Zoom lens
This is a continuation of application Ser. No. 511,674, filed July 7, 1983 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to zoom lenses suited for still cameras, cine-cameras and video cameras, and more particularly to zoom lenses of a high relative aperture and a large zoom ratio while still permitting good stability of aberration correction throughout the entire focusing range. 
     2. Description of the Prior Art 
     In many conventional zoom lenses, the front or first lens component has been constructed from three lenses of negative, positive and positive powers in this order from front, or from a doublet of lenses of negative and positive powers and a single of positive power. 
     And the first component has been made either to move as a member of the zoom section during zooming, or to remain stationary during zooming as it is exclusively devoted to focusing, for zooming is performed by second and third components arranged in rear of the first component. 
     The use of such construction and arrangement of the elements of the first component in the zoom lens design has generally resulted in a tendency that as the object distance changes, aberrations varies to large extent, particularly the ranges of variation of spherical aberration and astigmatism being increased when zoomed to telephoto positions. 
     Another tendency was that the diameter of the first component becomes relatively large, causing the weight of the complete zoom lens to increase, and prejudicing the manageability. For this reason, it has been proposed in Japanese Laid-Open Patent Publication No. Sho 57-53718 that some of the elements of the first component are made up of plastic material with an advantage of decreasing the weight of the first component. 
     However, upon consideration of the removal of an influence of change of the ambient temperature, two plastic lenses of opposite power must be employed. Because of the distribution of the negative, positive and positive powers over the first component, the frontmost or negative first lens must be of plastic material. Since this plastic lens is exposed to the open air, its front surface is liable to be scratched and also to be tarnished yellow. These constitute a serious problem. 
     As the conventional lens system having the first component constructed with three lenses, mention may be made of those disclosed in U.S. Pat. Nos. 2,843,016, 2,937,572 and 2,746,350 and Re 29237. As for a distribution of positive, negative, positive and positive powers, there is U.S. Pat. No. 4,113,356. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a zoom lens of a high relative aperture and a large zoom ratio with good stability of aberration correction not only throughout the zooming range but also throughout the focusing range. 
     Another object of the present invention is to provide a zoom lens of good manageability due to a decrease in the weight of the first component thereof. 
     To achieve the objects of the present invention, a feature of the construction and arrangement and form of the elements of the zoom lens is that the lens system has as at least three lens groups, or, from front to rear, a first lens group of positive refractive power, a second lens group of negative refractive power and a third lens group with the air separations between the successive two of the three lens groups made variable to effect zooming, and the first lens group is made constructed with, from front to rear, a bi-convex first lens, a second lens of negative refractive power, and a third lens of positive refractive power. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1, 3 and 5 are lens block diagrams of embodiments 1, 2 and 3 of the present invention respectively. 
     FIGS. 2-1 through -9, 4-1 through -9 and 6-1 through -9 are graphic representations of the various aberrations of the lenses of FIGS. 1, 3 and 5 respectively. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As in the present invention, by constructing the first lens group with three members in the form of, from front to rear, a first lens with both of its surfaces being convex, a second lens of negative refractive power and a third lens of positive refractive power, and by employing the zoom type that the first lens group is made to move during zooming, or the separation between the first and second lens groups is made to move during zooming, it is made possible to reduce the range of variation with focusing of aberrations while still maintaining good stability of aberration correction throughout the zooming range. Particularly by arranging the positive first and the negative second lenses in a broken contact, as the air lens created there-between is given an appropriate refractive power, it is made possible to achieve a good compromise between the requirements that variation of aberrations for variation of the object distance is as far lessened as possible, and that good correction of aberrations is maintained throughout the entire zooming range. 
     Further, in the present invention, to facilitate an improvement of the present state of aberration correction, it is preferred to satisfy the following conditions: 
     That is, letting F1 denote the overall focal length of the first lens group, and f1, f2 and f3 the focal lengths of the first, second and third lenses respectively, 
     ν1, ν2 and ν3 the Abbe numbers of the media of the first, second and third lenses respectively, and 
     Ri the radius of curvature of the i-th lens surface counting from front in the first lens group, 
     
         0.5f3&lt;f1&lt;2f3                                               (1) ##EQU1## 
    
     
         F1&lt;|R2|                                  (3) 
    
     
         0.8|R2|&lt;|R3|&lt;1.5|R2 (4) 
    
     are satisfied. 
     Inequalities of condition (1) represent a proper range of refractive power of the first lens in terms of that of the third lens, and are set forth to as far reduce the residual aberrations of the first lens group as possible. When the lower limit of the inequalities of condition (1) is exceeded, the refractive power of the first lens becomes too large to stabilize the aberrations throughout the zooming and focusing ranges. When the upper limit is exceeded, as the refractive power of the first lens weakens, the third lens must be given a larger duty of refractive power, thereby the spherical aberration is objectionably increased. 
     Inequality of condition (2) is to well correct the first lens group for chromatic aberrations in itself. When the condition (2) is violated, particularly the longitudinal chromatic aberration becomes difficult to correct. Inequality of condition (3) is to well correct for image aberrations for the maximum image angle in the shortest focal length positions. 
     The first lens is, upon consideration of good correction for distortion in the wide angle positions, desired to take the bi-convex form, wherein the radius of curvature, R2, of the rear surface of the first lens is made larger than the focal length of the first lens group, whereby higher-order aberrations are reduced, and the distortion is well corrected. 
     When condition (3) is violated, the distortion is objectionably increased in the wide angle positions. 
     Inequalities of condition (4) represent a proper range of radii of curvature of the front surface of the second lens in terms of the radius of curvature of the rear surface of the first lens, by which the aberrations and particularly spherical aberration and astigmatism are well corrected. 
     When the lower limit of condition (4) is exceeded, insufficient correction of distortion for the wide angle positions results. When the upper limit is exceeded, insufficient correction of spherical aberration for the telephoto positions results. 
     Further, in the present invention, in order to reduce the weight of the zoom lens as a whole, it is preferred to use plastic in making up the third lens, while the first lens is made up of optical glass. 
     Also, to compensate for a change of the refractive index of the plastic material due to the change of the ambient temperature, it is preferred to use plastic material in making up the second lens. 
     And, to allow for the great increases in the relative aperture and zoom ratio of the lens system to be effectively achieved by minimizing the residual aberrations of the first lens group, it is preferred to make at least one of the lens surfaces in the first lens group aspherical. In this connection, it should be pointed out that particularly when the aspheric surface is formed by the plastic material, aspherical lenses of the desired shape with the prescribed tolerances of the figuring parameters can be economically manufactured. 
     It is to be noted that in the embodiments of the invention, the zoom lens is constructed with four lens groups, and the focusing provision is made at the first lens group. But, focusing may be otherwise performed by moving all the four lens groups, or by moving another part of the lens system. 
     With the entire system consisting of three lens groups, however, it is preferred to move the first lens group for focusing. 
     Next, specific numerical examples of the invention are shown. In the specific numerical examples, Ri is the radius of curvature of the i-th lens surface counting from front, Di the i-th lens thickness or air separation counting from front, and Ni and νi the refractive index and Abbe number of the glass of the i-th lens element counting from front respectively. 
     The aspherical surface is figured by an equation for the amount of deviation from a spherical surface having radius of curvature, R, equal to that of the paraxial region of the aspherical surface, in the optical axis taken as an x-axis at a height, h, in a y-axis taken in a direction perpendicular to the x-axis and passing through the vertex of the aspherical surface, expressed as: ##EQU2## where B, C and D are aspheric coefficients. 
     NUMERICAL EXAMPLE 1 
     
         ______________________________________                                    
F = 1.00- 5.95 FNO = 1:1.4 2ω = 51.1°- 9.2°           
______________________________________                                    
R1 = 6.818                                                                
          D1 = 0.74    N1 = 1.69680                                       
                                   ν1 = 55.5                           
R2 = -10.748                                                              
          D2 = 0.13                                                       
R3 = -9.585                                                               
          D3 = 0.26    N2 = 1.84666                                       
                                   ν2 = 23.9                           
R4 = 70.157                                                               
          D4 = 0.01                                                       
R5 = 4.587                                                                
          D5 = 0.52    N3 = 1.60311                                       
                                   ν3 = 60.7                           
R6 = 30.982                                                               
          D6 = Variable                                                   
R7 = 5.469                                                                
          D7 = 0.10    N4 = 1.77250                                       
                                   ν4 = 49.6                           
R8 = 1.455                                                                
          D8 = 0.43                                                       
R9 = -1.686                                                               
          D9 = 0.09    N5 = 1.69680                                       
                                   ν5 = 55.5                           
R10 = 2.542                                                               
          D10 = 0.27   N6 = 1.84666                                       
                                   ν6 = 23.9                           
R11 = -23.551                                                             
          D11 = Variable                                                  
R12 = -2.373                                                              
          D12 = 0.09   N7 = 1.69680                                       
                                   ν7 = 55.5                           
R13 = -33.501                                                             
          D13 =  Variable                                                 
R14 = Stop                                                                
          D14 = 0.09                                                      
R15 = 9.230                                                               
          D15 = 0.44   N8 = 1.63854                                       
                                   ν8 = 55.4                           
R16 = -1.961                                                              
          D16 = 0.09                                                      
R17 = 2.774                                                               
          D17 = 0.35   N9 = 1.60311                                       
                                   ν9 = 60.7                           
R18 = -7.043                                                              
          D18 = 0.18                                                      
R19 = -2.216                                                              
          D19 = 0.11   N10 = 1.75520                                      
                                   ν10 = 27.5                          
R20 = 16.712                                                              
          D20 = 0.01                                                      
R21 = 1.872                                                               
          D21 = 0.43   N11 = 1.60311                                      
                                   ν11 = 60.7                          
R22 = -46.772                                                             
          D22 = 1.47                                                      
R23 = -29.353                                                             
          D23 = 0.13   N12 = 1.51633                                      
                                   ν12 = 64.1                          
R24 = -3.739                                                              
          D24 = 0.01                                                      
R25 = 1.791                                                               
          D25 = 0.24   N13 = 1.60311                                      
                                   ν13 = 60.7                          
R26 = 14.077                                                              
          D26 = 0.17                                                      
R27 = -1.175                                                              
          D27 = 0.09   N14 = 1.80518                                      
                                   ν14 = 25.4                          
R28 = -2.544                                                              
          D28 = 0.17                                                      
R29 = Stop                                                                
          D29 =  0.52  N15 = 1.51633                                      
                                   ν15 = 64.1                          
R30 = Stop                                                                
______________________________________                                    
Variable   Focal Length                                                   
Separation 1.00          2.85   5.95                                      
______________________________________                                    
D6         0.145         2.013  2.812                                     
D11        2.877         0.740  0.256                                     
D13        0.296         0.564  0.249                                     
______________________________________                                    
f1 = 6.09                                                                 
f2 = -9.95           F1 = 5.72                                            
f3 = 8.86                                                                 
______________________________________                                    
 
    
     NUMERICAL EXAMPLE 2 
     
         ______________________________________                                    
F = 1.00- 5.95 FNO = 1:1.4 2ω = 51.1°- 9.2°           
______________________________________                                    
R1 = 7.687                                                                
          D1 = 0.74    N1 = 1.69680                                       
                                   ν1 = 55.5                           
R2 = -10.650                                                              
          D2 = 0.09                                                       
R3 = -9.538                                                               
          D3 = 0.17    N2 = 1.58349                                       
                                   ν2 = 29.8                           
R4 = 5.224                                                                
          D4 = 0.01                                                       
R5 = 3.070                                                                
          D5 = 0.91    N3 = 1.49171                                       
                                   ν3 = 57.4                           
R6 = -12.312                                                              
          D6 = Variable                                                   
R7 = 5.820                                                                
          D7 = 0.10    N4 = 1.77250                                       
                                   ν4 = 49.6                           
R8 = 1.439                                                                
          D8 = 0.45                                                       
R9 = -1.716                                                               
          D9 = 0.09    N5 = 1.69680                                       
                                   ν5 = 55.5                           
R10 = 2.132                                                               
          D10 = 0.27   N6 = 1.84666                                       
                                   ν6 = 23.9                           
R11 = -25.150                                                             
          D11 = Variable                                                  
R12 = -2.187                                                              
          D12 = 0.09   N7 = 1.69680                                       
                                   ν7 = 55.5                           
R13 = -15.366                                                             
          D13 = Variable                                                  
R14 = Stop                                                                
          D14 = 0.09                                                      
R15 = 9.432                                                               
          D15 = 0.44   N8 = 1.63854                                       
                                   ν8 = 55.4                           
R16 = -1.970                                                              
          D16 = 0.09                                                      
R17 = 2.777                                                               
          D17 = 0.35   N9 = 1.60311                                       
                                   ν9 = 60.7                           
R18 = -8.218                                                              
          D18 = 0.18                                                      
R19 = -2.171                                                              
          D19 = 0.11   N10 = 1.75520                                      
                                   ν10 = 27.5                          
R20 = 15.171                                                              
          D20 = 0.01                                                      
R21 = 1.860                                                               
          D21 = 0.43   N11 = 1.60311                                      
                                   ν11 = 60.7                          
R22 = -47.885                                                             
          D22 = 1.47                                                      
R23 = -28.974                                                             
          D23 = 0.13   N12 = 1.51633                                      
                                   ν12 = 64.1                          
R24 = -3.661                                                              
          D24 = 0.01                                                      
R25 = 1.731                                                               
          D25 = 0.24   N13 = 1.60311                                      
                                   ν13 = 60.7                          
R26 = 17.427                                                              
          D26 = 0.17                                                      
R27 = -1.219                                                              
          D27 = 0.09   N14 = 1.80518                                      
                                   ν14 = 25.4                          
R28 = -2.446                                                              
          D28 = 0.17                                                      
R29 = Stop                                                                
          D29 =  0.52  N15 = 1.51633                                      
                                   ν15 = 64.1                          
R30 = Stop                                                                
______________________________________                                    
Variable   Focal Length                                                   
Separation 1.00          2.85   5.95                                      
______________________________________                                    
D6         0.098         1.967  2.765                                     
D11        2.863         0.726  0.242                                     
D14        0.296         0.564  0.249                                     
______________________________________                                    
Sur-                                                                      
face  Aspheric Coefficient                                                
No.   B            C            D                                         
______________________________________                                    
5     -0.2953 × 10.sup.-2                                           
                   -0.3669 × 10.sup.-3                              
                                -0.1963 × 10.sup.-4                 
______________________________________                                    
f1 = 6.52                                                                 
f2 = -5.76           F1 = 5.72                                            
f3 = 5.10                                                                 
______________________________________                                    
 
    
     In this embodiment, the second and third lenses are made of plastic material. 
     NUMERICAL EXAMPLE 3 
     
         ______________________________________                                    
F = 1.00- 5.72 FNO = 1:1.4- 2.0 2ω = 51.9°-                  
______________________________________                                    
9.7°                                                               
R1 = 8.178                                                                
          D1 = 0.66    N1 = 1.60311                                       
                                   ν1 = 60.7                           
R2 = -7.584                                                               
          D2 = 0.03                                                       
R3 = -9.307                                                               
          D3 = 0.27    N2 = 1.58349                                       
                                   ν2 = 29.8                           
R4 = 5.377                                                                
          D4 = 0.03                                                       
R5 = 3.125                                                                
          D5 = 0.78    N3 = 1.49171                                       
                                   ν3 = 57.4                           
R6 = -14.861                                                              
          D6 = Variable                                                   
R7 = 7.882                                                                
          D7 = 0.11    N4 = 1.77250                                       
                                   ν4 = 49.6                           
R8 = 1.540                                                                
          D8 = 0.31                                                       
R9 = -1.642                                                               
          D9 = 0.09    N5 = 1.69680                                       
                                   ν5 = 55.5                           
R10 = 2.137                                                               
          D10 = 0.28   N6 = 1.84665                                       
                                   ν6 = 23.9                           
R11 = -19.496                                                             
          D11 = Variable                                                  
R12 = Stop                                                                
          D12 = Variable                                                  
R13 = 5.775                                                               
          D13 = 0.28   N7 = 1.49171                                       
                                   ν7 = 57.4                           
R14 = -7.014                                                              
          D14 = 0.01                                                      
R15 = 2.005                                                               
          D15 = 0.51   N8 = 1.60311                                       
                                   ν8 = 60.7                           
R16 = -3.179                                                              
          D16 = 0.01                                                      
R17 = 9.057                                                               
          D17 = 0.13   N9 = 1.58349                                       
                                   ν9 = 29.8                           
R18 = 1.992                                                               
          D18 = Variable                                                  
R19 = 1.055                                                               
          D19 = 0.38   N10 = 1.62299                                      
                                   ν10 = 58.2                          
R20 = 1.778                                                               
          D20 = 0.24                                                      
R21 = 3.988                                                               
          D21 = 0.12   N11 = 1.84665                                      
                                   ν11 = 23.9                          
R22 = 0.882                                                               
          D22 = 0.27                                                      
R23 = 3.742                                                               
          D23 = 0.32   N12 = 1.60311                                      
                                   ν12 = 60.7                          
R24 = -15.322                                                             
          D24 = 0.01                                                      
R25 = 1.735                                                               
          D25 = 0.31   N13 = 1.77250                                      
                                   ν13 = 49.6                          
R26 = -10.003                                                             
          D26 = 0.35                                                      
R27 = Stop                                                                
          D27 = 0.53   N14 = 1.51633                                      
                                   ν14 = 64.1                          
R28 = Stop                                                                
______________________________________                                    
Variable   Focal Length                                                   
Separation 1.00          3.69    5.72                                     
______________________________________                                    
D6         0.075         2.322  2.711                                     
D11        1.324         0.548  0.215                                     
D12        1.412         0.569  0.208                                     
D18        0.177         1.020  1.382                                     
______________________________________                                    
Sur-                                                                      
face  Aspheric Coefficient                                                
No.   B            C            D                                         
______________________________________                                    
 5    -0.2913 × 10.sup.-2                                           
                   -0.4691 × 10.sup.-3                              
                                -0.9809 × 10.sup.-5                 
14     0.3340 × 10.sup.-1                                           
                    0.5166 × 10.sup.-2                              
                                 0.1111 × 10.sup.-1                 
18     0.2596 × 10.sup.-1                                           
                    0.6554 × 10.sup.-2                              
                                 0.1146 × 10.sup.-1                 
______________________________________                                    
f1 = 6.630                                                                
f2 = -5.80           F1 = 6.03                                            
f3 = 5.23                                                                 
______________________________________                                    
 
    
     Though the present invention has been described in connection with the embodiments where each lens system consists of four lens groups of which the first three are moved to effect zooming, the zoom lens of the invention may be in the form of three lens groups, as the fourth lens group is not always necessary. 
     As in the above, according to the present invention, a zoom lens of a high relative aperture and a large zoom ratio will stabilized for all aberrations can be realized.