Patent Publication Number: US-4545654-A

Title: Bright telephoto lens

Description:
This is a continuation of application Ser. No. 425,582 filed 09/28/82 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a bright telephoto lens having a small telephoto ratio with a secondary spectrum properly compensated for which would largely affect the image forming ability of the telephoto lens. 
     It is known in the art of telephoto lenses that even if chromatic aberrations are compensated for with respect to light rays of two particular wavelengths, chromatic aberrations remain for light rays of other wavelengths, that is, the amount of the secondary spectrum influences the image forming ability to a large degree. Such secondary spectrum can be reduced in telephoto lenses having a convergent lens group and a divergent lens group arranged in order from the object by using a special optical material of anomalous dispersion such as an optical crystal such as fluorite, or glass of anomalous dispersion for positive lenses in the convergent lens group. The optical material of anomalous dispersion used is located most closely to the object since the more apart the light ray is spaced from the optical axis, the more effective against the chromatic aberration. 
     The optical material of anomalous dispersion, however, is soft, has a surface refractive index that varies widely with temperature, and is of a high coefficient of thermal expansion, so that the material is liable to be injured when placed most closely to the object in direct contact with ambient air, and to get damaged or have its image forming ability reduced due to temperature changes. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide a bright telephoto lens having a small telephoto ratio which will eliminate the above difficulty by using a first lens made of an optical material having a relatively small partial dispersion ratio, a refractive index less variable with temperature, being less subjected to being injured, and having a smaller coefficient of thermal expansion as compared with a special optical material of anomalous dispersion, the telephoto lens including a convergent lens group having lenses distributed and shaped so as to reduce a secondary spectrum to a large degree and correct or compensate for various aberrations such as spherical aberration, coma and the like properly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1, 3, 5, 7, 9 and 11 are cross-sectional views of lenses according to Examples 1, 2, 3, 4, 5 and 6, respectively; and 
     FIGS. 2, 4, 6, 8, 10 and 12 are diagrams showing abberations of the lenses according to the Examples 1, 2, 3, 4, 5, and 6, respectively. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described in detail. 
     According to the present invention, as shown in the embodiments of FIGS. 1, 3, 5, 7, 9 and 11, a telephoto lens includes a convergent lens group C p  on the object side and a divergent lens group G n  on the image side, the convergent lens group G p  being composed of first, second and third positive lenses and a single negative lens, the first positive lens being closest to the object and made of an optical material having a partial dispersion ratio ##EQU2## in the range of from 1.22 to 1.29 with respect to g-line and d-line but not made of an optical material of anomalous dispersion, the negative lens being disposed more closely to the image than the first positive lens, said second and third positive lenses being disposed more closely toward the image than the first positive lens and made of an optical material of anomalous dispersion having a partial dispersion ratio θ in the range of from 1.22 to 1.24  with respect to g-line and d-line and having an Abbe number ν d  which ranges from 80 to 95.5, the telephoto lens satisfying the following conditions: ##EQU3## where ψ 1 , ψ 2 , ψ 3  are the refractive powers of the first, second and third positive lenses, respectively, ν dn  is the Abbe number of the negative lens, ψ p  is the sum of the surface refractive powers of the three positive lenses on their image sides, ψ n  is the surface refractive power of the negative lens on the image side, with ψ n  &lt;0. 
     The above conditional expressions will now be described. The condition (1) determines the ratio of the refractive power of the first positive lens on the object side to the refractive powers of the second and third positive lenses located more closely than the first positive lens to the image and made of an optical material of anomalous dispersion. If the upper limit of the condition (1) were exceeded, the advantage of the anomalous dispersion could not sufficiently be utilized failing to reduce the secondary spectrum. If the lower limit of the condition (1) were exceeded, the secondary spectrum could be improved effectively. However, if the refractive power of the convergent lens group were increased to gain a desired telephoto ratio, the proportion of the refractive powers of the second and third positive lenses would be excessively large, thereby increasing aberrations of higher degrees such as spherical aberration, coma and the like, so that a bright lens with the aberrations properly compensated for could not be available. 
     The condition (2) is required for the negative lens in the convergent lens group for reducing the secondary spectrum and preventing the various aberrations from getting worse. If the negative lens exceeded the upper limit of the condition (2), the refractive powers of the lenses in the convergent lens group would be increased in need of meeting an achromatic requirement, with the result that a bright lens with a small telephoto ratio could not be obtained which has aberrations of higher degree compensated for. If the negative lens went beyond the lower limit of the condition (2), it would be possible to correct the aberrations while meeting the achromatic requirement, but the secondary spectrum could not be reduced since the partial dispersion ratio becomes greater as the Abbe number is smaller. 
     The condition (3) is set for the shape of the lenses in the convergent lens group to provide a bright telephoto lens having a small secondary spectrum and a small telephoto ratio while meeting the foregoing conditions. If the upper limit of the condition (3) were exceeded, the aberrations of higher degree such as spherical aberrations and coma of the negative lens would be increased, and hence a bright lens could not be provided. If the lower limit of the condition (3) were exceeded, correction of the aberrations at the lens surfaces becomes out of balance, or the spherical aberration, coma and astigmatism could not be compensated for in balanced relation. 
     Futhermore, the telephoto lens so constructed to meet the above conditions according to the present invention includes a divergent lens group G n  composed of a first divergent lens group G n1  and a second divergent lens group G n2 . The first divergent lens group G n1  comprises lens surfaces r 9  -r 12  in the examples 1, 2, 3, 5 and 6, and comprises lens surface r 9  to r 14  in the example 4. The first divergent lens group G n1  is movable on the optical axis toward the image for enabling the lens to focus onto a closer object from a remoter object. 
     Data items on the examples of the present invention will be given below. Designated in the data items at r i  is the radius of curvature of the ith lens surface, d i  the thickness of the ith lens or the aerial space between adjacent lenses, n the refractive index with respect to d-line, ν the Abbe number, and θ g ,d the partial dispersion ratios with respect to g-line and d-line. 
     EXAMPLE 1 
     
         ______________________________________                                    
Focal length F = 100, F number 1:2.9, Angle of                            
view 2ω = 6.27 degrees                                              
r.sub.i       d.sub.i                                                     
                     n         ν θ.sub.g,d                       
______________________________________                                    
1       98.111    4.897  1.48749 70.1 1.22100                             
2       -78.667   0.385                                                   
3       47.937    4.488  1.49700 81.6 1.23153                             
4       -569.918  2.013                                                   
5       -82.115   2.436  1.76200 40.1                                     
6       86.052    0.511                                                   
7       44.901    4.998  1.49700 81.6 1.23153                             
8       -109.361  5.004                                                   
9       -86.907   2.562  1.80610 40.9                                     
10      81.705    8.209                                                   
11      -5912.352 2.566  1.72151 29.2                                     
12      54.626    7.712                                                   
13      90.599    1.794  1.51112 60.5                                     
14      -3569.930 9.415                                                   
15      34.579    1.282  1.51633 64.1                                     
16      13.108                                                            
Overall length (from the first lens plane to the image                    
plane): 89.139                                                            
 ##STR1##                                                                 
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     EXAMPLE 2 
     
         ______________________________________                                    
Focal length F = 100, F number 1:2.9, Angle of                            
view 2ω = 6.26 degrees                                              
r.sub.i      d.sub.i n         ν θ.sub.g,d                       
______________________________________                                    
1       185.319  4.926   1.48749 70.1 1.22100                             
2       -56.145  0.384                                                    
3       41.750   5.167   1.49700 81.6 1.23153                             
4       -178.364 1.365                                                    
5       -71.199  2.430   1.74320 49.3                                     
6       40.589   0.458                                                    
7       30.174   5.344   1.49700 81.6 1.23153                             
8       -299.029 4.264                                                    
9       -108.619 2.299   1.80610 40.9                                     
10      159.203  11.260                                                   
11      -441.007 2.558   1.72151 29.2                                     
12      -68.092  12.358                                                   
13      41.764   2.078   1.51112 60.5                                     
14      646.371  6.727                                                    
15      35.068   1.279   1.51633 64.1                                     
16      12.132                                                            
Overall length: 92.989                                                    
 ##STR2##                                                                 
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     EXAMPLE 3 
     
         ______________________________________                                    
Focal length F = 100, F number 1:2.9, Angle of                            
view 2ω = 6.25 degrees.                                             
r.sub.i      d.sub.i n         ν θ.sub.g,d                       
______________________________________                                    
1       91.304   4.117   1.54072 47.2 1.26463                             
2       -106.482 0.246                                                    
3       -223.623 2.435   1.78300 36.1                                     
4       49.623   0.812                                                    
5       59.130   4.802   1.49700 81.6 1.23153                             
6       -131.704 0.385                                                    
7       45.130   4.037   1.49700 81.6 1.23153                             
8       320.435  5.127                                                    
9       -382.985 1.946   1.80610 40.9                                     
10      77.324   0.590                                                    
11      86.236   2.346   1.72151 29.2                                     
12      696.091  12.456                                                   
13      21.973   2.535   1.54041 51.0                                     
14      36.600   21.140                                                   
15      18.449   1.282   1.58913 61.0                                     
16      10.554                                                            
Overall length: 95.222                                                    
 ##STR3##                                                                 
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     EXAMPLE 4 
     
         ______________________________________                                    
Focal length F = 100, F number 1:2.9, Angle of                            
view 2ω = 6.28 degrees.                                             
r.sub.i      d.sub.i n         ν θ.sub.g,d                       
______________________________________                                    
1       98.669   3.590   1.62004 36.3 1.28947                             
2       -221.024 0.279                                                    
3       36.366   4.798   1.43387 95.2 1.23246                             
4       742.199  0.478                                                    
5       65.203   3.569   1.49700 81.6 1.23153                             
6       -296.475 0.449                                                    
7       -248.696 3.085   1.80518 25.4                                     
8       65.468   10.700                                                   
9       -70.963  2.588   1.74950 35.3                                     
10      65.907   5.163                                                    
11      132.934  3.097   1.80518 25.4                                     
12      -55.244  0.513                                                    
13      306.877  2.052   1.67270 32.1                                     
14      72.011   18.461                                                   
15      11.824   1.862   1.80610 40.9                                     
16      10.145                                                            
Overall length: 90.826                                                    
 ##STR4##                                                                 
______________________________________                                    
 
    
     EXAMPLE 5 
     
         ______________________________________                                    
Focal length F = 100, F number 1:2.9, Angle of                            
view 2ω = 6.21 degrees                                              
r.sub.i       d.sub.i n         ν θ.sub.g,d                      
______________________________________                                    
1       67.602    4.952   1.48749 70.1 1.22100                            
2       -140.306  0.383                                                   
3       40.431    6.138   1.49700 81.6 1.23153                            
4       -103.959  0.554                                                   
5       -96.691   2.423   1.74320 49.3                                    
6       31.576    0.510                                                   
7       29.917    5.462   1.49700 81.6 1.23153                            
8       -1171.904 12.459                                                  
9       -60.204   1.469   1.80610 40.9                                    
10      136.017   5.446                                                   
11      -120.408  1.760   1.72151 29.2                                    
12      -39.924   11.883                                                  
13      25.068    1.656   1.69350 53.2                                    
14      75.232    6.319                                                   
15      29.476    1.148   1.65844 50.9                                    
16      12.495                                                            
Overall length: 92.506                                                    
 ##STR5##                                                                 
______________________________________                                    
 
    
     EXAMPLE 6 
     
         ______________________________________                                    
Focal length F = 100, F number 1:5.71, Angle of                           
view 2ω = 8.46 degrees                                              
r.sub.i      d.sub.i n         ν θ.sub.g,d                       
______________________________________                                    
1       66.521   1.553   1.48749 70.1 1.22100                             
2       -128.015 0.383                                                    
3       26.444   2.561   1.49700 81.6 1.23153                             
4       -73.748  0.414                                                    
5       -59.018  1.363   1.74320 49.3                                     
6       30.898   0.334                                                    
7       27.564   2.310   1.49700 81.6 1.23153                             
8       -70.358  6.045                                                    
9       -35.301  0.839   1.76200 40.1                                     
10      63.568   4.628                                                    
11      -106.265 1.234   1.72151 29.2                                     
12      -24.410  12.258                                                   
13      28.885   1.180   1.69350 53.2                                     
14      165.409  2.205                                                    
15      -59.655  0.676   1.56883 56.3                                     
16      17.731                                                            
Overall length: 79.353                                                    
 ##STR6##                                                                 
______________________________________                                    
 
    
     With the arrangement of the present invention, there is provided a bright telephoto lens having a small telephoto ratio and quite a small secondary spectrum with various aberrations such as spherical aberration and coma being corrected ro compensated for property, without using a special optical material of anomalous dispersion for the lens located most closely to the object. The telephoto lens of the invention is advantageous over telephoto lenses including a lens closest to the object which is made of an optical material in that the lens is less liable to get damaged and less subjected to poor performance due to temperature changes.