Abstract:
An image-erecting viewing optical system includes a first prism having an incident surface and at least two reflection surfaces, a front lens group of an objective optical system, a second prism having an incident surface and at least two reflection surfaces, a-rear lens group of the objective optical system, a field stop, and an eyepiece optical system, in this order from the object. One reflection surface of the first or second prism includes a roof-mirror surface. The following conditions are satisfied:  
     sin(θ−ω′)&gt;1/ n    (1)  
     6°&lt;ω&lt;16°  (2)  
     wherein  
     θ designates an angle between the incident surface and the first reflection surface of the first prism;  
     ω designates a real field-of-view ω (half amount);  
     ω′ designates an angle between a light ray of a real field-of-view ω (half amount) and the optical axis within the first prism; and  
     n designates the refractive index of the first prism.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an image-erecting viewing optical system having a large apparent visual angle and a low magnification.  
           [0003]    2. Description of the Prior Art  
           [0004]    An image-erecting viewing optical system used in, for example, a binocular or a monocular, generally includes an objective optical system, an image-erecting optical system, and an eyepiece optical system, in this order from the object. In an image-erecting optical system having a wide real field-of-view at a low magnification, there is a need to make the focal length of the objective optical system shorter; however, in an optical arrangement in which the objective optical system is provided in front of the image-erecting optical system, the focal length of the objective optical system has to be made longer by the optical path length of the image-erecting optical system. Consequently, the focal length of the eyepiece optical system has also to be made longer, so that miniaturization of the entire optical system becomes difficult.  
         SUMMARY OF THE INVENTION  
         [0005]    The present invention provides a miniaturized image-erecting optical system having a particularly wide real field-of-view and low magnification.  
           [0006]    As an aspect of the present invention, there is provided an image-erecting viewing optical system including a first prism having an incident surface and at least two reflection surfaces, a front lens group of an objective optical system, a second prism having an incident surface and at least two reflection surfaces, a rear lens group of the objective optical system, a field stop, and an eyepiece optical system, in this order from the object. One reflection surface of one of the first prism and second prism includes a roof-mirror surface, and satisfies the following conditions:  
           sin(θ−ω′)&gt;1 /n    (1)  
           6°&lt;ω&lt;16°  (2)  
           [0007]    wherein  
           [0008]    θ designates an angle between the incident surface and the first reflection surface of the first prism;  
           [0009]    ω designates a real field-of-view ω (half amount)  
           [0010]    ω′ designates an angle between a light ray of a real field-of-view ω (half amount) and the optical axis in the first prism; and  
           [0011]    n designates the refractive index of the first prism.  
           [0012]    On the other hand, in the case where the incident surface and the reflection surface of the first prism are the same surface, the first prism satisfies the following conditions:  
           sin(2α−ω′)&gt;1 /n    (3)  
           6°&lt;ω&lt;16°  (2)  
           [0013]    wherein  
           [0014]    α designates an angle between the incident surface and the first reflection surface of the first prism;  
           [0015]    ω designates a real field-of-view ω (half amount);  
           [0016]    ω′ designates an angle between a light ray of a real field-of-view ω (half amount) and the optical axis in the first prism; and  
           [0017]    n designates the refractive index of the first prism.  
           [0018]    In the above optical arrangement, focusing is performed by moving at least one of the rear lens group of the objective optical system and the eyepiece optical system.  
           [0019]    As another aspect of the present invention, there is provided an image-erecting viewing optical system including a first prism, a front lens group of an objective optical system, a second prism, a rear lens group of the objective optical system, a field stop, and an eyepiece optical system, in this order from the object. The image-erecting viewing optical system satisfies the following condition in the case where the rear lens group of the objective optical system is a focusing lens group:  
           0 &lt;fO/f 2&lt;0.5   (4)  
           [0020]    wherein  
           [0021]    f2 designates the focal length of the rear lens group of the objective optical system; and  
           [0022]    fO designates the focal length of the entire the objective optical system.  
           [0023]    The present disclosure relates to subject matter contained in Japanese Patent Application No. 2001-370584 (filed on Dec. 4, 2001) which is expressly incorporated herein in its entirety. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    The present invention will be discussed below in detail with reference to the accompanying drawings, in which:  
         [0025]    [0025]FIG. 1 is an optical structure of an image-erecting optical system according to a first embodiment of the present invention;  
         [0026]    [0026]FIG. 2 is an optical structure of the image-erecting optical system according to a second embodiment of the present invention;  
         [0027]    [0027]FIG. 3 is an optical structure of the image-erecting optical system according to a third embodiment of the present invention;  
         [0028]    [0028]FIG. 4 is an optical structure of the image-erecting optical system according to a fourth embodiment of the present invention;  
         [0029]    [0029]FIG. 5 is an optical structure of the image-erecting optical system according to a fifth embodiment of the present invention;  
         [0030]    [0030]FIG. 6 is an optical structure of the image-erecting optical system according to a sixth embodiment of the present invention;  
         [0031]    [0031]FIG. 7 is a sectional view along the arrows VII-VII shown in FIGS. 1, 2,  3 ,  4 ,  5  and  6 .  
         [0032]    [0032]FIG. 8 is a lens arrangement of the image-erecting optical system according to a first numerical embodiment;  
         [0033]    [0033]FIGS. 9A, 9B,  9 C and  9 D show aberrations occurred in the lens arrangement shown in FIG. 8, when an object at an infinite distance is in an in-focus state;  
         [0034]    [0034]FIGS. 10A, 10B,  10 C and  10 D show aberrations occurred in the lens arrangement shown in FIG. 8, when an object at a distance of 1.2 m is in an in-focus state;  
         [0035]    [0035]FIG. 11 is a lens arrangement of the image-erecting optical system according to a second numerical embodiment;  
         [0036]    [0036]FIGS. 12A, 12B,  12 C and  12 D show aberrations occurred in the lens arrangement shown in FIG. 11, when an object at an infinite distance is in an in-focus state;  
         [0037]    [0037]FIGS. 13A, 13B,  13 C and  13 D show aberrations occurred in the lens arrangement shown in FIG. 11, when an object at a distance of 1.5 m is in an in-focus state;  
         [0038]    [0038]FIG. 14 is a lens arrangement of the image-erecting optical system according to a third numerical embodiment;  
         [0039]    [0039]FIGS. 15A, 15B,  15 C and  15 D show aberrations occurred in the lens arrangement shown in FIG. 14, when an object at an infinite distance is in an in-focus state; and  
         [0040]    [0040]FIGS. 16A, 16B,  16 c and  16 D show aberrations occurred in the lens arrangement shown in FIG. 14, when an object at a distance of 1.5 m is in an in-focus state. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0041]    [0041]FIGS. 1 through 7 show the optical structure of the image-erecting optical system according to the first through sixth embodiments. More specifically, in each of the first through sixth embodiments, the image-erecting optical system includes a first prism  10  ( 110 ,  210 ,  310 ,  410 ,  510 ), a front lens group  21  of an objective optical system  20  ( 120 ,  220 ,  320 ,  420 ,  520 ), a second prism  30  ( 130 ,  230 ,  330 ,  430 ,  530 ), a rear lens group (focusing lens group)  22  of the objective optical system  20  ( 120 ,  220 ,  320 ,  420 ,  520 ), a field stop  40 , and an eyepiece optical system  50 , in this order from the object.  
         [0042]    In the first through sixth embodiments, a reflection surface of either first prism  10  ( 110 ,  210 ,  310 ,  410 ,  510 ) or the second prism  30  ( 130 ,  230 ,  330 ,  430 ,  530 ) is a roof-mirror surface, i.e., either of the first prism  10  or the second prism  30  is a roof prism. Furthermore, in each of the first through sixth embodiments, there are five reflection surfaces, in total, in the first prism  10  ( 110 ,  210 ,  310 ,  410 ,  510 ) and the second prism  30  ( 130 ,  230 ,  330 ,  430 ,  530 ); and one of the five reflection surfaces is a roof-mirror surface. The field stop  40  is provided at a position where an image of an object at infinity is formed through the objective optical system  20  ( 120 ,  220 ,  320 ,  420 ,  520 ).  
         [0043]    Specifically, in the first embodiment shown in FIG. 1, the first prism  10  includes an incident surface  10   i , a first reflection surface  10 - 1   r , a second reflection surface  10 - 2   r , and an exit surface  10   e . The second prism  30  includes an incident surface  30   i , a first reflection surface  30 - 1   r , a second reflection surface (roof-mirror surface)  30 - 2   r  (D), a third reflection surface  30 - 3   r , and an exit surface  30   e.    
         [0044]    The exit surface  10   e  and the first reflection surface  10 - 1   r  are the same surface. The incident surface  30   i  and the third reflection surface  30 - 3   r  are the same surface; and likewise, the first reflection surface  30 - 1   r  and the exit surface  30   e  are the same surface.  
         [0045]    In the second embodiment shown in FIG. 2, a first prism  110  includes an incident surface  110   i , a first reflection surface  110 - 1   r , a second reflection surface (roof-mirror surface)  110 - 2   r  (D), a third reflection surface  110 - 3   r , and an exit surface  110   e . The second prism  130  includes a incident surface  130   i , a first reflection surface  130 - 1   r , a second reflection surface  130 - 2   r , and an exit surface  130   e.    
         [0046]    The incident surface  110   i  and the third reflection surface  110 - 3   r  are the same surface; and the exit surface  110   e  and the first reflection surface  110 - 1   r  are the same surface. Likewise, the incident surface  130   i  and the second reflection surface  130 - 2   r  are the same surface.  
         [0047]    In the third embodiment shown in FIG. 3, a first prism  210  includes an incident surface  210   i , a first reflection surface  210 - 1   r , a second reflection surface (roof-mirror surface)  210 - 2   r  (D), and an exit surface  210   e . A second prism  230  includes an incident surface  230   i , a first reflection surface  230 - 1   r , a second reflection surface  230 - 2   r , a third reflection surface  230 - 3   r , and an exit surface  230   e.    
         [0048]    The first reflection surface  210 - 1   r  and the exit surface  210   e  are the same surface; and the incident surface  230   i  and the second reflection surface  230 - 2   r  are the same surface.  
         [0049]    In the fourth embodiment shown in FIG. 4, a first prism  310  includes an incident surface  310   i , a first reflection surface  310 - 1   r , a second reflection surface (roof-mirror surface)  310 - 2   r  (D), a third reflection surface  310 - 3   r , and an exit surface  310   e . A second prism  330  includes an incident surface  330   i , a first reflection surface  330 - 1   r , a second reflection surface  330 - 2   r , and an exit surface  330   e.    
         [0050]    The incident surface  310   i  and the third reflection surface  310 - 3   r  are the same surface; and the first reflection surface  310 - 1   r  and the exit surface  310   e  are the same surface.  
         [0051]    In the fifth embodiment shown in FIG. 5, a first prism  410  includes an incident surface  410   i , a first reflection surface  410 - 1   r , a second reflection surface  410 - 2   r , and an exit surface  410   e . A second prism  430  includes an incident surface  430   i , a first reflection surface  430 - 1   r , a second reflection surface (roof-mirror surface)  430 - 2   r  (D), a third reflection surface  430 - 3   r , and an exit surface  430   e.    
         [0052]    The incident surface  410   i  and the second reflection surface  410 - 2   r  are the same surface. The incident surface  430   i  and the third reflection surface  430 - 3   r  are the same surface. The first reflection surface  430 - 1   r  and the exit surface  430   e  are the same surface.  
         [0053]    In the sixth embodiment shown in FIG. 6, a first prism  510  includes an incident surface  510   i , a first reflection surface (roof-mirror surface)  510 - 1   r  (D), a second reflection surface  510 - 2   r , and an exit surface  510   e . A second prism  530  includes an incident surface  530   i , a first reflection surface  530 - 1   r , a second reflection surface  530 - 2   r , a third reflection surface  530 - 3   r , and an exit surface  530   e.    
         [0054]    The incident surface  510   i  and the second reflection surface  510 - 2   r  are the same surface. The incident surface  530   i  and the second reflection surface  530 - 2   r  are the same surface.  
         [0055]    In each of the first through sixth embodiments, the incident surfaces and the exit surfaces extend perpendicularly to the page on which FIGS. 1 through 6 are shown; and likewise, the reflection surfaces other than the roof-mirror surfaces extend perpendicularly to the page on which FIGS. 1 through 6 are shown.  
         [0056]    The first through sixth embodiments can be divided into two groups, i.e., (i) a first group: the first through fourth embodiments shown in FIGS. 1 through 4; and (ii) a second group: the fifth and sixth embodiments shown in FIGS. 5 and 6.  
         [0057]    The first group, i.e., the first through fourth embodiments, satisfies the following conditions:  
         sin(θ−ω′)&lt;1 /n    (1)  
         6°&lt;ω&lt;16°  (2)  
         [0058]    wherein  
         [0059]    θ designates an angle between the incident surface and the first reflection surface of the first prism;  
         [0060]    ω designates a real field-of-view ω (half amount);  
         [0061]    ω′ designates an angle between a light ray of the real field-of-view ω (half amount) and the optical axis in the first prism; and  
         [0062]    n designates the refractive index of the first prism.  
         [0063]    By satisfying conditions (1) and (2), light rays do not pass through the reflection surfaces in the first prism, so that reduction of peripheral illumination at the peripheral area of the view field, and ghosting caused by transmitting light rays can be avoided.  
         [0064]    In the second group, i.e., the fifth and sixth embodiments, the incident surface  410   i  ( 510   i ) and the second reflections surface  410 - 2   r  ( 510 - 2   r ) of the first prism  410  ( 510 ) are the same surface, and the first prism  410  ( 510 ) satisfies the following conditions:  
         sin(2α−ω′)&gt;1 /n    (3)  
         6°&lt;ω&lt;16°  (2)  
         [0065]    wherein  
         [0066]    α designates an angle between the incident surface and the first reflection surface of the first prism;  
         [0067]    ω designates a real field-of-view ω (half amount);  
         [0068]    ω′ designates an angle between a light ray of a real field-of-view ω (half amount) and the optical axis in the first prism; and  
         [0069]    n designates the refractive index of the first prism.  
         [0070]    By satisfying conditions (3) and (2), light rays do not pass through the reflection surfaces in the first prism, so that reduction of peripheral illumination at the peripheral area of the view field, and ghosting caused by transmitting light rays can be avoided.  
         [0071]    In the above-described embodiments, focusing is carried out by moving the rear lens group  22  of the objective optical system  20  ( 120 ,  220 ,  320 ,  420 ,  520 ) in a forward or rearward direction along the optical axis. Alternatively, the eyepiece optical system can be used as a focusing lens group.  
         [0072]    According to such an internal-focusing arrangement, no movable members are exposed to outside, so that strength of a lens-frame structure can be increased with ease; and the volume of a member in which the image-erecting viewing optical system is contained does not vary, so that a water-proof structure can be easily achieved.  
         [0073]    Furthermore, in the objective optical system  20  ( 120 ,  220 ,  320 ,  420 ,  520 ) including the front lens group  21  and the rear lens group  22 , it is preferable that the rear lens group  22  have a weak optical power for the purpose of reducing fluctuations of aberrations.  
         [0074]    Condition (4) specifies the power of both the front lens group  21  and the rear lens group  22 .  
         [0075]    If fO/f2 exceeds the lower limit of condition (4), the traveling distance of the rear lens group  22  upon focusing becomes longer, so that the size of the optical system increases in order to secure enough space to cover the traveling distance of the rear lens group  22  .  
         [0076]    If fO/f2 exceeds the upper limit of condition (4), it becomes difficult to correct the change in aberrations upon focusing, and especially difficult to correct field curvature.  
         [0077]    Specific numerical data of the embodiments will be described hereinafter. In the diagrams of chromatic aberration (on-axis chromatic aberration) represented by spherical aberration, the solid line and the two types of dotted lines respectively indicate spherical aberrations with respect to the d, g and C lines. Also, in the diagrams of lateral chromatic aberration, the two types of dotted lines respectively indicate magnification with respect Lo the g and C lines; however, the a line as the base line coincides with the ordinate. Furthermore, S designates the sagittal image, and M designates the meridional image; ER designates the diameter of the exit pupil, and B designates the apparent visual angle (°). In the tables, r designates the radius of curvature, d designates the lens-element thickness or distance between lens elements, Nd designates the refractive index of the d-line, and ν designates the Abbe number.  
         [0078]    In addition to the above, an aspherical surface which is symmetrical with respect to the optical axis is defined as follows:  
           x=cy   2 /(1+[1−{1+ K}c   2   y   2 ] 1/2 )+ A 4 y   4   +A 6 y   6   +A 8 y   8   +A 10 y   10    
         [0079]    wherein:  
         [0080]    c designates a curvature of the aspherical vertex (1/r);  
         [0081]    y designates a distance from the optical axis;  
         [0082]    x designates an amount of change in the optical axis direction with respect to the distance “y” from the optical axis;  
         [0083]    K designates the conic coefficient; and  
         [0084]    A4 designates a fourth-order aspherical coefficient;  
         [0085]    A6 designates a sixth-order aspherical coefficient;  
         [0086]    A8 designates a eighth-order aspherical coefficient; and  
         [0087]    A10 designates a tenth-order aspherical coefficient.  
         [0088]    [Numerical Embodiment 1] 
         [0089]    [0089]FIGS. 8 through 10D show the image-erecting optical system according to the first numerical embodiment of the present invention. FIG. 8 is a lens arrangement of the first numerical embodiment. FIGS. 9A through 9D show aberrations occurred in the lens arrangement shown in FIG. 8, when an object at an infinite distance is in an in-focus state. FIGS. 10A through 10D show aberrations occurred in the lens arrangement shown in FIG. 8, when an object at a distance of 1.2 m is in an in-focus state. Table 1 shows the numerical data of the first numerical embodiment. D7 (14.94;26.89) designates the distance d7 (FIG. 8) between the most object-side surface of the rear lens group  22  and the exit surface of the second prism  30 , when an object at an infinite distance is in an in-focus state, and when an object at a distance of 1.2 m is in an in-focus state. D9 (24.80; 12.85) designates the distance d9 (FIG. 8) between the most image-side surface of the rear lens group  22  and the most object-side surface of the eyepiece optical system  50 , when an object at an infinite distance is in an in-focus state, and when an object at a distance of 1.2 m is in an in-focus state.  
                                                     TABLE 1                       Surface No.   r   d   Nd   ν                                 1   ∞   31.28   1.56883   56.3        2   ∞   1.00   —   —        3   59.264   3.87   1.51633   64.1        4   −26.478   1.60   1.62004   36.3        5   −67.614   3.00   —   —        6   ∞   54.21   1.51633   64.1        7   ∞   D7   —   —        8   −14.300   8.00   1.78472   25.7        9   −16.720   D9   —   —        10*   −250.000   5.50   1.49176   57.4       11   −27.363   0.50   —   —       12   116.372   1.50   1.84666   23.8       13   24.611   10.96   1.58913   61.2       14   −24.611   0.50   —   —       15   23.839   5.61   1.58913   61.2       16   −150.000   —   —   —                          
 
         [0090]    Aspherical surface data (the aspherical surface coefficients not indicated are zero (0.00)):  
                                                             Surf.No.   K   A4   A6   A8                   10   0.00   −0.81400 × 10 −4     −0.25300 × 10 −6     0.11400 × 10 8              ω = 8°               D7 = 14.94   26.89       D9 = 24.80   12.85                  
 
         [0091]    [Numerical Embodiment 2] 
         [0092]    [0092]FIGS. 11 through 13D show the image-erecting optical system according to the second numerical embodiment of the present invention. FIG. 11 is a lens arrangement of the second numerical embodiment. FIGS. 12A through 12D show aberrations occurred in the lens arrangement shown in FIG. 11, when an object at an infinite distance is in an in-focus state. FIGS. 13A through 13D show aberrations occurred in the lens arrangement shown in FIG. 11, when an object at a distance of 1.5 m is in an in-focus state. Table 2 shows the numerical data of the second numerical embodiment. D7 (14.18; 27.10) designates the distance d7 (FIG. 8) between the most object-side surface of the rear lens group  22  and the exit surface of the second prism  30 , when an object at an infinite distance is in an in-focus state, and when an object at a distance of 1.5 m is in an in-focus state. D9 (20.95; 8.03) designates the distance d9 (FIG. 8) between the most image-side surface of the rear lens group  22  and the most object-side surface of the eyepiece optical system  50 , when an object at an infinite distance is in an in-focus state, and when an object at a distance of 1.5 m is in an in-focus state.  
                                                     TABLE 2                       Surface No.   r   d   Nd   ν                                 1   ∞   31.28   1.56883   56.3        2   ∞   1.00   —   —        3   47.000   3.87   1.51633   64.1        4   −29.750   1.60   1.62004   36.3        5   −92.300   3.00   —   —        6   ∞   54.21   1.51633   64.1        7   ∞   D7   —   —        8   −16.408   8.00   1.78472   25.7        9   −18.244   D9   —   —        10*   −60.000   5.50   1.49176   57.4       11   −18.140   0.50   —   —       12   ∞   1.50   1.84666   23.8       13   24.140   10.70   1.58913   61.2       14   −24.140   0.50   —   —       15   20.388   6.25   1.58913   61.2       16   −150.000   —   —   —                          
 
         [0093]    Aspherical surface data (the aspherical surface coefficients not indicated are zero (0.00)):  
                                                                             Surf.No.   K   A4   A6                           10   0.00   −0.12800 × 10 −3     0.13000 × 10 −6                  ω = 8°                   D7 = 14.18   27.10           D9 = 20.95    8.03                      
 
         [0094]    [Numerical Embodiment 3] 
         [0095]    [0095]FIGS. 14 through 16 D show a third numerical embodiment of an image-erecting optical system according to the present invention FIG. 14 is a lens arrangement of the third numerical embodiment. FIGS. 15A through 15D show aberrations occurred in the lens arrangement shown in FIG. 14, when an object at an infinite distance is in an in-focus state. FIGS. 16A through 16D show aberrations occurred in the lens arrangement shown in FIG. 14, when an object at a distance of 1.5 m is in an in-focus state.  
         [0096]    Table 3 shows the numerical data of the third numerical embodiment. D7 (13.89; 27.21) designates the distance d7 (FIG. 8) between the most object-side surface of the rear lens group  22  and the exit surface of the second prism  30 , when an object at an infinite distance is in an in-focus state, and when an object at a distance of 1.5 m is in an in-focus state. D9 (19.48; 6.16) designates the distance d9 (FIG. 8) between the most image-side surface of the rear lens group  22  and the most object-side surface of the eyepiece optical system  50 , when an object at an infinite distance is in an in-focus state, and when an object at a distance of 1.5 m is in an in-focus state.  
                                                     TABLE 3                       Surface No.   r   d   Nd   ν                                 1   ∞   31.28   1.56883   56.3        2   ∞   3.00   —   —        3   48.796   3.87   1.51633   64.1        4   −28.615   1.60   1.62004   36.3        5   −84.842   4.00   —   —        6   ∞   54.21   1.51633   64.1        7   ∞   D7   —   —        8   −17.193   8.00   1.78472   25.7        9   −18.656   D9   —   —        10*   −120.000   6.00   1.49176   57.4       11   −19.212   0.96   —   —       12   ∞   1.50   1.84666   23.8       13   25.307   8.70   1.58913   61.2       14   −25.307   1.04   —   —       15   20.414   5.80   1.58913   61.2       16   −100.000   —   —   —                          
 
         [0097]    Aspherical surface data (the aspherical surface coefficients not indicated are zero (0.00)):  
                                                                     Surf.No.   K   A4                           10   0.00   −0.94605 × 10 −4                  ω = 8°               D7 = 13.89   27.21           D9 = 19.48    6.16                      
 
         [0098]    Table 4 shows the numerical values of each condition for each numerical embodiment.  
                                                 TABLE 4                                   Num.Embod.1   Num.Embod.2   Num.Embod.3                                    Cond. (1)   θ = 48   θ = 48   θ = 48 (Embods.1˜4)       Cond. (2)   ω = 8.0   ω = 8.0   ω = 8.0       Cond. (3)   α = 24   α = 24   α = 24 (Embods.5˜6)       Cond. (4)   0.31   0.36   0.40                  
 
         [0099]    According to the above description, a miniaturized image-erecting optical system having a wide field-of-view and a low magnification can be achieved.