Zoom lens
This application is a continuation of application Ser. No. 07/957,009 filed Oct. 6, 1992, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a zoom lens, and more particularly, to a zoom lens which is suitable for a still-picture camera, which includes a wide-angle range, and which has a high varifocal ratio. 
     2. Description of the Prior Art 
     Recently, zoom lenses have generally been used as interchangeable lenses for single-lens reflex cameras. This tendency is a result of the high performance, size reduction and low price of zoom lenses. Zoom lenses which have greater varifocal ratios, and which are even more compact, hereafter also will be required. In the field of compact cameras, where lenses are built into the camera, zoom lenses which have greater varifocal ratios and which are more compact also will be required. 
     Furthermore, in accordance with the popularity of autofocus cameras, optical systems suitable for an autofocusing operation also are required. Hence, it has become required that the amount of movement, weight, diameter and the like of a focus-moving lens have smaller values. 
     Zoom lenses having high varifocal ratios mainly are used for single-lens reflex cameras and have been disclosed, for example, in Japanese Patent Publication No. 58-33531 (1983) and Japanese Patent Laid-open Application (Kokai) Nos. 59-198214 (1984) and 60-39613 (1985). So-called rear-focusing zoom lenses, in which focusing is performed by lens groups other than the front lens group also have been disclosed, for example, in Japanese Patent Laid-open Application (Kokai) Nos. 58-136012 (1983); 58-143312 (1983); 61-138228 (1986); 63-205628 (1988); 63-205629 (1988); 63-266414 (1988); 63-266415 (1988) and 1-154014 (1989). 
     These zoom lenses have the configuration of five groups having positive, negative, positive, negative and positive refractive powers, arranged from the object side, in order to efficiently perform a varifocal operation and to provide a small size. However, these efficiencies are still insufficient. While the diameter of a front lens can be reduced by adopting a rear-focusing configuration, such a configuration is so complicated that the response speed of an autofocusing operation becomes insufficient. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to overcome the above-described problems in the prior art. 
     It is another object of the present invention to provide a zoom lens which has a high varifocal ratio including a wide-angle range, which is compact, and which has a high optical performance. 
     It is still another object of the present invention to provide a zoom lens which can perform a prompt autofocusing operation. 
     According to one embodiment, the present invention achieves these objectives through a five group zoom lens comprising in order from the object side, a first lens group having a positive refractive power; a second lens group having a negative refractive power; a third lens group having a positive refractive power; a fourth lens group having a negative refractive power; and a fifth lens group having a positive refractive power, wherein a varifocal zooming operation is performed by changing a distance interval between respective lens groups, and wherein certain conditional expressions (to be described later) also are satisfied. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a lens according to a first embodiment of the present invention; 
     FIG. 2 is a diagram illustrating various kinds of aberrations at a wide-angle end of the lens of the first embodiment; 
     FIG. 3 is a diagram illustrating various kinds of aberrations at an intermediate focal length of the lens of the first embodiment; 
     FIG. 4 is a diagram illustrating various kinds of aberrations at a telesphoto end of the lens of the first embodiment; 
     FIG. 5 is a cross-sectional view of a lens according to a second embodiment of the present invention; 
     FIG. 6 is a diagram illustrating various kinds of aberrations at a wide-angle end of the lens of the second embodiment; 
     FIG. 7 is a diagram illustrating various kinds of aberrations at an intermediate focal length of the lens of the second embodiment; 
     FIG. 8 is a diagram illustrating various kinds of aberrations at a telephoto end of the lens of the second embodiment; 
     FIG. 9 is a cross-sectional view of a lens according to a third embodiment of the present invention; 
     FIG. 10 is a diagram illustrating various kinds of aberrations at a wide-angle end of the lens of the third embodiment; 
     FIG. 11 is a diagram illustrating various kinds of aberrations at an intermediate focal length of the lens of the third embodiment; and 
     FIG. 12 is a diagram illustrating various kinds of aberrations at a telephoto end of the lens of the third embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An explanation will now be provided of five group zoom lenses according to preferred embodiments of the present invention, with reference to the accompanying drawings. 
     In each of FIGS. 1, 5 and 9, the symbols I, II, III, IV, V and SP represent a first lens group having a positive refractive power; a second lens group having a negative refractive power; a third lens group having a positive refractive power; a fourth lens group having a negative refractive power; a fifth lens group having a positive refractive power, and a diaphragm, respectively, in the order from the object side. Zooming from a wide-angle end to a telephoto end is performed by changing a distance interval between respective lens groups. Focusing is performed by moving the second lens group, which has a relatively small diameter and also is light in weight. 
     According to the invention, a five group zoom lens also satisfies the following conditional expressions: 
     
         0.3&lt;f.sub.1 /f.sub.T &lt;0.8                                  (1) 
    
     
         1.2&lt;Z.sub.2 /Z.sub.3 &lt;3.0                                  (2) 
    
     
         0.5&lt;f.sub.3 /f.sub.W &lt;0.8                                  (3), 
    
     where f 1  and f 3  represent the focal lengths of the first and third lens groups, respectively; f W  and f T  represent the focal lengths at a wide-angle end and at a telephoto end of the lens, respectively; and Z 2  and Z 3  represent a sharing value for a varying magnification of the second and third lens groups, respectively, that is defined by a ratio of the paraxial lateral magnification at the telephoto end over the paraxial lateral magnification at the wide-angle end. 
     The meaning of each of these conditional expressions now will be explained. 
     Expression (1) provides a condition for making the focal length of the first lens group relatively short, and for reducing the entire length of the lens. Expression (1) also indicates a condition for reducing the outer diameter of the lens particularly by reducing the diameter of a light beam incident from the second lens group onto the side of the image at the telephoto end, and for reducing the diameter of the diaphragm. If the focal length of the first lens group is reduced so that the ratio f 1  /f T  becomes less than the lower limit value, aberrations, in particular a spherical aberration, at the telephoto end generated in this lens group greatly increase, whereby it becomes difficult to mutually correct the aberrations by the lens groups after the second lens group. If the ratio exceeds the upper limit value, the above-described object of providing a compact lens cannot be achieved. Moreover, the amount of movement of the lens for obtaining a desired varifocal zoom ratio must be increased, whereby it becomes difficult to provide a structure for holding the lens barrel. 
     Expression (2) indicates that the sharing value of a varying magnification for a second lens group at zooming must be relatively greater than that of a third lens group. This expression has the purpose of increasing the varifocal effect by the second lens group, by providing a short focal length for the first lens group. This condition also has the effects of reducing the burden of the third lens group, which can generate a rather large amount of aberrations, and of not deteriorating the performance of the lens even if the size of the lens is reduced. If the ratio Z 2  /Z 3  is less than the stated lower limit value, the above-described effects will be reduced. If the ratio Z 2  /Z 3  exceeds the stated upper limit value, the share of the second lens group becomes too large, and it becomes difficult to correct variations in aberrations during a focusing operation. 
     Expression (3) indicates a condition for providing an excellent balance between the amount of generation of aberrations and the total length of the lens by providing an appropriate focal length of the third lens group. If the focal length of the third lens group is reduced so that the ratio f 3  /f W  becomes less than the stated lower limit value, it becomes difficult to correct generation of aberrations by the third lens group. If the ratio f 3  /f W  exceeds the stated upper limit value, the back focus becomes too long, whereby it becomes difficult to reduce the total length of the lens. 
     In the present embodiments, zooming from the wide-angle end to the telephoto end is performed by moving at least the first, third and fifth lens group as arranged from the object side, and the imaging magnification of the second lens group is set to be greater than -1, so that the second lens group can perform a focusing operation. 
     Embodiments provided with numerical values meeting the conditions for the present invention now will be shown. In the following embodiments, ri represents the radius of curvature of the i-th lens surface from the side of the object; di is the thickness of the lens or the interval of the air of the i-th item, measured from the object side, and ni and νi are the refractive index and the Abbe number of the i-th lens measured from the object side, respectively. 
     
         __________________________________________________________________________
First Embodiment                                                          
f = 29-101.53                                                             
           fn0 = 1:3.5-4.6                                                
                     2w = 73.4°-24.1°                       
__________________________________________________________________________
r1 =                                                                      
    100.573                                                               
           d1 =                                                           
               2.00  n1 =                                                 
                         1.84666                                          
                             ν1 =                                      
                                 23.8                                     
r2 =                                                                      
    50.509 d2 =                                                           
               5.70  n2 =                                                 
                         1.69680                                          
                             ν2 =                                      
                                 55.5                                     
r3 =                                                                      
    233.318                                                               
           d3 =                                                           
               0.12                                                       
r4 =                                                                      
    45.952 d4 =                                                           
               4.80  n3 =                                                 
                         1.71300                                          
                             ν3 =                                      
                                 53.8                                     
r5 =                                                                      
    130.547                                                               
           d5 =                                                           
               variable                                                   
r6 =                                                                      
    58.722 d6 =                                                           
               1.20  n4 =                                                 
                         1.83400                                          
                             ν4 =                                      
                                 37.2                                     
r7 =                                                                      
    13.172 d7 =                                                           
               5.73                                                       
r8 =                                                                      
    -70.164                                                               
           d8 =                                                           
               1.10  n5 =                                                 
                         1.80400                                          
                             ν5 =                                      
                                 46.6                                     
r9 =                                                                      
    33.861 d9 =                                                           
               0.10                                                       
r10 =                                                                     
    22.714 d10 =                                                          
               4.35  n6 =                                                 
                         1.84666                                          
                             ν6 =                                      
                                 23.9                                     
r11 =                                                                     
    -52.072                                                               
           d11 =                                                          
               0.60                                                       
r12 =                                                                     
    -30.560                                                               
           d12 =                                                          
               1.10  n7 =                                                 
                         1.83481                                          
                             ν7 =                                      
                                 42.7                                     
r13 =                                                                     
    345.585                                                               
           d13 =                                                          
               variable                                                   
r14 =                                                                     
    (diaphragm)                                                           
           d14 =                                                          
               0.00                                                       
r15 =                                                                     
    24.449 d15 =                                                          
               1.20  n8 =                                                 
                         1.84666                                          
                             ν8 =                                      
                                 23.8                                     
r16 =                                                                     
    13.714 d16 =                                                          
               5.85  n9 =                                                 
                         1.60311                                          
                             ν9 =                                      
                                 60.7                                     
r17 =                                                                     
    -42.361                                                               
           d17 =                                                          
               0.12                                                       
r18 =                                                                     
    29.244 d18 =                                                          
               2.05  n10 =                                                
                         1.77250                                          
                             ν10 =                                     
                                 49.6                                     
r19 =                                                                     
    72.044 d19 =                                                          
               variable                                                   
r20 =                                                                     
    -51.388                                                               
           d20 =                                                          
               3.35  n11 =                                                
                         1.75520                                          
                             ν11 =                                     
                                 27.5                                     
r21 =                                                                     
    -12.692                                                               
           d21 =                                                          
               1.10  n12 =                                                
                         1.80400                                          
                             ν12 =                                     
                                 46.6                                     
r22 =                                                                     
    87.855 d22 =                                                          
               variable                                                   
r23 =                                                                     
    214.120                                                               
           d23 =                                                          
               5.45  n13 =                                                
                         1.48749                                          
                             ν13 =                                     
                                 70.2                                     
r24 =                                                                     
    -17.471                                                               
           d24 =                                                          
               0.12                                                       
r25 =                                                                     
    70.487 d25 =                                                          
               2.60  n14 =                                                
                         1.69680                                          
                             ν14 =                                     
                                 55.5                                     
r26 =                                                                     
    -89.327                                                               
           d26 =                                                          
               3.04                                                       
r27 =                                                                     
    -18.092                                                               
           d27 =                                                          
               1.40  n15 =                                                
                         1.84666                                          
                             ν15 =                                     
                                 23.8                                     
r28 =                                                                     
    -58.272                                                               
__________________________________________________________________________
Focal length                                                              
           29.00      50.00                                               
                         101.53                                           
Variable interval                                                         
d5         1.91       14.12                                               
                         29.26                                            
d13        15.46      8.85                                                
                         2.13                                             
d19        2.20       5.15                                                
                         7.88                                             
d22        6.97       4.02                                                
                         1.29                                             
__________________________________________________________________________
 
    
     
         __________________________________________________________________________
Second Embodiment                                                         
f = 29-132.58                                                             
           fn0 = 1:4-5.6                                                  
                     2w = 73.4°-18.5°                       
__________________________________________________________________________
r1 =                                                                      
    128.622                                                               
           d1 =                                                           
               2.00  n1 =                                                 
                         1.84666                                          
                             ν1 =                                      
                                 23.8                                     
r2 =                                                                      
    59.168 d2 =                                                           
               6.40  n2 =                                                 
                         1.65160                                          
                             ν2 =                                      
                                 58.5                                     
r3 =                                                                      
    6013.165                                                              
           d3 =                                                           
               0.10                                                       
r4 =                                                                      
    42.330 d4 =                                                           
               5.50  n3 =                                                 
                         1.69680                                          
                             ν3 =                                      
                                 55.5                                     
r5 =                                                                      
    100.826                                                               
           d5 =                                                           
               variable                                                   
r6 =                                                                      
    58.583 d6 =                                                           
               1.20  n4 =                                                 
                         1.83400                                          
                             ν4 =                                      
                                 37.2                                     
r7 =                                                                      
    12.947 d7 =                                                           
               6.00                                                       
r8 =                                                                      
    -78.767                                                               
           d8 =                                                           
               1.10  n5 =                                                 
                         1.80400                                          
                             ν5 =                                      
                                 46.6                                     
r9 =                                                                      
    52.766 d9 =                                                           
               0.10                                                       
r10 =                                                                     
    21.318 d10 =                                                          
               4.20  n6 =                                                 
                         1.84666                                          
                             ν6 =                                      
                                 23.9                                     
r11 =                                                                     
    -52.841                                                               
           d11 =                                                          
               1.10                                                       
r12 =                                                                     
    -31.884                                                               
           d12 =                                                          
               1.10  n7 =                                                 
                         1.83481                                          
                             ν7 =                                      
                                 42.7                                     
r13 =                                                                     
    51.054 d13 =                                                          
               variable                                                   
r14 =                                                                     
    (diaphragm)                                                           
           d14 =                                                          
               0.00                                                       
r15 =                                                                     
    25.604 d15 =                                                          
               1.20  n8 =                                                 
                         1.84666                                          
                             ν8 =                                      
                                 23.8                                     
r16 =                                                                     
    13.831 d16 =                                                          
               5.60  n9 =                                                 
                         1.60311                                          
                             ν9 =                                      
                                 60.7                                     
r17 =                                                                     
    -33.304                                                               
           d17 =                                                          
               0.12                                                       
r18 =                                                                     
    25.084 d18 =                                                          
               2.20  n10 =                                                
                         1.69680                                          
                             ν10 =                                     
                                 55.5                                     
r19 =                                                                     
    53.196 d19 =                                                          
               variable                                                   
r20 =                                                                     
    -72.832                                                               
           d20 =                                                          
               3.18  n11 =                                                
                         1.75520                                          
                             ν11 =                                     
                                 27.5                                     
r21 =                                                                     
    -13.495                                                               
           d21 =                                                          
               1.10  n12 =                                                
                         1.80400                                          
                             ν12 =                                     
                                 46.6                                     
r22 =                                                                     
    63.626 d22 =                                                          
               variable                                                   
r23 =                                                                     
    102.100                                                               
           d23 =                                                          
               5.46  n13 =                                                
                         1.48749                                          
                             ν13 =                                     
                                 70.2                                     
r24 =                                                                     
    -17.528                                                               
           d24 =                                                          
               0.12                                                       
r25 =                                                                     
    66.393 d25 =                                                          
               3.01  n14 =                                                
                         1.60311                                          
                             ν14 =                                     
                                 60.7                                     
r26 =                                                                     
    349.091                                                               
           d26 =                                                          
               2.90                                                       
r27 =                                                                     
    -15.781                                                               
           d27 =                                                          
               1.39  n15 =                                                
                         1.84666                                          
                             ν15 =                                     
                                 23.8                                     
r28 =                                                                     
    - 33.988                                                              
__________________________________________________________________________
Focal length                                                              
           29.00      70.03                                               
                         132.58                                           
Variable interval                                                         
d5         1.23       21.38                                               
                         33.23                                            
d13        18.16      8.22                                                
                         1.85                                             
d19        1.37       5.76                                                
                         8.11                                             
d22        8.60       4.22                                                
                         1.87                                             
__________________________________________________________________________
 
    
     
         __________________________________________________________________________
Third Embodiment                                                          
f = 29-191.1                                                              
           fn0 = 1:4-5.6                                                  
                     2w = 73.4°-12.9°                       
__________________________________________________________________________
r1 =                                                                      
    103.188                                                               
           d1 =                                                           
               2.00  n1 =                                                 
                         1.84666                                          
                             ν1 =                                      
                                 23.8                                     
r2 =                                                                      
    59.995 d2 =                                                           
               7.00  n2 =                                                 
                         1.65160                                          
                             ν2 =                                      
                                 58.5                                     
r3 =                                                                      
    889.650                                                               
           d3 =                                                           
               0.10                                                       
r4 =                                                                      
    42.569 d4 =                                                           
               5.60  n3 =                                                 
                         1.69680                                          
                             ν3 =                                      
                                 55.5                                     
r5 =                                                                      
    104.164                                                               
           d5 =                                                           
               variable                                                   
r6 =                                                                      
    61.312 d6 =                                                           
               1.20  n4 =                                                 
                         1.83400                                          
                             ν4 =                                      
                                 37.2                                     
r7 =                                                                      
    12.504 d7 =                                                           
               6.30                                                       
r8 =                                                                      
    -61.700                                                               
           d8 =                                                           
               1.10  n5 =                                                 
                         1.83481                                          
                             ν5 =                                      
                                 42.7                                     
r9 =                                                                      
     47.146                                                               
           d9 =                                                           
               0.10                                                       
r10 =                                                                     
    21.303 d10 =                                                          
               4.70  n6 =                                                 
                         1.84666                                          
                             ν6 =                                      
                                 23.9                                     
r11 =                                                                     
    -38.509                                                               
           d11 =                                                          
               1.10                                                       
r12 =                                                                     
    -25.283                                                               
           d12 =                                                          
               1.10  n7 =                                                 
                         1.83481                                          
                             ν7 =                                      
                                 42.7                                     
r13 =                                                                     
    60.506 d13 =                                                          
               variable                                                   
r14 =                                                                     
    (diaphragm)                                                           
           d14 =                                                          
               0.00                                                       
r15 =                                                                     
    22.060 d15 =                                                          
               1.20  n8 =                                                 
                         1.84666                                          
                             ν8 =                                      
                                 23.8                                     
r16 =                                                                     
    12.687 d16 =                                                          
               5.00  n9 =                                                 
                         1.60311                                          
                             ν9 =                                      
                                 60.7                                     
r17 =                                                                     
    -33.066                                                               
           d17 =                                                          
               0.10                                                       
r18 =                                                                     
    33.615 d18 =                                                          
               2.20  n10 =                                                
                         1.69680                                          
                             ν10 =                                     
                                 55.5                                     
r19 =                                                                     
    79.777 d19 =                                                          
               variable                                                   
r20 =                                                                     
    -85.039                                                               
           d20 =                                                          
               3.18  n11 =                                                
                         1.75520                                          
                             ν11 =                                     
                                 27.5                                     
r21 =                                                                     
    -13.516                                                               
           d21 =                                                          
               1.10  n12 =                                                
                         1.80400                                          
                             ν12 =                                     
                                 46.6                                     
r22 =                                                                     
    55.715 d22 =                                                          
               variable                                                   
r23 =                                                                     
    43.214 d23 =                                                          
               5.30  n13 =                                                
                         1.51633                                          
                             ν13 =                                     
                                 64.2                                     
r24 =                                                                     
    -18.478                                                               
           d24 =                                                          
               0.12                                                       
r25 =                                                                     
    -965.113                                                              
           d25 =                                                          
               1.40  n14 =                                                
                         1.77250                                          
                             ν14 =                                     
                                 49.6                                     
r26 =                                                                     
    29.121 d26 =                                                          
               2.20  n15 =                                                
                         1.60311                                          
                             ν15 =                                     
                                 60.7                                     
r27 =                                                                     
    1153.094                                                              
           d27 =                                                          
               3.60                                                       
r28 =                                                                     
    -14.543                                                               
           d28 =                                                          
               1.39  n16 =                                                
                         1.84666                                          
                             ν16 =                                     
                                 23.8                                     
r29 =                                                                     
    -25.140                                                               
__________________________________________________________________________
Focal length                                                              
           29.00      76.91                                               
                         191.11                                           
Variable interval                                                         
d5         1.08       21.90                                               
                         36.35                                            
d13        18.16      10.56                                               
                         1.80                                             
d19        1.61       6.89                                                
                         9.73                                             
d22        10.00      4.71                                                
                         1.88                                             
__________________________________________________________________________
 
    
     As described above, according to the present invention, a high varifocal ratio is provided, and the following advantages, for example, still can be obtained: 
     1. The total zoom lens length is reduced. 
     2. The outer diameter of the zoom lens is reduced. 
     3. By making the second lens group a focusing lens, the weight of the focus lens can be reduced, and a high-speed focusing operation can be performed. 
     4. Since no moving lens group portion is present outside the lens, an accidental, or unintentional, focus state change by a hand contact will not occur. 
     5. Since the first lens group does not rotate during a focusing operation, the user can easily operate, for example, a polarizing filter. 
     6. A petal-type lens hood can be mounted. 
     7. Deterioration of optical performance of the zoom lens is suppressed, even though the size of the lens has been reduced. 
     While the present invention has been described with respect to what presently are considered to be the preferred embodiments, it is understood that the invention is not limited to the disclosed embodiments. To the contrary, the present invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. The following claims are to be accorded with a broad interpretation, so as to encompass all such modifications and equivalent structures and functions.