Lens system having large relative aperture and long focus
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a compact lens system which has a large relative aperture and long focus but short total length and in which all aberrations are well corrected. 
     2. Description of the Prior Art 
     Among the conventional lenses which are constructed to have a large relative aperture are lenses known by the trade names Ernostar and Telezonar. In lenses of these types, aberrations such as coma, astigmatism, distortion, and axial and lateral chromatic aberrations are well corrected, but spherical aberration for short wavelengths near the g-line (λ = 435.8 mμ) is extremely over-corrected. As a result, the performance of these lenses at full aperture is not completely satisfactory. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a lens system of large relative aperture and long focus which is well corrected for aberrations including spherical aberration for short wavelengths. 
     A lens system constructed in accordance with the invention comprises, as viewed from the object side, a first positive lens member having its convex surface facing the object side of the lens system, a second positive lens member having its convex surface facing the object side, a third lens member, and a fourth positive lens member, the distance between the surface of the first lens member which faces the object side and the surface of the third lens member which faces the image side being from 0.4 times to 0.53 times the distance between the surface of the first lens members which faces the object side and the surface of the fourth lens members which faces the image side of the lens system. The third lens members may comprise a positive lens and a negative lens which are joined together and which satisfy the following conditions: 
     
         0.4f ≦ | r6| ≦ 0.7f, 
    
     
         n3 &gt; n4, and 
    
     
         32 &gt; ν4 &gt; ν3, 
    
     where f is the composite focal length of the entire lens system, r6 is the radius of curvature of the lens surfaces at the junction of the positive and negative lenses, n3 and n4 are the indices of refraction for the positive and negative lenses, respectively, and ν3 and ν4 are the Abbe numbers for the positive and negative lenses, respectively. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be further described with reference to the accompanying drawings which illustrate preferred and exemplary embodiments, and wherein: 
     FIG. 1 is a cross-sectional view of the lens system constructed in accordance with the present invention; 
     FIGS. 2(a), 2(b) and 2(c) are graphs illustrating the aberrations of a first example of the lens system; FIG. 2(a) illustrating spherical aberration and sine condition; FIG. 2(b), astigmatism; and FIG. 2(c), distortion; 
     FIGS. 3(a), 3(b) and 3(c) illustrate the aberrations for a second example of the lens system; and 
     FIGS. 4(a), 4(b) and 4(c) illustrate the aberrations for a third example of the lens system. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, the preferred lens system in accordance with the present invention has a four-member, five-lens construction. As viewed from the object side, the first lens member comprises a positive meniscus lens L1 having its convex surface facing the object side, the second lens member comprises a positive meniscus lens L2 having its convex surface facing the object side, the third lens member comprises a biconvex positive lens L3 and a biconcave negative lens L4 which are joined together by cement, and the fourth member comprises a positive meniscus lens L5 which is separated from the lens L4 by a large air space, this lens having its convex surface facing the object side. 
     For reasons which will be discussed hereinafter, the components of the lens system in accordance with a preferred embodiment of the invention are arranged to satisfy both of the following two conditions: 
     
         (I) 0.4 Σ d ≦ [d1 + d2 + d3 + d4 + d5 + d6] ≦ 0.53 Σ d, and 
    
     
         (II) 0.4f ≦ |r6| ≦ 0.7f, 
    
     
         n3 &gt; n4, 32 &gt; ν4&gt;ν3, 
    
     where d ADSCRIPTS are the center thicknesses of the respective lenses and the air spaces between adjacent lenses, Σ d is the total optical length of the lens system, f is the composite focal length of the lens system, r ADSCRIPTS are the radii of curvature of the surfaces of respective lenses, n ADSCRIPTS are the indices of refraction for the glass used to make the respective lenses, and ν ADSCRIPTS are Abbe numbers for the respective lenses. 
     The above conditions will now be discussed. In order to reduce the over-correction of the spherical aberration for the short-wavelength light rays near the g-line (λ = 435.8 mμ), it is preferred in principle to use, as the negative third lens member, a negative meniscus lens member (e.g., the combination of biconvex lens L3 and biconcave lens L4) having its concave surface looking toward the image side rather than to use a biconcave lens or a plano-concave lens, because a negative meniscus lens provides a more aplanatic condition than either of the other two types of lenses. 
     However, the declination at which the principal ray in an oblique beam directed toward the marginal area of the third lens member passes through the lens member gradually approaches the minimum level so that the desired effect of negating distortion produced in the positive direction by lenses L1 and L2 is reduced, with a result that most of the distortion generally remains in the positive direction. For the correction of such distortion, it is therefore desirable to minimize the value of (d1 + d2 + d3 + d4 + d5 + d6) with respect to Σ d (the optical length of the entire system) so that the air space d7 between the lenses L4 and L5 has the greatest possible value. By providing such a large air space, the refractive power of the lens L5 is increased and the principal ray in the oblique beam can be made to enter the lens L5 at a high position for refraction therethrough. Thus, the distortion in the positive direction will be directed toward the negative direction. 
     However, if the value of (d1 + d2 + d3 + d4 + d5 + d6) were reduced below the lower limit of 0.4 Σ d set forth in condition I, the rearward principal point of the entire system would approach the image side, greatly increasing the backfocus. As a result, it would be difficult to provide a compact lens system. Moreover, the distortion in the positive direction as well as other aberrations would be aggravated. Conversely, if the value of (d1 + d2 + d3 + d4 + d5 + d6) should exceed the upper limit of 0.53 Σ d recited in condition I so that the air space between the lenses L4 and L5 is reduced, the desired effect of negating the distortion in the positive direction would be reduced and at the same time a coma would be created which could hardly be corrected by subsequent lens members. 
     By further increasing the degree of bending of the negative meniscus third lens member to provide a more aplanatic condition, the spherical aberration produced in the negative third lens member may be further reduced. However, the degree of bending is limited by the need to correct the other types of aberrations. Also, the complete correction of the spherical aberration by this means alone would be difficult to accomplish. 
     In accordance with condition II, the spherical aberration can be further corrected with respect to the short-wavelength rays near the g-line without increasing the various aberrations. More specifically, the overcorrection of the spherical aberration with respect to the short-wavelength rays may be further improved by employing, as the third lens member, a doublet comprising lenses L3 and L4 which are selected so that the radius of curvature r6 of the lens surfaces at the junction of the two lenses, the indices of refraction of the two lenses and the Abbe numbers of the glass materials used all satisfy condition II. The biconvex lens L3 is provided with the effect of directing the spherical aberration near the g-line toward the direction of under-correction, whereas the biconcave lens L4 has the counter effect to that of the biconvex lens L3. Therefore, in combination of the biconvex and biconcave lenses L3 and L4, the deterioration of such effect of directing the spherical aberration near the g-line toward the direction of under-correction tends to occur. In order to prevent such tendency, it is necessary to make use of the condition n3 &gt;  n4 as well as ν4 &gt; ν3. Thus, the spherical aberration near the g-line may be corrected. 
     As |r6| approaches the upper limit of 0.7f set forth in condition II, the desired effect of directing the spherical aberration for light having wavelengths near the g-line toward the direction of under-correction is reduced. Therefore, this must be compensated for by selecting lenses L3 and L4 which have high indices of refraction n3 and n4, respectively. If |r6| were to exceed 0.7f, selection of glass materials having the necessary indices of refraction would be difficult. 
     Conversely, as |r6| approaches the lower limit of 0.4f recited in condition II, the spherical aberration would generally be directed toward the direction of under-correction. Consequently, this must be compensated for by selecting glass materials having low indices of refraction n3 and n4. However, the Abbe numbers ν3 and ν4 of such materials are increased to values which make them unsuitable for the correction of the axial and lateral chromatic aberrations. If |r6| were selected below the lower limit of 0.4f, it would be difficult to correct the balance between the spherical aberration and the chromatic aberration or the like. Furthermore, if Abbe numbers ν3 and ν4 were selected to be greater than 32, selection of a combination of glass materials from among the existing glass materials for the first and second lens groups would be impossible because Abbe numbers ν1 and ν2 are greater than 65. Thus, the correction of the axial and lateral chromatic aberrations would become extremely difficult. 
     According to the above-described lens construction and the given conditions, a compact lens system having a large relative aperture and long focus is obtained in which the aberrations, including spherical aberration for short wavelengths, are highly corrected. 
     The lens system of the invention will be described in greater detail with reference to the following three examples, each of which provides a lens system having a total composite focal length f of 100 mm., a relative aperture of 1:2.8, and an angle of view of 18.1°. FIGS. 2(a), 2(b) and 2(c) illustrate the spherical aberration, sine condition, astigmatism and distortion for example 1, while FIGS. 3(a), 3(b) and 3(c) and FIGS. 4(a), 4(b) and 4(c) illustrate the same aberrations for examples 2 and 3, respectively. 
     
                       EXAMPLE I                                                   
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r1 = 40.444                                                               
          d1 = 5.407 n1 = 1.6031 ν 1 = 65.41                           
r2 = 289.580                                                              
          d2 =0.444                                                       
r3 = 30.222                                                               
          d3 = 9.630 n2 = 1.6031 ν 2 = 65.41                           
r4 = 67.912                                                               
          d4 = 2.074                                                      
r5 = 173.333                                                              
          d5 = 3.481 n3 = 1.69895                                         
                                 ν 3 = 30.0                            
r6 = - 40.741                                                             
          d6 = 1.111 n4 = 1.68893                                         
                                 ν 4 = 31.1                            
r7 = 19.884                                                               
          d7 = 30.000                                                     
r8 = 62.222                                                               
          d8 = 1.630 n5 = 1.7495 ν 5 = 35.0                            
r9 = 195.494                                                              
 ##STR1##                                                                 
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                       EXAMPLE 2                                                   
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r1 = 41.630                                                               
          d1 = 5.407 n1 = 1.62041                                         
                                 ν 1 = 60.3                            
r2 = 300.296                                                              
          d2 = 0.444                                                      
r3 = 28.778                                                               
          d3 = 9.630 n2 = 1.62041                                         
                                 ν 2 = 60.3                            
r4 = 61.593                                                               
          d4 = 2.074                                                      
r5 = 157.876                                                              
          d5 = 3.481 n3 = 1.78470                                         
                                 ν 3 = 26.1                            
r6 = - 58.637                                                             
          d6 = 1.111 n4 = 1.74000                                         
                                 ν 4 = 28.2                            
r7 = 19.616                                                               
          d7 = 25.704                                                     
r8 = 62.222                                                               
          d8 = 1.630 n5 = 1.72825                                         
                                 ν5= 28.3                              
r9 = 186.713                                                              
 ##STR2##                                                                 
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                       EXAMPLE 3                                                   
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r1 = 41.407                                                               
          d1 = 5.407 n1 = 1.60881                                         
                                 ν 1 = 58.9                            
r2 = 288.507                                                              
          d2 = 0.444                                                      
r3 = 28.000                                                               
          d3 = 9.630 n2 = 1.60881                                         
                                 ν 2 = 58.9                            
r4 = 59.138                                                               
          d4 = 2.074                                                      
r5 = 191.111                                                              
          d5 = 3.481 n3 = 1.91761                                         
                                 ν 3 = 21.4                            
r6 = -68.889                                                              
          d6 = 1.111 n4 = 1.80518                                         
                                 ν 4 = 25.5                            
r7 = 20.221                                                               
          d7 = 18.000                                                     
r8 = 76.296                                                               
          d8 = 1.630 n5 = 1.72825                                         
                                 ν 5 = 28.3                            
r9 = 1750.370                                                             
 ##STR3##                                                                 
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     It is believed that the advantages and improved results furnished by the lens system of the invention will be apparent from the foregoing description of several preferred embodiments thereof. 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.