Abstract:
A focusing method for a zoom lens system, including at least two lens groups, enables a portion of the zoom lens system to function as a focusing lens group over the entire focal length constituted by discrete focal length ranges. The portion of the zoom lens system, functioning as the focusing lens group, is changed to another portion thereof, in accordance with to the discrete focal length ranges.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a focusing method for a zoom lens system.  
           [0003]    2. Description of the Related Art  
           [0004]    In conventional zoom lens systems, a front-focusing zoom lens system, in which the frontmost lens group out of a plurality of lens groups functions as a focusing lens group, has been widely used. In such a front-focusing zoom lens system, there is an advantage, i.e., the traveling distances of the focusing lens group are substantially the same regardless of the focal lengths. However, the diameter of the frontmost lens group is generally large, so that the frontmost lens group inevitably becomes heavy. Therefore the front-focusing zoom lens system is unsuitable for an AF camera in which the focusing lens group is driven through an electric motor. In the case of a mechanism for automatic focusing (AF mechanism) contained in the front-focusing zoom lens system, a driving mechanism for interconnecting the focusing lens group and a driving motor provided in the camera body becomes complicated (becoming large in size).  
           [0005]    In an AF camera, if an attempt is made to miniaturize an electric drive mechanism and to increase the focusing speed, a lens group, which (i) is positioned close to the camera body, and (ii) generally has a small diameter, can advantageously be utilized as the focusing lens group. Accordingly, an inner-focusing lens system or a rear-focusing lens system have been often employed in AF cameras.  
           [0006]    However, in conventional zoom lens systems, only a predetermined lens group is used, over the entire focal length constituted by a plurality of discrete focal length ranges, as the focusing lens group, no matter which focusing method is employed, i.e., any one of the front-focusing method, the inner-focusing method and the rear-focusing method. In other words, the above conventional zoom lens systems are required to move zoom lens groups along predetermined lens-group-moving paths at the time of zooming, and also required, at any discrete focal length ranges, to secure a space for the traveling distance of a lens group functioning as the focusing lens group from infinity to the shortest photographing distance. These requirements have to be satisfied even for discrete focal length ranges where distances between lens groups are short, i.e., where there is low freedom of movement of the lens groups. Consequently, these requirements have been obstacles when an attempt is made to miniaturize the zoom lens system, or to design a miniaturized zoom lens system with a high zoom ratio.  
           [0007]    Furthermore, the zoom lens systems are classified into the two types, i.e., the telephoto zoom lens system including the positive lens group and the negative lens group in this order from the object, and the retrofocus zoom lens system including the negative lens group and the positive lens group in this order from the object. In the retrofocus zoom lens system, among the lens group from the second one or therebehind, there is a lens group, which has the transverse magnification, varying from a low magnification less than life-size magnification (m=−1) to a high magnification greater than life-size magnification (i.e., in a range including life-size magnification), with respect to an object at infinity in accordance with the change of the focal length of the entire zoom lens system. More concretely, the above lens group (hereinafter, the lens group with the life-size transverse magnification) can vary the transverse magnification of the lens group itself from a low magnification of less than the life-size magnification, passing through the life-size magnification (m=−1), to a high magnification of greater than the life-size magnification. A lens group with the life-size transverse magnification cannot be used as the focusing lens group. Accordingly, in the conventional retrofocus zoom lens system, since only a predetermined lens group is arranged to be the focusing lens group over the entire focal length, a lens group other than the lens group with the life-size magnification has to be used as the focusing lens group. This requirement has also been an obstacle when an attempt is made to miniaturize the zoom lens system, or to design a miniaturized zoom lens system with a high zoom ratio.  
         SUMMARY OF THE INVENTION  
         [0008]    It is an object of the present invention to provide a focusing method for a zoom lens system, by which a miniaturized zoom lens system with a high zoom ratio can be obtained.  
           [0009]    The present invention is based on a conception that a traveling distance for focusing, which is required to the focusing lens group, can easily be secured without causing interference between the focusing lens group and other lens groups, if one lens group to be the focusing lens group, which has conventionally been limited to a predetermined lens group only, is changed to another lens group, in accordance with the change of the discrete focal length ranges.  
           [0010]    Furthermore, in regard to the retrofocus zoom lens system, the present invention is based on a conception that the lens group with the life-size transverse magnification, which has not been able to be used as the focusing lens group, can be used as the focusing lens group, if the lens group with the life-size transverse magnification is, not used as the focusing lens group at a focal length where the transverse magnification m becomes −1.  
           [0011]    In order to achieve the above-mentioned object, there is provided a focusing method for a zoom lens system including at least two lens groups.  
           [0012]    According to the focusing method, a portion of the zoom lens system is arranged to function as a focusing lens group over the entire focal length constituted by a plurality of discrete focal length ranges, and the portion functioning as the focusing lens group is changed to another portion, in accordance with to the discrete focal length ranges.  
           [0013]    More concretely, in the case of the two-lens-group zoom lens system including a first lens group and a second lens group in this order from the object, in a predetermined discrete focal length range, the first lens group functions as the focusing lens group, whereas, in another predetermined discrete focal length range, the second lens group functions as the focusing lens group.  
           [0014]    Furthermore, in the case of the three-lens-group zoom lens system including a first lens group, a second lens group and a third lens group in this order from the object, in a predetermined discrete focal length range, the second lens group functions as the focusing lens group, whereas, in another predetermined discrete focal length range, the third lens group functions as the focusing lens group. In this case, the first lens group can be a stationary lens group which is made immovable upon both zooming and focusing, or can be a zoom lens group which is made movable upon zooming.  
           [0015]    Still further, in the case of the four-lens-group zoom lens system including a first lens group, a second lens group, a third lens group and a fourth lens group in this order from the object, in a predetermined discrete focal length range, the second lens group functions as the focusing lens group, whereas, in another predetermined discrete focal length range, the second and fourth lens groups are made integrally movable so that these two lens groups function as the focusing lens group. As an alternative, in a predetermined discrete focal length range, the second, third and fourth lens groups are made integrally movable so that these three lens groups function as the focusing lens group, whereas, in another predetermined discrete focal length range, the third and fourth lens groups are made integrally movable so that these two lens groups function as the focusing lens group.  
           [0016]    In any of the above cases, according to the present invention, (i) the lens group with the life-size transverse magnification can be a part of the focusing lens group, and (ii) in that case, the lens group with the life-size transverse magnification can function as the focusing lens group in a predetermined discrete focal length range other than a discrete focal length range where the transverse magnification m becomes −1.  
           [0017]    The present disclosure relates to subject matter contained in Japanese Patent Application No. 2000-097601 (filed on Mar. 31, 2000) which is expressly incorporated herein by reference in its entirety. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    The invention will be discussed below in detail with reference to the accompanying drawings, in which:  
         [0019]    [0019]FIG. 1 is an optical diagram of a first embodiment of the focusing method, according to the present invention, which is applied to a two-lens-group zoom lens system;  
         [0020]    [0020]FIG. 2 is a diagram for indicating the symbols of the two-lens-group zoom lens system shown in FIG. 1;  
         [0021]    [0021]FIG. 3 is an optical diagram of a second embodiment of the focusing method, according to the present invention, which is applied to a three-lens-group zoom lens system;  
         [0022]    [0022]FIG. 4 is a diagram for indicating the symbols of the three-lens-group zoom lens system shown in FIG. 3;  
         [0023]    [0023]FIG. 5 is an optical diagram of a third embodiment of the focusing method, according to the present invention, which is applied to the three-lens-group zoom lens system;  
         [0024]    [0024]FIG. 6 is an optical diagram of a fourth embodiment of the focusing method, according to the present invention, which is applied to a four-lens-group zoom lens system;  
         [0025]    [0025]FIG. 7 is a diagram for indicating the symbols of the four-lens-group zoom lens system shown in FIG. 6; and  
         [0026]    [0026]FIG. 8 is an optical diagram of a fifth embodiment of the focusing method, according to the present invention, which is applied to the four-lens-group zoom lens system. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]    [Embodiment 1] 
         [0028]    [0028]FIGS. 1 and 2 show the first embodiment of the focusing method, according to the present invention, which is applied to a two-lens-group zoom lens system. This two-lens-group zoom lens system is a retrofocus zoom lens system including a negative first lens group  10  and a positive second lens group  20  in this order from the object. In the zoom lens system, upon zooming from the short focal length extremity S toward the long focal length extremity L, the first lens group  10 , at first, moves toward the image, thereafter moves toward the object, while the second lens group  20  linearly moves toward the object.  
         [0029]    According to the first embodiment, in a discrete focal length range from the short focal length extremity to an intermediate focal length (intermediate switching focal length) M, the second lens group  20  functions as the focusing lens group; and  
         [0030]    in a discrete focal length range from the intermediate focal length M to the long focal length extremity L, the first lens group  10  functions as the focusing lens group.  
         [0031]    The second lens group  20  corresponds to a lens group which can vary the transverse magnification of the lens group (i.e., the second lens group itself) from a low magnification of less than a life-size magnification, passing through the life-size magnification (m=−1), to a high magnification of greater than the life-size magnification (i.e., the lens group with the life-size transverse magnification); however, the second lens group  20  is not used as the focusing lens group in a discrete focal length range in which the transverse magnification m of the second lens group becomes −1.  
         [0032]    Tables 1a, 1b and 2 show numerical values for the two-lens-group zoom lens system according to the first embodiment shown FIG. 1. FIG. 2 is a diagram for indicating the symbols shown in tables 1a, 1b and 2, i.e., the first principal point H1 and the second principal point H2 with respect to the first lens group  10  and the second lens group  20 ; the distance HH between the first principal point H1 and the second principal point H2; the distance D1 between the first lens group  10  and the second lens group  20 ; the back focal distance fB.  
         [0033]    The focal length is made variable from 5.1 (S) to 19.5 (L). At the focal length of 9.3 (M), the focusing lens group is changed from the second lens group  20  to the first lens group  10 . In this first embodiment, the transverse magnification m of the second lens group  20  with respect to an object at infinity becomes the life-size magnification (m=−1) when the focal length of the entire zoom lens system is 10.98. Accordingly, the second lens group  20  functions as the focusing lens group in the discrete focal length range (5.1-9.3) where the transverse magnification m does not become −1. On the other hand, the first lens group  10  functions as the focusing lens group in the discrete focal length range (9.3 -19.5), i.e., other than the discrete focal length range (5.1-9.3).  
                                             TABLE 1a                           Object Distance ODIS   ∞            FNO   2.8   3.5   4.2   4.8   5.4       Focal Length f   5.1   9.3   12.7   16.1   19.5       Magnification (Mag) m   —   —   —   —   —       Half Angle of View ω   35.8   20.8   15.5   12.3   10.2       D1   20.09   9.70   6.27   4.28   2.99       fB   9.12   13.23   16.59   19.95   23.32       Mag of 1 st  Lens Group   0.000   0.000   0.000   0.000   0.000       Mag of 2 nd  L/G   −0.468   −0.847   −1.156   −1.466   −1.777       1 st  L/G: Traveling   —   —   —   —   —       Distance for Focusing       2 nd  L/G: Traveling   —   —   —   —   —       Distance for Focusing                  
 
         [0034]    [0034]                                             TABLE 1b                           Object Distance ODIS   100           (Shortest Photographing Distance)            FNO   2.8   3.5   4.2   4.8   5.4       Focal Length f   —   —   —   —   —       Magnification (Mag) m   −0.048   −0.081   −0.111   −0.141   −0.171       Half Angle of View ω   —   —   —   —   —       D1   19.78   10.75   7.32   5.34   4.04       fB   9.43   13.23   16.59   19.95   23.32       1 st  L/G: Traveling   0   −1.06   −1.06   −1.06   −1.06       Distance for Focusing       2 nd  L/G: Traveling   −0.031   0   0   0   0       Distance for Focusing                    
         [0035]    [0035]                                                             TABLE 2                                   f   H1   HH   H2                                        First Lens Group   −10.98   3.38   2.36   4.48           Second Lens Group   10.85   −1.49   3.99   6.82                                                                        
         [0036]    [Embodiment 2] 
         [0037]    [0037]FIG. 3 and  4  show the second embodiment of the focusing method, according to the present invention, which is applied to a three-lens-group zoom lens system. This three-lens-group zoom lens includes a positive first lens group  30 , a negative second lens group  40  and a positive third lens group  50  in this order from the object. In the zoom lens system, upon zooming from the short focal length extremity S toward the long focal length extremity L, the second lens group  40  linearly moves toward the image, and the third lens group  50  linearly moves toward the object. The first lens group  30  is made stationary.  
         [0038]    According to the second embodiment, in the discrete focal length range from the short focal length extremity S to the intermediate focal length (intermediate switching focal length) M, the third lens group  50  functions as the focusing lens group; and  
         [0039]    in the discrete focal length range from the intermediate focal length M to the long focal length extremity L, the second lens group  40  functions as the focusing lens group. Tables 3a, 3b and 4 show numerical values for the three-lens-group zoom lens system according to the second embodiment shown FIG. 3. FIG. 4 is a diagram for indicating the symbols shown in tables 3a, 3b and 4.  
         [0040]    The focal length is made variable from 6.0 (S) to 22.8 (L). At the focal length of 16.3 (M), the focusing lens group is changed from the third lens group 50 to the second lens group  40 .  
         [0041]    In this embodiment, in a discrete focal length range where the distance between the second lens group  40  and the third lens group  50  is sufficiently long, the third lens group  50  functions as the focusing lens group, while in a discrete focal length range where the distance between the second lens group  40  and the third lens group  50  is short, the second lens group  40  functions as the focusing lens group.  
                                             TABLE 3a                           Object Distance ODIS   ∞            FNO   2.8   2.9   3.1   3.3   3.5       Focal Length f   6.0   8.4   11.7   16.3   22.8       Magnification (Mag) m   —   —   —   —   —       Half Angle of View ω   32.4   23.2   16.8   12.2   8.83       D1   2.95   6.73   9.99   12.54   14.7       D2   19.80   15.22   11.11   7.35   3.83       fB   8.34   9.14   10.08   11.19   9.14       Mag of 1 st  Lens Group   0.000   0.000   0.000   0.000   0.000       Mag of 2 nd  L/G   −0.366   −0.436   −0.518   −0.615   −0.726       Mag of 3 rd  L/G   −0.414   −0.486   −0.570   −0.671   −0.793       2 nd  L/G: Traveling   —   —   —   —   —       Distance for Focusing       3 rd  L/G: Traveling   —   —   —   —   —       Distance for Focusing                  
 
         [0042]    [0042]                                             TABLE 3b                           Object Distance ODIS   100           (Shortest Photographing Distance)            FNO   2.8   2.9   3.1   3.3   3.5       Focal Length f   —   —   —   —   —       Magnification (Mag) m   −0.051   −0.068   −0.100   −0.087   −0.105       Half Angle of View ω   —   —   —   —   —       D1   2.95   6.73   9.90   8.95   10.2       D2   19.43   14.43   9.06   10.95   8.34       fB   7.97   7.56   6.29   11.19   9.14       2 nd  L/G: Traveling   0   0   0   −3.59   −4.50       Distance for Focusing       3 rd  L/G: Traveling   −0.37   −1.58   −3.79   0   0       Distance for Focusing                    
         [0043]    [0043]                                                             TABLE 4                                   f   H1   HH   H2                                        First Lens Group   39.58   1.43   3.57   4.05           Second Lens Group   −8.69   0.18   1.43   5.48           Third Lens Group   11.11   0.81   2.34   7.37                        
         [0044]    [Embodiment 3] 
         [0045]    [0045]FIG. 5 shows the third embodiment of the focusing method, according to the present invention, which is applied to a three-lens-group zoom lens system. This three-lens-group zoom lens includes a positive first lens group  30 , a negative second lens group  40  and a positive third lens group  50  in this order from the object. According to the second embodiment shown in FIG. 3, the first lens group  30  is made stationary upon zooming. On the other hand, as shown in FIG. 5, in the third embodiment, the first lens group  30  is a zoom lens group which is made moveable upon zooming.  
         [0046]    In the zoom lens system of the third embodiment, upon zooming from the short focal length extremity S toward the long focal length extremity L, the first lens group  30  linearly moves toward the object, the second lens group  40  linearly moves toward the image, and the third lens group  50  linearly moves toward the object.  
         [0047]    Similar to the second embodiment shown in FIG. 3, the switching of the focusing lens group in the third embodiment is carried out according to the following:  
         [0048]    in the discrete focal length range from the short focal length extremity S to the intermediate focal length (intermediate switching focal length) M, the third lens group  50  functions as the focusing lens group; and  
         [0049]    in the discrete focal length range from the intermediate focal length M to the long focal length extremity L, the second lens group  40  functions as the focusing lens group.  
         [0050]    The third lens group  50  corresponds to the lens group with the life-size transverse magnification; however, the third lens group  50  is not used as the focusing lens group in the discrete focal length range where the transverse magnification m of the third lens group becomes −1.  
         [0051]    Tables 5a, 5b and 6 show numerical values for the three-lens-group zoom lens system according to the third embodiment shown in FIG. 5. The symbols of shown in tables 5a, 5b and 6 are defined in the same manner as the second embodiment (refer to FIG. 4). More concretely, the focal length is made variable from 5.5 (S) to 22.0 (L). At the focal length of 8.2 (M), the focusing lens group is changed from the third lens group  50  to the second lens group  40 . In the third embodiment, the transverse magnification m of the third lens group  50  with respect to an object at infinity becomes the life-size magnification (m=−1) when the focal length of the entire zoom lens system is 20.5.  
                                             TABLE 5a                           Object Distance ODIS   ∞            FNO   2.8   3.0   3.2   3.6   3.9       Focal Length f   5.5   8.2   11.0   16.0   22.0       Magnification (Mag) m   —   —   —   —   —       Half Angle of View ω   33.8   23.6   18   12.6   9.25       D1   2.50   8.071   11.61   15.57   18.56       D2   17.76   12.52   9.284   5.79   3.28       fB   10.05   11.39   12.61   14.49   16.40       Mag of 1 st  Lens Group   0.000   0.000   0.000   0.000   0.000       Mag of 2 nd  L/G   −0.219   −0.258   −0.292   −0.341   −0.39       Mag of 3 rd  L/G   −0.465   −0.587   −0.698   −0.868   −1.042       2 nd  L/G: Traveling   —   —   —   —   —       Distance for Focusing       3 rd  L/G: Traveling   —   —   —   —   —       Distance for Focusing                  
 
         [0052]    [0052]                                             TABLE 5b                           Object Distance ODIS   100           (Shortest Photographing Distance)            FNO   2.8   3.0   3.2   3.6   3.9       Focal Length f   —   —   —   —   —       Magnification (Mag) m   −0.047   −0.058   −0.071   −0.092   −0.114       Half Angle of View ω   —   —   —   —   —       D1   2.5   6.73   9.90   8.95   10.2       D2   17.42   14.43   9.06   10.95   8.34       fB   10.39   11.39   12.61   11.19   9.14       2 nd  L/G: Traveling   0   −1.34   −1.71   −6.62   −8.36       Distance for Focusing       3 rd  L/G: Traveling   −0.34   0   0   0   0       Distance for Focusing                    
         [0053]    [0053]                                                         TABLE 6                                   f   H1   HH   H2                                    First Lens Group   54.06   −0.57   3.98   5.91       Second Lens Group   −8.02   1.04   1.90   5.19       Third Lens Group   11.00   1.93   0.81   6.06                    
         [0054]    [Embodiment 4] 
         [0055]    [0055]FIGS. 6 and 7 show the fourth embodiment of the focusing method, according to the present invention, which is applied to a four-lens-group zoom lens system. This four-lens-group zoom lens system includes a positive first lens group  60  and a negative second lens group  70 , a positive third lens group  80  and a positive fourth lens group  90  in this order from the object. In the zoom lens system, upon zooming from the short focal length extremity S toward the long focal length extremity L, the second lens group  70  linearly moves toward the image, and the fourth lens group  90  linearly moves toward the object. The first lens group  60  and the third lens group  80  are made stationary.  
         [0056]    According to the fourth embodiment, in a discrete focal length range from the short focal length extremity S to a predetermined focal length (M1), only the fourth lens group  90  functions as the focusing lens group;  
         [0057]    in a discrete focal length range from the predetermined focal length (M1)to another predetermined focal length (M2), only the second lens group  70  functions as the focusing lens group; and  
         [0058]    in a discrete focal length range from the other predetermined focal length (M2) to the long focal length extremity L, the second lens group  70  and the fourth lens group  90  are made integrally movable so that these two lens groups function as the focusing lens group.  
         [0059]    The second lens group  70  corresponds to the lens group with the life-size transverse magnification; however, the second lens group  70  alone is not used as the-focusing lens group in the discrete focal length range in which the transverse magnification m of the second lens group becomes −1.  
         [0060]    Tables 7a, 7b and 8 show numerical values for the four-lens-group zoom lens system according to the fourth embodiment shown FIG. 6. FIG. 7 is a diagram for indicating the symbols shown in tables 7a, 7b and 8.  
         [0061]    The focal length is made variable from 5.9 (S) to 22.4 (L). In a discrete focal length range from 5.9-8.23 (M1), only the fourth lens group  90  functions as the focusing lens group;  
         [0062]    in a discrete focal length range from 8.23 (M1)-16.1 (M2), only the second lens group  70  functions as the focusing lens group.  
         [0063]    in a discrete focal length range from 16.1 (M2)to 22.4 (L), the second lens group  70  and the fourth lens group  90  are made integrally movable so that these two lens groups function as the focusing lens group.  
         [0064]    In the fourth embodiment, the transverse magnification m of the second lens group 70 with respect to an object at infinity becomes the life-size magnification (m=−1) when the focal length of the entire zoom lens system is 19.7. 
                                             TABLE 7a                           Object Distance ODIS   ∞            FNO   2.8   2.9   2.9   3   3.0       Focal Length f   5.90   8.23   11.5   16.1   22.4       Magnification (Mag) m   —   —   —   —   —       Half Angle of View ω   31.7   22.5   16.4   11.9   8.5       D1   2.01   4.92   7.44   9.67   11.76       D2   12.57   9.66   7.14   4.91   2.82       D3   4.52   3.71   2.98   2.45   2.4       fB   9.38   10.19   11.73   13.81   15.94       Mag of 1 st  Lens Group   0.000   0.000   0.000   0.000   0.000       Mag of 2 nd  L/G   −0.411   −0.509   −0.640   −0.831   −1.152       Mag of 3 rd  L/G   −2.821   −4.501   −8.031   −15.515   −16.879       Mag of 4 th  L/G   0.195   0.138   0.086   0.048   0.044       2 nd  L/G: Traveling   —   —   —   —   —       Distance for Focusing       4th L/G: Traveling   —   —   —   —   —       Distance for Focusing                  
 
         [0065]    [0065]                                             TABLE 7b                               100 (Shortest Photographing       Object Distance ODIS   Distance)            FNO   2.8   2.9   2.9   3.0   3.0       Focal Length f   —   —   —   —   —       Magnification (Mag) m   −0.049   −0.064   −0.082   −0.079   −0.080       Half Angle of View ω   —   —   —   —   —       D1   1.78   4.52   6.77   6.49   6.59       D2   12.8   10.05   7.81   8.09   7.98       D3   4.28   3.32   2.31   2.45   2.4       fB   9.61   10.57   11.58   11.46   11.51       2 nd  L/G: Traveling   0   −0.40   −0.67   −1.14   −1.99       Distance for Focusing       4 th  L/G: Traveling   −0.24   0   0   −1.14   −1.99       Distance for Focusing                    
         [0066]    [0066]                                                             TABLE 8                                   f   H1   HH   H2                                        First Lens Group   26.12   4.17   3.51   2.22           Second Lens Group   −6.23   0.51   1.84   3.41           Third Lens Group   17.39   −1.21   1.58   2.93           Fourth Lens Group   14.08   0.83   1.51   1.96                        
         [0067]    [Embodiment 5] 
         [0068]    [0068]FIG. 8 is an optical diagram of a fifth embodiment of the focusing method, according to the present invention, which is applied to the four-lens-group zoom lens system.  
         [0069]    The fifth embodiment is different from the fourth embodiment on the following points:  
         [0070]    (i) the positive fourth lens group  90  is changed to the negative fourth lens group  90 N; and  
         [0071]    (ii) upon zooming from the short focal length extremity (S) to the long focal length extremity (L), the positive third lens group  80  linearly moves toward the object.  
         [0072]    According to the fifth embodiment, in the discrete focal length range from the short focal length extremity S to the intermediate focal length (intermediate switching focal length) M, the third lens group  80  and the fourth lens group  90  are made integrally movable so that these two lens group function as the focusing lens group; and  
         [0073]    in the discrete focal length range from the intermediate focal length M to the long focal length extremity L, the second lens group  70 , the third lens group  80  and the fourth lens group  90  are made integrally movable so that these three lens groups function as the focusing lens group.  
         [0074]    The second lens group  70  corresponds to the lens group with the life-size transverse magnification; however, the second lens group  70  alone is not used as the focusing lens group in the focal length range in which the transverse magnification m of the second lens group is −1.  
         [0075]    Tables 9a, 9b and 10 show numerical values for the four-lens-group zoom lens system according to the fifth embodiment shown FIG. 8. The symbols shown in the tables 9a, 9b and 10 are defined in the same manner as FIG. 7.  
         [0076]    The focal length is made variable from 5.9 (S) to 22.8 (L). In a discrete focal length range from 5.9-16.3 (M), the third lens group  80  and the fourth lens group  90 N which are made integrally movable so that these two lens groups function as the focusing lens group; and  
         [0077]    in a discrete focal length range from 16.3-22.8, the second lens group  70 , the third lens group  80  and the fourth lens group  90 N are made integrally movable so that these three lens groups function as the focusing lens group.  
         [0078]    In the fifth embodiment, the transverse magnification m of the second lens group  70  with respect to an object at infinity becomes the life-size magnification (m=−1) when the focal length of the entire zoom lens system is 20.7. 
                                             TABLE 9a                           Object Distance ODIS   ∞            FNO   2.8   2.9   3   3.1   3.2       Focal Length f   5.90   8.24   11.5   16.3   22.8       Magnification (Mag) m   —   —   —   —   —       Half Angle of View ω   31.9   23.2   16.8   12.1   8.7       D1   2.29   5.64   8.55   11.2   13.7       D2   14.68   10.8   7.35   4.25   1.57       D3   1.20   1.31   1.41   1.51   1.53       fB   6.91   7.33   7.77   8.14   8.25       Mag of 1 st  Lens Group   0.000   0.000   0.000   0.000   0.000       Mag of 2 nd  L/G   −0.419   −0.511   −0.632   −0.804   −1.090       Mag of 3 rd  L/G   −0.295   −0.334   −0.375   −0.409   −0.419       Mag of 4 th  L/G   1.430   1.441   1.453   1.463   1.466       2 nd  L/G: Traveling   —   —   —   —   —       Distance for Focusing       3 rd  L/G: Traveling   —   —   —   —   —       Distance for Focusing       4 th  L/G: Traveling   —   —   —   —   —       Distance for Focusing                  
 
         [0079]    [0079]                                             TABLE 9b                               100 (Shortest Photographing       Object Distance ODIS   Distance)            FNO   2.8   2.9   3.0   3.1   3.2       Focal Length f   —   —   —   —   —       Magnification (Mag) m   −0.050   −0.067   −0.096   −1.080   −0.139       Half Angle of View ω   —   —   —   —   —       D1   2.29   5.64   8.55   9.24   9.89       D2   14.31   10.04   5.52   4.25   1.57       D3   1.20   1.31   1.41   1.51   1.53       fB   7.27   8.51   10.46   8.50   8.61       2 nd  L/G: Traveling   0   0   0   3.17   5.17       Distance for Focusing       3 rd  L/G: Traveling   −0.36   −1.18   −2.69   −3.17   −5.17       Distance for Focusing       4 th  L/G: Traveling   −0.36   −1.18   −2.69   −3.17   −5.17       Distance for Focusing                    
         [0080]    [0080]                                                             TABLE 10                                   f   H1   HH   H2                                        First Lens Group   33.43   1.37   3.90   4.56           Second Lens Group   −7.78   0.23   1.03   6.36           Third Lens Group   7.81   2.25   1.32   2.80           Fourth Lens Group   −36.36   −4.81   −0.04   8.71                        
         [0081]    A practical structure for driving each lens group has been known to those who are skilled in the art. In recent cameras, particularly in digital cameras, it is a general practice for those who are skilled in the art to digitally control the position of each lens group. In such a digitally controlled camera, the switching of the focusing lens groups can be easily carried out. Furthermore, it is common for the zoom lens system to control the focal length according to predetermined number of discrete steps. In such a stepwise zoom lens system, the position data of each lens group for each discrete focal length range can be stored in a table, so that the amount of data thereof is reduced. Accordingly, the switching of the focusing lens groups can more easily be carried out.  
         [0082]    According to the focusing method of the present invention, a lens group to function as the focusing lens group can be changed to another lens group, in accordance with discrete focal length ranges, thereby the zoom lens system can be miniaturized, or a miniaturized zoom lens system with a high zoom ratio can be obtained.