Abstract:
An ocular system is disclosed which is composed of at least three lens elements for constituting, in order from the eye side, at least first and second lens units. At least one of the lens elements is made of resin. Two of the lens elements are substantially cemented together. In addition, an aspherical plastic lens is effectively used in the system.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an ocular lens for binoculars, astronomical telescopes and the like, which may be made in low cost with a wide viewing angle. 
     The following three-element types of ocular lens systems for binoculars or the like are known; 
     i) A three-lens element system for an ocular lens for binoculars (for example, Japanese Patent Unexamined Publication No. 51-120231); 
     ii) A three-unit and three-element system for an ocular lens for viewfinders, which uses a plastic material as an optical material with an aspherical surface (for example, Japanese Patent Unexamined Publication Nos. 60-227215 and 63-318514); and 
     iii) A two-unit and three-element system for a viewfinder ocular system, which uses only optical glass for lens elements (for example, Japanese Patent Unexamined Publication No. 61-48810). 
     However, the foregoing systems suffer from the following disadvantages: 
     The ocular system i) suffers from a disadvantage that a curvature of field, an astigmatism and the like could not sufficiently be compensated for in a wide viewing angle. The ocular system ii) suffers from a disadvantage that it would be difficult to compensate for aberrations in a wide viewing angle due to the generation of the transversal chromatic aberration and the like although the curvature of field would be small. Also, the ocular system ii) and iii) have a small viewing angle. It should be also noted that any of the systems i) to iii) could not be manufactured at a low cost. 
     On the other hand, there have been proposed ocular systems each of which includes four to six lens elements in order to carry out a good aberration compensation covering a marginal portion of the view field (see Japanese Patent Examined Publication Nos. 63-6851 and 60-48014). 
     However, in the types having four or more lens elements, there is a serious problem that the manufacturing cost therefor becomes increased and a physical size would be increased. In order to perform a satisfactory aberration compensation with the ocular system which consists of four lens elements, it is necessary that an optical glass having a high refractive index be used for each lens and a radius of curvature be shortened. As a result, it is difficult to carry out the machining so that the manufacturing cost is increased. In particular, it is very difficult to make an aspherical surface on a glass lens, resulting in increase of manufacturing cost. In order to cope with these problems, it is proposed to use a plastic lens element. However, in general, the refractive index of the plastic is low so that it would be difficult to fully compensate for the aberration, particularly, coma aberration and spherical aberration. 
     SUMMARY OF THE INVENTION 
     Accordingly, in view of the foregoing disadvantages inherent in the prior art, an object of the invention is to provide an ocular lens system which may be manufactured at low cost, and in which a satisfactory aberration compensation is effected even in a wide viewing field of 50° to 60° in appearance. 
     In order to meet this object, according to the present invention, there is provided a two-unit and three-element ocular system which is composed, in order from the eye side, of a positive first lens unit and a positive second lens unit, the first lens unit consisting of a single positive lens element, the second lens unit consisting of a cemented lens composed of a positive lens element and a negative lens element or of a negative lens element and a positive lens element, and at least one of the three lens elements being made of optical material of resin. 
     The above-specified ocular lens system, preferably, meets the following conditions: 
     
         |r.sub.4 /r.sub.n |&lt;0.47                 (1) 
    
     
         0.10&lt;f/f.sub.II &lt;0.65                                      (2) 
    
     where r 4  is the radius of curvature of the bonded surface of the second lens unit, r n  is the radius of cuvature of the surface other than the bonded surface of the negative lens element of the second lens unit, f is the overall focal length, and f II  is the focal length of the second lens unit. 
     It is more preferable to provide an aspherical surface, given by the following condition, to the ocular lens system in order to compensate for the curvature of field and astigmatism. ##EQU1## where X is an amount of deviation from an apex of a lens in the axial direction, y is an amount of deviation from the apex of the lens in a vertical direction relative to the axial direction, C is the inverted number of the radius of curvature, K is a conical constant, and A 4 , A 6 , . . . are the aspherical suface coefficients. 
     The following conditions should be met in order to facilitate the manufacturing of the ocular lens at low cost. 
     
         |r.sub.p /r.sub.n |&lt;1.55                 (3) 
    
     
         n&lt;1.60                                                     (4) 
    
     where r p  is the radius of curvature other than the bonded surface of the positive lens element of the second lens unit, and n is the average value of the refractive indexes of the three lens elements on d-line. 
     Furthermore, in order to keep the various aberrations at reasonable values, the following condition should be met. 
     
         -1.70&lt;r.sub.2 /r.sub.1 ≦0                           (5) 
    
     where r 1  and r 2  are the radii of curvature of the surfaces, on the eye side and the image side, of the first lens unit. 
     According to the present invention, since the resinous optical material is used for at least one of the lens elements of the two-unit and three-element system, it is possible to manufacture the system at low cost. 
     According to another aspect of the present invention, there is provided an ocular optical system which is composed, in order from the eye side, of a positive first lens element, a negative meniscus second lens element, a positive third lens element and a negative fourth lens element. The first and second lens elements are made of optical glass or resinous optical material, and the third and fourth lens elements are made of resinous optical material. At least one of the lens surfaces is preferably made as an aspherical surface as described before. 
     In the ocular system including four lens elements, the average refractive index condition (4) should also be met. In addition, the following condition should be met. 
     
         -4.0&lt;f.sub.4 /f&lt;-0.5                                       (6) 
    
     where f 4  is the focal length of the fourth lens element. 
     According to the present invention, the resinous optical material is used for the ocular lens system so that it is easy to machine the resinous material, which is benefitial for mass-production. In addition, the present invention compensates for the various aberrations without increasing the number of the lens elements. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
     FIG. 1 is a cross-sectional view showing a first example of an ocular lens system according to the invention; 
     FIG. 2 is a plurality of graphs showing various aberrations generated in the first example; 
     FIG. 3 is a cross-sectional view showing a second example of an ocular lens system according to the invention; 
     FIG. 4 is a plurality of graphs showing various aberrations generated in the second example; 
     FIG. 5 is a cross-sectional view showing a third example of an ocular lens system according to the invention; 
     FIG. 6 is a plurality of graphs showing various aberrations generated in the third example; 
     FIG. 7 is a cross-sectional view showing a fourth example of an ocular lens system according to the invention; 
     FIG. 8 is a plurality of graphs showing various aberrations generated in the fourth example; 
     FIG. 9 is a cross-sectional view showing a fifth example of an ocular lens system according to the invention; 
     FIG. 10 is a plurality of graphs showing various aberrations generated in the fifth example; 
     FIG. 11 is a cross-sectional view showing a sixth example of an ocular lens system according to the invention; 
     FIG. 12 is a plurality of graphs showing various aberrations generated in the sixth example; 
     FIG. 13 is a cross-sectional view showing a seventh example of an ocular lens system according to the invention; 
     FIG. 14 is a plurality of graphs showing various aberrations generated in the seventh example; 
     FIG. 15 is a cross-sectional view showing a eighth example of an ocular lens system according to the invention; 
     FIG. 16 is a plurality of graphs showing various aberrations generated in the eighth example; 
     FIG. 17 is a cross-sectional view showing a ninth example of an ocular lens system according to the invention; 
     FIG. 18 is a plurality of graphs showing various aberrations generated in the ninth example; 
     FIG. 19 is a cross-sectional view showing a tenth example of an ocular lens system according to the invention; 
     FIG. 20 is a plurality of graphs showing various aberrations generated in the tenth example; 
     FIG. 21 is a cross-sectional view showing a eleventh example of an ocular lens system according to the invention; 
     FIG. 22 is a plurality of graphs showing various aberrations generated in the tenth example; 
     FIG. 23 is a cross-sectional view showing a twelfth example of an ocular lens system according to the invention; 
     FIG. 24 is a plurality of graphs showing various aberrations generated in the twelfth example; 
     FIG. 25 is a cross-sectional view showing a thirteenth example of an ocular lens system according to the invention; 
     FIG. 26 is a plurality of graphs showing various aberrations generated in the thirteenth example; 
     FIG. 27 is a cross-sectional view showing a fourteenth example of an ocular lens system according to the invention; 
     FIG. 28 is a plurality of graphs showing various aberrations generated in the fourteenth example; 
     FIG. 29 is a cross-sectional view showing a fifteenth example of an ocular lens system according to the invention; and 
     FIG. 30 is a plurality of graphs showing various aberrations generated in the fifteenth example. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The condition (1) defined the radius of curvature of the bonded surface. If the upper limit of the condition (1) were exceeded, the radius of curvature of the bonded surface of the second lens unit would be increased. As a result, it would be difficult to compensate for transversal chromatic aberration. 
     The condition (2) relates to the focal length of the second lens unit. If the upper limit of the second condition (2) were increased, a refractive power of the second lens unit would be too large. As a result, the Petzval&#39;s sum would be increased so that the compensation of astigmatism would be unsatisfactory. If the lower limit were exceeded, the spherical aberration and the coma aberration would be generated. 
     It is also important to provide the aspherical surface to at least one surface in order to correct the transversal chromatic aberration and astigmatism. With the aspherical surface, it is possible to ensure an image surface stable up to a wide viewing angle. 
     The condition (3) defined the radius of curvature, other than the bonded surface, of the second lens unit. If the upper limit of the condition (3) were exceeded, the radius of curvature of the second lens unit would be shortened, and it would be difficult to meet the requirement of condition (1). 
     The condition (4) defined the average value of refractive indexes of the respective lens elements. As specified in this condition, the optical material having a low refractive index should be used to reduce the manufacturing cost. 
     In addition, the condition (5) defined the radii of curvature of the first and second surfaces of the first lens elements. If the upper limit of the condition (5) were exceeded, it is difficult to compensate for the spherical aberration and coma aberration. Inversely, if the lower limit exceeded, the distortion would be insufficiently corrected. 
     It should be noted that at least one of the lens elements is made of resinous optical material. It is preferable for use the glass material to a lens close to the eye. Namely, since the lens element on the eye side is exposed to the outside, the lens element is likely to be subjected to foreign matter or dust. If the lens is made of glass, then the lens is hard in comparison with the resinous lens. In addition, it is easier to apply a hard coating to the glass lens. 
     If the upper limit of the condition (6) were exceeded, the refractive power of the fourth lens element would be too large. As a result, the marginal light flux is unlikely to pass through the lens system. thus, it would be difficult to increase the viewing angle. Also, if the lower limit were exceeded, the Petzval&#39;s sum would be increased. 
     EXAMPLES 
     The specific examples of the present invention will be listed, in which f is the overall focal length, F NO  is the F-number, 2ω is the viewing angle, r is the radius of curvature of each lens surface, d is the lens thickness or the interval between the adjacent lenses, n is the refractive index of each lens at d-line, and ν is the Abbe number. 
     EXAMPLE 1 
     
         ______________________________________f = 1.0  F.sub.NO = 1:4.0 2ω = 56°Surface No.     r         d       n       v______________________________________1         1.901     0.340    1.51633                               64.12         -1.162    0.0173         1.675     0.531   1.492   57.4 (resin)4         -0.632    0.169   1.585   29.9 (resin)5         -2.301______________________________________    aspherical third surface______________________________________    K = 5.138    A.sub.4 = -2.17588 × 10.sup.-1    A.sub.6 = -3.22044 × 10.sup.-1    (1) |r.sub.4 /r.sub.n | = 0.275    (2) f/f.sub.II = 0.385    (3) |r.sub.p /r.sub.n | = 0.728    (4) - n = 1.531    (5) r.sub.2 /r.sub.1 = -0.611______________________________________ 
    
     EXAMPLE 2 
     
         ______________________________________f = 1.0  F.sub.NO = 1:4.0 2ω = 56°Surface No.     r         d       n       v______________________________________1         5.255     0.318    1.51633                               64.12         -1.031    0.0173         0.943     0.548   1.492   57.4 (resin)4         -0.842    0.168   1.585   29.9 (resin)5         8.573______________________________________    aspherical third surface______________________________________    K = 0.414    A.sub.4 = -1.06217 × 10.sup.-1    A.sub.6 = -1.38306 × 10.sup.-1    (1) |r.sub.4 /r.sub.n | = 0.098    (2) f/f.sub.II = 0.380    (3) |r.sub.p /r.sub.n | = 0.110    (4) - n = 1.531    (5) r.sub.2 /r.sub.1 = -0.196______________________________________ 
    
     EXAMPLE 3 
     
         ______________________________________f = 1.0  F.sub.NO = 1:4.0 2ω = 56°Surface No.     r         d       n       v______________________________________1         1.523     0.382    1.51633                               64.12         -1.433    0.0173         1.011     0.604   1.492   57.4 (resin)4         -0.752    0.126   1.585   29.9 (resin)5         4.216______________________________________    aspherical third surface______________________________________    K = 0.619    A.sub.4 = -1.05992 × 10.sup.-1    A.sub.6 = -3.44424 × 10.sup.-1    (1) |r.sub.4 /r.sub.n | = 0.178    (2) f/f.sub.II = 0.279    (3) |r.sub.p /r.sub.n | = 0.240    (4) - n = 1.531    (5) r.sub.2 /r.sub.1 = -0.941______________________________________ 
    
     EXAMPLE 4 
     
         ______________________________________f = 1.0  F.sub.NO = 1:4.0 2ω = 56°Surface No.     r         d       n       v______________________________________1         1.276     0.415    1.51633                               64.12         -1.276    0.0173         3.212     0.126   1.585   29.9 (resin)4         0.730     0.573   1.492   57.4 (resin)5         -1.340______________________________________    aspherical third surface______________________________________    K = -16.561    A.sub.4 = -2.26060 × 10.sup.-1    A.sub.6 = -4.32136 × 10.sup.-1    (1) |r.sub.4 /r.sub.n | = 0.227    (2) f/f.sub.II = 0.410    (3) |r.sub.p /r.sub.n | = 0.417    (4) - n = 1.531    (5) r.sub.2 /r.sub.1 = -1.000______________________________________ 
    
     EXAMPLE 5 
     
         ______________________________________f = 1.0  F.sub.NO = 1:4.0 2ω = 56°Surface No.     r         d       n       v______________________________________1         1.427     0.422   1.492   57.42         -0.796    0.0463         9.313     0.126   1.585   29.9 (resin)4         0.632     0.560   1.492   57.4 (resin)5         -1.446______________________________________    aspherical second surface______________________________________    K = -0.064    A.sub.4 =  5.46435 × 10.sup.-1    A.sub.6 =  2.80405 × 10.sup.-1    (1) |r.sub.4 /r.sub.n | = 0.068    (2) f/f.sub.II = 0.265    (3) |r.sub.p /r.sub.n | = 0.155    (4) - n = 1.523    (5) r.sub.2 /r.sub.1 = -0.558______________________________________ 
    
     EXAMPLE 6 
     
         ______________________________________f = 1.0  F.sub.NO = 1:4.0 2ω = 56°Surface No.     r         d       n       v______________________________________1         1.276     0.415   1.51633 64.12         -1.276    0.0173         3.212     0.126   1.585   29.9 (resin)4         0.730     0.600   1.51633 64.15         -1.586______________________________________    aspherical third surface______________________________________    K = -16.561    A.sub.4 = -2.26060 × 10.sup.-1    A.sub.6 = -4.32136 × 10.sup.-1    (1) |r.sub.4 /r.sub.n | = 0.227    (2) f/f.sub.II = 0.398    (3) |r.sub.p /r.sub.n | = 0.494    (4) - n = 1.539    (5) r.sub.2 /r.sub.1 = -1.000______________________________________ 
    
     EXAMPLE 7 
     
         ______________________________________f = 1.0  F.sub.NO = 1:4.0 2ω = 56°Surface No.     r         d       n       v______________________________________1         1.427     0.422   1.492   57.4 (resin)2         -0.796    0.0463         9.313     0.126   1.69895 30.14         0.800     0.560   1.51633 64.15         -1.208______________________________________    aspherical second surface______________________________________    K = -0.070    A.sub.4 =  5.46435 × 10.sup.-1    A.sub.6 =  4.00000 × 10.sup.-1    (1) |r.sub.4 /r.sub.n | = 0.086    (2) f/f.sub.II = 0.297    (3) |r.sub.p /r.sub.n | = 0.130    (4) - n = 1.569    (5) r.sub.2 /r.sub.1 = -0.558______________________________________ 
    
     EXAMPLE 8 
     
         ______________________________________f = 1.0  F.sub.NO = 1:4.0 2ω = 56°Surface No.     r         d       n       v______________________________________1         1.427     0.422   1.492   57.4 (resin)2         -0.796    0.0463         9.313     0.126    1.69895                               30.14         0.800     0.560   1.492   57.4 (resin)5         -1.042______________________________________    aspherical second surface______________________________________    K = -0.070    A.sub.4 =  5.46435 × 10.sup.-1    A.sub.6 =  6.00000 × 10.sup.-1    (1) |r.sub.4 /r.sub.n | = 0.086    (2) f/f.sub.II = 0.319    (3) |r.sub.p /r.sub.n | = 0.112    (4) - n = 1.561    (5) r.sub.2 /r.sub.1 = -0.558______________________________________ 
    
     EXAMPLE 9 
     
         ______________________________________f = 1.0  F.sub.NO = 1:4.0 2ω = 56°Surface No.     r         d       n       v______________________________________1         1.427     0.422   1.492   57.4 (resin)2         -0.796    0.0463         9.313     0.126   1.585   29.9 (resin)4         0.632     0.560    1.51633                               64.15         -1.851______________________________________    aspherical second surface______________________________________    K = -0.064    A.sub.4 =  5.46435 × 10.sup.-1    A.sub.6 =  2.80405 × 10.sup.-1    (1) |r.sub.4 /r.sub.n | = 0.068    (2) f/f.sub.II = 0.236    (3) |r.sub.p /r.sub.n | = 0.199    (4) - n = 1.531    (5) r.sub.2 /r.sub.1 = -0.558______________________________________ 
    
     EXAMPLE 10 
     
         ______________________________________f = 1.0  F.sub.NO = 1:4.0 2ω = 56°Surface No.     r         d       n       v______________________________________1         1.901     0.340   1.51633 64.12         -1.162    0.0173         1.675     0.531   1.492   57.4 (resin)4         -0.680    0.169   1.69895 30.15         -1.636______________________________________    aspherical third surface______________________________________    K = 5.138    A.sub.4 = -2.17588 × 10.sup.-1    A.sub.6 = -2.00000 × 10.sup.-1    (1) |r.sub.4 /r.sub.n | = 0.406    (2) f/f.sub.II = 0.403    (3) |r.sub.p /r.sub.n | = 0.977    (4) - n = 1.569    (5) r.sub.2 /r.sub.1 = -0.611______________________________________ 
    
     EXAMPLE 11 
     
         ______________________________________f = 1.0  F.sub.NO = 1:4.0 2ω = 56°Surface No.     r         d       n       v______________________________________1          1.531    0.464    1.58913                               61.22         -0.816    0.0123         -0.795    0.084   1.585   29.9 (resin)4         -1.539    0.0175          1.085    0.411   1.492   57.4 (resin)6         -1.444    0.0277         -1.220    0.118   1.585   29.9 (resin)8          8.585______________________________________    aspherical fifth surface______________________________________    K = -0.487    A.sub.4 = -3.62572 × 10.sup.-2    (1) - n = 1.563    (2) f.sub.4 /f = -1.816______________________________________ 
    
     EXAMPLE 12 
     
         ______________________________________f = 1.0  F.sub.NO = 1:4.0 2ω = 56°Surface No.     r         d       n       v______________________________________1          1.239    0.528    1.51633                               64.12         -0.759    0.084    1.64769                               33.83         -1.328    0.0174          0.969    0.460   1.492   57.4 (resin)5         -1.286    0.0346         -1.079    0.121   1.585   29.9 (resin)7          4.744______________________________________    aspherical third surface______________________________________    K = -0.946    A.sub.4 = 2.30838 × 10.sup.-2    (1) - n = 1.560    (2) f.sub.4 /f = -1.490______________________________________ 
    
     EXAMPLE 13 
     
         ______________________________________f = 1.0  F.sub.NO = 1:4.0 2ω = 56°Surface No.     r         d       n       v______________________________________1          1.316    0.494   1.492   57.4 (resin)2         -0.770    0.0153         -0.744    0.084   1.585   29.9 (resin)4         -1.205    0.0175          1.065    0.443   1.492   57.4 (resin)6         -1.108    0.0267         -0.977    0.118   1.585   29.9 (resin)8         -37.946______________________________________    aspherical first surface______________________________________    K =  0.215    A.sub.4 = 3.72173 × 10.sup.-2______________________________________    aspherical fifth surface______________________________________    K = -0.765    A.sub.4 = -8.73876 × 10.sup.-2    (1) - n = 1.539    (2) f.sub.4 /f = -1.714______________________________________ 
    
     EXAMPLE 14 
     
         ______________________________________f = 1.0  F.sub.NO = 1:4.0 2ω = 56°Surface No.     r         d       n       v______________________________________1          1.671    0.642   1.51633 64.12         -0.675    0.084   1.64769 33.83         -1.049    0.0174          0.888    0.574   1.492   57.4 (resin)5         -1.945    0.188   1.585   29.9 (resin)6          1.889______________________________________    aspherical fourth surface______________________________________    K = -1.015    A.sub.4 = 9.16712 × 10.sup.-2    (1) - n = 1.560    (2) f.sub.4 /f = -1.059______________________________________ 
    
     EXAMPLE 15 
     
         ______________________________________f = 1.0  F.sub.NO = 1:4.0 2ω = 56°Surface No.     r         d       n       v______________________________________1          1.239    0.500   1.492   57.4 (resin)2         -0.759    0.084    1.64769                               33.83         -1.328    0.0174          0.969    0.460   1.492   57.4 (resin)5         -1.286    0.0346         -1.079    1.021   1.585   29.9 (resin)7          41.319______________________________________    aspherical fourth surface______________________________________    K = -0.946    A.sub.4 = 2.30838 × 10.sup.-2    (1) - n = 1.554    (2) f.sub.4 /f = -1.795______________________________________