Abstract:
A zoom lens system with a new structure is disclosed which realizes high optical performance by appropriately setting the structure of each lens unit and using a material with anomalous dispersion in an appropriate lens unit. Specifically, according to several aspect herein disclosed, the zoom lens system includes, in order from an object side to an image side, lens units having a positive, a negative, and a positive optical powers in which the structure of each lens unit is specified or a third lens unit employs a lens element made of a material with anomalous dispersion.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a zoom lens system preferable for use in a digital still camera, a video camera or the like.  
           [0003]    2. Description of Related Art  
           [0004]    A known zoom lens which has a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, a third lens unit having a positive refractive power, and a fourth lens unit having a positive refractive power in order from an object side achieves zooming by moving the first, second, third, and fourth lens units (for example, see Patent Documents 1 to 3).  
           [0005]    Another known zoom lens has a four lens unit structure as described above including a third lens unit formed to have a plurality of lenses disposed with a large air spacing (for example, see Patent Documents 4, 5).  
           [0006]    In addition, a known zoom lens of a rear focus type achieves focusing by moving lens units other than a first lens unit on an object side as a means for realizing reductions in overall length of the zoom lens and a front element diameter (Patent Documents 6, 7).  
           [0007]    Generally, in the zoom lens of the rear focus type, the effective diameter of a first lens unit can be reduced as compared with a zoom lens which achieves focusing by moving a first lens unit, so that the size of the overall lens system can be easily reduced. Also, close-up, especially image-taking at the closest focusing distance is easily performed. Since small and lightweight lens units are moved, only a small driving force is needed for the lens units to allow fast focusing.  
           [0008]    (Patent Document 1)  
           [0009]    Japanese Patent Application Laid-Open No. H08(1996)- 50244   
           [0010]    (Patent Document 2)  
           [0011]    U.S. Pat. No. 4,632,519  
           [0012]    (Patent Document 3)  
           [0013]    Japanese Patent Application Laid-Open No. 2001-194586 (corresponding to U.S. Pat. No. 6,456,441)  
           [0014]    (Patent Document 4)  
           [0015]    Japanese Patent Application Laid-Open No. 2001-242379  
           [0016]    (Patent Document 5)  
           [0017]    Japanese Patent Application Laid-Open No. 2001-356269  
           [0018]    (Patent Document 6)  
           [0019]    Japanese Patent Application Laid-Open No. H11(1999)-305124 (corresponding to U.S. Pat. No. 6,166,864)  
           [0020]    (Patent Document 7)  
           [0021]    Japanese Patent Application Laid-Open No. H10(1998)- 62687  (corresponding to U.S. Pat. No. 6,016,228)  
           [0022]    In a zoom lens disclosed in Japanese Patent Application Laid-Open No. H08(1996)- 50244 , a third lens unit is formed of a single positive lens in which the effect of variable magnification is partially provided by moving the third lens unit. When a high zoom ratio is provided, variations in aberration of the third lens present a problem.  
           [0023]    In U.S. Pat. No. 4,632,519, the zoom ratio is approximately 5.7. The spacing between a third lens unit and a fourth lens unit becomes smaller from the wide-angle end to the telephoto end to reduce the action of variable magnification provided by the third lens unit. Especially for a high zoom ratio of 6 or more, the overall length of the zoom lens is increased at the telephoto end to present a problem in terms of a reduction in size.  
           [0024]    In Japanese Patent Application Laid-Open Nos. 2001-242379 and 2001-356269, the variable magnification ratio is approximately 3, and a first lens unit is formed of a single positive lens. Thus, when a higher zoom ratio is provided, variations in aberration occurring in the first lens unit during variation of magnification are not easily canceled by the other lens units.  
           [0025]    On the other hand, in a camera such as a video camera and a digital still camera, a solid-state image-pickup device with a number of pixels (a multi-pixel image-pickup device) is often used. A high-performance zoom lens is required as an optical system for use in such a camera.  
           [0026]    It is necessary, especially for a zoom lens for a multi-pixel image-pickup device, not only to correct monochromatic aberration but also to sufficiently correct chromatic aberration in a wide wavelength range. Generally, when a zoom lens with a high zoom ratio has a large focal length of the overall system at a zoom position on the telephoto side, a reduced secondary spectrum is highly demanded in chromatic aberration in addition to primary achromatism.  
           [0027]    Conventionally, a number of zoom lenses have been known which employ a lens made of glass with anomalous dispersion to correct a secondary spectrum of axial chromatic aberration (longitudinal chromatic aberration) at a zoom position on the telephoto side. In addition, as a structure of a zoom lens suitable for providing a high zoom ratio, a positive lead type zoom lens is an example which includes a lens unit having a positive refractive power closest to an object.  
           [0028]    A known zoom lens has a three lens unit structure with a positive, a negative, and a positive refractive powers in order from an object side in which a lens made of glass with anomalous dispersion is used (For example, Patent Documents 8 to 10).  
           [0029]    Another known zoom lens has a four lens unit structure with a positive, a negative, a positive, and a positive refractive powers in order from an object side in which a lens made of glass with anomalous dispersion is used (For example, Patent Documents 11 to 15).  
           [0030]    Another known zoom lens has a five lens unit structure with a positive, a negative, a positive, a negative, and a positive refractive powers in order from an object side in which a lens made of glass with anomalous dispersion is used (For example, Patent Documents 12, 15 to 17).  
           [0031]    (Patent Document 8)  
           [0032]    Japanese Patent No. 3008580 (corresponding to U.S. Pat. No. 5,257,134)  
           [0033]    (Patent Document 9)  
           [0034]    Japanese Patent Application Laid-Open No. H06(1994)- 43363   
           [0035]    (Patent Document 10)  
           [0036]    Japanese Patent Publication No. H03(1991)- 58490  (corresponding to U.S. Pat. No. 4,709,997)  
           [0037]    (Patent Document 11)  
           [0038]    Japanese Patent No. 3097399  
           [0039]    (Patent Document 12)  
           [0040]    Japanese Patent Application Laid-Open No. 2002-62478 (corresponding to U.S. Pat. No. 6,594,087)  
           [0041]    (Patent Document 13)  
           [0042]    Japanese Patent Application Laid-Open No. 2000-321499 (corresponding to U.S. Pat. No. 6,414,799)  
           [0043]    (Patent Document 14)  
           [0044]    Japanese Patent Application Laid-Open No. H08(1996)-248317  
           [0045]    (Patent Document 15)  
           [0046]    Japanese Patent Application Laid-Open No. 2001-194590 (corresponding to U.S. Pat. No. 6,404,561)  
           [0047]    (Patent Document 16)  
           [0048]    Japanese Patent Application Laid-Open No. H09(1997)- 5624  (corresponding to U.S. Pat. No. 5,760,966)  
           [0049]    (Patent Document 17)  
           [0050]    Japanese Patent Application Laid-Open No. 2001-350093 (corresponding to U.S. Pat. No. 6,449,433)  
           [0051]    In the positive lead type zoom lens, the secondary spectrum of axial chromatic aberration at a zoom position on the telephoto side is likely to occur in the first lens unit having a positive refractive power in which the height of axial rays is high. Thus, the positive lens of the first lens unit is often made of glass with anomalous dispersion to reduce the secondary spectrum. However, the glass with anomalous dispersion is typically difficult to process as compared with normal glass, and especially when it is used for the first lens unit having a large effective diameter, it is difficult to provide a lens with high processing accuracy.  
           [0052]    In a zoom lens formed of three lens units having a positive, a negative, and a positive refractive powers in order from an object side, the height of axial rays is high in the third lens unit, so that a significant effect of correcting the secondary spectrum of axial chromatic aberration is achieved by using glass with anomalous dispersion for a material of a positive lens of the third lens unit. This is advantageous in manufacture since the third lens unit has a smaller lens effective diameter than a first lens unit.  
           [0053]    In Patent Documents 14, 15, and 17, a first lens unit having a positive refractive power includes a lens made of glass with anomalous dispersion. However, a third lens unit has no lens made of glass with anomalous dispersion, and correction of chromatic aberration is not necessarily sufficient.  
           [0054]    In Patent Document 13, a fourth lens unit has a lens made of glass with anomalous dispersion, but any of a first lens unit and a third lens unit has no lens made of glass with anomalous dispersion. The lens made of the glass with anomalous dispersion in the fourth lens unit effectively reduces the secondary spectrum of chromatic aberration of magnification (lateral chromatic aberration). The zoom ratio in an embodiment is approximately 4, but when the zoom ratio is increased, correction of the axial secondary spectrum is not necessarily sufficient on the telephoto side.  
           [0055]    Generally, in a zoom lens having a three lens unit structure with a positive, a negative, and a positive refractive powers in order from an object side or having a four lens unit structure, the lens arrangement in which a first lens unit is moved during variation of magnification is suitable for reducing the overall length of the zoom lens at the wide-angle end at a high zoom ratio of 7 or more.  
           [0056]    In Patent Documents 8 to 11 and 16 described above, however, the first lens unit is fixed during variation of magnification. Thus, it is difficult to achieve both of a smaller overall length of the zoom lens and high variable magnification.  
           [0057]    In Patent Documents 10 and 11, each of a first lens unit and a third lens unit employs a lens made of lens with anomalous dispersion to favorably correct chromatic aberration. However, the first lens unit employs the lens, so that the lens tends to have a large effective diameter to cause difficulty in manufacturing the lens.  
           [0058]    In Patent Document 11, since an aperture stop is provided in the fourth lens unit, the diameter of a front element is increased when the focal length at the wide-angle end is reduced to obtain a wider angle. In each embodiment of Patent Document 11, half of the field angle at the wide-angle end is as narrow as 16.7 degrees to make the lens inappropriate for specifications including half of the field angle larger than 30 degrees.  
           [0059]    Generally, the use of a number of lenses made of a glass material with high anomalous dispersion is effective in enhancing the effect of correcting the secondary spectrum. A particularly effective glass material has an Abbe number larger than 90 and a partial dispersion ratio Θg, F lager than 0.53 (for example, fluorite). In any of the conventional examples, such a glass material with high anomalous dispersion is not used. Thus, the correction of the secondary spectrum is not sufficient when the number of pixels is increased to cause a smaller pitch in a solid-state image-pickup device in a digital camera or the like.  
         SUMMARY OF THE INVENTION  
         [0060]    It is an object of the present invention to provide a zoom lens system with a new structure which is different from those in the aforementioned conventional examples. The present invention realizes a zoom lens system which has favorable optical performance by appropriately setting the structure of each lens unit and using a material with anomalous dispersion in an appropriate lens unit. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0061]    [0061]FIG. 1 is an explanatory view of a paraxial refractive power arrangement in a zoom lens of Embodiment 1 according to the present invention;  
         [0062]    [0062]FIG. 2 is a section view of a zoom lens of Numerical Example 1 at the wide-angle end;  
         [0063]    [0063]FIG. 3 shows various types of aberration in the zoom lens of Numerical Example 1 at the wide-angle end;  
         [0064]    [0064]FIG. 4 shows various types of aberration in the zoom lens of Numerical Example 1 at the intermediate zoom position;  
         [0065]    [0065]FIG. 5 shows various types of aberration in the zoom lens of Numerical Example 1 at the telephoto end;  
         [0066]    [0066]FIG. 6 is a section view of a zoom lens of Numerical Example 2 at the wide-angle end;  
         [0067]    [0067]FIG. 7 shows various types of aberration in the zoom lens of Numerical Example 2 at the wide-angle end;  
         [0068]    [0068]FIG. 8 shows various types of aberration in the zoom lens of Numerical Example 2 at the intermediate zoom position;  
         [0069]    [0069]FIG. 9 shows various types of aberration in the zoom lens of Numerical Example 2 at the telephoto end;  
         [0070]    [0070]FIG. 10 is a section view of a zoom lens of Numerical Example 3 at the wide-angle end;  
         [0071]    [0071]FIG. 11 shows various types of aberration in the zoom lens of Numerical Example 3 at the wide-angle end;  
         [0072]    [0072]FIG. 12 shows various types of aberration in the zoom lens of Numerical Example 3 at the intermediate zoom position;  
         [0073]    [0073]FIG. 13 shows various types of aberration in the zoom lens of Numerical Example 3 at the telephoto end;  
         [0074]    [0074]FIG. 14 is a section view of a zoom lens of Numerical Example 4 at the wide-angle end;  
         [0075]    [0075]FIG. 15 shows various types of aberration in the zoom lens of Numerical Example 4 at the wide-angle end;  
         [0076]    [0076]FIG. 16 shows various types of aberration in the zoom lens of Numerical Example 4 at the intermediate zoom position;  
         [0077]    [0077]FIG. 17 shows various types of aberration in the zoom lens of Numerical Example 4 at the telephoto end;  
         [0078]    [0078]FIG. 18 is a section view of a zoom lens of Numerical Example 5 at the wide-angle end;  
         [0079]    [0079]FIG. 19 shows various types of aberration in the zoom lens of Numerical Example 5 at the wide-angle end;  
         [0080]    [0080]FIG. 20 shows various types of aberration in the zoom lens of Numerical Example 5 at the intermediate zoom position;  
         [0081]    [0081]FIG. 21 shows various types of aberration in the zoom lens of Numerical Example 5 at the telephoto end;  
         [0082]    [0082]FIG. 22 is an explanatory view of a paraxial refractive power arrangement in a zoom lens of Embodiment 2 according to the present invention;  
         [0083]    [0083]FIG. 23 is a section view of a zoom lens of Numerical Example 6 at the wide-angle end;  
         [0084]    [0084]FIG. 24 shows various types of aberration in the zoom lens of Numerical Example 6 at the wide-angle end;  
         [0085]    [0085]FIG. 25 shows various types of aberration in the zoom lens of Numerical Example 6 at the intermediate zoom position;  
         [0086]    [0086]FIG. 26 shows various types of aberration in the zoom lens of Numerical Example 6 at the telephoto end;  
         [0087]    [0087]FIG. 27 is a section view of a zoom lens of Numerical Example 7 at the wide-angle end;  
         [0088]    [0088]FIG. 28 shows various types of aberration in the zoom lens of Numerical Example 7 at the wide-angle end;  
         [0089]    [0089]FIG. 29 shows various types of aberration in the zoom lens of Numerical Example 7 at the intermediate zoom position;  
         [0090]    [0090]FIG. 30 shows various types of aberration in the zoom lens of Numerical Example 7 at the telephoto end;  
         [0091]    [0091]FIG. 31 is a section view of a zoom lens of Numerical Example 8 at the wide-angle end;  
         [0092]    [0092]FIG. 32 shows various types of aberration in the zoom lens of Numerical Example 8 at the wide-angle end;  
         [0093]    [0093]FIG. 33 shows various types of aberration in the zoom lens of Numerical Example 8 at the intermediate zoom position;  
         [0094]    [0094]FIG. 34 shows various types of aberration in the zoom lens of Numerical Example 8 at the telephoto end;  
         [0095]    [0095]FIG. 35 is a section view of a zoom lens of Numerical Example 9 at the wide-angle end;  
         [0096]    [0096]FIG. 36 shows various types of aberration in the zoom lens of Numerical Example 9 at the wide-angle end;  
         [0097]    [0097]FIG. 37 shows various types of aberration in the zoom lens of Numerical Example 9 at the intermediate zoom position;  
         [0098]    [0098]FIG. 38 shows various types of aberration in the zoom lens of Numerical Example 9 at the telephoto end;  
         [0099]    [0099]FIG. 39 is a section view of a zoom lens of Numerical Example 10 at the wide-angle end;  
         [0100]    [0100]FIG. 40 shows various types of aberration in the zoom lens of Numerical Example 10 at the wide-angle end;  
         [0101]    [0101]FIG. 41 shows various types of aberration in the zoom lens of Numerical Example 10 at the intermediate zoom position;  
         [0102]    [0102]FIG. 42 shows various types of aberration in the zoom lens of Numerical Example 10 at the telephoto end;  
         [0103]    [0103]FIG. 43 is a graph showing the relationship between the Abbe number νd and the partial dispersion ratio Θg, F; and  
         [0104]    [0104]FIG. 44 is a schematic diagram showing main portions of a digital camera having a zoom lens system according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0105]    Preferred embodiments of a zoom lens system and a camera having the zoom lens system according to the present invention are hereinafter described with reference to the drawings.  
         [0106]    (Embodiment 1)  
         [0107]    [0107]FIG. 1 is an explanatory view of a paraxial refractive power arrangement in a zoom lens of Embodiment 1 corresponding to Numerical Examples 1 to 5, later described. FIG. 2 is a section view of main portions of a zoom lens of Numerical Example 1. FIGS.  3  to  5  show various types of aberration at the wide-angle end, intermediate focal length, and telephoto end of the zoom lens of Numerical Example 1.  
         [0108]    [0108]FIG. 6 is a section view of main portions of a zoom lens of Numerical Example 2. FIGS.  7  to  9  show various types of aberration at the wide-angle end, intermediate focal length, and telephoto end of the zoom lens of Numerical Example 2.  
         [0109]    [0109]FIG. 10 is a section view of main portions of a zoom lens of Numerical Example 3. FIGS.  11  to  13  show various types of aberration at the wide-angle end, intermediate focal length, and telephoto end of the zoom lens of Numerical Example 3.  
         [0110]    [0110]FIG. 14 is a section view of main portions of a zoom lens of Numerical Example 4. FIGS.  15  to  17  show various types of aberration at the wide-angle end, intermediate focal length, and telephoto end of the zoom lens of Numerical Example 4.  
         [0111]    [0111]FIG. 18 is a section view of main portions of a zoom lens of Numerical Example 5. FIGS.  19  to  21  show various types of aberration at the wide-angle end, intermediate focal length, and telephoto end of the zoom lens of Numerical Example 5.  
         [0112]    In the section view of each zoom lens, L 1  shows a first lens unit having a positive refractive power (an optical power or the reciprocal of a focal length) and L 2  shows a second lens unit having a negative refractive power. L 3  shows a third lens unit having a positive refractive power which has a first lens sub-unit L 3   a  having a positive refractive power and a second lens sub-unit L 3   b  having a positive refractive power with the largest air spacing between them. L 4  shows a fourth lens unit having a positive refractive power. SP shows an aperture stop located in front of the third lens unit L 3 . G shows an optical block which corresponds to an optical filter, a faceplate or the like and is provided in design. IP shows an image plane at which an image-pickup surface of a solid-state image-pickup device (a photoelectrical conversion element) such as a CCD sensor and a CMOS sensor is located.  
         [0113]    In each aberration diagram, d and g show a d-line and a g-line, respectively. AM and AS show a meridional image plane and a sagittal image plane, respectively. The chromatic difference of magnification is represented by the g-line.  
         [0114]    In the zoom lens of Embodiment 1, the respective lens units are moved as shown by arrows in zooming from the wide-angle end to the telephoto end. The wide-angle end and the telephoto end refer to zoom positions when lens units for varying magnification are positioned at two ends of a mechanically movable range in an optical axis direction, respectively.  
         [0115]    The respective lens units are moved in zooming such that the spacing between the first lens unit L 1  and the second lens unit L 2  is larger, the spacing between the second lens unit L 2  and the third lens unit L 3  is smaller, and the spacing between the third lens unit L 3  and the fourth lens unit L 4  is larger at the telephoto end than at the wide-angle end.  
         [0116]    Specifically, in zooming from the wide-angle end to the telephoto end, the first lens unit L 1  is moved towards an object along part of a convex track towards an image. The movement of the first lens unit L 1  in zooming can reduce the overall length of the zoom lens at the wide-angle end to achieve a reduction in size in the optical axis direction. In addition, the smaller spacing between the first lens unit L 1  and the aperture stop SP at the wide-angle end reduces the effective diameter of the first lens unit L 1  to realize a reduction in diameter of the front element.  
         [0117]    The second lens unit L 2  is moved along a convex track towards the image or linearly towards the image.  
         [0118]    The third lens unit L 3  is moved towards the object from the wide-angle end to the telephoto end, and the moving track is set such that the spacing between the third lens unit L 3  and the fourth lens unit L 4  is larger at the telephoto end than at the wide-angle end to cause the third lens unit L 3  to make a contribution to the variable magnification action. This can diminish the variable magnification action provided by changing the spacing between the first lens unit L 1  and the second lens unit L 2 , so that it is possible to set a smaller spacing between the first lens unit L 1  and the second lens unit L 2  at the telephoto end. Consequently, the overall length of the zoom lens is reduced at the telephoto end and the diameter of the front element is reduced.  
         [0119]    The aperture stop SP may be moved together with the third lens unit L 3  or may be moved separately from the third lens unit L 3  during zooming. If the aperture stop SP is moved together with the third lens unit L 3 , the number of movable lens units can be reduced to facilitate simplification of the mechanical structure. If the aperture stop SP is moved separately from the third lens unit L 3 , the diameter of the front element can be easily reduced especially when the aperture stop SP is moved along a convex track towards the object.  
         [0120]    The fourth lens unit L 4  is moved along a convex track towards the object or moved towards the image to correct image plane variations associated with variations of magnification.  
         [0121]    In Embodiment 1, the first lens sub-unit L 3   a  consists of two positive lenses and two negative lenses. Specifically, the third lens unit L 3  is formed, in order from the object side to the image side, a first cemented lens formed by cementing a positive lens having a convex surface towards the object side to a negative lens having a concave surface towards the image, a second cemented lens formed by cementing a negative lens to a positive lens, and a positive lens. A part system of the third lens unit L 3  consisting of the first and second cemented lenses constitutes the first lens sub-unit L 3   a , and the other part of the third lens unit L 3  consisting the remaining single positive lens constitutes the second lens sub-unit L 3   b . The first lens sub-unit L 3   a  is spaced from the second lens sub-unit L 3   b  to some degree to locate the second lens sub-unit L 3   b  away from the aperture stop SP.  
         [0122]    When the aperture stop SP is moved during zooming, the exit pupil easily varies. Especially when the aperture stop SP is moved towards the object side in zooming from the wide-angle end to the telephoto end the exit pupil is likely to vary from negative to positive. Thus, the second lens sub-unit L 3   b  is disposed close to the fourth lens unit L 4  at the wide-angle end to cause the combination system of the second lens sub-unit L 3   b  and the fourth lens unit L 4  to have the action of locating the exit pupil away from the image plane. At the telephoto end, the third lens unit L 3  is moved towards the object to locate the second lens sub-unit L 3   b  away from the image plane, and the fourth lens unit L 4  is mainly responsible for the action of locating the exit pupil away from the image plane. The action of locating the exit pupil away from the image plane by the second lens sub-unit L 3   b  is particularly provided at a zoom position on the wide-angle side in this manner to cancel variations in the exit pupil due to the movement of the aperture stop SP. As a result, variations in the exit pupil are small even when the aperture stop SP is moved during zooming. When the zoom lens is applied to an image-taking apparatus which employs a solid-state image pickup device having a microlens arranged in each pixel, shading can be reduced over the entire zoom range.  
         [0123]    The first lens sub-unit L 3   a  is formed of the two cemented lenses to favorably correct various types of aberration. When the third lens unit L 3  is moved to make a contribution to variable magnification, it is necessary to satisfactorily correct various types of aberration occurring in the third lens unit L 3  including variations due to variations of magnification. If the third lens unit L 3  has a lateral magnification close to one, well-balanced correction can be made to various types of aberration by forming the first lens sub-unit L 3   a  in a symmetric form. A triplet is a representative example of a symmetric lens arrangement. In Embodiment 1, the negative and positive refractive powers of the triplet are divided into two components to increase flexibility in correction of aberration, thereby more favorably correcting various types of aberration such as spherical aberration, comatic aberration, and curvature of field.  
         [0124]    In addition, the first lens unit L 1  is formed to include at least one positive lens and negative lens to reduce variations in chromatic aberration during zooming. If two or more positive lenses may be included to share the refractive power, it is possible to reduce spherical aberration on the telephoto side and an axial secondary spectrum.  
         [0125]    In an optical system which requires-high resolution such as an image-taking lens for a digital camera or a video camera employing a multi-pixel solid-state image-pickup device, variations in chromatic aberration of magnification associated with variations of magnification need to be sufficiently corrected. Thus, the second lens unit L 2  is formed to include three or more negative lenses and one or more positive lenses.  
         [0126]    When only two negative lenses are included, correction of chromatic aberration of magnification is difficult if the refractive power of the second lens unit L 2  is increased to reduce the moving amounts of the first lens unit L 1  and the second lens unit L 2  in an attempt to reduce the overall length of the zoom lens. The second lens unit L 2  is formed of, in order from the object side, a meniscus negative lens having a concave surface towards the image, a negative lens, a positive lens having a convex surface towards the object, and a negative lens to reduce symmetry of a front side and a rear side of the second lens unit L 2 . This enhances the achromatism effect at the principal point to effectively correct chromatic aberration of magnification.  
         [0127]    In Embodiment 1, the fourth lens unit L 4  or the second lens sub-unit L 3   b  is used to achieve focusing. When the fourth lens unit L 4  is used for focusing, the relatively small and lightweight lens unit is moved as compared with front focusing, so that only a small driving power is required. In addition, it is compatible with an autofocus system due to the ability to perform fast focusing.  
         [0128]    When the second lens sub-unit L 3   b  is used for focusing, an additional mechanism is required for driving. However, as compared with the case where the fourth lens unit L 4  is used for focusing, the spacing between the second lens sub-unit L 3   b  and the fourth lens unit L 4  can be reduced at the wide-angle end to achieve a reduction in the overall length of the zoom lens. In addition, while no image is taken, a collapsible mechanism can be used to reduce the spacing between the lens units, thereby realizing a smaller size of the whole image-taking apparatus. When the second lens sub-unit L 3   b  has a moving mechanism, the second lens sub-unit L 3   b  is moved towards the object and collapsed in that state to allow a further reduction in the collapsed lens length.  
         [0129]    When the zoom lens in Embodiment 1 is applied to an image-taking apparatus, an optical filter in a flat shape may be disposed between the first lens sub-unit L 3   a  and the second lens sub-unit L 3   b . This arrangement advantageously improves the use of the spacing between the first lens sub-unit L 3   a  and the second lens sub-unit L 3   b  to eliminate the need to provide additional space for disposing a filter. As the optical filter, an ND filter for reducing an amount of light, an infrared cut filter for absorbing or reflecting light in the near-infrared region or the like can be used. Any of the filters may be fixed between the first lens sub-unit L 3   a  and the second lens sub-unit. L 3   b , or may be removably provided on the optical path. The ND filter is typically disposed near the aperture stop SP, but the filter disposed between the first lens sub-unit L 3   a  and the second lens sub-unit L 3   b  is advantageous in reducing the overall length of the zoom lens since the spacing between the second lens unit L 2  and the third lens unit L 3  can be reduced accordingly at the telephoto end zoom position. The infrared cut filter is typically disposed between an image-taking lens and a solid-state image-pickup device, but the filter disposed between the first lens sub-unit L 3   a  and the second lens sub-unit L 3   b  can reduce the effective range of rays to achieve smaller outer dimensions of the filter. Especially when the filter is removably provided, the overall image-taking apparatus can be reduced in size. Since the rays passing between the first lens sub-unit L 3   a  and the second lens sub-unit L 3   b  are relatively close to be afocal, the insertion or removal of the filter involves small variations in focus.  
         [0130]    In Embodiment 1, the following conditions are satisfied:  
         (0.6 ×D   3   b )&lt; d&lt;D   3   a   (1)  
         0.7 &lt;f   3   b/f   3   a&lt; 1.3  (2)  
         1.0 &lt;f   1 / ft&lt; 2.5  (3)  
         0.01 &lt;d   23 / ft&lt; 0.20  (4)  
         0.5&lt;(β3 t /β3 w )/(β2 t /β2 w )&lt;1.0  (5)  
         60&lt;ν3 b   (6)  
         [0131]    where D 3   a  represents a distance on the optical axis from a lens surface closest to the object to a lens surface closest to the image in the first lens sub-unit L 3   a , D 3   b  a distance on the optical axis from a lens surface closest to the object to a lens surface closest to the image in the second lens sub-unit L 3   b, d  an air spacing between the first lens sub-unit L 3   a  and the second lens sub-unit L 3   b , f 3   a  and f 3   b  focal lengths of the first lens sub-unit L 3   a  and the second lens sub-unit L 3   b , respectively, f 1  a focal length of the first lens unit L 1 , ft a focal length of the entire system at the telephoto end, d 23  a spacing between a lens surface of the second lens unit L 2  closest to the image and the lens surface of the third lens unit L 3  closest to the object at the telephoto end (a spacing between the second lens unit L 2  and the third lens unit L 3  at the telephoto end), β2 w and β3 w lateral magnifications of the second lens unit L 2  and the third lens unit L 3  at the wide-angle end, respectively, β2 t and β3 t lateral magnifications of the second lens unit L 2  and the third lens unit L 3  at the telephoto end, respectively, and ν3 b an Abbe number of the material of the single positive lens of the second lens sub-unit L 3   b.    
         [0132]    It is necessary only that at least one of the conditions is satisfied. The effect in association with the satisfied expression can be provided.  
         [0133]    Next, each of the conditions is described from a technical viewpoint.  
         [0134]    The conditional expression (1) defines the spacing between the first lens sub-unit L 3   a  and the second lens sub-unit L 3   b . A large spacing between the first lens sub-unit L 3   a  and the second lens sub-unit L 3   b , which exceeds the upper limit, is not preferable since the third lens unit L 3  is moved in the optical axis direction to increase the overall length of the zoom lens. On the other hand, a smaller spacing than the lower limit is not preferable since such a small spacing diminishes the action of locating the second lens sub-unit L 3   b  away from the image plane to reduce variations in the exit pupil during zooming.  
         [0135]    The conditional expression (2) defines the ratio of the refractive powers of the first lens sub-unit L 3   a  and the second lens sub-unit L 3   b . If the focal length of the second lens sub-unit L 3   b  is so large relative to the focal length of the first lens sub-unit L 3   a  as to result in the value of f 3   b /f 3   a  larger than the upper limit, that is, if the refractive power of the second lens sub-unit L 3   b  is too low relative to the refractive power of the first lens sub-unit L 3   a , the action of bending off-axis luminous flux is extremely small. Thus, such a large focal length is not preferable since the action of locating the exit pupil away from the image plane at the wide-angle end is diminished even when first lens sub-unit L 3   a  is spaced from the second lens sub-unit L 3   b  to some degree. If the refractive power of the second lens sub-unit L 3   b  is so high relative to the refractive power of the first lens unit L 3   a  as to result in the value of f 3   b /f 3   a  less than the lower limit, the second lens sub-unit L 3   b  must make a larger contribution to the variable magnification action in the third lens L 3 , and it is difficult for the second lens sub-unit L 3   b  formed of only one lens to correct aberration. Especially the Petzval sum is too large, and correction of curvature of field is difficult.  
         [0136]    The conditional expression (3) defines the focal length of the first lens unit L 1 . If the focal length of the first lens unit L 1  is so large as to result in the value of f 1 /ft exceeding the upper limit, that is, if the refractive power of the first lens unit L 1  is too low, the overall length of the zoom lens is too large especially at the telephoto end. On the other hand, if the focal length of the first lens unit L 1  is so small as to result in the value of fl/ft less than the lower limit, that is, if the refractive power of the first lens unit L 1  is too high, spherical aberration is unpreferably increased at the telephoto end.  
         [0137]    The conditional expression (4) defines the distance between the second lens unit L 2  and the third lens unit L 3  at the telephoto end. A long distance between the units L 2  and L 3 , which causes the value of d 23 /ft to exceed the upper limit, is not preferable since the overall length of the zoom lens is long at the telephoto end and the spacing between the aperture stop SP and the first lens unit L 1  is increased at the telephoto end to result in an increased diameter of the front element. If the distance is so short as to cause the value of d 23 /ft to be less than the lower limit, it is difficult to dispose the aperture stop unit between the second lens unit L 2  and the third lens unit L 3 .  
         [0138]    The conditional expression (5) defines the contribution to variable magnification of the second lens unit L 2  and the third lens unit L 3 . If the contribution to variable magnification of the third lens unit L 3  is so large relative to the second lens unit L 2  as to result in the value of (β3 t/β3 w)/(β2 t/β2 w) exceeding the upper limit, the third lens unit L 3  involves large variations in aberration such as spherical aberration, comatic aberration, and astigmatism during variation of magnification to cause difficulty in achieving favorable optical performance over the entire zoom range. On the other hand, a small contribution to variable magnification of the third lens unit L 3  relative to the second lens unit L 2 , which causes the value of (β3 t/β3 w)/(β2 t/β2 w) to be less than the lower limit, is not preferable since the variable magnification ratio of the entire system needs to be ensured by increasing the spacing between the first lens unit L 1  and the second lens unit L 2  at the telephoto end, leading to an increase in the overall length of the zoom lens.  
         [0139]    The conditional expression (6) defines the Abbe number of the material of the positive lens of the second lens sub-unit L 3   b . If the Abbe number is so small as to be less than the lower limit, the secondary component of chromatic aberration of magnification on the wide-angle side is too large. This is not preferable since the secondary component needs to be corrected as much as possible to reduce color spreading in the peripheral portion of the image when the image of a high-contrast object is taken.  
         [0140]    In Embodiment 1, it is more preferable to set the numerical values in the conditional expressions (1) to (6) as follows.  
         (0.65 ×D   3   b )&lt; d&lt; 0.8 ×D   3   a   (1 a)  
         0.8 &lt;f   3   b&lt;/f   3   a&lt; 1.2 (2 a)  
         1.0 &lt;f   1 / ft&lt; 2.3  (3 a)  
         0.02 &lt;d   23 / ft&lt; 0.15  (4 a)  
         0.6&lt;(β3 t /β3 w )/(β2 t/β 2 w )&lt;0.9 ( 5   a )  
         75&lt;ν3 b   (6 a)  
         [0141]    Next, numerical value data of Numerical Examples 1 to 5 are shown. In each Numerical Example, i shows the order of an optical surface from the object side, Ri the radius of curvature of an ith optical surface (an ith surface), Di a spacing between the ith surface and the i+1 surface, Ni and νi the refractive index and the Abbe number of the material of the ith optical member for the d-line. An aspheric shape is represented by:  
       x   =           (     1   /   R     )          h   2         1   +       {     1   -       (     1   +   k     )                       (     h   /   R     )     2         }           +     Bh   4     +     Ch   6     +     Dh   8     +     Eh   10                             
 
         [0142]    where k represents the conic constant, B, C, D, E aspheric coefficients, x a displacement in the optical axis direction at a height h from the optical axis relative to the plane vertex, and R a radius of curvature. For example, “e-Z” means “10 −z .” Table 1 shows values calculated with the aforementioned conditional expressions in the respective Numerical Examples. In addition, f represents a focal length, Fno an F number, and ω half of the field angle.  
         [0143]    In Numerical Examples, R 26  and R 27  represent the optical block G.  
         [0144]    Table 1 also shows values of the exit pupil distance at the wide-angle end and the telephoto end.  
       NUMERICAL EXAMPLE 1 
       [0145]    [0145]                                                                                                                         f = 1˜6.81 Fno = 2.44˜3.60 2ω = 74.2°˜12.7°                                    R1 = 9.527   D1 = 0.24   N1 = 1.846660   ν1 = 23.9           R2 = 5.713   D2 = 0.72   N2 = 1.603112   ν2 = 60.6           R3 = 76.147   D3 = 0.03           R4 = 5.157   D4 = 0.47   N3 = 1.603112   ν3 = 60.6           R5 = 15.192   D5 = Variable           R6 = 6.490   D6 = 0.15   N4 = 1.772499   ν4 = 49.6           R7 = 1.281   D7 = 0.72           R8 = −5.831   D8 = 0.12   N5 = 1.719995   ν5 = 50.2           R9 = 2.786   D9 = 0.12           R10 = 3.332   D10 = 0.43   N6 = 1.833100   ν6 = 23.9           R11 = −4.317   D11 = 0.09           R12 = −2.477   D12 = 0.11   N7 = 1.772499   ν7 = 49.6           R13 = −9.143   D13 = Variable           R14 = Stop   D14 = 0.11           R15 = 1.493   D15 = 0.54   N8 = 1.743300   ν8 = 49.3           R16 = −13.935   D16 = 0.58   N9 = 1.647689   ν9 = 33.8           R17 = 1.282   D17 = 0.19           R18 = 9.211   D18 = 0.11   N10 = 1.846660   ν10 = 23.9           R19 = 1.768   D19 = 0.49   N11 = 1.701536   ν11 = 41.2           R20 = −6.078   D20 = 0.49           R21 = 2.591   D21 = 0.43   N12 = 1.433870   ν12 = 95.1           R22 = −7.077   D22 = Variable           R23 = 3.491   D23 = 0.35   N13 = 1.804000   ν13 = 46.6           R24 = −13.483   D24 = 0.12   N14 = 1.761821   ν14 = 26.5           R25 = 14.406   D25 = 0.27           R26 = ∞   D26 = 0.32   N15 = 1.516800   ν15 = 64.2           R27 = ∞                            Variable   Focal length                spacing   1.00   2.34   6.81                       D5   0.14   1.99   4.26           D13   3.08   1.28   0.35           D22   0.38   1.52   3.37                        Aspheric coefficient                    R10   k = 6.34820e+00   B = −3.81929e−04   C = −1.75966e−02   D = 1.32512e−02   E = −1.59689e−02       R11   k = −6.36293e−01   B = −7.10089e−04   C = −2.66405e−03   D = −1.75694e−03   E = −1.86916e−03       R15   k = −4.21759e−01   B = −8.89717e−03   C = −2.34403e−03   D = 1.33403e−02   E = −1.47831e−02                    
       NUMERICAL EXAMPLE 2 
       [0146]    [0146]                                                                                                                         f = 1˜4.69 Fno = 2.47˜3.22 2ω = 74.1°˜18.3°                                    R1 = 7.891   D1 = 0.24   N1 = 1.846660   ν1 = 23.9           R2 = 4.867   D2 = 0.81   N2 = 1.487490   ν2 = 70.2           R3 = 63.560   D3 = 0.03           R4 = 5.039   D4 = 0.53   N3 = 1.696797   ν3 = 55.5           R5 = 18.362   D5 = Variable           R6 = 6.932   D6 = 0.15   N4 = 1.772499   ν4 = 49.6           R7 = 1.319   D7 = 0.60           R8 = −6.059   D8 = 0.12   N5 = 1.743997   ν5 = 44.8           R9 = 2.470   D9 = 0.27           R10 = 3.727   D10 = 0.40   N6 = 1.846660   ν6 = 23.9           R11 = −6.832   D11 = 0.09           R12 = −2.684   D12 = 0.11   N7 = 1.487490   ν7 = 70.2           R13 = −13.218   D13 = Variable           R14 = Stop   D14 = 0.11           R15 = 1.369   D15 = 0.54   N8 = 1.806100   ν8 = 40.7           R16 = −5.163   D16 = 0.32   N9 = 1.701536   ν9 = 41.2           R17 = 1.247   D17 = 0.19           R18 = 12.634   D18 = 0.11   N10 = 1.846660   ν10 = 23.9           R19 = 1.113   D19 = 0.54   N11 = 1.701536   ν11 = 41.2           R20 = −4.872   D20 = 0.51           R21 = 2.516   D21 = 0.47   N12 = 1.438750   ν12 = 95.0           R22 = −5.733   D22 = Variable           R23 = 3.020   D23 = 0.43   N13 = 1.696797   ν13 = 55.5           R24 = −8.912   D24 = 0.12   N14 = 1.846660   ν14 = 23.8           R25 = 17.380   D25 = 0.27           R26 = ∞   D26 = 0.40   N15 = 1.516800   ν15 = 64.2           R27 = ∞                            Variable   Focal length                spacing   1.00   1.82   4.69                       D5   0.13   1.42   3.40           D13   2.69   1.52   0.35           D22   0.43   1.30   2.33                        Aspheric coefficient                    R10   k = 1.06605e+01   B = −9.04390e−03   C = −2.33522e−02   D = 2.08204e−02   E = −2.70310e−02       R15   k = −4.60017e−01   B = −6.96826e−03   C = 7.14048e−03   D = −1.29140e−02   E = 1.39686e−02                    
       NUMERICAL EXAMPLE 3 
       [0147]    [0147]                                                                                                                         f = 1˜7.80 Fno = 2.47˜3.96 2ω = 74.1°˜11.1°                                    R1 = 8.961   D1 = 0.24   N1 = 1.846660   ν1 = 23.9           R2 = 5.891   D2 = 0.69   N2 = 1.603112   ν2 = 60.6           R3 = 92.370   D3 = 0.03           R4 = 5.125   D4 = 0.49   N3 = 1.496999   ν3 = 81.5           R5 = 16.053   D5 = Variable           R6 = 7.039   D6 = 0.15   N4 = 1.772499   ν4 = 49.6           R7 = 1.272   D7 = 0.62           R8 = −30.079   D8 = 0.12   N5 = 1.719995   ν5 = 50.2           R9 = 2.303   D9 = 0.13           R10 = 4.119   D10 = 0.43   N6 = 1.833100   ν6 = 23.9           R11 = −3.544   D11 = 0.08           R12 = −1.879   D12 = 0.11   N7 = 1.804000   ν7 = 46.6           R13 = −4.765   D13 = Variable           R14 = Stop   D14 = 0.11           R15 = 1.436   D15 = 0.54   N8 = 1.743300   ν8 = 49.3           R16 = 7.412   D16 = 0.39   N9 = 1.603420   ν9 = 38.0           R17 = 1.298   D17 = 0.19           R18 = 8.642   D18 = 0.11   N10 = 1.761821   ν10 = 26.5           R19 = 1.722   D19 = 0.49   N11 = 1.603112   ν11 = 60.6           R20 = −5.108   D20 = 0.40           R21 = 3.197   D21 = 0.47   N12 = 1.433870   ν12 = 95.1           R22 = −5.439   D22 = Variable           R23 = 3.081   D23 = 0.40   N13 = 1.804000   ν13 = 46.6           R24 = −11.224   D24 = 0.13   N14 = 1.761821   ν14 = 26.5           R25 = 8.659   D25 = 0.27           R26 = ∞   D26 = 0.32   N15 = 1.516800   ν15 = 64.2           R27 = ∞                            Variable   Focal length                spacing   1.00   2.56   7.80                       D5   0.13   1.93   4.43           D13   2.94   0.98   0.12           D22   0.32   1.90   4.00                        Aspheric coefficient                    R10   k = 1.12024e+01   B = −5.81964e−03   C = −2.24651e−02   D = 6.23755e−03   E = −3.89114e−02       R11   k = 5.28620e+00   B = −7.37130e−03   C = 1.85877e−03   D = −3.94514e−02   E = 6.11034e−03       R15   k = −3.70683e−01   B = −1.24747e−02   C = 3.21885e−03   D = −8.05549e−04   E = −3.79733e−03                    
       NUMERICAL EXAMPLE 4 
       [0148]    [0148]                                                                                                                         f = 1˜4.71 Fno = 2.47˜4.00 2ω = 74.1°˜18.2°                                    R1 = 7.966   D1 = 0.24   N1 = 1.846660   ν1 = 23.9           R2 = 5.275   D2 = 0.76   N2 = 1.487490   ν2 = 70.2           R3 = 49.266   D3 = 0.03           R4 = 6.732   D4 = 0.47   N3 = 1.696797   ν3 = 55.5           R5 = 23.986   D5 = Variable           R6 = 4.391   D6 = 0.15   N4 = 1.772499   ν4 = 49.6           R7 = 1.320   D7 = 0.59           R8 = −4.289   D8 = 0.12   N5 = 1.743997   ν5 = 44.8           R9 = 2.223   D9 = 0.24           R10 = 3.619   D10 = 0.40   N6 = 1.846660   ν6 = 23.9           R11 = −7.062   D11 = 0.11           R12 = −2.602   D12 = 0.11   N7 = 1.487490   ν7 = 70.2           R13 = −6.600   D13 = Variable           R14 = Stop   D14 = 0.11           R15 = 1.388   D15 = 0.54   N8 = 1.806100   ν8 = 40.7           R16 = −8.832   D16 = 0.36   N9 = 1.701536   ν9 = 41.2           R17 = 1.287   D17 = 0.19           R18 = 28.474   D18 = 0.11   N10 = 1.846660   ν10 = 23.9           R19 = 1.109   D19 = 0.54   N11 = 1.701536   ν11 = 41.2           R20 = −4.886   D20 = 0.51           R21 = 2.849   D21 = 0.47   N12 = 1.516330   ν12 = 64.1           R22 = −5.357   D22 = Variable           R23 = 2.474   D23 = 0.47   N13 = 1.696797   ν13 = 55.5           R24 = −8.219   D24 = 0.12   N14 = 1.846660   ν14 = 23.8           R25 = 10.966   D25 = 0.27           R26 = ∞   D26 = 0.40   N15 = 1.516800   ν15 = 64.2           R27 = ∞                            Variable   Focal length                spacing   1.00   1.88   4.71                       D5   0.13   1.71   4.24           D13   2.55   1.32   0.30           D22   0.82   1.89   3.08                        Aspheric coefficient                    R10   k = 1.01253e+01   B = −7.06875e−03   C = −2.52368e−02   D = 2.56657e−02   E = −3.27006e−02       R15   k = −4.81691e−01   B = −5.92118e−03   C = 5.95247e−03   D = −1.24807e−02   E = 2.03412e−02                    
       NUMERICAL EXAMPLE 5 
       [0149]    [0149]                                                                                                                         f = 1˜6.72 Fno = 2.45˜3.61 2ω = 74.2°˜12.8°                                    R1 = 8.366   D1 = 0.24   N1 = 1.846660   ν1 = 23.9           R2 = 5.350   D2 = 0.72   N2 = 1.603112   ν2 = 60.6           R3 = 51.245   D3 = 0.03           R4 = 6.215   D4 = 0.43   N3 = 1.603112   ν3 = 60.6           R5 = 19.963   D5 = Variable           R6 = 8.195   D6 = 0.15   N4 = 1.772499   ν4 = 49.6           R7 = 1.304   D7 = 0.62           R8 = −8.120   D8 = 0.12   N5 = 1.712995   ν5 = 53.9           R9 = 2.792   D9 = 0.18           R10 = 4.160   D10 = 0.43   N6 = 1.846660   ν6 = 23.9           R11 = −3.386   D11 = 0.06           R12 = −2.239   D12 = 0.12   N7 = 1.882997   ν7 = 40.8           R13 = −6.742   D13 = Variable           R14 = Stop   D14 = 0.11           R15 = 1.445   D15 = 0.54   N8 = 1.743300   ν8 = 49.3           R16 = −17.513   D16 = 0.54   N9 = 1.647689   ν9 = 33.8           R17 = 1.293   D17 = 0.19           R18 = 10.929   D18 = 0.11   N10 = 1.603420   ν10 = 38.0           R19 = 1.366   D19 = 0.62   N11 = 1.496999   ν11 = 81.5           R20 = −3.616   D20 = 0.27           R21 = 2.461   D21 = 0.41   N12 = 1.433870   ν12 = 95.1           R22 = −19.494   D22 = Variable           R23 = 3.253   D23 = 0.38   N13 = 1.772499   ν13 = 49.6           R24 = −12.739   D24 = 0.12   N14 = 1.846660   ν14 = 23.9           R25 = 28.016   D25 = 0.27           R26 = ∞   D26 = 0.32   N15 = 1.516330   ν15 = 64.1           R27 = ∞                            Variable   Focal length                spacing   1.00   2.29   6.72                       D5   0.20   2.10   4.48           D13   2.98   1.35   0.35           D22   0.40   1.76   3.63                        Aspheric coefficient                    R10   k = −2.51350e+00   B = 2.56313e−02   C = −8.97160e−03   D = 3.53085e−03   E = −1.09185e−02       R11   k = 1.34390e+00   B = 8.55219e−04   C = −8.88809e−03   D = −2.99691e−03   E = −5.04686e−03       R15   k = −4.68634e−01   B = −7.76638e−03   C = 7.44966e−04   D = 0.00000e+00   E = 0.00000e+00                    
         [0150]    [0150]                                                                                       TABLE 1                                       Conditional   Numerical example                Expression   1   2   3   4   5                    (1)   0.6 × D3b   0.26   0.28   0.28   0.28   0.25           d   0.49   0.51   0.40   0.51   0.27           D3a   1.91   1.70   1.72   1.74   2.00       (2)   t3b/f3a   1.06   1.02   1.13   0.85   1.23       (3)   f1/ft   1.27   1.69   1.14   2.07   1.35       (4)   d23/ft   0.067   0.098   0.029   0.087   0.068       (5)   (β3t/β3w)/(β2t/β2w)   0.66   0.75   0.74   0.83   0.71       (6)   ν3b   95.1   95.0   95.1   64.1   95.1           Distance to exit   −12.6   −11.3   −9.3   −26.9   −11.7           pupil at wide-angle           end           Distance to exit   29.8   −92.0   25.9   22.4   18.5           pupil at telephoto           end                    
         [0151]    According to the zoom lens of Embodiment 1 described above, the third lens unit consists of the front lens unit and the rear lens unit disposed with a certain air spacing between them. The rear lens unit of the third lens unit serves to a field lens particularly on the wide-angle side to locate the exit pupil on the wide-angle side away from the image plane and reduce variations in the exit pupil in zooming, thereby realizing the zoom lens which is compatible with a solid-state image-pickup device over the entire zoom range.  
         [0152]    (Embodiment 2)  
         [0153]    Next, description is made for a zoom lens of Embodiment 2 corresponding to Numerical Examples 6 to 10, later described. FIG. 22 is an explanatory view of a paraxial refractive power arrangement in the zoom lens of Embodiment 2.  
         [0154]    [0154]FIG. 23 is a section view of main portions of a zoom lens of Numerical Example 6. FIGS.  24  to  26  show various types of aberration at the wide-angle end, intermediate focal length, and telephoto end of the zoom lens of Numerical Example 6.  
         [0155]    [0155]FIG. 27 is a section view of main portions of a zoom lens of Numerical Example 7. FIGS.  28  to  30  show various types of aberration at the wide-angle end, intermediate focal length, and telephoto end of the zoom lens of Numerical Example 7.  
         [0156]    [0156]FIG. 31 is a section view of main portions of a zoom lens of Numerical Example 8. FIGS.  32  to  34  show various types of aberration at the wide-angle end, intermediate focal length, and telephoto end of the zoom lens of Numerical Example 8.  
         [0157]    [0157]FIG. 35 is a section view of main portions of a zoom lens of Numerical Example 9. FIGS.  36  to  38  show various types of aberration at the wide-angle end, intermediate focal length, and telephoto end of the zoom lens of Numerical Example 9.  
         [0158]    [0158]FIG. 39 is a section view of main portions of a zoom lens of Numerical Example 10. FIGS.  40  to  42  show various types of aberration at the wide-angle end, intermediate focal length, and telephoto end of the zoom lens of Numerical Example 10.  
         [0159]    [0159]FIG. 43 is a graph for explaining the relationship between the Abbe number νd and the partial dispersion ratio Θg, F.  
         [0160]    In the explanatory view of the paraxial refractive power arrangement of FIG. 22 and the section view of the zoom lens in each of Numerical Examples, L 1  shows a first lens unit having a positive refractive power, L 2  shows a second lens unit having a negative refractive power, L 3  shows a third lens unit having a positive refractive power, and L 4  shows a fourth lens unit having a positive refractive power. SP shows an aperture stop located in front of the third lens unit L 3 .  
         [0161]    G shows an optical block which corresponds to an optical filter, a faceplate or the like and is provided in design. IP shows an image plane at which an image-pickup surface of a solid-state image-pickup device such as a CCD sensor and a CMOS sensor is located.  
         [0162]    In each aberration diagram, d and g show a d-line and a g-line, respectively. AM and AS show a meridional image plane and a sagittal image plane, respectively. The chromatic aberration of magnification is represented by the g-line.  
         [0163]    In Embodiment 2, the first, second, third lens units L 1 , L 2 , L 3  are moved as shown by arrows in zooming from the wide-angle end to the telephoto end.  
         [0164]    The first, second, third lens units L 1 , L 2 , L 3  are moved for zooming such that the spacing between the first lens unit L 1  and the second lens unit L 2  is larger, the spacing between the second lens unit L 2  and the third lens unit L 3  is smaller, and the spacing between the third lens unit L 3  and the fourth lens unit L 4  is larger at the telephoto end than at the wide-angle end.  
         [0165]    Specifically, in zooming from the wide-angle end to the telephoto end, the first lens unit L 1  and the second lens unit L 2  are moved towards an object along part of a convex track towards an image. The third lens unit L 3  is moved towards the object. The fourth lens unit L 4  is not moved for zooming. Focusing is achieved by the second lens unit L 2  or the fourth lens unit L 4 .  
         [0166]    The first lens unit L 1  is moved in zooming to reduce the overall length of the zoom lens at the wide-angle end zoom position to achieve a reduction in size in the optical axis direction. In addition, the spacing between the first lens unit L 1  and the aperture stop SP is reduced at a zoom position on the wide-angle side to reduce the effective diameter of the first lens unit L 1 , leading to a reduced diameter of the front element.  
         [0167]    The third lens unit L 3  is moved towards the object in zooming from the wide-angle end to the telephoto end, and the moving track is set such that the spacing between the third lens unit L 3  and the fourth lens unit L 4  is increased in zooming from the wide-angle end to the telephoto end to cause the third lens unit L 3  to make a contribution to the variable magnification action. This can diminish the variable magnification action provided by changing the spacing between the first lens unit L 1  and the second lens unit L 2 , so that it is possible to set a smaller spacing between the first lens unit L 1  and the second lens unit L 2  at the telephoto end zoom position. Consequently, the overall length of the zoom lens at the telephoto end and the diameter of the front element are reduced.  
         [0168]    The aperture stop SP may be moved together with the third lens unit L 3  or may be moved separately from the third lens unit L 3  in zooming. If the aperture stop SP is moved together with the third lens unit L 3 , the number of movable lens units can be reduced to facilitate simplification of the mechanical structure. If the aperture stop SP is moved separately from the third lens unit L 3 , the diameter of the front element is advantageously reduced especially when the aperture stop SP is moved along a convex track towards the object.  
         [0169]    The second lens unit L 2  is moved along the convex track towards the image during zooming from the wide-angle end to the telephoto end. This favorably achieves primary achromatism at an intermediate zoom position to satisfactorily correct chromatic aberration over the entire zoom range.  
         [0170]    In the zoom lens of Embodiment 2, the first lens unit L 1  consists of, in order from the object side, a cemented lens formed of a negative lens and a positive lens (or a cemented lens formed of a positive lens and a negative lens may be used) and a positive lens. The minimum number of constituent lenses are used to provide a high zoom ratio and correct chromatic aberration such as axial chromatic aberration and chromatic aberration of magnification as well as spherical aberration. The use of glass with low dispersion and anomalous dispersion for a positive lens is effective in correcting a secondary spectrum at a zoom position on the telephoto side in the lens structure described above. However, a glass material with anomalous dispersion is difficult to process, and if it is used in the first lens unit L 1  with a large effective diameter, manufacturing thereof is difficult. In addition, anomalous dispersion glass with low dispersion typically has a low refractive index, so that the glass has a larger curvature (a smaller radius of curvature) to provide a desired refractive power, thereby making it difficult to correct spherical aberration at a zoom position on the telephoto-side. Especially, if the first lens unit L 1  has a higher refractive power, correction of spherical aberration is more difficult.  
         [0171]    In view of the foregoing, the zoom lens in Embodiment 2 employs a lens made of anomalous dispersion glass in the third lens unit L 3 , not in the first lens unit L 1 , to correct the secondary spectrum of axial chromatic aberration at a zoom position on the telephoto side. The use of the anomalous dispersion glass in the lens unit in which the height of axial rays is high is effective in correcting the secondary spectrum of the axial chromatic aberration. In Embodiment 2, the glass is used for positive lenses of the third lens unit L 3  through which axial rays pass at the second highest levels next to the first lens unit L 1  to make the correction. This can reduce the outside diameter of the lens to the half or less as compared with the case where the glass is used in the first lens unit L 1  to solve the processing problem.  
         [0172]    For example, in Numerical Example 6 shown in FIG. 23, a fourth lens G 34  and a fifth lens G 35  of the third lens unit L 3  in order from the object side are positive lenses with anomalous dispersion. The positive lens G 34  on the object side is a lens with the trade name of S-FPL51 manufactured by Ohara Inc. (with a refractive index of 1.49700 and an Abbe number of 81.5). The positive lens G 35  on the image side is a lens with the trade name of CAF2 manufactured by Optron Inc. (with a refractive index of 1.43387 and an Abbe number of 95.1).  
         [0173]    It should be noted that, in each of Numerical Examples 7 to 10, anomalous dispersion glass is used for a positive lens G 34  and a positive lens G 35  in a third lens unit L 3 , although the values of the Abbe number are somewhat different.  
         [0174]    In Embodiment 2, two positive lenses G 12  and G 13  of the first lens unit L 1  are lenses with the trade name of S-BSM14 manufactured by Ohara Inc. (with a refractive index of 1.60311 and an Abbe number of 60.6) with no anomalous dispersion. Since each of them has a higher refractive index than anomalous dispersion glass, the curvature of each lens surface is reduced (the radius of curvature is increased) to suppress occurrence of spherical aberration in the first lens unit L 1  on the telephoto side.  
         [0175]    In Embodiment 2, the first lens unit L 1  and the third lens unit L 3  are formed in this manner to favorably correct the secondary spectrum of the axial chromatic aberration and spherical aberration at a zoom position on the telephoto side.  
         [0176]    To enhance the effect of correcting the secondary spectrum by the lenses made of anomalous dispersion glass of the third lens unit L 3 , the refractive powers of the lens surfaces need to be increased to some extent. When a cemented lens is not used but a single lens is used, the refractive index of the lens itself may be increased. When anomalous dispersion glass is used for a cemented lens, the correction effect is enhanced by increasing the curvature of the cemented lens surface (reducing the radius of curvature). Especially for the cemented lens surface, the secondary spectrum can be corrected without significantly causing higher order components of spherical aberration and comatic aberration. In addition, the curvature of the cemented lens surface can be increased to make the correction without considerably increasing the refractive power of the whole cemented lens. Thus, the refractive power of the lens made of anomalous dispersion glass can be increased without increasing the refractive power of the whole third lens unit L 3  more than necessary. For example, when the correction effect is enhanced by increasing the refractive power of any of a plurality of lenses made of anomalous dispersion glass in Embodiment 2, the use of such a cemented lens is effective.  
         [0177]    The positive lenses made of anomalous dispersion glass of the third lens unit L 3  disposed at positions somewhat away from the aperture stop SP in the optical axis direction are effective in correcting the secondary spectrum of chromatic aberration of magnification since off-axis principal rays are bent at positions away from the optical axis. For example, in Embodiment 2, both of the positive lenses G 34  and G 35  are disposed on the image side in the third lens unit L 3 . Especially the positive lens G 35  serves as a field lens together with the fourth lens unit L 4  at a zoom position on the wide-angle side to lead off-axis rays telecentrically. When off-axis rays are bent, primary achromatism in chromatic aberration of magnification can be achieved by using a negative lens made of a high dispersion glass material, but correction of a secondary spectrum is effectively realized by using anomalous dispersion glass for a positive lens. The positive lens G 35  has such an effect and particularly contributes to correction of the secondary spectrum of chromatic aberration of magnification at a zoom position on the wide-angle side.  
         [0178]    In Embodiment 2, the third lens unit L 3  includes two cemented lenses. When the third lens unit L 3  is moved to make a contribution to variable magnification, it is necessary to satisfactorily correct various types of aberration occurring in the third lens unit L 3  including variation components due to variations of magnification. When the third lens unit L 3  has a lateral magnification close to one, well-balanced correction can be made to various types of aberration by forming the third lens unit L 3  in a symmetric form. A triplet is a representative example as a symmetric lens arrangement. In Embodiment 2, the negative and positive lens components of the triplet are divided into two components to increase flexibility in correction of aberration, thereby more favorably correcting various types of aberration such as spherical aberration, comatic aberration and curvature of field.  
         [0179]    The zoom lens of Embodiment 2 achieves focusing by the fourth lens unit L 4  or the second lens unit L 2 . When the fourth lens unit L 4  is used for focusing, rear focusing is performed in which the relatively small and lightweight lens unit is moved as compared with focusing achieved by the front element. Thus, the rear focusing advantageously requires only a small driving force. Also, it is compatible with an autofocus system due to fast focusing. When the second lens unit L 2  is used for focusing, focus sensitivity is high at a zoom position on the telephoto side, so that the extending amount of the lens can be advantageously reduced to cause smaller variations in aberration for an object at a short distance.  
         [0180]    Next, description is made for characteristics other than the foregoing in each Embodiment.  
         [0181]    The third lens unit L 3  includes one or more positive lenses, and at least one positive lens satisfies the following conditions:  
         νd&gt;80 (7)  
         Θg, F&gt;0.530 (8)  
         [0182]    where νd is defined as:  
       vd   =       Nd   -   1       NF   -   NC                             
 
         [0183]    and Θg, F is defined as:  
         Θ                 g     ,     F   =       Ng   -   NF       NF   -   NC                               
 
         [0184]    where Nd, NF, NC, and Ng represent refractive indexes of the material for a d-line, an F-line, a C-line, and a g-line of Fraunhofer lines, vd the Abbe number of the material, and Θg, F the partial dispersion ratio.  
         [0185]    The conditional expressions (7) and (8) are provided for favorably correcting the secondary spectrum of axial chromatic aberration at a zoom position on the telephoto side. When primary achromatism has been performed on axial chromatic aberration at a zoom position on the telephoto side, overcorrection typically occurs on the short wavelength side such as the g-line to cause the focusing position for short wavelengths to be located beyond the focusing position for the reference wavelength. Generally, a glass medium tends to have a higher refractive index for a shorter wavelength. Thus, the displacement of the focusing position for the short wavelengths located beyond the focusing position for the reference wavelength is reduced by using a material with a much higher refractive index on the shorter wavelength side for a positive lens. A large partial dispersion ratio Θg, F, that is, a larger value of (Ng-NF) than the value of primary dispersion of (NF-NC) means that the difference in the refractive index between the F-line and the g-line is larger relative to the difference in the refractive index between the F-line and the C-line. When glass with a large partial dispersion ratio Θg, F is used for a positive lens, the displacement of the focusing position for the g-line beyond the focusing position is reduced. Thus, in a low dispersion range in which the conditional expression (7) is satisfied, the focusing position for short wavelengths is effectively brought close to the focusing position for the reference wavelength at a partial dispersion ratio which satisfies the conditional expression (8) to reduce the secondary spectrum. Since such anomalous dispersion characteristics are inadequate out of the ranges defined by the conditional expressions (7) and (8), the secondary spectrum is not corrected sufficiently.  
         [0186]    In the zoom lens of Embodiment 2, the effect of correcting the secondary spectrum is enhanced by limiting the conditional expression (7) to the following range:  
         νd&gt;90 (7 a)  
         [0187]    [0187]FIG. 43 is a graph showing the relationship between the Abbe number νd and the partial dispersion ratio (Θg, F. In FIG. 43, a point A represents a product with the trade name of PBM2 manufactured by Ohara Inc. (νd equal to 36.26 and Θg, F equal to 0.5828). A point B represents a product with the trade name of NSL7 manufactured by Ohara Inc. (νd equal to 60.49 and Θg, F equal to 0.5436). When the line connecting the point A with the point B is defined as a reference line, optical glass distribution is roughly seen such that, in many cases, high dispersion glass with an Abbe number νd of approximately 35 or smaller is positioned over the reference line, low dispersion glass with an Abbe number νd of approximately 35 to 65 is positioned under the reference line, and anomalous dispersion glass is positioned over the reference line at an Abbe number νd of 60 or larger. The use of low dispersion glass positioned over the reference line is effective in correcting the secondary spectrum, and the correction effect is enhanced as it is positioned away from the reference line. If the conditional expression (7) is limited to the range of the conditional expression (7 a), glass in a limited range further away from the reference line thereover in FIG. 43 is used to enhance the effect of correcting the secondary spectrum.  
         [0188]    The following conditions are satisfied:  
         1.0 &lt;f   3   a/f   3 &lt;3.0  (9)  
         0.3 &lt;f   3 / ft&lt; 0 . 5     (10)  
         [0189]    where f 3   a  represents a focal length of the positive lens with the largest Abbe number of the positive lenses of the third lens unit L 3 , f 3  a focal length of the third lens unit L 3 , and ft a focal length of the entire system at the telephoto end.  
         [0190]    The conditional expression (9) defines the refractive power of the positive lens with anomalous dispersion forming part of the third lens unit L 3 . A low refractive power of the positive lens, which causes the value of f 3   a /f 3  to exceed the upper limit, is not preferable since the effect of reducing the secondary spectrum by anomalous dispersion is diminished. In the zoom lens of Embodiment 2, the lens made of anomalous dispersion glass needs to have a certain refractive power. A high refractive power, which causes the value of f 3   a /f 3  to be less than the lower limit of the conditional-expression (9), is not preferable since extreme spherical aberration occurs on the underfocus side although the effect of reducing the secondary spectrum is enhanced.  
         [0191]    The conditional expression (10) defines the refractive power of the third lens unit L 3 . If the refractive power of the third lens unit L 3  is so low as to cause the value of f 3 /ft to exceed the upper limit, a negative lens with a high refractive power needs to be used for the third lens unit L 3  in order to increase the refractive power of the positive lens with anomalous dispersion of the third lens unit L 3 . The negative lens is made of relatively high dispersion glass for primary achromatism, which is detrimental to correction of the secondary spectrum. Thus, to achieve primary achromatism and enhance the effect of correcting the secondary spectrum in the third lens unit L 3 , it is preferable that the third lens unit L 3  has an increased refractive power to some extent, and in addition, the anomalous dispersion glass included therein has an increased refractive power. These conditions are difficult to satisfy if the value of f 3 /ft in the conditional expression (10) exceeds the upper limit. On the other hand, if the refractive power is so high as to cause the value of f 3 /ft to be less than the lower limit, large variations occur in aberration such as spherical aberration and comatic aberration in the third lens unit L 3  during zooming to cause difficulty in maintaining favorable performance over the entire zoom range.  
         [0192]    More preferably, the numerical values in the conditional expressions (9) and (10) are set as follows:  
         1.2 &lt;f   3   a/f   3 &lt;2.6  (9 a)  
         0.33 &lt;f   3 / ft&lt; 0.46  (10 a)  
         [0193]    The aperture stop SP is provided on the object side of the third lens unit L 3 . The following condition is satisfied:  
         1.8 &lt;L   3   a/fw&lt; 3.0  (11)  
         [0194]    where L 3   a  represents a distance on the optical axis from the aperture stop SP to the lens surface on the image side of the positive lens closest to the image among the positive lenses made of a material with an Abbe number larger than 80 in the third lens unit L 3 , and fw a focal length of the entire system at the wide-angle end.  
         [0195]    The conditional expression (11) defines the position of the positive lens with anomalous dispersion forming part of the third lens unit L 3  from the aperture stop SP at the wide-angle end zoom position. If the distance of the positive lens from the aperture stop SP is so large as to cause the value of L 3   a /fw to exceed the upper limit, the lens diameter is increased. On the other hand, a small distance of the positive lens from the aperture stop SP, which causes the value of L 3   a /fw to be less than the lower limit, is not preferable since the correction effect of the secondary spectrum of chromatic aberration of magnification is diminished at the wide-angle end zoom position.  
         [0196]    More preferably, the numerical values in the conditional expression (11) are set as follows:  
         2.0 &lt;L   3   a/fw&lt; 2.9 ( 11   a )  
         [0197]    The first lens unit L 1  and the third lens unit L 3  are moved towards the object in zooming from the wide-angle end to the telephoto end. The following condition is satisfied:  
         0.2&lt;(β3 t /β3 w )/( ft/fw )&lt;0.4  (12)  
         [0198]    where β3 w and β3 t represent magnifications of the third lens unit L 3  at the wide-angle end and telephoto end, respectively, and fw and ft focal lengths of the entire system at the wide-angle end and telephoto end, respectively.  
         [0199]    The conditional expression (12) defines the contribution to variable magnification of the third lens unit L 3 . If the contribution to variable magnification of the third lens unit L 3  is so large as to cause the value of (β3 t/β3 w)/(ft/fw) to exceed the upper limit, large variations occur in aberration such as spherical aberration, comatic aberration, and astigmatism in the third lens unit during zooming to make it difficult to provide favorable optical performance over the entire zoom range. On the other hand, a small contribution to variable magnification of the third lens unit L 3 , which causes the value of (β3 t/β3 w)/(ft/fw) to be less than the lower limit, is not preferable since it is necessary to increase the spacing between the first lens unit L 1  and the second lens unit L 2  at the telephoto end zoom position to ensure the zoom ratio of the entire system, resulting in an increase in the overall length of the zoom lens.  
         [0200]    More preferably, the numerical values in the conditional expression (12) are set as follows:  
         0.23&lt;(β3 t /β3 w )/( ft/fw )&lt;0.35  (12 a)  
         [0201]    The following condition is satisfied:  
         0.4 &lt;|R   3   c|f   3 &lt;0.6  (13)  
         [0202]    where R 3   c  represents a radius of curvature of the cemented lens surface of the positive lens in the cemented lens forming part of the third lens unit L 3 , and f 3  a focal length of the third lens unit L 3 .  
         [0203]    The conditional expression (13) defines the curvature of the cemented lens surface of the cemented lens including the positive lens with anomalous dispersion forming part of the third lens unit L 3 . If the radius of curvature of the cemented lens surface is so large as to cause the value of |R 3   c |/f 3  to exceed the upper limit, that is, when the curvature is too small, the effect of reducing the secondary spectrum by anomalous dispersion is diminished. On the other hand, if the radius of curvature of the cemented lens surface is so small as to cause the value of |R 3   c |/f 3  to be less than the lower limit, that is, the curvature is too large, the effect of reducing the secondary spectrum is enhanced, but higher order components of spherical aberration and comatic aberration cannot be ignored even in the cemented lens surface to make the correction difficult.  
         [0204]    More preferably, the numerical values in the conditional expression (13) are set as follows:  
         0.42  |R   3   c|/f   3 &lt;0.59  (13 a)  
         [0205]    Next, numerical data in Numerical Examples 6 to 10 are shown. In Embodiment 2, the partial dispersion ratio Θg, F is also shown in addition to the refractive index Ni and the Abbe number νi as parameters of an optical material.  
       NUMERICAL EXAMPLE 6 
       [0206]    [0206]                                                                                                                             f = 1˜6.76 Fno = 2.45˜3.39 2ω = 74.2°˜12.8°                                    R1 = 8.373   D1 = 0.24   N1 = 1.846660   ν1 = 23.9   Θg, F1 = 0.610           R2 = 5.321   D2 = 0.72   N2 = 1.603112   ν2 = 60.6   Θg, F2 = 0.542           R3 = 60.225   D3 = 0.03           R4 = 6.229   D4 = 0.43   N3 = 1.603112   ν3 = 60.6   Θg, F3 = 0.542           R5 = 19.667   D5 = Variable           R6 = 8.261   D6 = 0.15   N4 = 1.772499   ν4 = 49.6   Θg, F4 = 0.552           R7 = 1.310   D7 = 0.62           R8 = −7.767   D8 = 0.12   N5 = 1.712995   ν5 = 53.9   Θg, F5 = 0.546           R9 = 2.874   D9 = 0.18           R10 = 4.280   D10 = 0.43   N6 = 1.846660   ν6 = 23.9   Θg, F6 = 0.610           R11 = −3.407   D11 = 0.06           R12 = −2.237   D12 = 0.12   N7 = 1.882997   ν7 = 40.8   Θg, F7 = 0.567           R13 = −7.136   D13 = Variable           R14 = Stop   D14 = 0.11           R15 = 1.448   D15 = 0.54   N8 = 1.743300   ν8 = 49.3   Θg, F8 = 0.553           R16 = −22.067   D16 = 0.54   N9 = 1.647689   ν9 = 33.8   Θg, F9 = 0.594           R17 = 1.292   D17 = 0.19           R18 = 10.762   D18 = 0.11   N10 = 1.603420   ν10 = 38.0   Θg, F10 = 0.584           R19 = 1.382   D19 = 0.62   N11 = 1.496999   ν11 = 81.5   Θg, F11 = 0.538*           R20 = −3.599   D20 = 0.27           R21 = 2.462   D21 = 0.41   N12 = 1.433870   ν12 = 95.1   Θg, F12 = 0.537*           R22 = −19.831   D22 = Variable           R23 = 3.293   D23 = 0.38   N13 = 1.77249   ν13 = 49.6   Θg, F13 = 0.552           R24 = −11.26   D24 = 0.12   N14 = 1.846660   ν14 = 23.9   Θg, F14 = 0.61           R25 = 32.531   D25 = 0.27           R26 = ∞   D26 = 0.32   N15 = 1.516330   ν15 = 64.1   Θg, F15 = 0.535           R27 = ∞                            Variable   Focal length                spacing   1.00   2.21   6.76                       D5   0.20   2.00   4.46           D13   2.96   1.40   0.35           D22   0.41   1.71   3.64                        Aspheric coefficient                    R10   k = −2.36901e+00   B = 2.48109e−02   C = −9.68342e−03   D = 3.45879e−03   E = −1.21645e−02       R11   k = 1.26563e+00   B = 4.66823e−04   C = −5.96445e−03   D = −1.03149e−02   E = −1.44692e−03       R15   k = −4.68634e−01   B = −7.76053e−03   C = 7.44031e−04   D = 0.00000e+00   E = 0.00000e+00                    
       NUMERICAL EXAMPLE 7 
       [0207]    [0207]                                                                                                                             f = 1˜6.72Fno = 2.45˜3.612ω = 74.2°˜12.8°                                    R1 = 8.284   D1 = 0.24   N1 = 1.846660   ν1 = 23.9   Θg, F1 = 0.610           R2 = 5.299   D2 = 0.72   N2 = 1.603112   ν2 = 60.6   Θg, F2 = 0.542           R3 = 57.734   D3 = 0.03           R4 = 6.373   D4 = 0.43   N3 = 1.603112   ν3 = 60.6   Θg, F3 = 0.542           R5 = 22.15   D5 = Variable           R6 = 8.435   D6 = 0.15   N4 = 1.772499   ν4 = 49.6   Θg, F4 = 0.552           R7 = 1.290   D7 = 0.65           R8 = −6.665   D8 = 0.12   N5 = 1.712995   ν5 = 53.9   Θg, F5 = 0.546           R9 = 2.549   D9 = 0.20           R10 = 4.066   D10 = 0.43   N6 = 1.846660   ν6 = 23.9   Θg, F6 = 0.610           R11 = −3.351   D11 = 0.06           R12 = −2.205   D12 = 0.12   N7 = 1.88299   ν7 = 40.8   Θg, F7 = 0.567           R13 = −6.318   D13 = Variable           R14 = Stop   D14 = 0.11           R15 = 1.472   D15 = 0.54   N8 = 1.743300   ν8 = 49.3   Θg, F8 = 0.553           R16 = 42.174   D16 = 0.54   N9 = 1.698947   ν9 = 30.1   Θg, F9 = 0.603           R17 = 1.380   D17 = 0.15           R18 = 4.552   D18 = 0.11   N10 = 1.603420   ν10 = 38.0   Θg, F10 = 0.584           R19 = 1.606   D19 = 0.62   N11 = 1.455999   ν11 = 90.3   Θg, F11 = 0.534*           R20 = −3.059   D20 = 0.27           R21 = 2.754   D21 = 0.35   N12 = 1.433870   ν12 = 95.1   Θg, F12 = 0.537*           R22 = 26.997   D22 = Variable           R23 = 2.789   D23 = 0.38   N13 = 1.696797   ν13 = 55.5   Θg, F13 = 0.543           R24 = −14.208   D24 = 0.12   N14 = 1.728250   ν14 = 28.5   Θg, F14 = 0.608           R25 = 17.535   D25 = 0.27           R26 = ∞   D26 = 0.32   N15 = 1.516330   ν15 = 64.1   Θg, F15 = 0.535           R27 = ∞                            Variable   Focal length                spacing   1.00   2.14   6.72                       D5   0.20   1.95   4.50           D13   2.85   1.48   0.51           D22   0.44   1.74   3.67                        Aspheric coefficient                    R10   k = 7.84930e+00   B = 6.56602e−03   C = 2.69281e−03   D = −3.54250e−02   E = 1.33648e−02       R11   k = −1.19232e+00   B = −7.92173e−03   C = 8.07633e−03   D = −4.31266e−02   E = 2.39379e−02       R15   k = −4.68634e−01   B = −7.78692e−03   C = 7.48254e−04   D = 0.00000e+00   E = 0.00000e+00                    
       NUMERICAL EXAMPLE 8 
       [0208]    [0208]                                                                                                                             f = 1˜6.72Fno = 2.45˜3.612ω = 74.2°˜12.8°                                    R1 = 8.284   D1 = 0.24   N1 = 1.846660   ν1 = 23.9   Θg, F1 = 0.610           R2 = 5.299   D2 = 0.72   N2 = 1.603112   ν2 = 60.6   Θg, F2 = 0.542           R3 = 57.734   D3 = 0.03           R4 = 6.373   D4 = 0.43   N3 = 1.603112   ν3 = 60.6   Θg, F3 = 0.542           R5 = 22.156   D5 = Variable           R6 = 8.435   D6 = 0.15   N4 = 1.772499   ν4 = 49.6   Θg, F4 = 0.552           R7 = 1.290   D7 = 0.65           R8 = −6.665   D8 = 0.12   N5 = 1.712995   ν5 = 53.9   Θg, F5 = 0.546           R9 = 2.549   D9 = 0.20           R10 = 4.066   D10 = 0.43   N6 = 1.846660   ν6 = 23.9   Θg, F6 = 0.610           R11 = −3.351   D11 = 0.06           R12 = −2.205   D12 = 0.12   N7 = 1.882997   ν7 = 40.8   Θg, F7 = 0.567           R13 = −6.318   D13 = Variable           R14 = Stop   D14 = 0.11           R15 = 1.472   D15 = 0.54   N8 = 1.743300   ν8 = 49.3   Θg, F8 = 0.553           R16 = 42.174   D16 = 0.54   N9 = 1.698947   ν9 = 30.1   Θg, F9 = 0.594           R17 = 1.380   D17 = 0.15           R18 = 4.552   D18 = 0.11   N10 = 1.603420   ν10 = 38.0   Θg, F10 = 0.584           R19 = 1.606   D19 = 0.62   N11 = 1.455999   ν11 = 90.3   Θg, F11 = 0.538*           R20 = −3.059   D20 = 0.27           R21 = 2.754   D21 = 0.35   N12 = 1.433870   ν12 = 95.1   Θg, F12 = 0.537*           R22 = 26.997   D22 = Variable           R23 = 2.789   D23 = 0.38   N13 = 1.696797   ν13 = 55.5   Θg, F13 = 0.552           R24 = −14.208   D24 = 0.12   N14 = 1.728250   ν14 = 28.5   Θg, F14 = 0.610           R25 = 17.535   D25 = 0.27           R26 = ∞   D26 = 0.32   N15 = 1.516330   ν15 = 64.1   Θg, F15 = 0.535           R27 = ∞                            Variable   Focal length                spacing   1.00   2.14   6.72                       D5   0.20   1.95   4.50           D13   2.85   1.48   0.51           D22   0.44   1.74   3.67                        Aspheric coefficient                    R10   k = 7.84930e+00   B = 6.56602e−03   C = 2.69281e−03   D = −3.54250e−02   E = 1.33648e−02       R11   k = −1.19232e+00   B = −7.92173e−03   C = 8.07633e−03   D = −4.31266e−02   E = 2.39379e−02       R15   k = −4.68634e−01   B = −7.78692e−03   C = 7.48254e−04   D = 0.00000e+00   E = 0.00000e+00                    
       NUMERICAL EXAMPLE 9 
       [0209]    [0209]                                                                                                                             f = 1˜6.98Fno = 2.83˜4.292ω = 64.4°˜10.3°                                    R2 = 4.268   D2 = 0.52   N2 = 1.603112   ν2 = 60.6   Θg, F2 = 0.542           R3 = 76.821   D3 = 0.02           R4 = 5.371   D4 = 0.32   N3 = 1.603112   ν3 = 60.6   Θg, F3 = 0.542           R5 = 16.265   D5 = Variable           R6 = 4.985   D6 = 0.12   N4 = 1.772499   ν4 = 49.6   Θg, F4 = 0.552           R7 = 1.032   D7 = 0.41           R8 = −7.497   D8 = 0.10   N5 = 1.712995   ν5 = 53.9   Θg, F5 = 0.546           R9 = 2.499   D9 = 0.11           R10 = 3.293   D10 = 0.36   N6 = 1.846660   ν6 = 23.9   Θg, F6 = 0.610           R11 = −2.648   D11 = 0.03           R12 = −1.697   D12 = 0.10   N7 = 1.882997   ν7 = 40.8   Θg, F7 = 0.567               R13 = −6.832   D13 = Variable           R14 = Stop   D14 = 0.09           R15 = 1.187   D15 = 0.45   N8 = 1.743300   ν8 = 49.3   Θg, F8 = 0.553           R16 = −18.097   D16 = 0.45   N9 = 1.647689   ν9 = 33.8   Θg, F9 = 0.594           R17 = 1.033   D17 = 0.16           R18 = 4.821   D18 = 0.09   N10 = 1.620041   ν10 = 36.3   Θg, F10 = 0.588           R19 = 1.424   D19 = 0.52   N11 = 1.455999   ν11 = 90.3   Θg, F11 = 0.534*           R20 = −3.144   D20 = 0.23           R21 = 2.110   D21 = 0.34   N12 = 1.433870   ν12 = 95.1   Θg, F12 = 0.537*           R22 = −13.275   D22 = Variable           R23 = 3.173   D23 = 0.32   N13 = 1.772499   ν13 = 49.6   Θg, F13 = 0.552           R24 = −8.027   D24 = 0.10   N14 = 1.846660   ν14 = 23.9   Θg, F14 = 0.610           R25 = 60.072   D25 = 0.23           R26 = ∞   D26 = 0.27   N15 = 1.516330   ν15 = 64.1   Θg, F15 = 0.535           R27 = ∞                            Variable   Focal length                spacing   1.00   2.03   6.98                       D5   0.17   1.60   3.90           D13   2.27   1.25   0.29           D22   0.69   1.69   3.29                        Aspheric coefficient                    R10   k = −1.93131e+01   B = 5.98657e−02   C = −5.06889e−02   D = −1.65751e−01   E = −4.12139e−02       R11   k = 2.59883e+00   B = −3.36767e−02   C = −1.31189e−02   D = −2.03192e−01   E = 5.42095e−02       R15   k = −4.68634e−01   B = −1.34349e−02   C = 1.85707e−03   D = 0.00000e+00   E = 0.00000e+00                    
       NUMERICAL EXAMPLE 10 
       [0210]    [0210]                                                                                                                             f = 1˜6.73 Fno = 2.45˜3.37 2 ω = 74.2°˜12.8°                                    R1 = 8.901   D1 = 0.24   N1 = 1.846660   ν1 = 23.9   Θg, F1 = 0.610           R2 = 5.482   D2 = 0.72   N2 = 1.603112   ν2 = 60.6   Θg, F2 = 0.542           R3 = 78.169   D3 = 0.03           R4 = 6.265   D4 = 0.43   N3 = 1.603112   ν3 = 60.6   Θg, F3 = 0.542           R5 = 20.239   D5 = Variable           R6 = 8.498   D6 = 0.15   N4 = 1.772499   ν4 = 49.6   Θg, F4 = 0.552           R7 = 1.322   D7 = 0.58           R8 = −14.403   D8 = 0.12   N5 = 1.712995   ν5 = 53.9   Θg, F5 = 0.546           R9 = 2.823   D9 = 0.15           R10 = 4.397   D10 = 0.43   N6 = 1.846660   ν6 = 23.9   Θg, F6 = 0.610           R11 = −3.570   D11 = 0.07           R12 = −2.099   D12 = 0.12   N7 = 1.882997   ν7 = 40.8   Θg, F7 = 0.567           R13 = −6.241   D13 = Variable           R14 = Stop   D14 = 0.11           R15 = 1.457   D15 = 0.54   N8 = 1.743300   ν8 = 49.3   Θg, F8 = 0.553           R16 = 38.450   D16 = 0.54   N9 = 1.647689   ν9 = 33.8   Θg, F9 = 0.594           R17 = 1.354   D17 = 0.19           R18 = 7.487   D18 = 0.11   N10 = 1.603420   ν10 = 38.0   Θg, F10 = 0.584           R19 = 1.400   D19 = 0.62   N11 = 1.438750   ν11 = 95.0   Θg, F11 = 0.534*           R20 = −3.120   D20 = 0.27           R21 = 2.491   D21 = 0.41   N12 = 1.487490   ν12 = 70.2   Θg, F12 = 0.530           R22 = −72.068   D22 = Variable           R23 = 3.504   D23 = 0.38   N13 = 1.772499   ν13 = 49.6   Θg, F13 = 0.552           R24 = −6.057   D24 = 0.12   N14 = 1.846660   ν14 = 23.9   Θg, F14 = 0.610           R25 = 51.349   D25 = 0.27           R26 = ∞   D26 = 0.32   N15 = 1.516330   ν15 = 64.1   Θg, F15 = 0.535           R27 = ∞                            Variable   Focal length                spacing   1.00   2.24   6.73                       D5   0.20   2.06   4.53           D13   3.06   1.45   0.40           D22   0.28   1.59   3.51                        Aspheric coefficient                    R10   κ = 3.97923e+00   B = 1.51786e−02   C = −1.99898e−02   D = 2.62113e−03   E = −7.06732e−03       R11   κ = −4.91450e−01   B = −8.91258e−03   C = −1.67061e−02   D = −1.27744e−02   E = 5.98699e−03       R15   κ = −4.68634e−01   B = −7.76890e−03   C = 7.45369e−04   D = 0.00000e+00   E = 0.00000e+00                    
         [0211]    [0211]                                   TABLE 2                           Numerical   Numerical   Numerical   Numerical   Numerical           example 7   example 8   example 9   example 10   example 11                   Conditional Expression (7), (8)   81.5, 0.538   90.3, 0.534   81.5, 0.538   90.3, 0.534   95.0, 0534       Conditional Expression (7), (8)   95.1, 0.537   95.1, 0.537   95.1, 0.537   95.1, 0.537   —       Conditional Expression (9)   1.719   2.401   1.482   1.731   1.678       Conditional Expression (10)   0.437   0.436   0.442   0.350   0.438       Conditional Expression (11)   2.788   2.686   2.788   2.322   2.111       Conditional Expression (12)   0.310   0.297   0.300   0.280   0.316       Conditional Expression (13)   0.468   0.548   0.441   0.583   0.475                    
         [0212]    (Embodiment of Camera)  
         [0213]    Next, description is made for Embodiment of a digital still camera (an image-taking apparatus) which has the zoom lens described in Embodiments 1 and 2 with reference to FIG. 44.  
         [0214]    [0214]FIG. 44A is a front view of the digital still camera, and FIG. 44B shows a cross section thereof. In FIGS. 44A and 44B, reference numeral 10 shows a camera body (a housing),  11  an image-taking optical system which employs any of the zoom lenses of Embodiments 1 and 2, 12 a viewfinder optical system, and 13 a solid-state image-pickup device (a photoelectrical conversion element) such as a CCD sensor or a CMOS sensor. The solid-state image-pickup device  13  receives an image of an object formed by the image-taking optical system  11  and converts it into electric information. The image information of the object converted into electric information is recorded in a storing section, not shown.  
         [0215]    The zoom lenses described in Embodiments 1 and 2 can be applied to the image-taking optical system of the digital still camera to realize a compact image-taking apparatus.  
         [0216]    While preferred embodiments have been described, it is to be understood that modification and variation of the present invention may be made without departing from scope of the following claims.