Semi-wide angle photographic lens having short overall length
BACKGROUND OF THE INVENTION 
     This invention relates to a novel photographic lens system having a relatively wide view angle of ±24° and an aperture ratio of 1:2.8. The lens is designed so as to have a short overall length. The length of the lens is approximately 0.5 F, where F is the overall focal length. This length is very short in such a class of lenses. However, the lens has a sufficiently long backfocal length with good compensation of various aberrations. 
     SUMMARY OF THE INVENTION 
     In view of the deficiencies in the prior art it is an object of this invention to provide for a wide angle lens having a short overall length. 
     It is another object of this invention to provide a wide angle lens of short overall length having a sufficiently long backfocal length with good compensation of various aberations. 
     Still another object of this invention is to provide a wide angle lens of compact design yet offering improved optical characteristics. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
     FIG. 1 is a lens construction according to the first Example of the invention; 
     FIG. 2 are graphs showing the various aberration curves for the lens of the first Example; 
     FIG. 3 is a lens construction according to the second Example of the invention; 
     FIG. 4 are graphs showing the various aberration curves for the lens of the second Example; 
     FIG. 5 is a lens construction according to the third Example of the invention; and 
     FIG. 6 are graphs showing the various aberration curves for the lens of the third Example. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the lens construction views of the lenses according to Examples 1 to 3, shown in FIGS. 1, 3 and 5, respectively, the individual lens elements are positioned along the optical axis. The lenses are shown such that the object side is on the left and the image side is on the right. The lens surfaces are designated by r 1 , r 2 , r 3 , . . . , r 11 , and the distance between two adjacent surfaces along the optical axis is designated by d j , where d j  is the distance between adjacent surfaces r j  and r j+1 . 
     The lens construction according to the invention will be hereinafter described. The lens is composed of six lens elements grouped into five components. A first component is a negative meniscus lens L 1  having a convex surface r 1  directed toward the object. A second component is a positive lens L 2 . A third component is a negative lens L 3 . A fourth component is composed of negative and positive lenses L 4 , L 5  cemented together along surface r 8 . The fourth component has a positive focal length. A fifth component is a positive lens L 6 . 
     The photographic lens of the above described construction is specifically designed to satisfy the following conditions: 
     
         (F/1.7)&lt;F.sub.1·2 &lt;(F/1.1)                        (1) 
    
     
         0.005 F&lt;d.sub.2 &lt;0.1 F                                     (2) 
    
     
         |F.sub.1·2·3 |&gt;(F/0.3) (3) 
    
     
         0.05 F&lt;d.sub.4 &lt;0.15 F                                     (4) 
    
     
         0.4&lt;Σ.sub.d &lt;0.7 F                                   (5) 
    
     where, 
     F is the overall focal length; 
     F 1 ·2·3 . . . i is the resultant focal length from the first lens component to the i-th lens component; 
     r j  is the radius of curvature of the j-th lens surface; 
     d j  is the lens thickness or the aerial space between the j-th surface and the (j+1)-th surface; 
     ν k  is the Abbe number of the k-th lens; and Σ d  is the overall lens length. 
     Each of the conditions will be described in detail. 
     Condition (1) of (F/1.7)&lt;F 1 ·2 &lt;(F/1.1) is required to obtain a sufficient backfocal length and to compensate for coma aberration and distortion aberration. When F 1 ·2 exceeds the lower limit of (F/1.7), cooperating with conditions (3) and (4), it is difficult to obtain a sufficient backfocal length. Conversely, when the upper limit of (F/1.1) is exceeded, though available to obtain a sufficient backfocal length, the radius of curvature becomes longer than required and also cooperating with condition (3), the focal length of the negative lens of the third component becomes too large. As a result, it is difficult to compensate for coma aberration and distortion aberration. 
     Condition (2) of 0.005 F&lt;d 2  &lt;0.1 F is required to miniaturize the lens system. When the upper limit of 0.1 F is exceeded, the overall length of the lens becomes disadvantageously large. Conversely, when the lower limit of 0.005 F is exceeded, though the lens overall length can be shortened, it is impossible to produce such a lens. 
     Condition (3) of |F 1 ·2·3 |&gt;(F/0.3) is required to determine the focal length of the negative lens L 3  of the third component cooperating with condition (1), to obtain a sufficient backfocal length and to compensate for the coma aberration. When |F 1 ·2·3 | is smaller than the lower limit and F 1 ·2·3 is positive, it is not only difficult to obtain a sufficient backfocal length but also the focal length of the negative lens L 3  of the third component increases so that Petzval&#39;s sum increases. This causes an increase of the field curvature. When F 1 ·2·3 is negative and the absolute value of F 1 ·2·3 is smaller than the lower limit value, though available to obtain a sufficient backfocal length, the focal lengths of the positive lenses disposed following the fourth lens component become too small so that coma aberration is deteriorated and at the same time this causes an increase of spherical aberration. 
     Condition (4) of 0.05 F&lt;d 4  &lt;0.15 F is required to maintain the size of the lens small and relates to the compensation of the chromatic aberration. When the upper limit of 0.15 F is exceeded, not only is the overall lens length increased, but the residual chromatic aberration of magnification remaining to the second lens component is also increased. It is difficult to compensate for it with the lens disposed following the second lens component. Conversely, when the lower limit of 0.05 F is exceeded, since the negative lens of the third lens component is defined by conditions (1) and (3), Petzval&#39;s sum increases. This causes an increase of the field curvature. 
     Condition (5) of 0.4 F&lt;Σ d  &lt;0.7 F is required to maintain the lens length small, which is one of the specific objects of the invention. When the upper limit of 0.7 F is exceeded, the overall length of the lens becomes excessive. This is inconsistent with the object of the invention. Conversely, when the lower limit of 0.4 F is exceeded, though the overall lens length is shortened, the end thicknesses of the lenses become smaller than required. It is also impossible to produce such a lens. 
     The semi-wide angle photographic lens according to the invention is designed to satisfy the above described conditions, having an aperture ratio 1:2.8 and a view angle of ±24°. Further, the overall length of the lens can be maintained very small. Further, the various aberrations can be well compensated. 
     The specific parameters for Examples 1 to 3 are given below. In the Examples, all linear dimensions are numerals such that the overall focal length F is estimated as 100. The aberration curves resulting from lenses according to Examples 1 to 3 are shown in FIGS. 2, 4 and 6, respectively. 
     EXAMPLE 1 
     
         ______________________________________                                    
OVERALL       APERTURE                                                    
FOCAL LENGTH  RATIO        VIEW ANGLE                                     
F = 100,      1 : 2.8      ± 24°                                
______________________________________                                    
      r.sub.1                                                             
            =   71.91 d.sub.1                                             
                          =   3.26 n.sub.1 = 1.72825                      
                                            ν.sub.1 = 28.5             
L.sub.1                                                                   
      r.sub.2                                                             
            =   42.26 d.sub.2                                             
                          =   1.59                                        
      r.sub.3                                                             
            =   43.90 d.sub.3                                             
                          =   9.18 n.sub.2 = 1.76200                      
                                            ν.sub.2 = 40.2             
L.sub.2                                                                   
      r.sub.4                                                             
            = -248.88 d.sub.4                                             
                          =   6.76                                        
      r.sub.5                                                             
            =  -59.63 d.sub.5                                             
                          =   4.11 n.sub.3 = 1.58913                      
                                            ν.sub.3 = 61.1             
L.sub.3                                                                   
      r.sub.6                                                             
            =   71.64 d.sub.6                                             
                          =   5.44                                        
      r.sub.7                                                             
            =  -47.40 d.sub.7                                             
                          =   3.36 n.sub.4 = 1.72825                      
                                            ν.sub.4 = 28.5             
L.sub.4                                                                   
      r.sub.8                                                             
            =    67.40                                                    
                      d.sub.8                                             
                          =   10.74                                       
                                   n.sub.5 = 1.72000                      
                                            ν.sub.5 = 50.2             
L.sub.5                                                                   
      r.sub.9                                                             
            =  -45.18 d.sub.9                                             
                          =   0.41                                        
      r.sub.10                                                            
            =  399.80 d.sub.10                                            
                          =   6.15 n.sub.6 = 1.76200                      
                                            ν.sub.6 = 40.2             
L.sub.6                                                                   
      r.sub.11                                                            
            =  -94.37                                                     
Backfocus = 95.967 = 0.96 F                                               
 ##STR1##                                                                 
 ##STR2##                                                                 
Σ.sub.d = 51.02 = 0.51 F                                            
Petzval&#39;s sum = 0.208                                                     
______________________________________                                    
 
    
     EXAMPLE 2 
     
         __________________________________________________________________________
OVERALL FOCAL LENGTH                                                      
                APERTURE RATIO                                            
                           VIEW ANGLE                                     
F = 100         1 : 2.8    ± 24°                                
__________________________________________________________________________
      r.sub.1 = 76.53                                                     
              d.sub.1 = 4.09                                              
                     n.sub.1 = 1.72825                                    
                             ν.sub.1 = 28.5                            
L.sub.1                                                                   
      r.sub.2 = 44.06                                                     
              d.sub.2 = 2.49                                              
      r.sub.3 = 49.22                                                     
              d.sub.3 = 9.67                                              
                     n.sub.2 = 1.80610                                    
                             ν.sub.2 = 40.9                            
L.sub.2                                                                   
      r.sub.4 = -173.55                                                   
              d.sub.4 = 6.20                                              
      r.sub.5 = -48.98                                                    
              d.sub.5 = 3.43                                              
                     n.sub.3 = 1.58913                                    
                             ν.sub.3 = 61.1                            
L.sub.3                                                                   
      r.sub.6 = 82.29                                                     
              d.sub.6 = 5.18                                              
      r.sub.7 = -64.49                                                    
              d.sub.7 = 3.35                                              
                     n.sub.4 = 1.76182                                    
                             ν.sub. 4 = 26.6                           
L.sub.4                                                                   
      r.sub.8 = 84.16                                                     
              d.sub.8 = 10.04                                             
                     n.sub.5 = 1.77250                                    
                             ν.sub.5 = 49.6                            
L.sub.5                                                                   
      r.sub.9 = -50.49                                                    
              d.sub.9 = 0.41                                              
      r.sub.10 = -175.36                                                  
              d.sub.10 = 5.76                                             
                     n.sub.6 = 1.80610                                    
                             ν.sub.6 = 40.9                            
L.sub.6                                                                   
      r.sub.11 = -63.53                                                   
__________________________________________________________________________
 Backfocus = 95.951 = 0.96 F                                              
 ##STR3##                                                                 
 ##STR4##                                                                 
 Σ.sub.d = 50.62 = 0.51 F                                           
 Petzval&#39;s sum = 0.195                                                    
 
    
     EXAMPLE 3 
     
         __________________________________________________________________________
OVERALL FOCAL LENGTH                                                      
                APERTURE RATIO                                            
                           VIEW ANGLE                                     
F = 100,        1 : 2.8    ± 24°                                
__________________________________________________________________________
      r.sub.1 = 73.13                                                     
              d.sub.1 = 3.44                                              
                     n.sub.1 = 1.72825                                    
                             ν.sub.1 = 28.5                            
L.sub.1                                                                   
      r.sub.2 = 40.94                                                     
              d.sub.2 = 1.69                                              
      r.sub.3 = 43.15                                                     
              d.sub.3 = 9.54                                              
                     n.sub.2 = 1.72342                                    
                             ν.sub.2 = 38.0                            
L.sub.2                                                                   
      r.sub.4 = -200.63                                                   
              d.sub.4 = 6.91                                              
      r.sub.5 = -61.94                                                    
              d.sub.5 = 4.85                                              
                     n.sub.3 = 1.58913                                    
                             ν.sub.3 = 61.1                            
L.sub.3                                                                   
      r.sub.6 = 84.07                                                     
              d.sub.6 = 5.49                                              
      r.sub.7 = -44.49                                                    
              d.sub.7 = 4.76                                              
                     n.sub.4 =  1.72825                                   
                             ν.sub.4 = 28.5                            
L.sub.4                                                                   
      r.sub.8 = 60.51                                                     
              d.sub.8 = 11.60                                             
                     n.sub.5 = 1.69350                                    
                             ν.sub.5 = 50.8                            
L.sub.5                                                                   
      r.sub.9 = -43.72                                                    
              d.sub.9 = 0.40                                              
      r.sub.10 = 241.43                                                   
              d.sub.10 = 6.43                                             
                     n.sub.6 = 1.72342                                    
                             ν.sub.6 = 38.0                            
L.sub.6                                                                   
      r.sub.11 = -102.08                                                  
__________________________________________________________________________
 Backfocus = 98.980 = 0.99 F                                              
 ##STR5##                                                                 
 ##STR6##                                                                 
 Σ.sub.d = 55.11 = 0.55 F                                           
 Petzval&#39;s sum = 0.244