Abstract:
A zoom lens system includes a positive first lens group, and a negative second lens. Zooming is performed by varying the distance between the first lens group and the second lens group. The first lens group includes a negative first sub lens group and a positive second sub lens group. The zoom lens system satisfies the following conditions: 
     5.0&lt; ft/f   1 &lt;7.0  (1) 
     0.9&lt;| fw/f   1   a |&lt;1.8  (2) 
     wherein  
     ft designates the focal length of the entire zoom lens system at the long focal length extremity;  
     f 1  designates the focal length of the first lens group;  
     fw designates the focal length of the entire zoom lens system at the short focal length extremity; and  
     f 1   a  designates the focal length of the first sub lens group of the first lens group.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a zoom lens system for a compact camera (lens-shutter camera).  
           [0003]    2. Description of the Prior Art  
           [0004]    In a zoom lens system of a compact camera, a long back focal distance is not necessary, unlike a zoom lens system for a single lens reflex camera (SLR camera) which requires a space behind the photographing lens system for providing a quick-return mirror. Accordingly, in a compact camera, a telephoto lens system, including either a positive lens group and a negative lens group, or a positive lens group, a positive lens group and a negative lens group, in this order from the object, is generally employed. On the other hand, in an SLR camera, a retrofocus lens system, including a negative lens group and a positive lens group, in this order from the object, is generally employed.  
           [0005]    Recently, in zoom lens systems for compact cameras, there have been increasing requirements for miniaturization, a higher zoom ratio, and a shorter focal length at the short focal length extremity. In order to satisfy the requirement for miniaturization, it is preferable to utilize a two-lens-group zoom lens system including a small number of lens elements. However, in the two-lens-group zoom lens systems of the prior art, if an attempt is made to obtain the zoom ratio of 3.5 or more, it is difficult to attain an adequate balance of aberrations from the short focal length extremity to the long focal length extremity. Furthermore, if an attempt is made to obtain a shorter focal length at the short focal length extremity, the diameter of the front lens group generally becomes larger, which makes miniaturization of the zoom lens system difficult, and makes the correcting of aberrations difficult. Due to these reasons, it has been difficult, in the prior art, to provide a two-lens-group zoom lens system which can attain miniaturization, a higher zoom ratio, and a shorter focal length at the short focal length extremity at the same time.  
         SUMMARY OF THE INVENTION  
         [0006]    An object of the present invention is to provide a small telephoto two-lens-group zoom lens system having a zoom ratio of 3.5 or more, and the half angle-of-view of 35 degrees or more at the short focal length extremity.  
           [0007]    In order to achieve the above-mentioned object, there is provided a zoom lens system including a positive first lens group, and a negative second lens, in this order from the object. Zooming is performed by varying the distance between the first lens group and the second lens group. The first lens group includes a negative first sub lens group ( 1   a  lens group) and a positive second sub lens group ( 1   b  lens group), in this order from the object. The zoom lens system satisfies the following conditions: 
           5.0 &lt;ft/f   1 &lt;7.0  (1) 
           0.9 &lt;|fw/f   1   a |&lt;1.8  (2) 
           [0008]    wherein  
           [0009]    ft designates the focal length of the entire zoom lens system at the long focal length extremity;  
           [0010]    f 1  designates the focal length of the first lens group;  
           [0011]    fw designates the focal length of the entire zoom lens system at the short focal length extremity; and  
           [0012]    f 1 a designates the focal length of the first sub lens group ( 1   a  lens group) of the first lens group.  
           [0013]    The most object-side lens element of the first lens group can be a negative lens element having a concave surface facing the object, and preferably satisfies the following condition: 
           0.1 &lt;|r   1   /ft 51 &lt;0.2  (3) 
           [0014]    wherein  
           [0015]    r 1  designates the radius of curvature of the object-side concave surface of the most object-side negative lens element of the first lens group.  
           [0016]    It is preferable that at least one surface of lens elements in the first lens group is made aspherical. Similarly, it is preferable that at least one surface of lens elements in the second lens group is made aspherical.  
           [0017]    The present disclosure relates to subject matter contained in Japanese Patent Application No. 2000-301732 (filed on Oct. 2, 2000) which is expressly incorporated herein 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 a lens arrangement of a first embodiment of a zoom lens system according to the present invention;  
         [0020]    [0020]FIGS. 2A, 2B,  2 C and  2 D show aberrations occur in the lens arrangement shown in FIG. 1 at the short focal length extremity;  
         [0021]    [0021]FIGS. 3A, 3B,  3 C and  3 D show aberrations occur in the lens arrangement shown in FIG. 1 at an intermediate focal length;  
         [0022]    [0022]FIGS. 4A, 4B,  4 C and  4 D show aberrations occur in the lens arrangement shown in FIG. 1 at the long focal length extremity;  
         [0023]    [0023]FIG. 5 is a lens arrangement of a second embodiment of a zoom lens system according to the present invention;  
         [0024]    [0024]FIGS. 6A, 6B,  6 C and  6 D show aberrations occur in the lens arrangement shown in FIG. 5 at the short focal length extremity;  
         [0025]    [0025]FIGS. 7A, 7B,  7 C and  7 D show aberrations occur in the lens arrangement shown in FIG. 5 at an intermediate focal length;  
         [0026]    [0026]FIGS. 8A, 8B,  8 C and  8 D show aberrations occur in the lens arrangement shown in FIG. 5 at the long focal length extremity;  
         [0027]    [0027]FIG. 9 is a lens arrangement of a third embodiment of a zoom lens system according to the present invention;  
         [0028]    [0028]FIGS. 10A, 10B,  10 C and  10 D show aberrations occur in the lens arrangement shown in FIG. 9 at the short focal length extremity;  
         [0029]    [0029]FIGS. 11A, 11B,  11 C and  11 D show aberrations occur in the lens arrangement shown in FIG. 9 at an intermediate focal length;  
         [0030]    [0030]FIGS. 12A, 12B,  12 C and  12 D show aberrations occur in the lens arrangement shown in FIG. 9 at the long focal length extremity;  
         [0031]    [0031]FIG. 13 is a lens arrangement of a fourth embodiment of a zoom lens system according to the present invention;  
         [0032]    [0032]FIGS. 14A, 14B,  14 C and  14 D show aberrations occur in the lens arrangement shown in FIG. 13 at the short focal length extremity;  
         [0033]    [0033]FIGS. 15A, 15B,  15 C and  15 D show aberrations occur in the lens arrangement shown in FIG. 13 at an intermediate focal length;  
         [0034]    [0034]FIGS. 16A, 16B,  16 C and  16 D show aberrations occur in the lens construction shown in FIG. 13 at the long focal length extremity; and  
         [0035]    [0035]FIG. 17 shows the lens-group moving paths of the zoom lens system according to the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0036]    The zoom lens system of the present invention, as shown in the lens-group moving paths in FIG. 17, is a telephoto zoom lens system including a positive first lens group  10  and a negative second lens group  20 , in this order from the object. The positive first lens group  10  includes a first sub lens group  10   a  ( 1   a  lens group) and a second sub lens group  10   b  ( 1   b  lens group). Upon zooming from the short focal length extremity to the long focal length extremity, the first lens group  10  and the second lens group  20  move toward the object while the distance therebetween is varied (reduced).  
         [0037]    Condition (1) specifies the refractive power of the first lens group  10  in order to achieve a higher zoom ratio.  
         [0038]    If ft/f 1  exceeds the lower limit of condition (1) to the extent that the refractive power of the first lens group  10  becomes too weak, the zoom ratio cannot be made higher.  
         [0039]    If ft/f 1  exceeds the upper limit of condition (1) to the extent that the refractive power of the first lens group  10  becomes too strong, it becomes difficult to reduce spherical aberration.  
         [0040]    Condition (2) specifies the negative refractive power of the first sub lens group  10   a  ( 1   a  lens group) of the first lens group  10  in order to maintain a balance between miniaturization of the zoom lens system and the correcting of aberrations.  
         [0041]    If |fw/f 1   a | exceeds the lower limit of condition (2) to the extent that the refractive power (absolute value) of the first sub lens group  10   a  ( 1   a  lens group) becomes too weak, it is difficult to secure a back focal distance at the short focal length extremity.  
         [0042]    If |fw/f 1   a | exceeds the upper limit of condition (2) to the extent that the refractive power (absolute value) of the first sub lens group  10   a  ( 1   a  lens group) becomes too strong, the back focal distance becomes longer, and the refractive power of each lens group becomes stronger. As a result, spherical aberration which occurs in the second sub lens group  10   b  ( 1   b  lens group) becomes larger, and spherical aberration is undercorrected over the entire zoom lens system.  
         [0043]    Condition (3) specifies the radius of curvature of a concave surface facing the object, in the case where a negative lens element having the concave surface facing the object is provided as the most object-side lens element of the first lens group. This condition is to attain miniaturization (i.e., a smaller diameter) of the first lens group  10 .  
         [0044]    If |r 1 /ft| exceeds the lower limit of condition (3) to the extent that the radius of curvature becomes too small, the correcting of distortion and astigmatism which occur on the concave surface become difficult.  
         [0045]    If |r 1 /ft| exceeds the upper limit of condition (3) to the extent that the radius of curvature becomes too large, the diameter of the first lens group  10  (the front lens group) becomes larger, in the case where the focal length at the short focal length extremity is made shorter.  
         [0046]    It is preferable that at least one surface of lens elements in the first lens group  10  is made aspherical. Similarly, it is preferable that at least one surface of lens elements in the second lens group  20  is made aspherical. An aspherical surface provided in the first lens group  10  is effective in mainly correcting spherical aberration over the entire zooming range, and an aspherical surface provided in the second lens group  20  is effective in correcting distortion on the side of shorter focal lengths.  
         [0047]    Specific numerical data of the embodiments will be described hereinafter. In the diagrams of chromatic aberration (axial chromatic aberration) represented by spherical aberration, the solid line and the two types of dotted lines respectively indicate spherical aberrations with respect to the d, g and C lines. Also, in the diagrams of lateral chromatic aberration, the two types of dotted lines respectively indicate magnification with respect to the g and C lines; however, the d line as the base line coincides with the ordinate. S designates the sagittal image, and M designates the meridional image. In the tables, Fno designates the F-number, f designates the focal length of the entire zoom lens system, W designates the half angle-of-view (°), f B  designates the back focal distance, r designates the radius of curvature, d designates the lens-element thickness or distance between lens elements, Nd designates the refractive index of the d-line, and v designates the Abbe number.  
         [0048]    In addition to the above, an aspherical surface which is symmetrical with respect to the optical axis is defined as follows: 
           x=cy   2 /(1+[1={1 +K}c   2   y   2 ] ½ ) +A   4   y   4   +A   6   y   6   +A   8   y   8   +A   10   y   10   
         [0049]    wherein:  
         [0050]    c designates a curvature of the aspherical vertex (1/r);  
         [0051]    y designates a distance from the optical axis;  
         [0052]    K designates the conic coefficient; and  
         [0053]    A 4  designates a fourth-order aspherical coefficient;  
         [0054]    A 6  designates a sixth-order aspherical coefficient;  
         [0055]    A 8  designates a eighth-order aspherical coefficient; and  
         [0056]    A 10  designates a tenth-order aspherical coefficient.  
         [0057]    [Embodiment 1] 
         [0058]    [0058]FIGS. 1 through 4D show the first embodiment of the present invention. FIG. 1 is the lens arrangement of the first embodiment of the zoom lens system according to the present invention. The first lens group  10  is composed of the first sub lens group  10   a  ( 1   a  lens group) including a negative single lens element with a concave surface facing the object, and the second sub lens group  10   b  ( 1   b  lens group) including cemented lens elements of a positive lens element and a negative lens element, and a positive lens element, in this order from the object. The second lens group  20  includes a positive lens element and a negative lens element, in this order from the object. Table 1 shows the numerical data of the first embodiment. FIGS. 2A through 2D show aberrations occur in the lens arrangement shown in FIG. 1 at the short focal length extremity. FIGS. 3A through 3D show aberrations occur in the lens arrangement shown in FIG. 1 at an intermediate focal length. FIGS. 4A through 4D show aberrations occur in the lens arrangement shown in FIG. 1 at the long focal length extremity. A diaphragm S is located 1.50 millimeters behind (on the image side) the first lens group  10  (surface No. 7 ).  
                                                           TABLE 1                           F NO  = 3.7 − 9.8 − 14.0       f = 29.44 − 77.00 − 110.00 (Zoom Ratio:3.74)       W = 35.8 − 15.7 − 11.1       f B = 7.94 − 49.67 − 78.62              Surf. No.   r   d   Nd   ν                     1   −16.294   2.53   1.83481   42.7        2   −56.654   0.25   —   —        3   18.014   4.91   1.48749   70.2        4   −9.275   1.38   1.83400   37.2        5   −50.347   0.44   —   —        6   41.926   3.10   1.73077   40.5        7*   −14.577   11.07 − 3.65 − 2.28        8*   −85.796   2.46   1.58547   29.9        9   −30.955   4.42   —   —       10   −9.400   1.50   1.83481   42.7       11   −53.508   —   —   —                          
 
         [0059]    Aspherical surface data (the aspherical surface coefficients not indicated are zero (0.00)):  
                                               Surf.No.   K   A4   A6   A8                   7   0.00   0.8633 × 10 −4     −0.4202 × 10 −7     0.1646 × 10 −8         8   0.00   0.8318 × 10 −4      0.1995 × 10 −6     0.7078 × 10 −8                    
 
         [0060]    [Embodiment 2] 
         [0061]    [0061]FIGS. 5 through 8D show the second embodiment of the present invention. FIG. 5 is the lens arrangement of the second embodiment of the zoom lens system according to the present invention. Table 2 shows the numerical data of the second embodiment. FIGS. 6A through 6D show aberrations occur in the lens arrangement shown in FIG. 5 at the short focal length extremity. FIGS. 7A through 7D show aberrations occur in the lens arrangement shown in FIG. 5 at an intermediate focal length. FIGS. 8A through 8D show aberrations occur in the lens arrangement shown in FIG. 5 at the long focal length extremity. The basic lens arrangement of the second embodiment is the same as the first embodiment except that the first sub lens group  10   a  ( 1   a  lens group) includes cemented lens elements of a negative lens element and a positive lens element. The diaphragm S is located 1.50 millimeters behind (on the image side) the first lens group  10  (surface No. 8 ).  
                                                           TABLE 2                           F NO  = 4.0 − 12.7 − 15.8       f = 25.07 − 80.20 − 100.30 (Zoom Ratio:4.00)       W = 39.9 − 15.3 − 12.3       f B  = 8.85 − 66.37 − 87.35            Surf. No.   r   d   Nd   ν                     1   −17.717   1.50   1.88300   40.8        2   8.718   4.01   1.66680   33.0        3   −74.897   0.13   —   —        4   16.369   4.86   1.50137   56.4        5   −8.874   1.50   1.85026   32.3        6   −31.967   0.13   —   —        7   32.984   3.26   1.69350   53.2        8*   −13.396   11.75 − 3.84 − 3.11   —   —        9*   −266.836   3.13   1.58547   29.9        10*   −23.319   2.69   —   —       11   −9.419   1.75   1.80400   46.6       12   973.898   —   —   —                          
 
         [0062]    Aspherical surface data (the aspherical surface coefficients not indicated are zero (0.00)):  
                                                                     Surf.No.   K   A4   A6   A8                                8   0.00   0.1099 × 10 −3     0.5383 × 10 −7     —       9   0.00   0.1281 × 10 −3     0.2719 × 10 −5     −0.3462 × 10 −8         10   0.00   0.5105 × 10 −5     0.3848 × 10 −5     —                  
 
         [0063]    [Embodiment 3] 
         [0064]    [0064]FIGS. 9 through 12D show the third embodiment of the present invention. FIG. 9 is the lens arrangement of the third embodiment of a zoom lens system according to the present invention. Table 3 shows the numerical data of the third embodiment. FIGS. 10A through 10D show aberrations occur in the lens arrangement shown in FIG. 9 at the short focal length extremity. FIGS. 11A through 11D show aberrations occur in the lens arrangement shown in FIG. 9 at an intermediate focal length. FIGS. 12A through 12D show aberrations occur in the lens arrangement shown in FIG. 9 at the long focal length extremity. The basic lens arrangement of the third embodiment is the same as the first embodiment. The diaphragm S is located 1.50 millimeters behind (on the image side) the first lens group  10  (surface No.  7 ).  
                                                           TABLE 3                           F NO  = 4.2 − 10.8 − 15.8       f = 29.83 − 83.40 − 122.50 (Zoom Ratio: 4.11)       W = 36.5 − 14.6 − 10.0       f B  = 8.77 − 57.28 − 92.68            Surf. No.   r   d   Nd   ν                     1   −14.393   1.88   1.83400   37.2        2   −37.704   0.13   —   —        3   26.728   5.39   1.48749   70.2        4   −9.400   1.50   1.83400   37.2        5   −57.488   0.13   —   —        6   30.060   3.38   1.73077   40.5        7*   −13.908   10.72 − 3.39 − 2.08   —   —        8*   −82.529   3.01   1.58547   29.9        9   −32.520   3.98   —   —       10   −9.400   1.75   1.80400   46.6       11   −56.559   —   —   —                          
 
         [0065]    Aspherical surface data (the aspherical surface coefficients not indicated are zero (0.00)):  
                                                   Surf.No.   K   A4   A6   A8       7   0.00   0.9680 × 10 −4      0.1374 × 10 −6     −0.9000 × 10 −9         8   0.00   0.9530 × 10 −4     −0.1674 × 10 −6     −0.1348 × 10 −7                    
 
         [0066]    [Embodiment 4] 
         [0067]    [0067]FIGS. 13 through 16D show the fourth embodiment of the present invention. FIG. 13 is the lens arrangement of the fourth embodiment of a zoom lens system according to the present invention. Table 4 shows the numerical data of the fourth embodiment. FIGS. 14A through 14D show aberrations occur in the lens arrangement shown in FIG. 13 at the short focal length extremity. FIGS. 15A through 15D show aberrations occur in the lens arrangement shown in FIG. 13 at an intermediate focal length. FIGS. 16A through 16D show aberrations occur in the lens construction shown in FIG. 13 at the long focal length extremity. The basic lens arrangement of the fourth embodiment is the same as the second embodiment. The diaphragm S is located 1.00 millimeters behind (on the image side) the first lens group  10  (surface No. 8 ).  
                                                                               F NO  = 3.9 − 9.1 − 15.8       f = 29.74 − 69.00 − 120.00 (Zoom Ratio:4.03)       W = 36.0 − 17.5 − 10.2       f B  = 9.88 − 46.13 − 93.22            Surf. No.   r   d   Nd   ν                     1*   15.311   1.25   1.83481   42.7        2   12.286   4.01   1.66446   35.8        3   37.835   0.12   —   —        4   15.811   4.86   1.48749   70.2        5   −11.020   1.13   1.80518   25.4        6   −41.270   0.13   —   —        7   50.708   3.01   1.71850   33.5        8*   −16.073   9.87 − 3.78 − 1.81   —   —        9*   −80.297   3.13   1.66680   33.0        10*   −34.442   4.90   —   —       11   −9.279   1.76   1.72916   54.7       12   −129.476                          
 
         [0068]    Aspherical surface data (the aspherical surface coefficients not indicated are zero (0.00)):  
                                                                     Surf.No.   K   A4   A6   A8                                1   0.00   −0.1209 × 10 −4     0.2090 × 10 −6     —       8   0.00    0.8490 × 10 −4     0.1831 × 10 −7     —       9   0.00    0.5765 × 10 −4     0.9375 × 10 −6     0.1138 × 10 −7         10   0.00   −0.3126 × 10 −4     0.1300 × 10 −5     0.1879 × 10 −7                    
 
         [0069]    Each condition of each embodiment is shown in the following Table 5.  
                                                         TABLE 5                                   Embod.1   Embod.2   Embod.3   Embod.4                                    Cond.(1)   5.480   6.031   6.318   6.461       Cond.(2)   −1.044   −1.623   −1.030   −1.421       Cond.(3)   0.148   0.177   0.117   0.128                  
 
         [0070]    As can be understood from Table 5, each embodiment satisfies each condition. Furthermore, as can be understood from the aberration diagrams, the aberrations are relatively well corrected.  
         [0071]    According to the above description, a small telephoto two-lens-group zoom lens system, which has a zoom ratio of 3.5 or more, and the half angle-of-view of 35 degrees or more at the short focal length extremity, can be obtained.