Inverted telephoto type wide angle lens system
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an inverted telephoto type wide angle lens system having a relatively high aperture ratio of greater than 1/2.8, a relatively wide field angle reaching 75 degrees and a back focal length greater than the focal length of the lens system. 
     2. Description of the Prior Art 
     Retrofocus lens systems have been suggested in the prior art with a negative front lens group to obtain a sufficiently long back focal length. For example, a negative front lens group formed from a combination of a pair of negative single lenses and a positive single lens is suggested in U.S. Pat. No. 3,862,794. 
     The prior art has strived to provide an increased field angle while still maintaining a light weight compact lens structure. In attempts to achieve this compactness, it has been recommended to locate a positive single lens, preferably in the form of a positive meniscus lens convex to the object side, at the front of the lens system and to arrange a pair of negative lenses, preferably each in front of a negative meniscus lens convex to the object side, at the image side of the positive single lens. Reference is made to U.S. Pat. No. 3,973,832 to disclose such a lens system. 
     The competitive rigors of the camera industry has placed a constant demand on the lens designers to provide ever increasing optical performances while further compacting and making light weight lens systems. Accordingly, even though the prior art is crowded in attempts to achieve improved optical performance, there is still a demand to provide an improved inverted telephoto type wide angle lens system. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to improve an inverted telephoto type wide angle lens system while maintaining a balanced correction of aberration. 
     It is another object of the present invention to provide a relatively high aperture ratio with a relatively wide field angle approximating 75° while maintaining sufficient illumination of the marginal areas of the image transmitted to the focal plane. 
     It is a further object of the present invention to improve such a lens system while maintaining a compactness in size. 
     The inverted telephoto type wide angle lens system of the present invention includes, from the object to image side, a first lens group having a positive single lens, a first negative meniscus single lens convex to the object side and a second negative meniscus single lens convex to the object side. A second lens group includes at least one positive lens while a third lens group includes, from the object to the image side, a first positive meniscus single lens convex to the image side, a biconcave single lens, a second positive meniscus single lens convex to the image side and a positive single lens. In addition, the inverted telephoto type wide angle lens system of the present invention fulfills the following conditions: 
     
         ______________________________________                                    
0.75f &lt;  |f.sub.1,2,3 |                                 
                     &lt; 1.25f    f.sub.1,2,3 &lt; 0                           
10f&lt;     f.sub.1     &lt; 20f                                                
6.5f &lt;   |r.sub.2 |                                     
                     &lt; 22f      r.sub.2 &lt; 0                               
1.3f &lt;   f.sub.5     &lt; 1.9f                                               
______________________________________                                    
 
    
     wherein: 
     f 1 , 2, 3 represents the focal length of the first lens group; 
     f 1  represents the focal length of the positive single lens in the first lens group; 
     r 2  represents the radius of curvature of the image side surface of the positive single lens in the first lens group; 
     f 5  represents the focal length of the first positive meniscus single lens in the third lens group; and 
     f represents the focal length of the whole lens system. 
     The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The present invention, both as to its organization and manner of operation, together with further objects and advantages thereof, may best be understood by reference to the following description, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 represents a schematic view of a first embodiment of the present invention; 
     FIGS. 2a, 2b and 2c represent graphic plots of the various aberrations of the first embodiment; 
     FIG. 3 represents a schematic view of a second embodiment of the present invention; 
     FIGS. 4a, 4b and 4c represent graphic plots of the various aberrations of the second embodiment; 
     FIG. 5 represents a schematic view of a third embodiment of the present invention; 
     FIGS. 6a, 6b and 6c represent graphic plots of the various aberrations of the third emobidiment; 
     FIG. 7 represents a schematic view of a fourth embodiment of the present invention, and 
     FIGS. 8a, 8b and 8c represent graphic plots of the various aberrations of the fourth embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description is provided to enable any person skilled in the optical and camera field to make and use the invention and sets forth the best modes contemplated by the inventor for carrying out his invention. Various modifications, however, will remain readily apparent to those skilled in these arts since the generic principles of the present invention have been defined herein specifically to provide a relatively economical and easily manufactured inverted telephoto type wide angle lens system. 
     The derivation of the formulae and the relation of the lens powers set forth herein can be accomplished with the assistance of a computer. The present invention represents the parameters of a compromise balance of acceptable aberrations which can be relatively easily manufactured to provide a low cost lens system for utilization with a 35 mm SLR camera. 
     As can be seen in each of the embodiments of the present invention disclosed in FIGS. 1, 3, 5 and 7, an inverted telephoto type wide angle lens system having three lens groups is provided. As can be appreciated, the symbols which designate the particular lenses should be understood in the specification and claims to be interpreted broadly enough with respect to certain lens components to cover either a single lens element or an equivalent comprising a group of two or more lens elements such as a doublet. 
     Referring to FIG. 1, a schematic lens diagram of a first embodiment of the present invention is disclosed. While the specific dimensions of the lens systems and individual lens elements will vary in the succeeding embodiments of the present invention, the general principles of the present invention can be understood with reference to FIG. 1. It should be noted that the embodiment disclosed in FIG. 7 differs in that the second lens group will comprise two separate lens elements. Also in FIG. 5, the second lens group consists of a doublet as opposed to a single lens element. 
     Taking these modifications into consideration, and referring to FIG. 1, the present invention can be generally defined, from the object to image side, as a first lens group (L 1 , L 2  and L 3 ) consisting of a positive single lens (L 1 ), a first negative meniscus single lens (L 2 ) convex to the object side and a second negative meniscus single lens (L 3 ) convex to the object side; a second lens group (L 4 ) including at least a positive lens (L 4 ), and a third lens group (L 5 , L 6 , L 7  and L 8 ) consisting, from the object to the image side, of a first positive meniscus single lens (L 5 ) convex to the image side, a biconcave single lens (L 6 ) and a second positive meniscus single lens (L 7 ) convex to the image side and a positive single lens (L 8 ). The lens system of the present invention fulfills the following conditions: 
     
         ______________________________________                                    
(1)      0.75f &lt;  |f.sub.1,2,3 |                        
                          &lt; 1.25f, f.sub.1,2,3 &lt; 0                        
(2)      10f &lt;    f.sub.1 &lt; 20f                                           
(3)      6.5f &lt;   |r.sub.2 |                            
                          &lt; 22f,   r.sub.2 &lt; 0                            
(4)      1.3f &lt;   f5      &lt; 1.9f                                          
______________________________________                                    
 
    
     wherein: 
     f 1 , 2, 3 represents the focal length of the first lens group (L 1 , L 2  and L 3 ); 
     f 1  represents the focal length of the positive single lens (L 1 ) in the first lens group; 
     r 2  represents the radius of curvature of the image side surface (r 2 ) of the positive single lens (L 1 ) in the first lens group; 
     f 5  represents the focal length of the first positive meniscus single lens (L 5 ) in third lens group; and 
     f represents the focal length of the whole lens system. 
     Above conditions (1), (2) and (3) relate to the first lens group located at the front of the lens system. According to the present invention, the front positive lens (L 1 ) is given a relatively weak positive refractive power in contrast with the prior art to lessen the negative refractive powers required of the subsequent pair of negative meniscus lenses (L 2  and L 3 ) for achieving a necessary negative refractive power of the first lens group (L 1 , L 2 , and L 3 ) as a whole. This design approach reduces the aberrations generated in the first lens group (L 1 , L 2  and L 3 ). The first lens group (L 1 , L 2  and L 3 ) of the present invention is further characterized by the specific shape of the front positive lens (L 1 ) to insure a sufficient illumination of the marginal area of an image in the focal plane. 
     Specifically, condition (1) defines the negative refractive power of the first lens group (L 1 , L 2  and L 3 ). If |f 1 , 2, 3 | is excessively reduced to violate the lower limit of condition (1), then the first lens group (L 1 , L 2  and L 3 ) will have an excessive negative refractive power which increases both the negative distortion and astigmatism to a degree that they cannot be practically corrected by the remaining image side lens group (L 4  to L 8 ). If the upper limit of the range is violated then a desirable long back focal length of the system cannot be obtained because of insufficient negative refractive power of the first lens group (L 1 , L 2  and L 3 ). 
     Conditions (2) and (3) define the refractive power and shape of the front lens (L 1 ). If f 1  is reduced below the lower limit of condition (2), the positive refractive power of the front lens (L 1 ) will be so strong that greater negative refractive powers would have to be provided to the pair of negative meniscus lenses (L 2  and L 3 ) to maintain the overall necessary negative refractive power of the first lens group (L 1 , L 2  and L 3 ). By providing such strong refractive powers for lenses (L 1 ), (L 2 ) and (L 3 ) will inevitably result in increased distortion and astigmatism. On other other hand, an excessively great focal length f 1  of the positive lens (L 1 ) will violate the upper limit of condition (2) and will lose an effective ability to correct distortion. 
     With respect to condition (3), the positive refractive power of surface (r 2 ) will be so strong that over-corrections of lateral chromatic aberration and distortion would result if |r 2  | is excessively reduced to violate the lower limit of condition (3). On the contrary, if |r 2  | is excessively increased by violating the upper limit of condition (3), the expected use of surface (r 2 ) to correct distortion will not be effective. Additionally, in case that |r 2  | is excessively increased, the illumination of the marginal area of the focal plane would be insufficient since the radius of curvature of the front surface (r 1 ) should be positive and inevitably for maintaining the necessary positive refractive power of the lens (L 1 ) required by condition (2). In practice, the combination of conditions (2) and (3) implies that the absolute value for the object side surface (r 1 ) should be greater than that of the image side surface (r 2 ) in the front positive lens (L 1 ). 
     Condition (4) insures that the positive refractive power of the first positive meniscus lens (L 5 ), which is located next to the aperture stop at the image side thereof, should be relatively great in contrast with the prior art. Lens (L 5 ) is expected to effectively correct spherical aberration and coma, which are principle problems tht occur with an increase in the aperture ratio of the lens system. If f 5  is excessively reduced to violate the lower limit of condition (4), an under-correction of spherical aberration and a negatively deviated field curvature result. On the other hand, if f 5  is excessively increased to violate the upper limit of condition (4), the correction of coma is difficult unless the total length of the system is increased which is at variance with the design objective of compactness. 
     The following Tables 1 to 4 show four separate embodiments of the present invention, respectively, wherein FIGS. 1 and 2a, 2b and 2c relate to Table 1; FIGS. 3 and 4a, 4b and 4c relate to Table 2; FIGS. 5 and 6a, 6b and 6c relate to Table 3 and FIGS. 7 and 8a, 8b, and 8c relate to Table 4. 
     In each Table, f equals the focal length, 2ω equals the field angle, r is the radius of curvature with respective subnumbers indicating the surfaces from the object to image side along the optical axis, d represents the axial distance and includes both the airspaces and the actual thicknesses of the lens components along the optical axis. N equals the refractive index and again the subnumbers refer to the particular optical element from the object to image side and finally, ν, equals the Abbe number and the subnumbers refer to the specific lens elements from the object to image side. 
     
                       TABLE 1                                                     
______________________________________                                    
(Embodiment 1)                                                            
f = 100 F.sub.No. = 2.8 12ω = 75°                            
Radius of   Axial      Refractive  Abbe                                   
Curvature   Distance   Index       Number                                 
______________________________________                                    
r.sub.1                                                                   
       -4488.98                                                           
                d.sub.1                                                   
                       13.50 N.sub.1                                      
                                  1.6935 ν.sub.1                       
                                              53.61                       
r.sub.2                                                                   
       -759.55                                                            
                d.sub.2                                                   
                       0.42                                               
r.sub.3                                                                   
       132.45                                                             
                d.sub.3                                                   
                       4.00  N.sub.2                                      
                                  1.5213 ν.sub.2                       
                                              51.19                       
r.sub.4                                                                   
       48.88                                                              
                d.sub.4                                                   
                       12.00                                              
r.sub.5                                                                   
       101.16                                                             
                d.sub.5                                                   
                       4.00  N.sub.3                                      
                                  1.5182 ν.sub.3                       
                                              59.11                       
r.sub.6                                                                   
       46.60                                                              
                d.sub.6                                                   
                       23.22                                              
r.sub.7                                                                   
       86.11                                                              
                d.sub.7                                                   
                       40.00 N.sub.4                                      
                                  1.7620 ν.sub.4                       
                                              40.33                       
r.sub.8                                                                   
       -309.71                                                            
                d.sub.8                                                   
                       10.97                                              
r.sub.9                                                                   
       -333.34                                                            
                d.sub.9                                                   
                       7.00  N.sub.5                                      
                                  1.7495 ν.sub.5                       
                                              50.41                       
r.sub.10                                                                  
       -93.53                                                             
                d.sub.10                                                  
                       8.60                                               
r.sub.11                                                                  
       -78.73                                                             
                d.sub.11                                                  
                       10.65 N.sub.6                                      
                                  1.8052 ν.sub.6                       
                                              25.21                       
r.sub.12                                                                  
       166.05                                                             
                d.sub.12                                                  
                       3.50                                               
r.sub.13                                                                  
       - 230.47                                                           
                d.sub.13                                                  
                       6.50  N.sub.7                                      
                                  1.7725 ν.sub.7                       
                                              50.14                       
r.sub.14                                                                  
       -61.73                                                             
                d.sub.14                                                  
                       0.42                                               
r.sub.15                                                                  
       1940.20                                                            
                d.sub.15                                                  
                       5.40  N.sub.8                                      
                                  1.6935 ν.sub.8                       
                                              50.47                       
r.sub.16                                                                  
       -168.51                                                            
______________________________________                                    
 
    
     
                       TABLE 2                                                     
______________________________________                                    
(Embodiment 2)                                                            
f = 100 F.sub.No. = 1.8 2ω = 75°                             
Radius of   Axial      Refractive  Abbe                                   
Curvature   Distance   Index       Number                                 
______________________________________                                    
r.sub.1                                                                   
       -1603.62                                                           
                d.sub.1                                                   
                       13.50 N.sub.1                                      
                                  1.6935 ν.sub.1                       
                                              53.61                       
r.sub.2                                                                   
       -702.33                                                            
                d.sub.2                                                   
                       0.52                                               
r.sub.3                                                                   
       104.85                                                             
                d.sub.3                                                   
                       6.00  N.sub.2                                      
                                  1.5168 ν.sub.2                       
                                              64.22                       
r.sub.4                                                                   
       54.70                                                              
                d.sub.4                                                   
                       12.00                                              
r.sub.5                                                                   
       102.95                                                             
                d.sub.5                                                   
                       6.00  N.sub.3                                      
                                  1.5168 ν.sub.3                       
                                              64.22                       
r.sub.6                                                                   
       51.73                                                              
                d.sub.6                                                   
                       30.72                                              
r.sub.7                                                                   
       128.36                                                             
                d.sub.7                                                   
                       40.00 N.sub.4                                      
                                  1.8061 ν.sub.4                       
                                              40.83                       
r.sub.8                                                                   
       -297.73                                                            
                d.sub.8                                                   
                       14.50                                              
r.sub.9                                                                   
       -233.61                                                            
                d.sub.9                                                   
                       10.97 N.sub.5                                      
                                  1.7725 ν.sub.5                       
                                              50.14                       
r.sub.10                                                                  
       -84.12                                                             
                d.sub.10                                                  
                       8.60                                               
r.sub.11                                                                  
       -79.66                                                             
                d.sub.11                                                  
                       10.65 N.sub.6                                      
                                  1.8052 ν.sub.6                       
                                              25.21                       
r.sub.12                                                                  
       191.70                                                             
                d.sub.12                                                  
                       7.00                                               
r.sub.13                                                                  
       - 291.20                                                           
                d.sub.13                                                  
                       12.00 N.sub.7                                      
                                  1.7725 ν.sub.7                       
                                              50.14                       
r.sub.14                                                                  
       -85.05                                                             
                d.sub.14                                                  
                       0.52                                               
r.sub.15                                                                  
       1349.13                                                            
                d.sub.15                                                  
                       13.04 N.sub.8                                      
                                  1.6968 ν.sub.8                       
                                              55.80                       
r.sub.16                                                                  
       -121.66                                                            
______________________________________                                    
 
    
     
                       TABLE 3                                                     
______________________________________                                    
(Embodiment 3)                                                            
f = 100 F.sub.No. = 2.0 2ω = 75°                             
Radius of  Axial      Refractive   Abbe                                   
Curvature  Distance   Index        Number                                 
______________________________________                                    
r.sub.1                                                                   
      2134.43                                                             
               d.sub.1                                                    
                      13.50 N.sub.1                                       
                                  1.6935 ν.sub.1                       
                                              53.61                       
r.sub.2                                                                   
      -2066.01                                                            
               d.sub.2                                                    
                      0.52                                                
r.sub.3                                                                   
      131.97                                                              
               d.sub.3                                                    
                      6.00  N.sub.2                                       
                                  1.5168 ν.sub.2                       
                                              64.22                       
r.sub.4                                                                   
      56.00                                                               
               d.sub.4                                                    
                      12.00                                               
r.sub.5                                                                   
      121.73                                                              
               d.sub.5                                                    
                      6.00  N.sub.3                                       
                                  1.5168 ν.sub.3                       
                                              64.22                       
r.sub.6                                                                   
      53.51                                                               
               d.sub.6                                                    
                      30.72                                               
r.sub.7                                                                   
      98.13                                                               
               d.sub.7                                                    
                      31.77 N.sub.4                                       
                                  1.6975 ν.sub.4                       
                                              48.32                       
r.sub.7&#39;                                                                  
      -110.52                                                             
               d.sub.7&#39;                                                   
                      9.41   N.sub.4&#39;                                     
                                  1.5176  ν.sub.4&#39;                     
                                              53.59                       
r.sub.8                                                                   
      -338.65                                                             
               d.sub.8                                                    
                      14.50                                               
r.sub.9                                                                   
      -315.46                                                             
               d.sub.9                                                    
                      10.97 N.sub.5                                       
                                  1.6676 ν.sub.5                       
                                              41.90                       
r.sub.10                                                                  
      -84.17                                                              
               d.sub.10                                                   
                      8.60                                                
r.sub.11                                                                  
      - 65.53                                                             
               d.sub.11                                                   
                      10.65 N.sub.6                                       
                                  1.8052 ν.sub.6                       
                                              25.21                       
r.sub.12                                                                  
      228.02                                                              
               d.sub.12                                                   
                      5.30                                                
r.sub.13                                                                  
      -261.01                                                             
               d.sub.13                                                   
                      11.00 N.sub.7                                       
                                  1.7570 Ξ.sub.7                       
                                              48.04                       
r.sub.14                                                                  
      -71.10                                                              
               d.sub.14                                                   
                      0.52                                                
r.sub.15                                                                  
      1231.72                                                             
               d.sub.15                                                   
                      13.00 N.sub.8                                       
                                  1.7050 ν.sub.8                       
                                              55.30                       
r.sub.16                                                                  
      -139.66                                                             
______________________________________                                    
 
    
     
                       TABLE 4                                                     
______________________________________                                    
(Embodiment 4)                                                            
f = 100 F.sub.No.  = 2.0 2ω = 75°                            
Radius of  Axial      Refractive   Abbe                                   
Curvature  Distance   Index        Number                                 
______________________________________                                    
r.sub.1                                                                   
      7518.80                                                             
               d.sub.1                                                    
                      13.50 N.sub.1                                       
                                  1.6935 ν.sub.1                       
                                              53.61                       
r.sub.2                                                                   
      -1494.77                                                            
               d.sub.2                                                    
                      0.52                                                
r.sub.3                                                                   
      117.92                                                              
               d.sub.3                                                    
                      6.00  N.sub.2                                       
                                  1.511  ν.sub.2                       
                                              60.49                       
r.sub.4                                                                   
      54.21                                                               
               d.sub.4                                                    
                      12.00                                               
r.sub.5                                                                   
      115.00                                                              
               d.sub.5                                                    
                      6.00  N.sub.3                                       
                                  1.5111 ν.sub.3                       
                                              60.49                       
r.sub.6                                                                   
      52.66                                                               
               d.sub.6                                                    
                      30.72                                               
r.sub.7                                                                   
      112.26                                                              
               d.sub.7                                                    
                      14.00 N.sub.4                                       
                                  1.7620 ν.sub.2                       
                                              40.38                       
r.sub.7&#39;                                                                  
      694.93                                                              
               d.sub.7&#39;                                                   
                      4.00                                                
r.sub.7&#34;                                                                  
      1254.71                                                             
               d.sub.7&#34;                                                   
                      22.00  N.sub.4&#39;                                     
                                  1.7620  ν.sub.4&#39;                     
                                              40.38                       
r.sub.8                                                                   
      -217.30                                                             
               d.sub.8                                                    
                      14.50                                               
r.sub.9                                                                   
      -427.72                                                             
               d.sub.9                                                    
                      10.97 N.sub.5                                       
                                  1.7495 ν.sub.5                       
                                              50.41                       
r.sub.10                                                                  
      -89.36                                                              
               d.sub.10                                                   
                      8.60                                                
r.sub.11                                                                  
      -74.06                                                              
               d.sub.11                                                   
                      10.65 N.sub.6                                       
                                  1.8052 ν.sub.6                       
                                              25.21                       
r.sub.12                                                                  
      192.94                                                              
               d.sub.12                                                   
                      7.00                                                
r.sub.13                                                                  
      -233.15                                                             
               d.sub.13                                                   
                      11.50 N.sub.7                                       
                                  1.7725 ν.sub.7                       
                                              50.14                       
r.sub.14                                                                  
      -75.97                                                              
               d.sub.14                                                   
                      0.52                                                
r.sub.15                                                                  
      1956.95                                                             
               d.sub.15                                                   
                      12.50 N.sub.8                                       
                                  1.6968 ν.sub.8                       
                                              55.80                       
r.sub.16                                                                  
      -141.52                                                             
______________________________________                                    
 
    
     As can be readily appreciated, it is possible to deviate from the above embodiments of the present invention and, as will be readily understood by those skilled in the optical art, the invention is capable of many modifications and improvements within the scope and spirit thereof. Accordingly, it will be understood that the invention is not to be limited by the specific enclosed embodiments but only by the scope and spirit of the appended claims.