Abstract:
An afocal zooming optical system includes a plurality of lens groups or components, in which some of the lens groups movable along the optical axis for zooming are included in the dioptry adjusting lens groups movable for dioptry adjustment, with others of the lens groups movable for zooming being excluded from said dioptry adjusting lens groups.

Description:
BACKGROUND OF THE INVENTION 
     This application is based on and claims priority from Japanese Application No. HEI 2-335935 filed Nov. 30, 1990, the disclosure of which is incorporated by reference herein. 
     The present invention relates to an afocal optical system for use with binoculars and monoculars, particularly to a zooming optical system capable of changing magnification. 
     Throughout this specification, the terms &#34;group&#34; and &#34;component&#34; are used interchangeably to refer to a structure comprising at least one lens element. 
     Eyepiece optical systems such as binoculars are often provided with a dioptry adjustment capability for adjusting the focus in accordance with the eyesight of the user. With zooming optical systems which are capable of adjusting the magnification (focal length), a reference dioptry which does not change with the magnification is determined at the design stage. 
     If there is even a partial overlap between the lens group movable for dioptry adjustment and the lens group movable for zooming, the change in dioptry due to zooming cannot be suppressed for all dioptries. Under these circumstances, the dioptry most commonly used is specified as a reference, and the system is designed such that zooming will cause no change in dioptry at least in the case where the system is set for the reference dioptry. Hence, there will be no change in dioptry due to magnification alterations if the system is set for the reference dioptry. However, if the system is set for values other than the reference dioptry, magnification alterations will cause changes in dioptry, and another dioptry adjustment becomes necessary. 
     FIG. 21 shows a conventional zooming optical system. An objective optical unit that is positioned closer to the object than a field ring S is composed of a fixed first lens group G1 and a second lens group G2 movable for zooming, whereas an eyepiece optical unit positioned closer to the eye than the field ring S is composed of a third lens group G3 movable for zooming and a fourth lens group G4 that is fixed during zooming. Dioptry adjustment is effected by moving altogether the three lens groups enclosed with a dashed line, i.e., the second lens group G2, the third lens group G3 and the fourth lens group G4. 
     A problem with this prior art zooming optical system is that the lens groups movable for dioptry adjustment experience a great change in focal length during zooming, since those dioptry adjusting lens groups contain all of the lens groups G2 and G3 which have a zooming capability. The eyesight mismatch decreases with decreasing change in the focal lengths of the dioptry adjusting groups, so a great eyesight mismatch will inevitably occur in the conventional system. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide and afocal zooming optical system that is capable of substantially preventing the occurrence of eyesight mismatch during zooming. 
     This and other objects of the invention are attained by an afocal zooming optical system comprising a plurality of lens groups or components, in which some of the lens groups movable along the optical axis for zooming are included in the dioptry adjusting lens groups movable for dioptry adjustment, and others of the lens groups movable for zooming being excluded from the dioptry adjusting lens groups. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be more clearly understood from the following description in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a schematic diagram illustrating the operation of an afocal zooming optical system according to the present invention; 
     FIG. 2 is a graph comparing eyesight mismatch for the case of a conventional lens system and the system of FIG. 1; 
     FIGS. 3, 5 and 7 illustrate the composition of the lens system according to a first embodiment of the present invention in low magnification, moderate magnification and high magnification positions, respectively; 
     FIGS. 4, 6 and 8 are graphs plotting the aberration curves for the lens systems as illustrated in FIGS. 3, 5 and 7, respectively; 
     FIGS. 9, 11 and 13 illustrate the composition of the lens system according to a second embodiment of the present invention in low magnification, moderate magnification and high magnification positions, respectively; 
     FIGS. 10, 12 and 14 are graphs plotting the aberration curves for the lens systems as illustrated in FIGS. 9, 11 and 13, respectively; 
     FIGS. 15, 17 and 19 illustrate the composition of the lens system according to a second embodiment of the present invention in low magnification, moderate magnification and high magnification positions, respectively; 
     FIGS. 16, 18 and 20 are graphs plotting the aberration curves for the lens systems as illustrated in FIGS. 15, 17 and 19, respectively; and 
     FIG. 21 is a schematic diagram showing a conventional afocal zooming optical system. 
    
    
     In the drawings, ER is the eye ring, ERφ means a diameter of the eye ring, B is the angle defined by the optical axis and the principal ray of the exit pupil light flux. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described with reference to FIGS. 1-20, with the operating theory of the present invention being first described. 
     An afocal zooming optical system such as a monocular or binocular having a zooming capability is composed of a plurality of lens groups with at least part of the system being moved along the optical axis for zooming, and with at least a part of the system being moved along the optical axis for dioptry adjustment. If the lens groups movable for zooming are defined as &#34;zooming lens groups&#34; whereas those movable for dioptry adjustment are defined as a &#34;dioptry adjusting lens groups&#34;, the two sets of groups overlap at least partially. In other words, at least one lens group serves both as a zooming group and as a dioptry adjusting group. 
     As already mentioned, a dioptry set for values other than a reference dioptry will change upon zooming. If the amount of change in dioptry (eyesight mismatch) that occurs upon zooming from a low to a high magnification is expressed by ΔDI, its value can be determined by: 
     
         ΔDI=(DIW-DIO)·(fxw/fxt).sup.2               (1) 
    
     where DIW is the dioptry set for low magnification, DIO is the reference dioptry, fxw is the composite focal length of the dioptry adjusting lens groups at low magnification, and fxt is the composite focal length of the dioptry adjusting lens group at high magnification. 
     As one can see from equation (1), the eyesight mismatch that occurs upon zooming can be suppressed or held at a low value by reducing the change in the composite focal length of the dioptry adjusting lens groups which are moved for dioptry adjustment. 
     Hence, in the afocal zooming optical system of the present invention, at least one of the zooming lens groups is included as a dioptry adjusting lens group movable for dioptry adjustment, whereas at least one of the zooming lens groups is not included as a dioptry adjusting lens group. Compared to the case where all of the zooming lens groups are included as dioptry adjusting lens groups, the system of the present invention has the advantage that the focal length of the dioptry adjusting lens group experiences only a small change upon zooming, whereby possible eyesight mismatch can be reduced to a small level. 
     It is possible to perform the dioptry adjustment only with the fourth lens group G4. It is, however, possible to move the third and fourth groups G3, G4 for the dioptry adjustment. In this case, the amount of movement of the lens groups for the adjustment may be small. 
     FIG. 1 shows a four-group zoom lens system as an embodiment of the present invention. An objective optical unit that is positioned closer to the object than a field ring S is composed of a fixed first lens group G1 and a second lens group G2 movable for zooming. An eyepiece optical unit that is positioned closer to the image than the field ring S is composed of a third lens group G3 movable for zooming and a fourth lens group G4 that is fixed during zooming. Dioptry adjustment is effected by moving in unison the two lens groups enclosed with a dashed line, i.e., the third lens group G3 and the fourth lens group G4. Hence, in accordance with the present invention, the third lens group G3 which is a zooming lens group is also included as a dioptry adjusting lens group, but the second lens group G2 which is also a zooming lens group is not included as a dioptry adjusting lens group. 
     FIG. 2 compares the eyesight mismatch between two cases: one being such that dioptry adjustment is effected with a four-group zoom lens system by the method illustrated in FIG. 1, and the other being the case where dioptry adjustment is effected with a zoom lens system of the same composition by the conventional method shown in FIG. 21. The result obtained by the embodiment of FIG. 1 is indicated by solid lines, with the lens configuration corresponding to Lens Composition 2 set forth later herein. The result obtained by the conventional method is indicated by dashed lines. 
     As one can see from FIG. 2, the system of the present invention in which at least one of the zooming lens groups is not included as a dioptry adjusting lens group has the advantage that the eyesight mismatch can be reduced to a smaller level than in the case where all of the zooming lens groups are included as dioptry adjusting lens groups. 
     Three specific examples of the lens composition that can be adopted to realize the optical system described above are set forth below. In these examples, the reference dioptry is set at -1 diopter. 
     Lens Composition 1 
     FIGS. 3, 5 and 7 show a first embodiment of the lens composition for the zooming optical system of the present invention, at low, moderate and high magnifications, respectively. 
     Specific numerical data for the system are given in Table 1, wherein r is the radius of curvature of an individual lens surface, d is the lens thickness or air space, n is the refractive index of a lens, and ν is the Abbe number of a lens. The fourth to the seventh surfaces are those of erecting prisms. 
     The magnification is altered by changing d7, d9 and d14, and the numerical data for the respective alterations are given in Table 2, wherein f0 is the focal length of the objective optical unit, fe is the focal length of the eyepiece optical unit, and ω is the half view angle. As already mentioned, the reference dioptry is -1 diopter. 
     The aberration curves obtained with lens composition 1 are plotted in FIG. 4 for low magnification, in FIG. 6 for moderate magnification and in FIG. 8 for high magnification. 
     
                       TABLE 1______________________________________SurfaceNo.       r        d           n     ν______________________________________1         63.811   4.79        1.51633                                64.12         -38.439  1.31        1.62004                                36.33         -117.465 23.764         ∞  33.26       1.56883                                56.35         ∞  1.586         ∞  30.09       1.56883                                56.37         ∞  variable8         -70.480  1.31        1.51633                                64.19         26.398   variable10        -322.719 1.31        1.80518                                25.411        11.419   5.82        1.71300                                53.812        -23.749  0.1713        55.560   3.72        1.51633                                64.114        -21.238  variable15        35.161   2.56        1.51633                                64.116        -54.737______________________________________ 
    
     
                       TABLE 2______________________________________Low Magni-      Moderate Mag-                       High Magni-fication        nification  fication______________________________________f0     100.000      133.95      158.93fe     16.69        15.02       13.36ω  3.50° 2.56°                           2.08°d7     23.27        14.41       10.30d9     7.03         21.13       31.71d14    12.28        7.04        0.57______________________________________ 
    
     When dioptry adjustment is effected by the method shown in FIG. 1, the dioptry present for the low magnification will vary at the moderate and high magnifications as shown in Table 3. Table 4 shows the numerical data obtained in the case where the same optical system is used, but where dioptry adjustment is by the prior art method shown in FIG. 21. In Tables 3 and 4, symbol fx denotes the composite focal length of the dioptry adjusting lens groups. 
     
                       TABLE 3______________________________________DIOPTRY fx(In Diopters)Low Magni-    Moderate Mag-                     High Magni-fication      nification  fication______________________________________-4.0          -4.7        -5.7-2.0          -2.2        -2.6-1.0          -1.0        -1.00.0           +0.2        +0.6+2.0          +2.7        +3.716.69         15.02       13.36______________________________________ 
    
     
                       TABLE 4______________________________________DIOPTRY fx(In Diopters)Low Magni-    Moderate Mag-                     High Magni-fication      nification  fication______________________________________-4.0          -7.7        -12.9-2.0          -3.2        -5.0-1.0          -1.0        -1.00.0           +1.2        +3.0+2.0          +5.7        +10.915.89         10.64       7.96______________________________________ 
    
     Lens Composition 2 
     FIGS. 9, 11 and 13 show a second embodiment of the lens composition for the zooming optical system of the present invention, at low, high and moderate magnifications, respectively. Specific numerical data for the system are given in Table 5. In Lens Composition 2, the magnification is altered by changing d7, d9 and d14, and the numerical data for the respective alterations are given in Table 6. 
     The aberration curves obtained with Lens Composition 2 are plotted in FIGS. 10, 12 and 14 for low, moderate and high magnifications, respectively. 
     
                       TABLE 5______________________________________SurfaceNo.       r        d           n     ν______________________________________1         57.232   4.99        1.51633                                64.12         -36.823  1.45        1.62004                                36.33         -116.590 25.894         ∞  29.93       1.56883                                56.35         ∞  1.816         ∞  29.93       1.56883                                56.37         ∞  variable8         -41.540  1.18        1.51633                                64.19         26.168   variable10        55.235   1.18        1.80518                                25.411        10.857   5.62        1.71300                                53.812        -22.194  0.3613        145.117  2.63        1.51633                                64.114        -26.719  variable15        19.391   2.36        1.51633                                64.116        141.943______________________________________ 
    
     
                       TABLE 6______________________________________Low Magni-      Moderate Mag-                       High Magni-fication        nification  fication______________________________________f0     100.00       131.84      155.83fe     16.45        14.68       13.14ω  2.67° 1.94°                           1.58°d7     14.31        7.77        4.61d9     10.23        22.23       31.36d14    13.37        7.90        1.93______________________________________ 
    
     When dioptry adjustment is effected by the method shown in FIG. 1, the dioptry preset for the low magnification will vary at the moderate and high magnifications as shown in Table 7. Table 8 shows the numerical data obtained in the case where the same optical system is used, but where dioptry is adjusted by the prior art method shown in FIG. 21. In Tables 7 and 8, symbol fx denotes the composite focal length of the dioptry adjusting lens groups. 
     
                       TABLE 7______________________________________DIOPTRY fx(In Diopters)Low Magni-    Moderate Mag-                     High Magni-fication      nification  fication______________________________________-4.0          -4.8        -5.7-2.0          -2.3        -2.6-1.0          -1.0        -1.00.0           +0.3        +0.6+2.0          +2.8        +3.716.45         14.68       13.14______________________________________ 
    
     
                       TABLE 8______________________________________DIOPTRY fx(In Diopters)Low Magni-    Moderate Mag-                     High Magni-fication      nification  fication______________________________________-4.0          -7.6        -12.5-2.0          -3.2        -4.8-1.0          -1.0        -1.00.0           +1.2        +2.8+2.0          +5.6        +10.514.52         9.80        7.41______________________________________ 
    
     Lens Composition 3 
     FIGS. 15, 17 and 19 show a third example of the lens composition for the zooming optical system of the present invention at low, moderate and high magnifications, respectively. Specific numerical data for the system are given in Table 9. In Lens Composition 3, two of the five lens elements in the eyepiece optical unit, namely, the first lens positioned closest to the object and the fourth lens, are plastic lenses. 
     The aberration curves obtained with Lens Composition 3 are plotted in FIGS. 16, 18 and 20 for low magnification, moderate magnification and high magnification, respectively. 
     
                       TABLE 9______________________________________SurfaceNo.       r        d           n     ν______________________________________1         64.573   5.24        1.51633                                64.12         -33.786  1.52        1.62004                                36.33         -95.352  28.574         ∞  32.38       1.56883                                56.35         ∞  1.916         ∞  28.57       1.56883                                56.37         ∞  variable8         -160.979 1.24        1.492 57.49         14.802   variable10        -63.592  1.24        1.80518                                25.411        14.263   6.57        1.71300                                53.812        -17.104  0.3813        50.744   3.52        1.492 57.414        -27.452  variable15        24.422   2.67        1.51633                                64.116        186.044______________________________________ 
    
     
                       TABLE 10______________________________________Low Magni-      Moderate Mag-                       High Magni-fication        nification  fication______________________________________f0     100.000      136.95      165.18fe     16.66        15.28       13.96ω  2.67° 1.94°                           1.58°d7     12.35        5.78        2.75d9     9.55         21.55       30.82d14    12.51        7.07        0.84.______________________________________ 
    
     When dioptry adjustment is effected by the method shown in FIG. 1, the dioptry present for the low magnification will vary at the moderate and high magnifications as shown in Table 11. Table 12 shows the numerical data obtained in the case where the same optical system is used, but where dioptry adjustment is by the prior art method shown in FIG. 21. In Tables 11 and 12, symbol fx denotes the collective focal length of the dioptry adjusting lens groups. 
     
                       TABLE 11______________________________________DIOPTRY fx(In Diopters)Low Magni-    Moderate Mag-                     High Magni-fication      nification  fication______________________________________-4.0          -4.6        -5.3-2.0          -2.2        -2.4-1.0          -1.0        -1.00.0           +0.2        +0.4+2.0          +2.6        +3.316.66         15.28       13.96______________________________________ 
    
     
                       TABLE 12______________________________________DIOPTRY fx(In Diopters)Low Magni-    Moderate Mag-                     High Magni-fication      nification  fication______________________________________-4.0          -7.7        -12.8-2.0          -3.2        -4.9-1.0          -1.0        -1.00.0           +1.2        +2.9+2.0          +5.7        +10.814.88         9.93        7.50______________________________________ 
    
     As described above, the variations in the focal length of the dioptry adjusting lens groups that occur during zooming can be held to small levels in the system of the present invention, which hence is characterized by a very small amount of eyesight mismatch that occurs during zooming.