Retrofocus lens
This application is a continuation-in-part of application Ser. No. 941,524, filed Sep. 11, 1978 and now abandoned. 
    
    
     This invention relates to lenses of the retrofocus type and more particularly relates to such a lens having a relatively fast speed and compact size. 
     Lenses of the retrofocus type having a large relative aperture and close focusing capability are presently available. Examples of such lenses of excellent quality are disclosed and claimed in U.S. Pat. No. 4,099,849 and one is marketed as a Vivitar Series I lens by Vivitar Corporation of Santa Monica, CA. However, this lens and others of high quality are relatively expensive and also are relatively large as compared to the normal 50mm equivalent focal length lens for a single lens reflex camera having a 24×36mm image frame. 
     Accordingly, the present invention provides a new and improved retrofocus lens of high quality, yet reduced cost, while being more compact and embodying the ability to focus on near objects with a magnification of essentially 0.1, while subtending a field of up to 84°. 
     The design and configuration of a lens embodying the invention results in a smaller overall length and small diameter front lens grouping which provides a more compact configuration than would normally be expected for a lens of comparable equivalent focal length and relative aperture. 
     A lens embodying the invention is well corrected over the focusing range normally available in a lens of this field angle and in the closer focusing distance provided by this lens. 
     Briefly stated, the invention in one form thereof comprises a lens of eight elements divided into front and rear groupings. The front grouping comprises a positive element followed by two negative menisci and is overall of negative power. The second grouping comprises a positive element, the aperture defining means, another positive element, followed by a concave element, a positive meniscus concave to the object, and a final bi-convex element. The second grouping is of positive power. Such a lens embodies certain parameters hereinafter pointed out. 
     During focusing, from infinity to near objects, all elements move forward while a portion of the rear grouping may move at a faster rate. 
     An object of this invention is to provide a new and improved lens of the retrofocus type. 
     Another object of this invention is to provide a retrofocus lens which is compact, has a large relative aperture, and will close focus to a magnification of 0.1 while subtending a field of up to 84° or larger. 
    
    
     The features of the invention which are believed to be novel are particularly pointed out and distinctly claimed in the concluding portion of this specification. The invention, however, both as to its organization and operation, together with further objects and advantages thereof may best be appreciated by reference to the following detailed description taken in conjunction with the drawings, wherein: 
     FIG. 1 is a side elevation of a lens form embodying the invention; 
     FIGS. 2A and 2B are graphical representations of astigmatic field curvature for a lens as set forth in Table 1, at infinity and at a magnification of 1.10, respectively; 
     FIGS. 3A and 3B are plots of the distortion curves of the lens of Table 1 at infinity and at a magnification of 1:10, respectively; 
     FIGS. 4A and 4B are graphical representations of astigmatic field curvature for a lens as set forth in Table 2, at infinity and at a magnification of 1.10, respectively; and 
     FIGS. 5A and 5B are plots of the distortion curves of the lens of Table 2 at infinity and at a magnification of 1:10,  respectively. 
    
    
     Referring now to FIG. 1, a lens embodying the invention may comprise eight air spaced elements L1-L8 having surfaces defined on radii R1-R16. 
     Elements L1-L3 form a first group G1 of negative overall power while elements L4-L8 define a second group G2 of positive overall power. 
     As will hereinafter be made apparent, during focusing from infinity to near distance to provide a one to ten magnification, there is differential movement of elements L1-L4 and L5-L8 to permit closer focusing. If close focusing is not an objective of the lens, the differential movement may not be utilized. 
     From the object side, the lens comprises a positive meniscus convex to the object L1, a second element L2 which is a negative meniscus convex to the object, and a third element L3 which is a negative meniscus convex to the object. Elements L1-L3 form front grouping G1 of negative power, having an equivalent focal length (EFL) F 1 . 
     The rear group G2 comprises a positive element L4 followed by an adjustable aperture defining means in the form of an iris I, an element L5 of positive power convex to the image, a bi-concave element L6, a positive meniscus L7 convex to the image, and an element L8 which is bi-convex. The elements L4-L8 comprise a second group G2 having an EEL F 2  . 
     A lens embodying the invention has the following parameters: 
     
         0.7F.sub.o &lt;F.sub.1 ≦1.0F.sub.o                     (1) 
    
     
         0.8F.sub.o &lt;F.sub.2 &lt;1.1F.sub.o                            (2) 
    
     
         52&lt;(V.sub.1 +V .sub.2 +V.sub.3)/3&lt;70                       (3) 
    
     
         23&lt;V.sub.4 &lt;33                                             (4) 
    
     where 
     F o  is the focus length of the overall lens, 
     F 1  is the equivalent focal length of the first grouping G1. 
     F 2  is the equivalent focal length of the second grouping G2, and 
     V 1 ,V 2 , V 3  and V 4  are the dispersions as measured by the Abbe numbers of elements L1, L2, L3 and L4. 
     The range selected under parameter (1) is such that the aberrations generated in the front grouping G1 may be well corrected in the second grouping G2. For example, if the absolute value of F 1  is smaller than the 0.7 F o , then the aberrations generated in the front group of the lens cannot be corrected without the rear grouping becoming unusually complex. Moreover, if F 1  should become larger than 1.0 F o , the overall dimensioning of the lens will increase and the object of compactness will be lost. 
     By maintaining the EFL F 2  of group G2 in the range specified in relation to F 1 , the EFL of group G1, the back focal length (BFL) of the lens is decreased, resulting in a shorter front vertex distance (VFD). If F 2  is held to be less than 1.1 F o , the power of the rear group will be sufficient to balance the aberrations generated in the strongly negative front group G1 and still maintain compactness of the lens. 
     In attempting to keep a wide angle lens compact, one of the most prevalent problems is the correction of distortion. Balancing the distortion may lead to chromatic variation of distortion. Therefore, the dispersion of the lens elements becomes critical. The parameters (3) and (4) relating to lens dispersion are necessary to favorably correct chromatic aberration, including lateral color and chromatic variation of distortion. 
     Prescriptions of six embodiments of the invention are set forth in Tables 1-6. In these embodiments, the lenses are scaled to a unity focal length and all dimensions are given as ratios thereof. The lens element thickness dimensions T are the thickness of the lens and the spacings S are the axial air spacings between the lens surfaces. The dispersion as measured by the Abbe number is given as V d , and the index of refraction is N d . Positive radii are struck from the right on the optical axis, while negative radii are struck from the left on the optical axis. 
     In the drawings, the lens surfaces are defined on radii R1-R16 from the object end. For simplicity, the facing surfaces of elements L4 and L5 are shown as plano in the drawings since these surfaces are defined on very large radii R8 and R9 which may be positive, negative, or plano, as shown by the examples. 
     
                       TABLE 1                                                     
______________________________________                                    
Relative Aperture = 2.0 F.sub.o = 1.0 Field Angle = 84°            
F.sub.1 = -0.79 BFL = 1.59 F.sub.2 = 0.99                                 
                             Spacing                                      
                   Lens      Between                                      
                   Element   Lens                                         
       Surface Radii                                                      
                   Thickness Elements                                     
Element                                                                   
       (R)         (T)       (S)     N.sub.d                              
                                          V.sub.d                         
______________________________________                                    
       R1 =    2.752                                                      
L1                     T.sub.1 = 0.167 1.697                              
                                            55.5                          
       R2 =    10.490                                                     
                               S.sub.1 = .008                             
       R3 =    1.421                                                      
L2                     T.sub.2 = 0.082 1.697                              
                                            55.5                          
       R4 =    .530                                                       
                               S.sub.2 = .387                             
       R5 =    1.340                                                      
L3                     T.sub.3 = 0.061 1.603                              
                                            60.7                          
       R6 =    0.580                                                      
                               S.sub.3 = .176                             
       R7 =    1.032                                                      
L4                     T.sub.4 = 0.245 1.717                              
                                            29.5                          
       R8 =    -9.947                                                     
                               S.sub.4 = .155                             
       R9 =    ∞                                                    
L5                     T.sub.5 = 0.221 1.603                              
                                            60.7                          
       R10 =   -0.799                                                     
                               S.sub.5 = .101                             
       R11 =   -0.780                                                     
L6                     T.sub.6  = 0.074                                   
                                       1.805                              
                                            25.5                          
       R12 =   2.379                                                      
                               S.sub.6 = .069                             
       R13 =   -2.431                                                     
L7                     T.sub.7 = 0.169 1.640                              
                                            60.2                          
       R14 =   -0.741                                                     
                               S.sub.7 = .008                             
       R15 =   4.377                                                      
L8                     T.sub.8 = 0.161 1.640                              
                                            60.2                          
       R16 =   -1.385                                                     
______________________________________                                    
 
    
     
                       TABLE 2                                                     
______________________________________                                    
Relative Aperture = 2.0 F.sub.o = 1.0 Field Angle = 72°            
F.sub.1 32  -0.996 BFL = 1.36 F.sub.2 = 0.926                             
                             Spacing                                      
                   Lens      Between                                      
                   Element   Lens                                         
       Surface Radii                                                      
                   Thickness Elements                                     
Element                                                                   
       (R)         (T)       (S)     N.sub.d                              
                                          V.sub.d                         
______________________________________                                    
       R1 =    1.976                                                      
L1                     T.sub.1 = 0.154 1.517                              
                                            64.2                          
       R2 =    10.526                                                     
                               S.sub.1 = 0.007                            
       R3 =    0.794                                                      
L2                     T.sub.2 = 0.070 1.678                              
                                            55.5                          
       R4 =    0.401                                                      
                               S.sub.2 = 0.203                            
       R5 =    2.113                                                      
L3                     T.sub.3 = 0.056 1.487                              
                                            70.4                          
       R6 =    0.717                                                      
                               S.sub.3 = 0.182                            
       R7 =    0.878                                                      
L4                     T.sub.4 = 0.193 1.755                              
                                            27.5                          
       R8 =    4.104                                                      
                               S.sub.4 = 0.132                            
       R9 =    ∞                                                    
L5                     T.sub.5 = 0.193 1.640                              
                                            60.2                          
       R10 =   -0.690                                                     
                               S.sub.5 = 0.085                            
       R11 =   -0.591                                                     
L6                     T.sub.6 = 0.053 1.805                              
                                            25.5                          
       R12 =   2.583                                                      
                               S.sub.6 = 0.066                            
       R13 =   -1.341                                                     
L7                     T.sub.7 = 0.140 1.658                              
                                            57.3                          
       R14 =   -0.610                                                     
                               S.sub.7 = 0.007                            
       R15 =   3.089                                                      
L8                     T.sub.8 = 0.193 1.697                              
                                            55.5                          
       R16 =   -1.236                                                     
______________________________________                                    
 
    
     
                       TABLE 3                                                     
______________________________________                                    
Relative Aperture = 2.0 F.sub.o = 1.0 Field Angle = 72°            
F.sub.1 = -0.94  BFL = 1.37 F.sub.2 = 0.93                                
                             Spacing                                      
                   Lens      Between                                      
                   Element   Lens                                         
       Surface Radii                                                      
                   Thickness Elements                                     
Element                                                                   
       (R)         (T)       (S)     N.sub.d                              
                                          V.sub.d                         
______________________________________                                    
       R1 =    1.552                                                      
L1                     T.sub.1 = 0.144 1.517                              
                                            64.2                          
       R2 =    8.704                                                      
                               S.sub.1 = 0.007                            
       R3 =    1.233                                                      
L2                     T.sub.2 = 0.070 1.667                              
                                            57.7                          
       R4 =    0.434                                                      
                               S.sub.2 = 0.290                            
       R5 =    1.782                                                      
L3                     T.sub.3 = 0.056 1.487                              
                                            70.4                          
       R6 =    0.754                                                      
                               S.sub.3 = 0.148                            
       R7 =    0.828                                                      
L4                     T.sub.4 = 0.193 1.757                              
                                            28.6                          
       R8 =    3.553                                                      
                               S.sub.4 = 0.133                            
       R9 =    3.592                                                      
L5                     T.sub.5 = 0.193 1.643                              
                                            59.8                          
       R10 =   -0.771                                                     
                               S.sub. 5 = 0.082                           
       R11 =   -0.616                                                     
L6                     T.sub.6 = 0.054 1.805                              
                                            25.5                          
       R12 =   1.955                                                      
                               S.sub.6 = 0.075                            
       R13 =   -1.320                                                     
L7                     T.sub.7 = 0.133 1.659                              
                                            58.4                          
       R14 =   -0.627                                                     
                               S.sub.7 = 0.007                            
       R15 =   3.027                                                      
L8                     T.sub.8 = 0.148 1.697                              
                                            55.5                          
       R16 =   -1.280                                                     
______________________________________                                    
 
    
     
                       TABLE 4                                                     
______________________________________                                    
Relative Aperture = 2.0 F.sub.o = 1.0 Field Angle = 72°            
F.sub.1 = -0.89 BFL = 1.39 F.sub.2 = 0.91                                 
                             Spacing                                      
                   Lens      Between                                      
                   Element   Lens                                         
       Surface Radii                                                      
                   Thickness Elements                                     
Element                                                                   
       (R)         (T)       (S)     N.sub.d                              
                                          V.sub.d                         
______________________________________                                    
       R1 =    1.736                                                      
L1                     T.sub.1 = 0.163 1.697                              
                                            55.5                          
       R2 =    7.514                                                      
                               S.sub.1 = 0.004                            
       R3 =    1.517                                                      
L2                     T.sub.2 = 0.082 1.697                              
                                            55.5                          
       R4 =    0.440                                                      
                               S.sub.2 = 0.221                            
       R5 =    1.288                                                      
L3                     T.sub.3 = 0.061 1.487                              
                                            70.4                          
       R6 =    0.652                                                      
                               S.sub.3 = 0.151                            
       R7 =    0.866                                                      
L4                     T.sub.4 = 0.193 1.744                              
                                            27.6                          
       R8 =    12.092                                                     
                               S.sub.4 = 0.132                            
       R9 =    5.954                                                      
L5                     T.sub.5 = 0.193 1.651                              
                                            59.1                          
       R10 =   -0.725                                                     
                               S.sub.5 = 0.049                            
       R11 =   -0.646                                                     
L6                     T.sub.6 = 0.097 1.805                              
                                            25.5                          
       R12 =   1.889                                                      
                               S.sub.6 = 0.067                            
       R13 =   -1.917                                                     
L7                     T.sub.7 = 0.169 1.686                              
                                            56.2                          
       R14 =   -0.661                                                     
                               S.sub.7 = 0.004                            
       R15 =   4.018                                                      
L8                     T.sub.8 = 0.166 1.640                              
                                            60.2                          
       R16 =   -1.432                                                     
______________________________________                                    
 
    
     
                       TABLE 5                                                     
______________________________________                                    
Relative Aperture = 2.0 F.sub.o = 1.0 Field Angle = 84°            
F.sub.1 = -0.81  BFL = 1.6 F.sub.2 = 0.97                                 
                             Spacing                                      
                   Lens      Between                                      
                   Element   Lens                                         
       Surface Radii                                                      
                   Thickness Elements                                     
Element                                                                   
       (R)         (T)       (S)     N.sub.d                              
                                          V.sub.d                         
______________________________________                                    
       R1 =    2.384                                                      
L1                     T.sub.1 = 0.201 1.542                              
                                            61.1                          
       R2 =    13.791                                                     
                               S.sub.1 = 0.008                            
       R3 =    1.354                                                      
L2                     T.sub.2 = 0.082 1.697                              
                                            55.5                          
       R4 =    0.510                                                      
                               S.sub.2 = 0.320                            
       R5 =    1.449                                                      
L3                     T.sub.3 = 0.061 1.487                              
                                            70.4                          
       R6 =    0.543                                                      
                               S.sub.3 = 0.199                            
       R7 =    1.042                                                      
L4                     T.sub.4 = 0.225 1.705                              
                                            29.5                          
       R8 =    -6.649                                                     
                               S.sub.4 = 0.157                            
       R9 =    -84.000                                                    
L5                     T.sub.5 = 0.225 1.614                              
                                            52.7                          
       R10 =   -0.743                                                     
                               S.sub.5 = 0.062                            
       R11 =   -0.727                                                     
L6                     T.sub.6 = 0.074 1.805                              
                                            25.5                          
       R12 =   2.171                                                      
                               S.sub.6 = 0.066                            
       R13 =   -3.165                                                     
L7                     T.sub.7 = 0.169 1.618                              
                                            61.1                          
       R14 =   -0.721                                                     
                               S.sub.7 = 0.008                            
       R15 =   6.386                                                      
L8                     T.sub.8 = 0.161 1.640                              
                                            60.2                          
       R16 =   -1.353                                                     
______________________________________                                    
 
    
     
                       TABLE 6                                                     
______________________________________                                    
Relative Aperture = 2.0 F.sub.o = 1.0 Field Angle = 84°            
F.sub.1 = -0.79  BFL = 1.59 F.sub.2 = 0.99                                
                             Spacing                                      
                   Lens      Between                                      
                   Element   Lens                                         
       Surface Radii                                                      
                   Thickness Elements                                     
Element                                                                   
       (R)         (T)       (S)     N.sub.d                              
                                          V.sub.d                         
______________________________________                                    
       R1 =    2.153                                                      
L1                     T.sub.1 = 0.203 1.487                              
                                            70.4                          
       R2 =    17.462                                                     
                               S.sub.1 = 0.004                            
       R3 =    1.481                                                      
L2                     T.sub.2 = 0.082 1.697                              
                                            55.5                          
       R4 =    0.520                                                      
                               S.sub.2 = 0.368                            
       R5 =    1.513                                                      
L3                     T.sub.3 = 0.061 1.487                              
                                            70.4                          
       R6 =    0.547                                                      
                               S.sub.3 = 0.163                            
       R7 =    1.231                                                      
L4                     T.sub.4 = 0.225 1.753                              
                                            27.3                          
       R8 =    -7.445                                                     
                               S.sub.4 = 0.201                            
       R9 =    ∞                                                    
L5                     T.sub.5 = 0.225 1.669                              
                                            57.6                          
       R10 =   -0.758                                                     
                               S.sub.5 = 0.065                            
       R11 =   -0.877                                                     
L6                     T.sub.6 = 0.174 1.805                              
                                            25.5                          
       R12 =   2.056                                                      
                               S.sub.6 = 0.057                            
       R13 =   -6.073                                                     
L7                     T.sub.7 = 0.169 1.607                              
                                            61.7                          
       R14 =   -0.836                                                     
                               S.sub.7 = 0.004                            
       R15 =   4.717                                                      
L8                     T.sub.8 = 0.154 1.640                              
                                            60.2                          
       R16 =   -1.710                                                     
______________________________________                                    
 
    
     In the foregoing embodiments, the last four elements may move differentially with respect to the first four elements, but at a linear rate. In Tables 1-6, the given spacing S4 is at infinity. The spacings S4 at a magnification of 1:10 are given below in Table 7, as well as the EFL&#39;s F 1  and F 2 . 
     
                       TABLE 7                                                     
______________________________________                                    
Table      S.sub.4    F.sub.1     F.sub.2                                 
______________________________________                                    
1          .135       -0.79       0.99                                    
2          .116        -0.996      0.926                                  
3          .117       -0.94       0.93                                    
4          .116       -0.89       0.91                                    
5          .135       -0.81       0.97                                    
6          .180       -0.79       0.99                                    
______________________________________                                    
 
    
     FIGS. 2A and 2B represent the astigmatic field curves of the lens of Table 1 at infinity and at 1:10 magnification, respectively. The ordinate is semi-field angle and the abscissi are deviations from the image plane in millimeters. FIGS. 3A and 3B represent the degree of correction of astigmatism of the lens of FIG. 1 at infinity and at a magnification of 1:10, respectively. The ordinates are semi-field angle and the abscissi are per cent distortion. The curves S and T represent sagittal and tangential sags of light, respectively. FIGS. 3A and 3B show the lens to be well corrected for astigmatism, a primary consideration in high speed lenses of large relative aperture. 
     FIGS. 4A and 4B represent the astigmatic field curves of the lens of Table 2 at infinity and at 1:10 magnification, respectively. The ordinate is semi-field angle and the abscissi are deviations from the image plane in millimeters. FIGS. 5A and 5B represent the degree of correction of astigmatism of the lens of FIG. 1 at infinity and at a magnification of 1:10,  respectively. The ordinates are semi-field angle and the abscissi are per cent distortion. The curves S and T represent sagittal and tangential sags of light, respectively. FIGS. 5A and 5B show the lens to be well corrected for astigmatism, a primary consideration in high speed lenses of large relative aperture. 
     It may thus be seen that the objects of the invention set forth as well as those made apparent from the foregoing description are efficiently attained. While preferred embodiments of the invention have been set forth for purposes of disclosure, modification to the disclosed embodiments of the invention as well as other embodiments thereof may occur to those skilled in the art. Accordingly, the appended claims are intended to cover all embodiments of the invention and modifications to the disclosed embodiments which do not depart from the spirit and scope of the invention.