 
                 1.51825 
                 64.14 
               
               
                   
                 13 
                 ∞ 
                 0.373 
               
               
                   
                 14 
                 129.693 
                 1.673 
                 1.48915 
                 70.23 
               
               
                   
                 15 
                 −1.473 
                 0.354 
                 1.85504 
                 23.78 
               
               
                   
                 16 
                 −2.037 
                 0.101 
               
               
                   
                 17 
                 −40.113 
                 0.928 
                 1.77621 
                 49.6 
               
               
                   
                 18 
                 −2.936 
                 0.243 
                 1.93429 
                 18.9 
               
               
                   
                 19 
                 −12.017 
                 0.198 
               
               
                   
                 20 
                 ∞ 
                 6.96 
                 2.01169 
                 28.27 
               
               
                   
                 21 
                 ∞ 
                 0.369 
               
               
                   
                 22 
                 ∞ 
                 0.9 
                 1.51825 
                 64.14 
               
               
                   
                 23 
                 ∞ 
                 0.7 
                 1.50801 
                 60 
               
               
                   
                 image plane 
                 ∞ 
                 0 
               
               
                   
                   
               
             
          
           
               
                 Aspherical Surface Data 
               
             
          
           
               
                   
                   
                 RDX 
                 RDY 
               
               
                   
                   
               
               
                   
                 third surface TOC 
                 2.45 
                 27.302 
               
               
                   
                 fourth surface TOC 
                 1.347 
                 1.194 
               
               
                   
                 seventh surface TOC 
                 1.781 
                 37.743 
               
               
                   
                 eighth surface TOC 
                 1.391 
                 7.041 
               
               
                   
                   
               
             
          
         
       
     
     Example 4 
     Lens configuration diagrams of the stereoscopic image-capturing objective optical system  1  according to Example 4 are shown in  FIGS. 14A and 14B , and the lens data thereof are shown below. Furthermore, aberration diagrams of the objective lens of this example are shown in  FIGS. 15A to 15E .  FIG. 14A  is a lens configuration diagram along the XZ plane, and  FIG. 14B  is a lens configuration diagram along the YZ plane. 
       FIG. 15A  is a spherical aberration diagram in the XZ cross-section,  FIG. 15B  is a spherical aberration diagram in the YZ cross-section, and  FIG. 15C  is a diagram of astigmatism, in which the solid line represents aberration in the sagittal direction (YZ direction), and the broken line represents aberration in the meridional direction (XZ direction),  FIG. 15D  is a distortion diagram in the diagonal direction, and  FIG. 15E  is a magnification chromatic aberration diagram in the diagonal direction. Furthermore, in  FIGS. 15A and 15E , the solid line represents aberration at the e-line (546.07 nm), the one-dot chain line represents aberration at the F-line (486.13 nm), and the broken line represents aberration at the C-line (656.27 nm). 
     
       
         
               
             
               
               
               
               
               
               
             
               
             
               
               
               
               
             
           
               
                   
               
             
             
               
                 Surface Data 
               
             
          
           
               
                   
                 surface number 
                 r 
                 d 
                 ne 
                 vd 
               
               
                   
                   
               
               
                   
                 object plane 
                 ∞ 
                 29.7259 
               
               
                   
                  1 
                 ∞ 
                 0.444 
                 1.88815 
                 40.76 
               
               
                   
                  2 
                 2.155 
                 0.393 
               
               
                   
                  3$ 
                 15.248 
                 0.207 
                 2.01169 
                 28.27 
               
               
                   
                  4$ 
                 4.296 
                 0.195 
               
               
                   
                  5 
                 ∞ 
                 2 
                 1.77621 
                 49.6 
               
               
                   
                  6 
                 ∞ 
                 0.069 
               
               
                   
                  7$ 
                 5.636 
                 0.347 
                 1.77621 
                 49.6 
               
               
                   
                  8$ 
                 2.994 
                 0.155 
               
               
                   
                  9 
                 3.498 
                 0.409 
                 1.93429 
                 18.9 
               
               
                   
                 10 
                 −13.631 
                 0.67 
               
               
                   
                 11 (stop) 
                 ∞ 
                 0.324 
               
               
                   
                 12 
                 ∞ 
                 0.343 
                 1.51825 
                 64.14 
               
               
                   
                 13 
                 ∞ 
                 0.329 
               
               
                   
                 14 
                 129.743 
                 1.485 
                 1.48915 
                 70.23 
               
               
                   
                 15 
                 −1.463 
                 0.228 
                 1.85504 
                 23.78 
               
               
                   
                 16 
                 −2.565 
                 0.048 
               
               
                   
                 17 
                 −84.942 
                 0.808 
                 1.77621 
                 49.6 
               
               
                   
                 18 
                 −3.054 
                 0.263 
                 1.93429 
                 18.9 
               
               
                   
                 19 
                 −4.823 
                 0.057 
               
               
                   
                 20 
                 ∞ 
                 8.4 
                 2.01169 
                 28.27 
               
               
                   
                 21 
                 ∞ 
                 0.369 
               
               
                   
                 22 
                 ∞ 
                 0.9 
                 1.51825 
                 64.14 
               
               
                   
                 23 
                 ∞ 
                 0.7 
                 1.50801 
                 60 
               
               
                   
                 image plane 
                 ∞ 
                 0 
               
               
                   
                   
               
             
          
           
               
                 Aspherical Surface Data 
               
             
          
           
               
                   
                   
                 RDX 
                 RDY 
               
               
                   
                   
               
               
                   
                 third surface TOC 
                 15.248 
                 −2.229 
               
               
                   
                 fourth surface TOC 
                 4.296 
                 1.829 
               
               
                   
                 seventh surface TOC 
                 5.636 
                 39.815 
               
               
                   
                 eighth surface TOC 
                 2.994 
                 46.6 
               
               
                   
                   
               
             
          
         
       
     
     Example 5 
     Lens configuration diagrams of the stereoscopic image-capturing objective optical system  1  according to Example 5 are shown in  FIGS. 16A and 16B , and the lens data thereof are shown below. Furthermore, aberration diagrams of the objective lens of this example are shown in  FIGS. 17A to 17E .  FIG. 16A  is a lens configuration diagram along the XZ plane, and  FIG. 16B  is a lens configuration diagram along the YZ plane. 
       FIG. 17A  is a spherical aberration diagram in the XZ cross-section,  FIG. 17B  is a spherical aberration diagram in the YZ cross-section, and  FIG. 17C  is a diagram of astigmatism, in which the solid line represents aberration in the sagittal direction (YZ direction), and the broken line represents aberration in the meridional direction (XZ direction),  FIG. 17D  is a distortion diagram in the diagonal direction, and  FIG. 17E  is a magnification chromatic aberration diagram in the diagonal direction. Furthermore, in  FIGS. 17A and 17E , the solid line represents aberration at the e-line (546.07 nm), the one-dot chain line represents aberration at the F-line (486.13 nm), and the broken line represents aberration at the C-line (656.27 nm). 
     
       
         
               
             
               
               
               
               
               
               
             
               
             
               
               
               
               
             
           
               
                   
               
             
             
               
                 Surface Data 
               
             
          
           
               
                   
                 surface number 
                 r 
                 d 
                 ne 
                 vd 
               
               
                   
                   
               
               
                   
                 object plane 
                 ∞ 
                 26.5 
               
               
                   
                  1 
                 29.051 
                 0.443 
                 1.88815 
                 40.76 
               
               
                   
                  2 
                 3.744 
                 0.296 
               
               
                   
                  3$ 
                 5.792 
                 0.24 
                 2.01169 
                 28.27 
               
               
                   
                  4$ 
                 2.457 
                 0.377 
               
               
                   
                  5 
                 ∞ 
                 6.2 
                 2.01169 
                 28.27 
               
               
                   
                  6 
                 ∞ 
                 0.01 
               
               
                   
                  7$ 
                 3.173 
                 0.332 
                 1.77621 
                 49.6 
               
               
                   
                  8$ 
                 9.972 
                 0.217 
               
               
                   
                  9 (stop) 
                 ∞ 
                 0.106 
               
               
                   
                 10 
                 −421.796 
                 0.286 
                 1.93429 
                 18.9 
               
               
                   
                 11 
                 −9.291 
                 0.425 
               
               
                   
                 12 
                 ∞ 
                 0.343 
                 1.51825 
                 64.14 
               
               
                   
                 13 
                 ∞ 
                 0.425 
               
               
                   
                 14 
                 129.676 
                 1.68 
                 1.48915 
                 70.23 
               
               
                   
                 15 
                 −1.471 
                 0.381 
                 1.85504 
                 23.78 
               
               
                   
                 16 
                 −5.445 
                 0.135 
               
               
                   
                 17 
                 32.806 
                 0.742 
                 1.77621 
                 49.6 
               
               
                   
                 18 
                 −4.321 
                 0.165 
               
               
                   
                 19 
                 ∞ 
                 4.2 
                 1.51825 
                 64.14 
               
               
                   
                 20 
                 ∞ 
                 0.369 
               
               
                   
                 21 
                 ∞ 
                 0.9 
                 1.51825 
                 64.14 
               
               
                   
                 22 
                 ∞ 
                 0.7 
                 1.50801 
                 60 
               
               
                   
                 image plane 
                 ∞ 
                 0 
               
               
                   
                   
               
             
          
           
               
                 Aspherical Surface Data 
               
             
          
           
               
                   
                   
                 RDX 
                 RDY 
               
               
                   
                   
               
               
                   
                 third surface TOC 
                 5.792 
                 −4.226 
               
               
                   
                 fourth surface TOC 
                 2.457 
                 1.899 
               
               
                   
                 seventh surface TOC 
                 3.173 
                 3.495 
               
               
                   
                 eighth surface TOC 
                 9.972 
                 92.384 
               
               
                   
                   
               
             
          
         
       
     
     The stereoscopic image-capturing objective optical system according to Examples 1 to 5 above satisfies the following Conditional Expressions (1) to (9).
 
0.4≦vertical focal length/horizontal focal length≦0.7   Conditional Expression (1)
 
−3≦first lens/horizontal focal length≦−1.5   Conditional Expression (2)
 
2≦2 −O  vertical/horizontal focal length≦7.5   Conditional Expression (3)
 
2.1≦5 −TO/ horizontal focal length≦6.6   Conditional Expression (4)
 
0.8≦second anamorphic surface  R  ratio(horizontal/vertical)≦2.75   Conditional Expression (5)
 
0≦fourth anamorphic surface  R  ratio(horizontal/vertical)≦1.6   Conditional Expression (6)
 
0.45≦first reflecting surface distance/horizontal focal length≦2.2   Conditional Expression (7)
 
0.7≦second reflecting surface distance/vertical focal length≦4.5   Conditional Expression (8)
 
1.2≦combined focal length behind stop/horizontal focal length≦2.8   Conditional Expression (9)
 
     Conditional Expression (1) is a conditional expression corresponding to the vertical and horizontal dimensions of the image-capturing surface. If the range of Conditional Expression (1) is exceeded, an unnatural image will result because the significance of the distortion differs between the vertical and horizontal directions. 
     Conditional Expression (2) is a conditional expression for correcting center astigmatism. The greater the power, the greater the center astigmatism, and the smaller the power, the greater the beam height at the first lens, leading to an increase in the size of the system. In addition, the pantoscopic lenses interfere with each other, making layout difficult. 
     Conditional Expression (3) is a conditional expression necessary for correcting vertical and horizontal coma and for matching the image plane positions by respectively controlling the vertical and horizontal coma, because the amount of field curvature in the vertical direction does not always agree with that in the horizontal direction. If the range of Conditional Expression (3) is exceeded, a change in the amount of field curvature cannot be compensated for by correction of coma, and thus, the vertical and horizontal image plane positions are not aligned. 
     Conditional Expression (4) corresponds to the power arrangement of a cemented lens and corrects axial chromatic aberration and magnification chromatic aberration. The smaller the power, the greater the magnification chromatic aberration, and the greater the power, the smaller the magnification chromatic aberration, which, however, makes correction of axial chromatic aberration difficult. 
     Conditional Expressions (5) and (6) are conditional expressions for correcting astigmatism, which represent the ranges of the R ratios of the anamorphic surfaces. If the ranges of Conditional Expressions (5) and (6) are exceeded, not only center astigmatism, but also significant peripheral astigmatism occurs. If the upper limits of Conditional Expressions (5) and (6) are exceeded, the sagittal image plane tends to be over in the horizontal direction, and the meridional image plane tends to be under. If the lower limits of Conditional Expressions (5) and (6) are exceeded, the sagittal image plane tends to be under, and the meridional image plane tends to be over in the same horizontal direction. 
     Conditional Expressions (7) and (8) are conditional expressions for introducing reflecting surfaces. At the lower limits of Conditional Expressions (7) and (8), a space big enough to introduce the reflecting surfaces, necessary in the layout, cannot be ensured. However, if the upper limits are exceeded, a space that is larger than required is used, making the overall length of the optical system too large, which is not preferable from the standpoint of the layout. 
     Conditional Expression (9) is a conditional expression for reducing the size of the entire system. If the upper limit of Conditional Expression (9) is exceeded and the power of the positive group behind the stop is small, the overall length increases, which is not preferable. If the lower limit of Conditional Expression (9) is exceeded and the power is large, the back focal length is short, making it difficult to ensure a space for providing the reflective member. 
     {Reference Signs List} 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 1: 
                 stereoscopic image-capturing objective optical system 
               
               
                 2: 
                 first lens group (a pair of negative lens groups) 
               
               
                 3:  
                 prism (first prisms) 
               
               
                 3c, 5c: 
                 reflecting surface 
               
               
                 4: 
                 second lens group (a pair of positive lens groups) 
               
               
                 5: 
                 prism (second prisms) 
               
               
                 5b: 
                 exit surface 
               
               
                 6a: 
                 image-capturing surface