Abstract:
A zoom lens having only four lens groups is disclosed. In order from the object side, these are: a first lens group having positive refractive power, a second lens group having negative refractive power that moves for zooming, a third lens group having negative refractive power that moves for correcting a shift in the image surface position when zooming, and a fourth lens group having positive refractive power and formed of front and rear subgroups, with the rear subgroup consisting of a single positive lens element that moves to correct for different back focus lengths that are required when the zoom lens is used in different television cameras. Various conditions are preferably satisfied in order to maintain favorable correction of spherical aberration and astigmatism even when the back focus of the zoom lens is adjusted slightly so as to compensate for manufacturing tolerances.

Description:
BACKGROUND OF THE INVENTION 
     In general, the back flange length of a mount in a television camera varies among individual cameras even though they satisfy the same standard. Therefore, after a lens is mounted in a television camera, the back focus is adjusted so that the image pick-up plane of the television camera coincides with the image plane of the lens. Back focus adjustment is performed by moving one or more lens elements of a lens group in order to adjust the image plane position. 
     PRIOR ART EXAMPLE 1 
     A prior art zoom lens used in a broadcast television camera that employs a tricolor separation prism is shown in FIG.  17 . This zoom lens includes an afocal part and a lens group having a complex structure that is moved for adjusting the axial position of the image plane. Therefore, aberrations are sufficiently corrected and lens performance is sufficiently maintained after the lens is moved in order to make the position of the image plane coincide with the image pick-up plane of the television camera. 
     As shown in FIG. 17, wherein X is the optical axis, the zoom lens of Prior Art Example 1 is formed of, in order from the object side, a first lens group I having a first lens L 1  through a fifth lens L 5 , a second lens group II having a sixth lens L 6  through a ninth lens L 9 , a third lens group III having a tenth lens L 10  and an eleventh lens L 11 , and a fourth lens group IV having a front subgroup IVa and a rear subgroup IVb. The front subgroup IVa is formed of a twelfth lens L 12  through a fifteenth lens L 15  and the rear subgroup IVb is formed of a sixteenth lens L16 through a twenty-first lens L 21 . A diaphragm stop  1  is positioned on the object side of front subgroup IVa of the fourth lens group. A tricolor separation prism  4  and a filter  2  are positioned between the rear subgroup IVb of the fourth lens group and the image plane R 42 . An image sensor  3 , in this instance a CCD, is provided at the image plane. The image sensor  3 , tricolor separation prism  4 , and filter  2  are provided in the television camera body. 
     Table 1 below shows the surface # in order from the object side, the radius of curvature R (in mm) of each surface, the on-axis spacing D (in mm) between surfaces, as well as the refractive index N e  and Abbe number ν e  (both at the e-line) of the lens elements of the zoom lens of Prior Art Example 1. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 # 
                 R 
                 D 
                 N e   
                 ν e   
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 −167.933 
                 2.506 
                 1.81264 
                 25.2 
               
               
                 2 
                 205.525 
                 5.917 
               
               
                 3 
                 ∞ 
                 8.182 
                 1.43496 
                 94.6 
               
               
                 4 
                 −123.571 
                 0.116 
               
               
                 5 
                 284.452 
                 7.902 
                 1.43496 
                 94.6 
               
               
                 6 
                 −212.171 
                 7.497 
               
               
                 7 
                 124.045 
                 9.502 
                 1.57098 
                 70.9 
               
               
                 8 
                 −302.956 
                 0.116 
               
               
                 9 
                 64.461 
                 6.206 
                 1.82016 
                 46.4 
               
               
                 10 
                 130.958 
                 D10 
               
               
                 11 
                 54.496 
                 0.771 
                 1.83932 
                 36.9 
               
               
                 12 
                 14.181 
                 6.852 
               
               
                 13 
                 −61.901 
                 0.771 
                 1.83945 
                 42.5 
               
               
                 14 
                 40.862 
                 1.368 
               
               
                 15 
                 25.631 
                 4.732 
                 1.85501 
                 23.7 
               
               
                 16 
                 −49.180 
                 0.665 
               
               
                 17 
                 −32.185 
                 0.771 
                 1.77620 
                 49.3 
               
               
                 18 
                 79.600 
                 D18 
               
               
                 19 
                 −27.304 
                 0.771 
                 1.75843 
                 52.1 
               
               
                 20 
                 41.637 
                 2.390 
                 1.85501 
                 23.7 
               
               
                 21 
                 ∞ 
                 D21 
               
               
                 22 
                 ∞ (stop) 
                 1.706 
               
               
                 23 
                 213.835 
                 4.828 
                 1.52033 
                 58.7 
               
               
                 24 
                 −34.837 
                 0.116 
               
               
                 25 
                 153.522 
                 3.267 
                 1.59143 
                 61.0 
               
               
                 26 
                 −2088.371 
                 0.116 
               
               
                 27 
                 51.765 
                 7.016 
                 1.52033 
                 58.7 
               
               
                 28 
                 −36.176 
                 1.205 
                 1.80810 
                 46.3 
               
               
                 29 
                 −518.539 
                 32.938 
               
               
                 30 
                 49.070 
                 4.655 
                 1.51825 
                 63.9 
               
               
                 31 
                 −82.602 
                 0.116 
               
               
                 32 
                 34.236 
                 6.180 
                 1.48914 
                 70.2 
               
               
                 33 
                 −41.312 
                 1.253 
                 1.83945 
                 42.5 
               
               
                 34 
                 21.220 
                 3.442 
               
               
                 35 
                 31.261 
                 8.163 
                 1.51825 
                 63.9 
               
               
                 36 
                 −22.118 
                 1.205 
                 1.83945 
                 42.5 
               
               
                 37 
                 −94.916 
                 0.116 
               
               
                 38 
                 84.626 
                 6.426 
                 1.48914 
                 70.2 
               
               
                 39 
                 −26.108 
                 10.665 
               
               
                 40 
                 ∞ 
                 33.000 
                 1.58565 
                 46.2 
               
               
                 41 
                 ∞ 
                 13.200 
                 1.51825 
                 63.9 
               
               
                 42 
                 ∞ (image) 
               
               
                   
               
             
          
         
       
     
     Table 2 below shows at both the wide-angle end (WIDE) and telephoto end (TELE) for Prior Art Example 1: the focal length f (in mm); the F-number F NO  with the diaphragm stop fully open; and the on-axis spacings D10, D18 and D21 (in mm) between the lens groups. Also listed, for the wide-angle end (WIDE) of Prior Art Example 1 are: the back focus Bf (in mm) of the zoom lens; the back focus Bf 4b  of the rear subgroup of the fourth lens group that is moveable in order to adjust the back focus; the ratio Bf 4b /Bf; the maximum ray height Ha on the image-side surface immediately before the movable component(s) of the fourth lens group (in this case, the surface of radius of curvature R 29 ) at the wide-angle end when the diaphragm stop is fully open; the maximum ray height Hb on the lens surface of the movable component(s) of the fourth lens group nearest the object side (in this case, the surface of radius of curvature R 30 ) at the wide-angle end when the diaphragm stop is fully open; the absolute value of Ha minus Hb; the value 0.02 Bf/F N ; and the amount of axial shift (in mm) in focal point position when the rear subgroup IVb is moved (from the position given in Table 1) 1 mm toward the image so as to adjust the back focus. In the bottom portion of the table are listed the image sensor diameter (hereinafter termed the “screen size”, in mm) for which the zoom lens of Prior Art Example 1 is designed, as well as the diameter of the diaphragm stop (in mm) when fully open. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 WIDE 
                 TELE 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 f 
                 8 
                 160 
               
               
                   
                 F NO   
                 1.65 
                 2.56 
               
               
                   
                 D10 
                 0.944 
                 54.595 
               
               
                   
                 D18 
                 55.218 
                 7.177 
               
               
                   
                 D21 
                 6.514 
                 0.905 
               
               
                   
                 Bf 
                 40.171 
               
               
                   
                 Bf 4b   
                 40.036 
               
               
                   
                 Bf 4b /Bf 
                 0.997 
               
               
                   
                 Ha 
                 15.649 
               
               
                   
                 Hb 
                 15.409 
               
               
                   
                 Ha − Hb 
                 0.24 
               
               
                   
                 0.02 Bf/F N   
                 0.487 
               
               
                   
                 Focal point shift 
                 1 
               
               
                   
                   
               
               
                   
                 Screen size: Φ = 11.0  
               
               
                   
                 Diaphragm stop diameter when fully open = 27.858  
               
             
          
         
       
     
     FIGS.  19 ( a ) and  19 ( b ) show the spherical aberration and the astigmatism, respectively, at the wide-angle end of the zoom lens of Prior Art Example 1 with the image plane position as designed (i.e., as given in Table 1 above). FIGS.  19 ( c ) and  19 ( d ) show the spherical aberration and astigmatism, respectively, at the wide-angle end of the zoom lens of Prior Art Example 1 after the rear subgroup IVb has been moved (from the position indicated in Table 1 above) 1 mm toward the image so as to adjust the back focus. In FIGS.  19 ( b ) and  19 ( d ) the astigmatism is shown for both the sagittal (S) and tangential (T) image planes. As is apparent from FIGS.  19 ( a )- 19 ( d ), the zoom lens of Prior Art Example 1 does not show significant differences in spherical aberration and astigmatism after the rear subgroup IVb has been moved (from its position indicated in Table 1) 1 mm toward the image so as to adjust the back focus. In fact, the lens performance changes very little when the rear subgroup IVb is moved 1 mm toward the image from its design position. 
     PRIOR ART EXAMPLE 2 
     Unlike the zoom lens of Prior Art Example 1 described above, a zoom lens used in a simple television camera such as a CCTV (hereinafter termed Prior Art Example 2) is formed of a simple and compact lens group that is moved for adjusting the back focus. Therefore, the lens performance will inevitably change in association with the back focus adjustment. The basic lens element configuration of the zoom lens of Prior Art Example 2 is shown in FIG.  18 . 
     As is shown in FIG. 18, the zoom lens of Prior Art Example 2 is formed of, in order from the object side, a first lens group I having a first lens L 1  through a third lens L 3 , a second lens group II having a fourth lens L 4  through a sixth lens L 6 , a third lens group III having a seventh lens L 7 , and a fourth lens group IV having an eighth lens L 8  through a fourteenth lens L 14 . The fourth lens group IV further is organized as a front subgroup IVa formed of an eighth lens L 8  through an eleventh lens L 11 , and a rear subgroup IVb formed of a twelfth lens L 12  through a fourteenth lens L 14 . 
     A diaphragm stop  1  is positioned between L 8  and L 9 . A filter  2  is positioned between the rear subgroup IVb and an image sensor  3  such as a CCD. The image sensor  3  and filter  2  are provided in the television camera body. X is the optical axis in FIG.  18 . 
     Table 3 below shows the surface # in order from the object side, the radius of curvature R (in mm) of each surface, the on-axis spacing D (in mm) between surfaces, as well as the refractive index N e  and Abbe number ν e  (both at the e-line) of the lens elements of the zoom lens of Prior Art Example 2. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                 # 
                 R 
                 D 
                 N e   
                 ν e   
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 101.143 
                 1.739 
                 1.79191 
                 25.5 
               
               
                 2 
                 47.046 
                 8.790 
                 1.59143 
                 61.0 
               
               
                 3 
                 −127.845 
                 0.098 
               
               
                 4 
                 36.375 
                 4.883 
                 1.59143 
                 61.0 
               
               
                 5 
                 74.271 
                 D5  
               
               
                 6 
                 100.166 
                 0.801 
                 1.72341 
                 50.1 
               
               
                 7 
                 17.576 
                 4.835 
               
               
                 8 
                 −21.703 
                 0.801 
                 1.72341 
                 50.1 
               
               
                 9 
                 19.417 
                 3.780 
                 1.81262 
                 25.3 
               
               
                 10 
                 −237.680 
                 D10 
               
               
                 11 
                 −33.107 
                 0.801 
                 1.72794 
                 37.7 
               
               
                 12 
                 −334.076 
                 D12 
               
               
                 13 
                 112.912 
                 5.411 
                 1.70558 
                 40.9 
               
               
                 14 
                 −30.760 
                 1.807 
               
               
                 15 
                 ∞ (stop) 
                 1.660 
               
               
                 16 
                 50.604 
                 4.542 
                 1.62508 
                 52.8 
               
               
                 17 
                 −75.801 
                 2.149 
               
               
                 18 
                 −27.512 
                 1.465 
                 1.81262 
                 25.3 
               
               
                 19 
                 −152.164 
                 0.098 
               
               
                 20 
                 24.115 
                 5.762 
                 1.64128 
                 55.2 
               
               
                 21 
                 −320.217 
                 12.736 
               
               
                 22 
                 −62.407 
                 1.270 
                 1.81262 
                 25.3 
               
               
                 23 
                 17.953 
                 1.465 
               
               
                 24 
                 46.397 
                 3.907 
                 1.64128 
                 55.2 
               
               
                 25 
                 −25.769 
                 0.098 
               
               
                 26 
                 16.223 
                 3.125 
                 1.62409 
                 36.1 
               
               
                 27 
                 72.764 
                 12.320 
               
               
                 28 
                 ∞ 
                 5.000 
                 1.51824 
                 63.9 
               
               
                 29 
                 ∞ (image) 
               
               
                   
               
             
          
         
       
     
     Table 4 below shows at both the wide-angle end (WIDE) and telephoto end (TELE) for Prior Art Example 2: the focal length f (in mm); the F-number F NO  with the diaphragm stop fully open; and the on-axis spacings D5, D10 and D12 (in mm) between the lens groups. Also listed, for the wide-angle end (WIDE) of Prior Art Example 2 are: the back focus Bf (in mm) of the zoom lens; the back focus Bf 4b  of the rear subgroup of the fourth lens group; the ratio Bf 4b /Bf; the maximum ray height Ha on the image-side surface immediately before the movable component(s) of the fourth lens group (in this case, the surface of radius of curvature R 21 ) at the wide-angle end when the diaphragm stop is fully open; the maximum ray height Hb on the lens surface of the movable component(s) of the fourth lens group nearest the object side (in this case, the surface of radius of curvature R 22 ) at the wide-angle end when the diaphragm stop is fully open; the absolute value of Ha minus Hb; the value 0.02 Bf/F N ; and the amount of axial shift (in mm) in focal point position when the rear subgroup IVb is moved (from the position given in Table 3) 1 mm toward the image so as to adjust the back focus. In the bottom portion of the table are listed the screen size (in mm) for which the zoom lens of Prior Art Example 2 is designed, as well as the diameter of the diaphragm stop (in mm) when fully open. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 WIDE 
                 TELE 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 f 
                 12.5 
                 75 
               
               
                   
                 F NO   
                 1.65 
                 1.66 
               
               
                   
                 D5 
                 1.221 
                 30.571 
               
               
                   
                 D10 
                 28.675 
                 3.127 
               
               
                   
                 D12 
                 4.288 
                 0.485 
               
               
                   
                 Bf 
                 15.613 
               
               
                   
                 Bf 4b   
                 50.163 
               
               
                   
                 Bf 4b /Bf 
                 3.213 
               
               
                   
                 Ha 
                 8.999 
               
               
                   
                 Hb 
                 5.744 
               
               
                   
                 Ha − Hb 
                 3.255 
               
               
                   
                 0.02 Bf/F N   
                 0.189 
               
               
                   
                 Focal point shift 
                 0.375 
               
               
                   
                   
               
               
                   
                 Screen size: Φ = 11.0  
               
               
                   
                 Diaphragm stop diameter when fully open = 19.613  
               
             
          
         
       
     
     FIGS.  20 ( a ) and  20 ( b ) show the spherical aberration and the astigmatism, respectively, at the wide-angle end of the zoom lens of Prior Art Example 2 when the rear subgroup IVb is positioned at its design position. FIGS.  20 ( c ) and  20 ( d ) show the spherical aberration and astigmatism, respectively, at the wide-angle end of the zoom lens of Prior Art Example 2 after the rear subgroup IVb has been moved (from the position given in Table 3 above) 1 mm toward the image so as to adjust the back focus. The curves shown in each of FIGS.  20 ( b ) and  20 ( d ) are for the astigmatism in the sagittal (S) and tangential (T) image planes. As is apparent from FIGS.  20 ( a )- 20 ( d ), the zoom lens of Prior Art Example 2 undergoes significant deterioration in spherical aberration and some deterioration in astigmatism as a result of the subgroup IVb being moved 1 mm toward the image so as to adjust the back focus. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention relates to a zoom lens for television cameras, especially to a zoom lens that can be mounted in a camera body such as a CCTV (Closed Circuit television) camera and that has an adjustable back focus. 
     The object of the present invention is to provide a zoom lens in which a lens component that is moved for adjusting the back focus has a simple and compact structure and the lens performance is maintained even when the back focus is adjusted, as is often necessary where the zoom lens, in use, may be mounted to various T.V. cameras of broadcast quality with the same nominal design, but the mounting flanges from camera to camera vary somewhat due to production tolerances. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given below and the accompanying drawings, which are given by way of illustration only and thus are not limitative of the present invention, wherein: 
     FIG. 1 shows the basic lens element configuration of Embodiment 1 of the present invention; 
     FIG. 2 shows the basic lens element configuration of Embodiment 2 of the present invention; 
     FIG. 3 shows the basic lens element configuration of Embodiment 3 of the present invention; 
     FIGS.  4 ( a ) and  4 ( b ) show the spherical aberration and astigmatism, respectively, of the zoom lens of Embodiment 1 at the wide-angle end; 
     FIG. 5 shows the coma of the zoom lens of Embodiment 1 at the wide-angle end for field angles ω equal to 0, 16.2, 21.4 and 26.3 degrees for both the tangential (left column) and sagittal (right column) image surfaces; 
     FIGS.  6 ( a )- 6 ( b ) show the spherical aberration and astigmatism, respectively, of the zoom lens of Embodiment 1 at the telephoto end; 
     FIG. 7 shows the coma of the zoom lens of Embodiment 1 at the telephoto end for field angles ω equal to 0, 0.8, 1.0 and 1.3 degrees for both the tangential (left column) and sagittal (right column) image surfaces; 
     FIGS.  8 ( a ) and  8 ( b ) show the spherical aberration and astigmatism, respectively, of the zoom lens of Embodiment 2 at the wide-angle end; 
     FIG. 9 shows the coma of the zoom lens of Embodiment 2 at the wide-angle end for field angles ω equal to 0, 16, 21.1, and 25.9 degrees for both the tangential (left column) and sagittal (right column) image surfaces; 
     FIGS.  10 ( a ) and  10 ( b ) show the spherical aberration and astigmatism, respectively, of the zoom lens of Embodiment 2 at the telephoto end; 
     FIG. 11 shows the coma of the zoom lens of Embodiment 2 at the telephoto end for field angles ω equal to 0, 0.8, 1.0, and 1.2 degrees for both the tangential (left column) and sagittal (right column) image surfaces; 
     FIGS.  12 ( a ) and  12 ( b ) show the spherical aberration and astigmatism, respectively, at the wide-angle end of the zoom lens of Embodiment 2 as designed, and FIGS.  12 ( c ) and  12 ( d ) show these same respective aberrations after a rear subgroup of the fourth lens group has been moved 1 mm toward the image from its design position so as to adjust the back focus; 
     FIGS.  13 ( a ) and  13 ( b ) show the spherical aberration and astigmatism, respectively, of the zoom lens of Embodiment 3 at the wide-angle end; 
     FIG. 14 shows the coma of the zoom lens of Embodiment 3 at the wide-angle end for field angles ω equal to 0, 15.8, 20.8, and 25.6 degrees for both the tangential (left column) and sagittal (right column) image surfaces; 
     FIGS.  15 ( a ) and  15 ( b ) show the spherical aberration and astigmatism, respectively, of the zoom lens of Embodiment 3 at the telephoto end; 
     FIG. 16 shows the coma of the zoom lens of Embodiment 3 at the telephoto end for field angles ω equal to 0, 0.7, 1.0, and 1.2 degrees for both the tangential (left column) and sagittal (right column) image surfaces; 
     FIG. 17 shows the basic lens element configuration of Prior Art Example 1; 
     FIG. 18 shows the basic lens element configuration of Prior Art Example 2; 
     FIGS.  19 ( a ) and  19 ( b ) show the spherical aberration and astigmatism, respectively, at the wide-angle end of the zoom lens of Prior Art Example 1 at the design position, and FIGS.  19 ( c ) and  19 ( d ) show the spherical aberration and astigmatism, respectively, after the rear subgroup IVb of the fourth lens group has been moved 1 mm toward the image so as to adjust the back focus; and 
     FIGS.  20 ( a ) and  20 ( b ) show the spherical aberration and astigmatism, respectively, at the wide-angle end of the zoom lens of Prior Art Example 2 at its design position, and FIGS.  20 ( c ) and  20 ( d ) show the spherical aberration and astigmatism, respectively, after the rear subgroup IV(b) of the fourth lens group has been moved 1 mm toward the image so as to adjust the back focus. 
    
    
     DETAILED DESCRIPTION 
     The zoom lens of the present invention is formed of, in order from the object side, a first lens group having positive refractive power, a second lens group having negative refractive power and that moves for changing the magnification when zooming, a third lens group having negative refractive power and that moves for correcting image plane shifts that otherwise would accompany magnification change during zooming, and a fourth lens group having positive refractive power. The fourth lens group is formed of a front subgroup and a rear subgroup with the rear subgroup consisting of a single positive lens element that moves so as to adjust the back focus (i.e., to position the image plane at a desired axial position, which varies due to manufacturing tolerances even among broadcast quality T.V. cameras that are built to a common specification). 
     It is preferred that the following Condition (1) is satisfied: 
     
       
         1.0 &lt;Bf   4b   /Bf&lt; 1.5  Condition (1) 
       
     
     where 
     Bf is the back focus of the zoom lens, and 
     Bf 4b  is the back focus of the rear subgroup of the fourth lens group. 
     Condition (1) limits the ratio Bf 4b /Bf to a specified range. This reduces the deviation in aberrations when the rear subgroup IVb of the fourth lens group is moved for adjusting the back focus. When the ratio Bf 4b /Bf exceeds the lower limit in Condition (1), the aberrations are not satisfactorily corrected. When the ratio Bf 4b /Bf exceeds the upper limit, the rear subgroup IVb has an excessive power, increasing the deviation in aberrations as the rear subgroup IVb is moved. 
     It is also preferred that the following Condition (2) is satisfied: 
     
       
         | Ha−Hb| &lt;0.02× Bf/Fn   Condition (2) 
       
     
     where 
     Bf is as defined above, 
     Fn is the F-number at the wide-angle end with the diaphragm stop fully open, 
     Ha is the maximum ray height on the image-side surface immediately before the movable component(s) of the fourth lens group, at the wide-angle end when the diaphragm stop is fully open; and 
     Hb is the maximum ray height on the lens surface of the movable component(s) of the fourth lens group nearest the object side, at the wide-angle end when the diaphragm stop is fully open. 
     Condition (2) limits the height difference to a specified range of the outermost rays on the image-side surface of L 15  versus the object-side surface of L 16  This reduces deviations in spherical aberration when the rear subgroup IVb of the fourth lens group is moved for adjusting the back focus. When the absolute value of Ha−Hb exceeds a predetermined value, the deviation in spherical aberration significantly increases as the rear subgroup is moved, causing the lens performance to degrade. 
     It is further preferred that the front subgroup of the fourth lens group is formed of, in order from the object side, a positive lens with a convex surface on the image side, a biconvex lens, a doublet having a positive lens element joined to a negative lens element, a positive lens with a convex surface on the object side, and a doublet having a positive lens element joined to a negative lens element. 
     It is further preferred that, the first lens group is formed of, in order from the object side, a doublet having a negative lens element joined to a positive lens element, and a positive meniscus lens with its convex surface on the object side. And, it is preferred that the second lens group is formed of, in order from the object side, a negative lens element, a negative lens element, and a positive meniscus lens element with its convex surface on the object side. Further it is preferred that the third lens group is formed of a doublet having a biconcave lens element joined to a positive lens element. 
     Three embodiments of the zoom lens of the present invention will now be described in detail. 
     Embodiment 1 
     FIG. 1 shows the basic lens element configuration of the zoom lens of Embodiment 1 of the present invention. The zoom lens of Embodiment 1 is formed of, in order from the object side, a first lens group I having positive refractive power, a second lens group II having negative refractive power and that moves for changing magnification when zooming, a third lens group III having negative refractive power and that moves for correcting image plane shifts that otherwise would accompany magnification change during zooming, and a fourth lens group IV having positive refractive power. The fourth lens group IV is formed of a front subgroup IVa and a rear subgroup IVb. 
     The first lens group I is formed of, in order from the object side, a doublet having a negative meniscus lens element L 1  with its convex surface on the object side that is joined to a biconvex lens element L 2 , and a positive meniscus lens element L 3  with its convex surface on the object side. 
     The second lens group II is formed of, in order from the object side, a negative meniscus lens element L 4  with its convex surface on the object side, a biconcave lens element L 5 , and a positive meniscus lens element L 6  with its convex surface on the object side. 
     The third lens group III is formed of a doublet having, in order from the object side, a biconcave lens element L 7  joined to a biconvex lens element L 8 . 
     The fourth lens group IV is formed of a front subgroup IVa and a rear subgroup IVb. The front subgroup IVa is formed of, in order from the object side, a biconvex lens element L 9  with different radii of curvature on its two sides, a biconvex lens element L 10 , a doublet having a biconvex lens element L 11  that is joined to a negative meniscus lens element L 12  with its convex surface on the image side, a biconvex lens element L 13  having different radii of curvature on its two sides, and a doublet formed of a biconvex lens element L 14  that is joined to a biconcave lens element L 15 . The rear subgroup IVb consists of a single biconvex lens element L 16 . 
     The biconvex lens element L 9  is positioned with its surface of smaller radius of curvature on the image side, and the biconvex lens element L 13  is positioned with its surface of smaller radius of curvature on the object side. A diaphragm stop  1  is positioned within the front subgroup IVa. A filter  2  is positioned between the rear subgroup IVb and an image sensor  3 , such as a CCD. The image sensor  3  and the filter  2  are provided in a television camera body. The optical axis in FIG. 1 is indicated by an X. In the zoom lens of Embodiment 1, the rear subgroup IVb is moved along the optical axis X in order to adjust the back focus so that the image pick-up plane (image sensor  3 ) of the television camera coincides with the image plane of the zoom lens. The zoom lens of Embodiment 1 satisfies both Condition (1) and Condition (2) above. 
     Table 5 below lists the surface number # in order from the object side, the radius of curvature R (in mm) of each surface, the on-axis spacing D (in mm) between surfaces, as well as the index of refraction N e  and the Abbe number ν e  (both at the e-line) of the lens elements of the zoom lens of Embodiment 1. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 5 
               
               
                   
               
               
                 # 
                 R 
                 D 
                 N e   
                 ν e   
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 164.503 
                 2.441 
                 1.81264 
                 25.2 
               
               
                 2 
                 75.787 
                 10.038 
                 1.62286 
                 60.1 
               
               
                 3 
                 −975.027 
                 0.117 
               
               
                 4 
                 73.436 
                 6.396 
                 1.71615 
                 53.7 
               
               
                 5 
                 194.680 
                 D5  
               
               
                 6 
                 94.382 
                 1.367 
                 1.77620 
                 49.3 
               
               
                 7 
                 16.011 
                 7.417 
               
               
                 8 
                 −69.864 
                 1.172 
                 1.77620 
                 49.3 
               
               
                 9 
                 69.864 
                 0.049 
               
               
                 10 
                 29.084 
                 3.799 
                 1.81264 
                 25.2 
               
               
                 11 
                 108.952 
                 D11 
               
               
                 12 
                 −35.768 
                 1.172 
                 1.80811 
                 46.3 
               
               
                 13 
                 64.907 
                 2.676 
                 1.81264 
                 25.2 
               
               
                 14 
                 −346.174 
                 D14 
               
               
                 15 
                 ∞ (stop) 
                 2.607 
               
               
                 16 
                 336.976 
                 5.400 
                 1.62286 
                 60.1 
               
               
                 17 
                 −65.101 
                 0.117 
               
               
                 18 
                 143.651 
                 4.560 
                 1.62286 
                 60.1 
               
               
                 19 
                 −143.651 
                 0.117 
               
               
                 20 
                 61.422 
                 11.093 
                 1.48914 
                 70.2 
               
               
                 21 
                 −37.634 
                 1.562 
                 1.81077 
                 40.7 
               
               
                 22 
                 −502.377 
                 1.094 
               
               
                 23 
                 45.077 
                 8.779 
                 1.62286 
                 60.1 
               
               
                 24 
                 −87.381 
                 0.137 
               
               
                 25 
                 464.633 
                 4.404 
                 1.48914 
                 70.2 
               
               
                 26 
                 −75.953 
                 1.367 
                 1.81077 
                 40.7 
               
               
                 27 
                 29.349 
                 12.337 
               
               
                 28 
                 90.467 
                 4.424 
                 1.62286 
                 60.1 
               
               
                 29 
                 −90.467 
                 62.212 
               
               
                 30 
                 ∞ 
                 5.000 
                 1.51824 
                 63.9 
               
               
                 31 
                 ∞ (image) 
               
               
                   
               
             
          
         
       
     
     Table 6 below lists, at both the wide-angle end (WIDE) and telephoto end (TELE), for Embodiment 1 of the present invention: the focal length f (in mm); the F-number F NO  with the diaphragm stop fully open; and the on-axis spacings D5, D11 and D14 (in mm) between the lens groups. Also listed, for the wide-angle end of Embodiment 1 are: the back focus Bf (in mm) of the zoom lens; the back focus Bf 4b  of the rear subgroup of the fourth lens group; the ratio Bf 4b /Bf; the maximum ray height Ha on the image-side surface immediately before the movable component(s) of the fourth lens group (in this case, the surface of radius of curvature R 27 ) at the wide-angle end when the diaphragm stop is fully open; the maximum ray height Hb on the lens surface of the movable component(s) of the fourth lens group nearest the object side (in this case, the surface of radius of curvature R 28 ) at the wide-angle end when the diaphragm stop is fully open; the absolute value of Ha minus Hb; the value 0.02 Bf/F N ; and the amount of shift (in mm) in focal point position when the rear subgroup IVb is moved (from the position given in Table 5) 1 mm toward the image in order to adjust the back focus. In the bottom portion of the table are listed the screen size (in mm) for which the zoom lens of Embodiment 1 is designed, as well as the diameter of the diaphragm stop (in mm) when fully open. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 6 
               
               
                   
                   
               
               
                   
                 WIDE 
                 TELE 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 f 
                 16.5 
                 330 
               
               
                   
                 F NO   
                 2.4 
                 5.18 
               
               
                   
                 D5 
                 0.976 
                 75.701 
               
               
                   
                 D11 
                 72.15 
                 10.509 
               
               
                   
                 D14 
                 17.195 
                 4.112 
               
               
                   
                 Bf 
                 65.505 
               
               
                   
                 Bf 4b   
                 71.934 
               
               
                   
                 Bf 4b /Bf 
                 1.098 
               
               
                   
                 Ha 
                 14.377 
               
               
                   
                 Hb 
                 14.304 
               
               
                   
                 Ha − Hb 
                 0.073 
               
               
                   
                 0.02 Bf/Fn 
                 0.546 
               
               
                   
                 Focal point shift 
                 0.992 
               
               
                   
                   
               
               
                   
                 Screen size: Φ = 16.0  
               
               
                   
                 Diaphragm stop diameter when fully open = 36.561  
               
             
          
         
       
     
     As is apparent from Table 6, the zoom lens of Embodiment 1 satisfies Conditions (1) and (2). 
     FIGS.  4 ( a ) to  7  show aberrations of the zoom lens of Embodiment 1 with the distance to an object being 12 m. FIGS.  4 ( a ) and  4 ( b ) show the spherical aberration and astigmatism, respectively, at the wide-angle end. FIG. 5 shows the coma at the wide-angle end at field angles ω of 0, 16.2, 21.4 and 26.3 degrees, for both the tangential image plane (left column) and the sagittal image plane (right column), with the vertical axis dimension being in mm. As the coma on axis (ω=0°) is the same for both the tangential and sagittal image planes, only the curve for the tangential image plane is illustrated in the bottom row. FIGS.  6 ( a ) and  6 ( b ) show the spherical aberration and astigmatism, respectively, at the telephoto end. In FIGS.  4 ( b ) and  6 ( b ), the astigmatism is shown for both the sagittal (S) and tangential (T) image planes. FIG. 7 shows the coma at the telephoto end at field angles of 0, 0.8, 1.0 and 1.3 degrees, for both the tangential image plane (left column) and the sagittal image plane (right column), with the vertical axis dimension being in mm. Once, again only a single curve is illustrated in the bottom row, for the reason discussed above. 
     As is apparent from FIGS.  4 ( a ) to  7 , the zoom lens of Embodiment 1, has its aberrations favorably corrected just as satisfactorily as the zoom lens of Prior Art Example 1. Moreover, whereas the zoom lens of Prior Art Example 1 has a complex configuration of lens elements that are moved in order to adjust the back focus of the zoom lens, Embodiment 1 of the present invention provides a simpler and more compact configuration of the lens element(s) that are moved for adjusting the back focus. 
     Embodiment 2 
     As shown in FIG. 2, the zoom lens of Embodiment 2 has nearly the same configuration as the zoom lens of Embodiment 1, except in this embodiment, the twelfth lens element L 12  is biconcave and the thirteenth lens element L 13  is a positive meniscus lens having a convex surface on the object side. 
     Table 7 below lists the surface number # in order from the object side, the radius of curvature R (in mm) of each surface, the on-axis spacing D (in mm) between surfaces, as well as the index of refraction N e  and the Abbe number ν e  (both at the e-line) of the lens elements of the zoom lens of Embodiment 2. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 7 
               
               
                   
               
               
                 # 
                 R 
                 D 
                 N e   
                 ν e   
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 165.822 
                 2.461 
                 1.81264 
                 25.2 
               
               
                 2 
                 76.395 
                 10.119 
                 1.62286 
                 60.1 
               
               
                 3 
                 −982.844 
                 0.118 
               
               
                 4 
                 74.025 
                 6.447 
                 1.71615 
                 53.7 
               
               
                 5 
                 196.241 
                 D5  
               
               
                 6 
                 95.139 
                 1.378 
                 1.77620 
                 49.3 
               
               
                 7 
                 16.139 
                 7.476 
               
               
                 8 
                 −70.424 
                 1.181 
                 1.77620 
                 49.3 
               
               
                 9 
                 70.424 
                 0.049 
               
               
                 10 
                 29.317 
                 3.829 
                 1.81264 
                 25.2 
               
               
                 11 
                 109.826 
                 D11 
               
               
                 12 
                 −36.055 
                 1.181 
                 1.80810 
                 46.3 
               
               
                 13 
                 65.427 
                 2.697 
                 1.81264 
                 25.2 
               
               
                 14 
                 −348.950 
                 D14 
               
               
                 15 
                 ∞ (stop) 
                 2.628 
               
               
                 16 
                 691.470 
                 6.506 
                 1.62286 
                 60.1 
               
               
                 17 
                 −47.321 
                 0.118 
               
               
                 18 
                 171.620 
                 4.262 
                 1.62286 
                 60.1 
               
               
                 19 
                 −171.620 
                 0.118 
               
               
                 20 
                 54.090 
                 10.660 
                 1.48914 
                 70.2 
               
               
                 21 
                 −41.450 
                 1.575 
                 1.81077 
                 40.7 
               
               
                 22 
                 88.043 
                 0.118 
               
               
                 23 
                 29.741 
                 8.997 
                 1.62286 
                 60.1 
               
               
                 24 
                 1148.88 
                 0.138 
               
               
                 25 
                 27.519 
                 8.347 
                 1.48914 
                 70.2 
               
               
                 26 
                 −132.735 
                 1.378 
                 1.81077 
                 40.7 
               
               
                 27 
                 18.577 
                 10.241 
               
               
                 28 
                 58.787 
                 4.380 
                 1.62286 
                 60.1 
               
               
                 29 
                 −58.787 
                 32.131 
               
               
                 30 
                 ∞ 
                 5.000 
                 1.51824 
                 63.9 
               
               
                 31 
                 ∞ (image) 
               
               
                   
               
             
          
         
       
     
     Table 8 below lists at both the wide-angle end (WIDE) and telephoto end (TELE) for Embodiment 2 of the present invention: the focal length f (in mm); the F-number F NO  with the diaphragm stop fully open; and the on-axis spacings D5, D11 and D14 (in mm) between the lens groups. Also, for the wide-angle end of Embodiment 2 are listed: the back focus Bf (in mm) of the zoom lens; the back focus Bf 4b  of the rear subgroup of the fourth lens group; the ratio Bf 4b /Bf; the maximum ray height Ha on the image-side surface immediately before the movable component(s) of the fourth lens group (in this case, the surface of radius of curvature R 27 ) at the wide-angle end when the diaphragm stop is fully open; the maximum ray height Hb on the lens surface of the movable component(s) of the fourth lens group nearest the object side (in this case, the surface of radius of curvature R 28 ) at the wide-angle end when the diaphragm stop is fully open; the absolute value of Ha minus Hb; the value 0.02 Bf/F N ; and the amount of shift (in mm) in focal point position when the rear subgroup IVb is moved (from the position given in Table 7 above) 1 mm toward the image in order to adjust the back focus. In the bottom portion of the table are listed the screen size (in mm) for which the zoom lens of Embodiment 2 is designed, as well as the diameter of the diaphragm stop (in mm) when fully open. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 8 
               
               
                   
                   
               
               
                   
                 WIDE 
                 TELE 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 f 
                 11.5 
                 230 
               
               
                   
                 F NO   
                 1.65 
                 3.58 
               
               
                   
                 D5 
                 0.984 
                 76.308 
               
               
                   
                 D11 
                 72.729 
                 10.593 
               
               
                   
                 D14 
                 17.333 
                 4.145 
               
               
                   
                 Bf 
                 35.423 
               
               
                   
                 Bf 4b   
                 46.507 
               
               
                   
                 Bf 4b /Bf 
                 1.313 
               
               
                   
                 Ha 
                 12.102 
               
               
                   
                 Hb 
                 11.794 
               
               
                   
                 Ha − Hb 
                 0.308 
               
               
                   
                 0.02 Bf/Fn 
                 0.429 
               
               
                   
                 Focal point shift 
                 0.946 
               
               
                   
                   
               
               
                   
                 Screen size: Φ = 11.0  
               
               
                   
                 Diaphragm stop diameter when fully open = 36.964  
               
             
          
         
       
     
     As is apparent from Table 8, the zoom lens of Embodiment 2 satisfies the above Conditions (1) and (2). 
     FIGS.  8 ( a ) to  11  show aberrations of the zoom lens of Embodiment 2 with the distance to an object being 12 m. FIGS.  8 ( a ) and  8 ( b ) show the spherical aberration and astigmatism, respectively, at the wide-angle end. FIG. 9 shows the coma at the wide-angle end at field angles ω of 0, 16, 21.1 and 25.9 degrees, for both the tangential image plane (left column) and the sagittal image plane (right column), with the vertical axis dimension being in mm. As the coma on axis (ω=0°) is the same for both the tangential and sagittal image planes, only the curve for the tangential image plane is illustrated in the bottom row. FIGS.  10 ( a ) and  10 ( b ) show the spherical aberration and astigmatism, respectively, at the telephoto end. In FIGS.  8 ( b ) and  10 ( b ), the astigmatism is shown for both the sagittal (S) and tangential (T) image planes. FIG. 11 shows the coma at the telephoto end at field angles of 0, 0.8, 1.0 and 1.2 degrees, for both the tangential image plane (left column) and the sagittal image plane (right column), with the vertical axis dimension being in mm. Once, again only a single curve is illustrated in the bottom row, for the reason discussed above. 
     FIGS.  12 ( a )- 12 ( d ) show the spherical aberration and the astigmatism at the wide angle end of the zoom lens of Embodiment 2. FIGS.  12 ( a ) and  12 ( b ) show the spherical aberration and the astigmatism, respectively, with the image plane position as designed, and FIGS.  12 ( c ) and  12 ( d ) show these aberrations after the rear subgroup IVb of the fourth lens group has been moved (from the position given in Table 7 above) 1 mm toward the image so as to adjust the back focus. In FIGS.  12 ( b ) and  12 ( d ), the astigmatism is shown for both the sagittal (S) and tangential (T) image planes. 
     As is apparent from comparing FIGS.  8 ( a ) to  12 ( d ) with FIGS.  19 ( a )-( b ), the zoom lens of Embodiment 2, has its aberrations corrected almost as satisfactorily as Prior Art Example 1. The astigmatism is somewhat improved, and the spherical aberration, though generally different, is not significantly degraded. Moreover, whereas Prior Art Example 1 has a complex configuration of lens elements that are moved in order to adjust the back focus of the zoom lens, the present invention provides a simpler and more compact configuration of the lens group that is moved for adjusting the back focus. As is apparent from FIG. 12, the zoom lens of Embodiment 2 does not show significant differences in spherical aberration and astigmatism after the rear subgroup IVb has been moved 1 mm toward the image in order to adjust the back focus. Indeed, the lens performance remains satisfactory as the back focus is adjusted despite the simpler and more compact configuration of the lens subgroup that is moved as compared to Prior Art Example 1 which has a more complex configuration of lens elements that are moved. 
     Embodiment 3 
     The zoom lens of Embodiment 3, as is shown in FIG. 3, has nearly the same configuration as the zoom lens of Embodiment 1 except, in this embodiment, the ninth lens element L 9  of the front subgroup IVa is a piano-convex lens with its convex surface on the image side, the twelfth lens element L 12  of the front subgroup IVa is a biconcave lens, and the thirteenth lens element L 13  of the front subgroup IVa is a piano-convex lens with its convex surface on the object side. 
     Table 9 below lists the surface number # in order from the object side, the radius of curvature R (in mm) of each surface, the on-axis spacing D (in mm) between surfaces, as well as the index of refraction N e  and the Abbe number ν e  (both at the e-line) of the lens elements of the zoom lens of Embodiment 3. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 9 
               
               
                   
               
               
                 # 
                 R 
                 D 
                 N e   
                 ν e   
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 158.290 
                 2.349 
                 1.81264 
                 25.2 
               
               
                 2 
                 72.924 
                 9.659 
                 1.62286 
                 60.1 
               
               
                 3 
                 −938.197 
                 0.113 
               
               
                 4 
                 70.662 
                 6.154 
                 1.71615 
                 53.7 
               
               
                 5 
                 187.327 
                 D5  
               
               
                 6 
                 90.817 
                 1.315 
                 1.77620 
                 49.3 
               
               
                 7 
                 15.406 
                 7.137 
               
               
                 8 
                 −67.225 
                 1.128 
                 1.77620 
                 49.3 
               
               
                 9 
                 67.225 
                 0.047 
               
               
                 10 
                 27.985 
                 3.655 
                 1.81264 
                 25.2 
               
               
                 11 
                 104.837 
                 D11 
               
               
                 12 
                 −34.417 
                 1.128 
                 1.80810 
                 46.3 
               
               
                 13 
                 62.455 
                 2.575 
                 1.81264 
                 25.2 
               
               
                 14 
                 −333.098 
                 D14 
               
               
                 15 
                 ∞ (stop) 
                 2.509 
               
               
                 16 
                 ∞ 
                 7.874 
                 1.62286 
                 60.1 
               
               
                 17 
                 −38.838 
                 0.263 
               
               
                 18 
                 136.253 
                 4.557 
                 1.51872 
                 64.0 
               
               
                 19 
                 −136.253 
                 0.113 
               
               
                 20 
                 55.611 
                 11.632 
                 1.48914 
                 70.2 
               
               
                 21 
                 −32.030 
                 1.503 
                 1.80922 
                 39.3 
               
               
                 22 
                 125.415 
                 6.013 
               
               
                 23 
                 27.815 
                 9.518 
                 1.62286 
                 60.1 
               
               
                 24 
                 ∞ 
                 0.132 
               
               
                 25 
                 27.019 
                 7.536 
                 1.48914 
                 70.2 
               
               
                 26 
                 −169.992 
                 1.503 
                 1.80922 
                 39.3 
               
               
                 27 
                 15.559 
                 10.167 
               
               
                 28 
                 38.291 
                 4.764 
                 1.62286 
                 60.1 
               
               
                 29 
                 −38.291 
                 19.213 
               
               
                 30 
                 ∞ 
                 5.000 
                 1.51824 
                 63.9 
               
               
                 31 
                 ∞ (image) 
               
               
                   
               
             
          
         
       
     
     Table 10 below lists at both the wide-angle end (WIDE) and telephoto end (TELE) for Embodiment 3 of the present invention: the focal length f (in mm); the F-number F NO  with the diaphragm stop fully open; and the on-axis spacings D5, D11 and D14 (in mm) between the lens groups. Also, for the wide-angle end of Embodiment 3 are listed: the back focus Bf (in mm) of the zoom lens; the back focus Bf 4b  of the rear subgroup of the fourth lens group; the ratio Bf 4b /Bf; the maximum ray height Ha on the image-side surface immediately before the movable component(s) of the fourth lens group (in this case, the surface of radius of curvature R 27 ) at the wide-angle end when the diaphragm stop is fully open; the maximum ray height Hb on the lens surface of the movable component(s) of the fourth lens group nearest the object side (in this case, the surface of radius of curvature R 28 ) at the wide-angle end when the diaphragm stop is fully open; the absolute value of Ha minus Hb; the value 0.02 Bf/F N ; and the amount of shift (in mm) in focal point position when the rear subgroup IVb is moved 1 mm toward the image so as to adjust the back focus. In the bottom portion of the table are listed the screen size (in mm) for which the zoom lens of Embodiment 3 is designed, as well as the diameter of the diaphragm stop (in mm) when fully open. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 10 
               
               
                   
                   
               
               
                   
                 WIDE 
                 TELE 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 f 
                 8.5 
                 170 
               
               
                   
                 F NO   
                 1.25 
                 2.77 
               
               
                   
                 D5 
                 0.94 
                 72.842 
               
               
                   
                 D11 
                 69.425 
                 10.112 
               
               
                   
                 D14 
                 16.546 
                 3.956 
               
               
                   
                 Bf 
                 22.507 
               
               
                   
                 Bf 4b   
                 29.986 
               
               
                   
                 Bf 4b /Bf 
                 1.332 
               
               
                   
                 Ha 
                 10.783 
               
               
                   
                 Hb 
                 10.557 
               
               
                   
                 Ha − Hb 
                 0.226 
               
               
                   
                 0.02 Bf/Fn 
                 0.360 
               
               
                   
                 Focal point shift 
                 0.943 
               
               
                   
                   
               
               
                   
                 Screen size: Φ = 8.0  
               
               
                   
                 Diaphragm stop diameter when fully open = 36.011  
               
             
          
         
       
     
     As is apparent from Table 10, the zoom lens of Embodiment 3 satisfies the above Conditions (1) and (2). 
     FIGS.  13 ( a ) to  16  show aberrations of the zoom lens of Embodiment 3 with the distance to an object being 12 m. FIGS.  13 ( a ) and  13 ( b ) show the spherical aberration and astigmatism, respectively, at the wide-angle end. FIG. 14 shows the coma at the wide-angle end at field angles ω of 0, 15.8, 20.8 and 25.6 degrees, for both the tangential image plane (left column) and the sagittal image plane (right column), with the vertical axis dimension being in mm. As the coma on axis (ω=0°) is the same for both the tangential and sagittal image planes, only the curve for the tangential image plane is illustrated in the bottom row. FIGS.  15 ( a ) and  15 ( b ) show the spherical aberration and astigmatism, respectively, at the telephoto end. In FIGS.  13 ( b ) and  15 ( b ), the astigmatism is shown for both the sagittal (S) and tangential (T) image planes. FIG. 16 shows the coma at the telephoto end at field angles of 0, 0.7, 1.0 and 1.2 degrees, for both the tangential image plane (left column) and the sagittal image plane (right column), with the vertical axis dimension being in mm. Once again, only a single curve is illustrated in the bottom row, for the reason discussed above. 
     As is apparent from FIGS.  13 ( a ) to  16 , the zoom lens of Embodiment 3, which has a simple and compact configuration of the lens subgroup that is moved for adjusting the back focus, has its aberrations corrected as satisfactorily as Prior Art Example 1 which has a more complex configuration. 
     As described above, each of Embodiments 1 to 3 (designed for screen sizes of 16.0 mm, 11.0 mm, and 8.0 mm, respectively) has its aberrations corrected as satisfactorily as that of Prior Art Example 1 which has a more complex lens configuration. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. For example, the shapes and number of lenses forming each lens group can be appropriately modified. Such variations are not to be regarded as a departure from the spirit and scope of the invention. Rather, the scope of the invention shall be defined as set forth in the following claims and their legal equivalents. All such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.