Abstract:
A symmetrical type four-component lens system comprises, in the direction of incidence of light, a first lens component, a second lens component, a stop, a third lens component and a fourth lens component. Each lens component is a combination of a positive and a negative lens element, the concave surface of each element facing the stop. The surfaces of the first and fourth components facing the stop have shorter radii of curvature than the surfaces of the second and third components facing away from the stop. The first and fourth components have greater focal lengths than the second and third components. The second and third components satisfy certain conditions relating to the materials forming them.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a symmetrical type four-component lens system for use as a photographic lens, reproduction lens or the like. 
     2. Description of the Prior Art 
     The four-component lenses of the prior art, particularly those which satisfy the conditions that the surface of each lens component facing a stop be concave with respect to the stop, that the surfaces of the first and fourth lens components facing the stop have radii of curvature less than the radii of curvature of the surfaces of the second and third lens components facing away from the stop, and that the first and fourth lens components have focal lengths greater in absolute value than the focal lengths of the second and third lens components, have the limitation that the width of the axial chromatic aberration in equimultiple magnification (1/1) can only be corrected, at best, to a value on the order of 4/1,000 of the total focal length. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a symmetrical four-component lens satisfying the above-mentioned conditions has second and third lens components each comprising a combination of a positive lens element and a negative lens element, the width of the axial chromatic aberration of the lens in equimultiple magnification (1/1) being corrected to a value on the order of 4/10,000 of the total focal length by imposing certain conditions upon the materials which form the positive and negative elements. 
     Description will now be made of how to remove chromatic aberration from a lens for light comprising wavelengths E (green: 546.1mμ), G (blue: 435.8mμ) and C (orange: 656.3mμ). Designating the refractive indices of the lens-forming materials for the wavelengths E, G and C as n E , n G  and n C , respectively, the value K in equation (1) below may conveniently be treated as representing the optical properties of the lens-forming materials. ##EQU1## 
     If all the lens-forming materials have equal values of K but different dispersive powers, it will be apparent that the chromatic aberration in the lens for light comprising wavelengths E, G and C can be completely removed. In practice, however, the materials generally used for forming lenses have the characteristic that as the dispersive power increases (or as the Abbe number ##EQU2## which is the reciprocal of the dispersive power, decreases), the value of K decreases. Combinations of materials which have equal values of K  but different dispersive powers are rare, and combinations of materials which have greatly different dispersive powers are extremely rare. Therefore, the degree of freedom with respect to the index of refraction of lens-forming materials is limited and, depending on the construction of the lens, correction of aberrations other than chromatic aberration is very difficult to attain. 
     In accordance with the invention, the chromatic aberration in a lens of the above-described type can be removed substantially completely by forming each of the second and third lens components as a combination of a positive element and a negative element, and by imposing certain conditions upon the materials which form those positive and negative elements. 
     The invention will become more fully apparent from the following detailed description taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of a four-component lens system in accordance with the invention; 
     FIGS. 2(A) and 2(B) show the aberration curves of a first embodiment of the invention; 
     FIGS. 3(A) and 3(B) show the aberration curves of a second embodiment of the invention; 
     FIGS. 4(A) - 4(C) show the aberration curves of a third embodiment of the invention; and 
     FIGS. 5(A) - 5(C) show the aberration curves of a fourth embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIG. 1, a symmetrical type four-component lens in accordance with the present invention comprises, in the direction of incidence of light, a first positive meniscus lens component L1, a second positive meniscus lens component L2, a stop S, a third positive meniscus lens component L3 and a fourth positive meniscus lens component L4, each lens component being a combination of a positive lens element and a negative lens element. In a lens of this type, it is desirable that the indices of refraction of the materials forming the first and fourth lens components L1 and L4 be greater than those of the materials forming the second and third lens components L2 and L3 and that the indices of refraction of the materials forming the negative lens elements be smaller than those of the materials forming the positive lens elements. However, no commercially available glass material provides equal values of K for the positive and negative lens elements forming the first and fourth components L1 and L4 in a lens having a construction which staisfies the foregoing conditions. Designating the values of K for the positive lens elements of the first and fourth components L1 and L4 as K p1  and K p4 , respectively, and the values of K for the negative lens elements of of the same components as K n1  and K n4 , respectively, the relationship of these values is generally expressed by equation (2) below. 
     
         K.sub.p1 - K.sub.n1 &gt; o                                    (2) 
    
     
         K.sub.p4 - K.sub.n4 &gt; 0 
    
     Consequently, if K p  is the sum of the K values in the glass materials forming the positive lens elements of the second and third components L2 and L3, if ν p  is the sum of the Abbe numbers of these glass materials, if K n  is the sum of the K values in the glass materials forming the negative lens elements of the second and third components L2 and L3, and if ν n  is the sum of the Abbe numbers of the latter glass materials, then the desired relationship of these values is expressed as follows: 
     
         K.sub.p - K.sub.n ≦ 0                               (3) 
    
     
         ν.sub.p - ν.sub.n &gt; 10 
    
     Thus, in accordance with the invention, a combination of glass materials is selected which will satisfy the two conditions in equation (3) simultaneously. If the conditions expressed by equation (3) were imposed upon only one of the second and third components L2 and L3, there would be provided a lens system having less chromatic aberration than conventional lenses, but having more chromatic aberration than a lens system wherein the conditions of equation (3) are imposed on both lens components. 
     With regard to the condition of equation (3) that ν p  - ν n  &gt; 10, it has been found that for ν p  - ν n  &lt; 10 the refractive powers of the positive and negative lens elements are too great to provide a lens of sufficient F-number. 
     The invention will be described in more detail referring to the following four examples, wherein 
     R = radius of curvature 
     d = center thickness of lens and lens spacing 
     ν = Abbe number of the glass used. 
     n = index of refraction of the respective lens elements measured at the designated wavelengths. 
     
         Example ITotal focal length f=100, F-number 1:5.6,Magnification M-1/1 (Perfect Symmetry)______________________________________R.sub.1  =  23.285   d.sub.1                  = 7.972                         ν.sub.1 = 49.5                                K.sub.p1 = 0.34551R.sub.2  =  -217.425 d.sub.2                  = 1.812                         ν.sub.2 = 44.3                                K.sub.n1 = 0.34429R.sub.3  =  17.394   d.sub.3                  = 1.051R.sub.4  =  26.816   d.sub.4                  = 2.682                         ν.sub.3 = 64.2                                K.sub.n2 = 0.35599R.sub.5  =  12.901   d.sub.5                  = 5.001                         ν.sub.4 = 81.5                                K.sub.p2 = 0.35403R.sub.6  =  46.384   d.sub.6                  = 6.378R.sub.7  =  -46.384  d.sub.7                  = 5.001                         ν.sub.5 = 81.5                                K.sub.p3 = 0.35403R.sub.8  =  -12.901  d.sub.8                  = 2.682                         ν.sub.6 = 64.2                                K.sub.n3 = 0.35599R.sub.9   =  -26.816  d.sub.9                  = 1.051R.sub.10 =  -17.394   d.sub.10                  = 1.812                         ν.sub.7 = 44.3                                K.sub.n4 = 0.34429R.sub.11 =  217.425   d.sub.11                  = 7.972                         ν.sub.8 = 49.5                                K.sub.p4 = 0.34451R.sub.12 =  -23.285K.sub.p - K.sub.n = - .00392              ν.sub.p - ν.sub.n = 34.6______________________________________Indices of Refraction for Example I______________________________________n.sub.E1 = 1.69091       n.sub.G1 = 1.70506                     n.sub.C1 = 1.68344n.sub.E2 = 1.61595       n.sub.G2 = 1.63012                     n.sub.C2 = 1.60851n.sub.E3 = 1.51871       n.sub.G3 = 1.52667                     n.sub.C3 = 1.51431n.sub.E4 = 1.48749       n.sub.G4 = 1.49342                     n.sub.C4 = 1.49424n.sub.E5 = 1.48749       n.sub.G5 = 1.49342                     n.sub.C5 = 1.48424n.sub.E6 = 1.51871       n.sub.G6 = 1.52667                     n.sub.C6 = 1.51431n.sub.E7 = 1.61595       n.sub.G7 = 1.63012                     n.sub.C7 = 1.60851n.sub.E8 = 1.69091       n.sub.G8 = 1.70506                     n.sub.C8 = 1.68344______________________________________ 
    
     
         Example IITotal focal length 9 100, F-number 1:5.6,Magnification M=1/1 (Perfect Symmetry)______________________________________R.sub.1  =  25.425    d.sub.1  =                   13.597                         ν.sub.1 = 58.6                                K.sub.p1 = 0.35087R.sub.2  =  -224.347  d.sub.2  =                   1.700 ν.sub.2 = 52.8                                K.sub.n1 = 0.34991R.sub.3  =  17.139    d.sub.3  =                   0.986R.sub.4  =  25.514    d.sub.4  =                   2.515 ν.sub.3 = 67.2                                K.sub.n2 = 0.35525R.sub.5  =  11.693    d.sub.5  =                   4.691 ν.sub.4 = 94.9                                K.sub.p2 = 0.35328R.sub.6  =  47.929    d.sub.6  =                   4.936R.sub.7  =  -47.929   d.sub.7  =                   4.691 ν.sub.5 = 94.9                                K.sub.p3 = 0.35328R.sub.8  =  -11.693   d.sub.8  =                   2.515 ν.sub.6 = 67.2                                K.sub.n3 =  0.35525R.sub.9  =  -25.514   d.sub.9  =                   0.986R.sub.10 =  -17.139   d.sub.10 =                   1.700 ν.sub.7 = 52.8                                K.sub.n4 = 0.34991R.sub.11 =  224.347   d.sub.11 =                   13.597                         ν.sub.8 = 58.6                                K.sub.p4 = 0.35087R.sub.12 =  -25.425K.sub.p - K.sub.n = - .00394              ν.sub.p - ν.sub.n = 55.4______________________________________Indices of Refraction for Example II______________________________________n.sub.E1 = 1.61521       n.sub.G1 = 1.62570                     n.sub.C1 = 1.60954n.sub.E2 = 1.52365       n.sub.G2 = 1.53359                     n.sub.C2 = 1.51830n.sub.E3 = 1.44786       n.sub.G3 = 1.45443                     n.sub.C3 = 1.44424n.sub.E4 = 1.43497       n.sub.G4 = 1.43951                     n.sub.C4 = 1.43249n.sub.E5 = 1.43497       n.sub.G5 = 1.43951                     n.sub.C5 =  1.43249n.sub.E6 = 1.44786       n.sub.G6 = 1.45443                     n.sub.C6 = 1.44424n.sub.E7 = 1.52365       n.sub.G7 = 1.53359                     n.sub.C7 = 1.51830n.sub.E8 = 1.61521       n.sub.G8 = 1.62570                     n.sub.C8 = 1.60954______________________________________ 
    
     
         Example IIITotal focal length f=100, F-number 1:5.6,Magnification M=1/2______________________________________R.sub.1  =  23.105   d.sub.1                  = 7.719                         ν.sub.1 = 49.5                                K.sub.p1 = 0.34551R.sub.2  =  -219.798 d.sub.2                  = 1.827                         ν.sub.2 = 44.3                                K.sub.n1 = 0.34429R.sub.3  =  17.383   d.sub.3                  = 1.061R.sub.4  =  27.478   d.sub.4                  =2.652 ν.sub.3 = 64.2                                K.sub.n2 = 0.35599R.sub.5  =  13.043   d.sub.5                  = 4.496                         ν.sub.4 = 81.5                                K.sub.p2 = 0.35403R.sub.6  =  46.285   d.sub.6                  = 6.447R.sub.7  =  -47.601  d.sub.7                  = 4.614                         ν.sub.5 = 81.5                                K.sub.p3 = 0.35403R.sub.8  =  -13.043  d.sub.8                  = 2.711                         ν.sub.6 = 64.2                                K.sub.n3 = 0.35599R.sub.9   =  -26.728  d.sub.9                  = 1.061R.sub.10 =  -17.383   d.sub.10                  = 1.827                         ν.sub.7 = 44.3                                K.sub.n4 = 0.34429R.sub.11 =  219.798   d.sub.11                  = 7.796                         ν.sub.8 = 49.5                                K.sub.p4 = 0.34551R.sub.12 =  -23.258K.sub.p - K.sub.n = - .00392              ν.sub.p - ν.sub.n = 34.6______________________________________Indices of Refraction for Example III______________________________________n.sub.E1 = 1.69091       n.sub.G1 = 1.70506                     n.sub.C1 = 1.68344n.sub.E2 = 1.61595       n.sub.G2 = 1.63012                     n.sub.C2 = 1.60851n.sub.E3 = 1.51871       n.sub.G3 = 1.52667                     n.sub.C3 = 1.51431n.sub.E4 = 1.48749       n.sub.G4 = 1.49342                     n.sub.C4 = 1.48424n.sub.E5 = 1.48749       n.sub.G5 = 1.49342                     n.sub.C5 = 1.48424n.sub.E6 =  1.51871       n.sub.G6 = 1.52667                     n.sub.C6 = 1.51431n.sub.E7 = 1.61595       n.sub.G7 = 1.63012                     n.sub.C7 = 1.60851n.sub.E8 = 1.69091       n.sub.G8 = 1.70506                     n.sub.C8 = 1.68344______________________________________ 
    
     
         Example IVTotal focal length f=100, F-number 1:5.6,Magnification M=1/2______________________________________R.sub.1    =     25.434   d.sub.1   =                    13.541                          ν.sub.1 = 58.6                                  K.sub.p1 = 0.35087R.sub.2    =     -224.569 d.sub.2    =                    1.648 ν.sub.2 = 52.8                                  K.sub.n1 = 0.34991R.sub.3    =     16.735   d.sub.3   =                    1.001R.sub.4    =     25.775   d.sub.4   =                    2.355 ν.sub.3 = 67.2                                  K.sub.n2 = 0.35525R.sub.5    =     12.010   d.sub.5   =                    4.828 ν.sub.4 = 94.9                                  K.sub.p2 = 0.35328R.sub.6    =     48.005   d.sub.6   =                    4.945R.sub.7    =     -48.954  d.sub.7   =                    5.534 ν.sub.5 = 94.9                                  K.sub.p3 = 0.35328R.sub.8    =     -11.728  d.sub.8   =                    1.707 ν.sub.6 = 67.2                                  K.sub.n3 = 0.35525R.sub.9    =     -25.993  d.sub.9   =                    1.001R.sub.10    =     -17.830  d.sub.10 =                    1.707 ν.sub.7 = 52.8                                  K.sub.n4 = 0.34991R.sub.11    =     227.977  d.sub.11 =                    13.423                          ν.sub.8 = 58.6                                  K.sub.p4 = 0.35087R.sub.12    =     -25.484______________________________________K.sub.p - K.sub.n = -.00394              ν.sub.p - ν.sub.n = 55.4______________________________________Indices of Refraction for Example IV______________________________________n.sub.E1 = 1.61521        n.sub.G1 = 1.62570                      n.sub.C1 = 1.60954n.sub.E2 = 1.52365        n.sub.G2 = 1.53359                      n.sub.C2 = 1.51830n.sub.E3 = 1.44786        n.sub.G3 = 1.45443                      n.sub.C3 = 1.44424n.sub.E4 = 1.43497        n.sub.G4 = 1.43951                      n.sub.C4 = 1.43249n.sub.E5 = 1.43497        n.sub.G5 = 1.43951                      n.sub.C5 = 1.43249n.sub.E6 = 1.44786        n.sub.G6 = 1.45443                      n.sub.C6 = 1.44424n.sub.E7 = 1.52365        n.sub.G7 = 1.53359                      n.sub.C7 = 1.51830n.sub.E8 = 1.61521        n.sub.G8 = 1.62570                      n.sub.C8 = 1.60954______________________________________ 
    
     The spherical aberration and astigmatism of the lenses of Examples I and II are shown in FIGS. 2(A) and 2(B) and 3(A) and 3(B), respectively, and the spherical aberration, astigmatism and distortion of the lenses of Examples III and IV are shown in FIGS. 4(A) - 4(C) and 5(A) - 5(C), respectively. It will be apparent that the various aberrations have been well corrected and, more particularly, that the chromatic aberration has been removed to a negligible extent, that is, the width of axial chromatic aberration has been reduced to a value on the order of 4/10,000 of the total focal length. 
     It is believed that the advantages and improved results furnished by the lens systems of the invention will be apparent from the foregoing description of several preferred embodiments of the invention. Various changes and modifications may be made without departing from the spirit and scope of the invention as sought to be defined in the following claims.