Wide angle photographic lens
BACKGROUND OF THE INVENTION 
     The present invention relates to a wide angle photographic lens, particularly to an inverted-telephoto or retrofocus photographic lens. 
     It is well known that the retrofocus lens was developed to provide a long back focus to small focal length lenses. In such lenses compensation of aberrations is difficult because of the asymmetrical arrangement of the divergent lens group at the object end of the convergent lens group. Correction of distortion, curvature of field and halo of the off-axial sagital rays is particularly difficult. The smaller the length of the lens, the more difficult is the compensation. 
     In a conventional retrofocus type lens, the number of the lenses is increased to overcome the difficulty of correcting aberrations. This, however, is undesirable because it raises manufacturing costs. 
     The inventors of the present invention have described retrofocus type lenses consisting of a small number of lenses German Patent Publications 1062028 and 1187393. An investigation has been made to decrease the total lens length to improve the aberrations and increase the picture angle and the back focus of a photographing lens, mentioned in the German Patent Publication 1187393, whose F-number is 1:2.8, picture angle 2ω is 62°, back focus is 1.053f and lens length (the distance between the first surface and the last surface) is 1.274f. 
     SUMMARY OF THE INVENTION 
     A purpose of the present invention is to provide a compact lens while limiting the disadvantages of prior art lenses. 
     Another purpose of the present invention is to correct various aberrations, especially distortion, curvature of field or halo of the off-axial sagital rays. 
     Still another purpose of the present invention is to provide a lens with a long back focus suitable for an interchangeable lens for a single lens reflex camera. 
     The lens in accordance with the present invention comprises 5 groups of 6 lenses in which the first group is a negative meniscus lens having a convex surface toward the object, the second group is a positive lens consisting of a bi-convex lens and a negative lens cemented to each other, the third group is a bi-concave lens, the fourth lens is a positive meniscus lens having a convex surface for facing the object, the fifth group is a positive lens having a convex surface with a small radius of to be directed toward the object, a diaphragm is arranged between the second and the third group, and in which the following conditions prevail: 
     
         0.90f&lt;L&lt;1.25f                                              (1) 
    
     
         1.5f&lt;|f.sub.1 |&lt;2.2f (f.sub.1 &lt;0)        (2) ##EQU1## 
    
     
         Nv&gt;Nc and Nv&gt;1.7                                           (5) 
    
     wherein; 
     f: Focal length of the whole system 
     L: Total length (distance from the first surface to the last surface along the axis) 
     f 1  : Focal length of the first group 
     R 1  : Radius of curvature of the surface of the lens toward the object, of the first group 
     R 2  : Radius of curvature of the surface of the lens toward the image, of the first group 
     R 4  : Radius of curvature of the surface of the cemented lens of the second group 
     N 2  : Refractive index of the material for the lens toward the object, of the second group 
     N 3  : Refractive index of the material for the lens toward the image, of the second group 
     Nv: Mean refractive index of the material for all of the positive lenses 
     Nc: Mean refractive index of the material for all of the negative lenses 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a section of a first example of a lens embodying the invention. 
     FIGS. 2A-2C show longitudinal aberration diagrams of the first example. 
     FIG. 2D shows lateral aberration diagram of the first example. 
     FIG. 3 shows a section of a second example of a lens embodying the invention. 
     FIGS. 4A-4C show longitudinal aberration diagrams of the second example. 
     FIG. 4D shows lateral aberration diagram of the second example. 
     FIG. 5 shows a section of a third example of a lens embodying the invention. 
     FIGS. 6A-6C show longitudinal aberration diagrams of the third example. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The first of the aforementioned prevailing conditions (0.90f&lt;L&lt;1.25f) refers to the total length of the lens. Here when L is larger than the upper limit (L&gt;1.25f) the total length can not be said to be shortened. When L is smaller than the lower limit (L&lt;0.90f), the correction of aberrations becomes difficult, so that it is hard to produce a lens with which a superior picture quality can be obtained. 
     When the total length has been selected in accordance with the first condition, it is essential that the first group be chosen with a high refractive power in comparison with the conventional lens of this type in order to obtain a sufficient back focus. 
     The second condition (1.5f&lt;|f 1  |&lt;2.2f) refers to the long back focus. Here when |f 1  | is larger than the upper limit (|f 1  |&gt;2.2f), the ultimate purpose can not be realized. When |f 1  | is smaller than the lower limit (|f 1  |&lt;1.5f), this means an increase of the refractive power. Hereby, if the refractive power is increased excessively, the superior correction of the aberration becomes difficult even when the condition to be explained later is satisfied. 
     On the other hand, when the total length as well as the refractive power of the first group are chosen within the above mentioned range, the aberration correction by means of a conventional lens arrangement is generally difficult. In accordance with the the present invention, this difficulty is overcome by giving a strong meniscus shape to the first group in comparison with such a first group in a conventional wide angle lens, and forming the cemented surface in the second group as a strong positive refractive surface. 
     The third condition (0.20&lt;(R 1  -R 2 )/(R 1  +R 2 )&lt;0.31) refers to the satisfactory correction of the distortion as well as the curvature of field by according to a strong meniscus shape to the first group, whereby when (R 1  -R 2 )/(R 1  +R 2 ) is larger than the upper limit (0.31), the distortion as well as the field curvature can not be corrected satisfactorily. When on the other hand, (R 1  -R 2 )/(R 1  +R 2 ) is smaller than the lower limit (0.20), the positive spherical aberration, the negative coma and the halo due to the off-axial sagital beam taking place in the first group are increased beyond the correction of the lens group arranged closer to the image side than the second group. 
     At this time generally, the spherical aberration is over-corrected, the coma under-corrected and the off-axial sagital beam over-corrected when the radius of curvature of the first group is chosen within the aforementioned ranges. 
     The fourth condition serves effectively to correct the aforementioned aberrations due to the above third condition by forming the cemented surface of the second group as a strongly positive refractive surface. When (N 3  -N 2 )/R 4  in the condition 0.34/f&lt;(N 3  -N 2 )/R 4  &lt;0.50/f is smaller than the lower limit (0.34/f), the correction of the above mentioned aberrations is difficult. When (N 3  -N 2 )/R 4  is larger than the upper limit (0.50/f) the spherical aberration of higher degree, the coma of higher degree and so on taking place at the cemented surface are increased. This is undesirable. 
     The fifth condition serves to decrease the arch in the middle part of the aberration diagram of the curvature of field due to the decreased total length. When the condition Nv&gt;Nc, Nv&gt;1.7 is not satisfied, the Petzval sum becomes large, which makes it difficult to correct the central arch. 
     FIG. 1 shows a first example which satisifies the afore mentioned conditions. FIGS. 2A-2D respectively show the spherical aberration, the astigmatism, the distortion and the lateral aberration of the embodiment in FIG. 1. FIGS. 3 and 5 respectively show the second and the third examples, while FIGS. 4A-4D and FIGS. 6A-6D show the respective aberrations. In FIGS. 2B, 4B and 6B, S is the sagital astigmatic line while M is the meridional astigmatic line. 
     EXAMPLE 1 
     
         ______________________________________                                    
f = 1  F/2.8    2ω = 64°  b.f = 1.1033                       
No.    R           D         Nd       Vd                                  
______________________________________                                    
1      0.6056      0.0507    1.51633  64.1                                
2      0.3534      0.4314                                                 
3      0.9877      0.1181    1.8061   40.9                                
4      -0.4909     0.0620    1.60342  38.0                                
5      4.0782      0.1857                                                 
6      -0.5410     0.0428    1.74077  27.8                                
7      2.2224      0.0225                                                 
8      -1.7714     0.0796    1.6935   53.3                                
9      -0.4768     0.0042                                                 
10     6.4449      0.0676    1.6935   53.3                                
11     -1.1735                                                            
ΣD = 1.0646 ( = L/f)                                                
 ##STR1##                                                                 
 ##STR2##                                                                 
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         __________________________________________________________________________
Aberration of Example 1                                                   
No.                                                                       
   I       II     III    P      V                                         
__________________________________________________________________________
1  1.011660                                                               
           0.112329                                                       
                  0.012472                                                
                         0.562382                                         
                                0.063829                                  
2  -10.020367                                                             
           1.499647                                                       
                  -0.224437                                               
                         -0.963721                                        
                                0.177820                                  
3  2.742492                                                               
           0.993804                                                       
                  0.360127                                                
                         0.451966                                         
                                0.294280                                  
4  6.179866                                                               
           -1.391036                                                      
                  0.313110                                                
                         0.142598                                         
                                -0.102576                                 
5  0.096242                                                               
           -0.182361                                                      
                  0.345538                                                
                         -0.092297                                        
                                -0.479844                                 
6  -6.675520                                                              
           1.594289                                                       
                  -0.380758                                               
                         -0.786735                                        
                                0.278828                                  
7  - 1.677963                                                             
           -1.393091                                                      
                  -1.156582                                               
                         -0.191515                                        
                                -1.119227                                 
8  0.080916                                                               
           0.210579                                                       
                  0.548017                                                
                         -0.231222                                        
                                0.824437                                  
9  5.295434                                                               
           -0.667528                                                      
                  0.084147                                                
                         0.859031                                         
                                -0.118895                                 
10 -0.000351                                                              
           0.005702                                                       
                  -0.092654                                               
                         0.063552                                         
                                0.472858                                  
11 3.649237                                                               
           -0.765887                                                      
                  0.160741                                                
                         0.349030                                         
                                -0.106989                                 
Σ                                                                   
   0.681647                                                               
           0.016448                                                       
                  -0.030277                                               
                         0.163069                                         
                                0.184521                                  
__________________________________________________________________________
 I: Spherical aberration                                                  
 II: Coma                                                                 
 III: Astigmatism                                                         
 P: Petzval sum                                                           
 V: Distortion                                                            
 
    
     EXAMPLE 2 
     
         ______________________________________                                    
f = 1  F/2.8    2ω = 64°  b.f = 1.1246                       
No.    R           D         Nd       Vd                                  
______________________________________                                    
1      0.7606      0.0507    1.58313  59.4                                
2      0.4085      0.4414                                                 
3      1.0123      0.2048    1.8061   40.9                                
4      -0.5400     0.0563    1.61293  37.                                 
5      8.9289      0.2140                                                 
6      -0.5269     0.0421    1.7552   27.5                                
7      2.7839      0.0225                                                 
8      -1.7639     0.0800    1.6935   53.2                                
9      -0.4769     0.0042                                                 
10     8.6391      0.0676    1.7725   49.6                                
11     -1.2443                                                            
ΣD = 1.1836 ( = L/f)                                                
 ##STR3##                                                                 
 ##STR4##                                                                 
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         __________________________________________________________________________
Aberration of Example 2                                                   
No.                                                                       
   I      II     III    P      V                                          
__________________________________________________________________________
1  0.528842                                                               
          0.130281                                                        
                 0.032095                                                 
                        0.484299                                          
                               0.127214                                   
2  -8.257174                                                              
          1.166295                                                        
                 -0.164735                                                
                        -0.901798                                         
                               0.150644                                   
3  3.210762                                                               
          0.992494                                                        
                 0.306794                                                 
                        0.440877                                          
                               0.231116                                   
4  4.606003                                                               
          -1.069221                                                       
                 0.248205                                                 
                        0.122797                                          
                               -0.086123                                  
5  0.170149                                                               
          -0.247523                                                       
                 0.360084                                                 
                        -0.042559                                         
                               -0.461918                                  
6  -7.079107                                                              
          1.477163                                                        
                 -0.308276                                                
                        -0.816523                                         
                               0.234740                                   
7  -1.636783                                                              
          -1.403723                                                       
                 -1.203849                                                
                        -0.154552                                         
                               -1.164980                                  
8  0.102679                                                               
          0.245284                                                        
                 0.585945                                                 
                        -0.232157                                         
                               0.845146                                   
9  5.366247                                                               
          -0.553558                                                       
                 0.057103                                                 
                        0.858628                                          
                               -0.094463                                  
10 -0.000559                                                              
          0.007023                                                        
                 -0.088272                                                
                        0.050448                                          
                               0.475391                                   
11 3.684435                                                               
          -0.724842                                                       
                 0.142599                                                 
                        0.350266                                          
                               -0.096962                                  
Σ                                                                   
   0.696494                                                               
          0.019673                                                        
                 -0.032306                                                
                        0.159724                                          
                               0.159805                                   
__________________________________________________________________________
 
    
     EXAMPLE 3 
     
         ______________________________________                                    
f = 1  F/2.8    2ω = 64°  b.f = 1.0522                       
No.    R           D         Nd       Vd                                  
______________________________________                                    
1      0.4858      0.0507    1.48749  70.1                                
2      0.3116      0.3595                                                 
3      0.8666      0.1153    1.8061   40.9                                
4      -0.4479     0.0563    1.59551  39.2                                
5      3.5840      0.1043                                                 
6      -0.6011     0.0918    1.7847   26.2                                
7      1.5842      0.0225                                                 
8      -2.4692     0.0800    1.7725   49.6                                
9      -0.4950     0.0042                                                 
10     -40.2080    0.0676    1.7725   49.6                                
11     -1.5202                                                            
ΣD = 0.9522 ( = L/f)                                                
 ##STR5##                                                                 
 ##STR6##                                                                 
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