Abstract:
A zoom lens system is disclosed, comprising, in order from an object side to an image side, a first lens unit of positive optical power, a second lens unit of negative optical power, a third lens unit of positive optical power, and a fourth lens unit of positive optical power, wherein the second and fourth lens units are moved during zooming. The third lens unit consists of a first lens subunit of positive optical power, an aperture stop, and a second lens subunit having one or more negative lens elements and one or more positive lens elements, in order from the object to the image sides. Optical parameters including the focal length of each lens unit, lens shapes, the lens structure of each lens unit are specified to realize a zoom lens system with a smaller size of the entire lens system and higher optical performance.

Description:
BACKGROUND OF THE INVENTION 
   Field of the Invention 
   The present invention relates to a zoom lens preferable for use in an electronic camera such as a video camera and a digital still camera, a film camera, a broadcasting camera and the like. 
   Recently, as enhanced functionality and a smaller size are achieved in video cameras, digital still cameras, personal digital assistants (PDAs), cellular phones, and conventional photographic cameras which use image-pickup elements, both of higher optical performance and a smaller size are needed in optical systems used in such optical apparatuses. 
   A known zoom lens used in such optical apparatuses has, in order from an object side, a first lens unit immovable during zooming and having a positive refractive power, a second lens unit moving for varying magnification and having a negative refractive power, a third lens unit immovable during zooming and having a positive refractive power, and a fourth lens unit moving on an optical axis for maintaining an image plane position changeable due to a variation of magnification and having a positive refractive power (for example, see Patent Documents 1 to 3). 
   Another known zoom lens has four lens units, that is, in order from an object side, a first lens unit having a positive refractive power, a second lens unit having an aspheric surface and a negative refractive power, a third lens unit having an aspheric surface and a positive refractive power, and a fourth lens unit having an aspheric surface and a positive refractive power. The second lens unit is moved to provide variable magnification, and the fourth lens unit is moved to correct image plane variations due to a variation of magnification and to achieve focusing (for example, see Patent Document 4). 
   (Patent Document 1) Japanese Patent Application Laid-Open No. H07(1995)-270684 (corresponding to U.S. Pat No. 5,963,378) 
   (Patent Document 2) 
   Japanese Patent Application Laid-Open No. H07(1995)-318804 (corresponding to U.S. Pat. No. 5,963,378) 
   (Patent Document 3) 
   Japanese Patent Application Laid-Open No. H11(1999)-305124 (corresponding to U.S. Pat. No. 6,166,864) 
   (Patent Document 4) 
   Japanese Patent Application Laid-Open No. H08(1996)-292369 (corresponding to U.S. Pat. No. 5,940,221) 
   In recent years, with extremely smaller pixels of image-pickup elements, a small zoom lens achieving both of high optical performance and a small overall length is desired for use in optical apparatuses such as digital cameras, video cameras, and PDAs. In addition, for higher image quality in video cameras, recording of still images is needed, and a lens system having high optical performance and a small size at the same time is required. 
   Generally, in a zoom lens, increasing a refractive power of each lens unit reduces an amount of movement of each lens unit for providing a predetermined zoom ratio to allow a reduction in the overall length of the lenses. 
   Simply increasing a refractive power of each lens unit, however, presents a problem that larger variations in aberration occur in association with zooming to cause difficulty in providing favorable optical performance over the entire zoom range. Also, exacting tolerances are needed in manufacture and thus mass production is difficult. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a zoom lens system of new structure which has a smaller size of the entire lens system and higher optical performance. 
   According to one aspect, the present invention provides a zoom lens system which comprises, in order from an object side to an image side, a first lens unit having a positive optical power, a second lens unit having a negative optical power, a third lens unit having a positive optical power, and a fourth lens unit having a positive optical power, wherein the second lens unit and the fourth lens unit are moved during zooming. The third lens unit of the zoom lens system consists of a first lens subunit having a positive optical power, an aperture stop, and a second lens subunit having one or more negative lens elements and one or more positive lens elements, in order from the object side to the image side. In the zoom lens system, optical parameters such as the focal length of each lens unit, lens shapes, the lens structure of each lens unit are specified to realize the aforementioned object. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a section view of lenses of a zoom lens of Embodiment 1; 
       FIG. 2  shows various types of aberration in the zoom lens of Embodiment 1 at the wide-angle end; 
       FIG. 3  shows various types of aberration in the zoom lens of Embodiment 1 at an intermediate zoom position; 
       FIG. 4  shows various types of aberration in the zoom lens of Embodiment 1 at the telephoto end; 
       FIG. 5  is a section view of lenses of a zoom lens of Embodiment 2; 
       FIG. 6  shows various types of aberration in the zoom lens of Embodiment 2 at the wide-angle end; 
       FIG. 7  shows various types of aberration in the zoom lens of Embodiment 2 at an intermediate zoom position; 
       FIG. 8  shows various types of aberration in the zoom lens of Embodiment 2 at the telephoto end; 
       FIG. 9  is a section view of lenses of a zoom lens of Embodiment 3; 
       FIG. 10  shows various types of aberration in the zoom lens of Embodiment 3 at the wide-angle end; 
       FIG. 11  shows various types of aberration in the zoom lens of Embodiment 3 at an intermediate zoom position; 
       FIG. 12  shows various types of aberration in the zoom lens of Embodiment 3 at the telephoto end; and 
       FIG. 13  is a schematic diagram showing main portions of a video camera. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In the following, description is made for embodiments of a zoom lens system of the present invention and an image-taking apparatus using the zoom lens system as an image-taking optical system with reference to the drawings. 
     FIG. 1  is a section view of lenses of a zoom lens of Embodiment 1 at the wide-angle end.  FIGS. 2 to 4  show various types of aberration in the zoom lens of Embodiment 1 at the wide-angle end, intermediate zoom position, and telephoto end, respectively. 
     FIG. 5  is a section view of lenses of a zoom lens of Embodiment 2 at the wide-angle end.  FIGS. 6 to 8  show various types of aberration in the zoom lens of Embodiment 2 at the wide-angle end, intermediate zoom position, and telephoto end, respectively. 
     FIG. 9  is a section view of lenses of a zoom lens of Embodiment 3 at the wide-angle end.  FIGS. 10 to 12  show various types of aberration in the zoom lens of Embodiment 3 at the wide-angle end, intermediate zoom position, and telephoto end, respectively. 
   In each of the section views of the lenses of  FIGS. 1 ,  5 , and  9 , L 1  shows a first lens unit having a positive refractive power (that is, an optical power which is the reciprocal of a focal length), L 2  shows a second lens unit having a negative refractive power, and L 3  shows a third lens unit having a positive refractive power. The third lens unit L 3  has a first lens subunit L 3   a  having a positive refractive power and a second lens subunit L 3   b  having a positive refractive power. L 4  shows a fourth lens unit having a positive refractive power. SP shows an aperture stop which is disposed between the first lens subunit L 3   a  and the second lens subunit L 3   b . G shows a glass block provided in design and corresponding to a color splitting prism, a face plate, or various optical filters. IP shows an image plane at which a solid-state image-pickup element such as a CCD sensor and a CMOS sensor is located. 
   In the aberration diagrams of  FIGS. 2 to 4 ,  FIGS. 6 to 8 , and  FIGS. 10 to 12 , d and g show a d-line and a g-line, respectively. ΔM and ΔS show a meridional image plane and a sagittal image plane, respectively. The chromatic aberration of magnification is represented by the g-line. In addition, fno represents an F number, and ω represents half of the field angle. 
   In each of the zoom lenses of Embodiments 1 to 3, the second lens unit L 2  is moved toward the image side to provide variable magnification as shown by an arrow during zooming from the wide-angle end to the telephoto end. In addition, the fourth lens unit L 4  is moved to have part of a convex track toward the object side to correct image plane variations associated with variations of magnification. 
   The zoom lenses of Embodiments 1 to 3 employ a rear focusing scheme in which the fourth lens unit L 4  is moved on an optical axis to achieve focusing. For example, to achieve focusing on an object at a short distance from an object at infinity at the telephoto end, the fourth lens unit L 4  is moved forward as shown by a line  4   c  in each of  FIGS. 1 ,  5 , and  9 . A solid curve line  4   a  and a dotted curve line  4   b  showing tracks of movement of the fourth lens unit L 4  in each of  FIGS. 1 ,  5 , and  9  represent tracks of movement thereof for correcting image plane variations associated with variations of magnification from the wide-angle end to the telephoto end when the zoom lens is focused on an object at infinity and an object at a short distance, respectively. The fourth lens unit L 4  is moved to have part of the convex track toward the object side to effectively use the space between the third lens unit L 3  and the fourth lens unit L 4  to advantageously achieve a reduction in the overall length of the zoom lens system. The movement track of the fourth lens unit L 4  depends on an object distance. 
   It should be noted that, while the first lens unit L 1  and the third lens unit L 3  are not moved during zooming and focusing in each of the zoom lenses of Embodiments 1 to 3, they may be moved as required. 
   Next, specific characteristics of the zoom lens systems of Embodiments 1 to 3 are described. 
   The following conditional expression is satisfied:
 
0.05 &lt;|f 2 /ft |&lt;0.081  (1)
 
where ft represents a focal length of the entire system at the telephoto end, and f 2  represents a focal length of the second lens unit L 2 .
 
   The conditional expression (1) is provided for the following reasons. When a reduced size of the entire lens system is intended in the zoom type lens of the aforementioned structure, it is necessary to reduce the focal length of the second lens unit L 2 . Simply reducing the focal length, however, places a significant burden of the refractive power on the second lens unit L 2  to cause difficulty in maintaining favorable optical performance. On the other hand, when the focal length at the telephoto end is large, aberration of the second lens unit L 2  at the telephoto end has a great influence. Thus, the focal length (the refractive power) of the second lens unit L 2  is set to the range defined by the conditional expression (1) as above to satisfactorily correct off-axis optical performance, especially flare. 
   More preferably, the numerical range of the conditional expression (1) may be set as follows:
 
0.06 &lt;|f 2 /ft |&lt;0.078  (1a)
 
   In addition, one or more of the following conditional expressions are satisfied:
 
0.34 &lt;f 1 /ft &lt;0.57  (2)
 
0.22 &lt;|f 2 /fA |&lt;0.34  (3)
 
0.74 &lt;f 3 /f 4&lt;1.2  (4)
 
0.48&lt;|β4 t |&lt;0.71  (5)
 
where fw and ft represent focal lengths of the entire system at the wide angle end and the telephoto end, respectively, fi represents a focal length of an i-th lens unit, β 4   t  represents an image-forming magnification of the fourth lens unit L 4  when the zoom lens is focused on an object at infinity at the telephoto end, and fA is represented by:
 
 fA=√{square root over (fw·ft)} 
 
   It should be noted that the wide-angle end and the telephoto end refer to zoom positions when a lens unit for variable magnification (the second lens unit L 2  in Embodiments 1 to 3) is positioned at two ends of a mechanically movable range on an optical axis. 
   The conditional expressions (2) to (5) are provided mainly for reducing the overall length of the zoom lens system and maintaining favorable optical performance. 
   The conditional expression (2) is provided for setting a proper focal length of the first lens unit L 1 . The first lens unit L 1  has a great influence on aberration on the telephoto end side. Thus, if the focal length of the first lens unit L 1  is so large as to result in the value of f 1 /ft larger than the upper limit of the conditional expression (1), the focal length of the entire lens system is large to cause difficulty in ensuring a desired focal length. On the other hand, a small value of f 1 /ft less than the lower limit of the conditional expression (1) is not preferable since the first lens unit L 1  is heavily burdened to prevent achievement of favorable optical performance, especially spherical aberration and chromatic aberration at the telephoto end. 
   The conditional expression (3) is provided for setting a proper focal length of the second lens unit L 2  (in other words, a power which is the reciprocal of the focal length). A larger focal length of the second lens unit L 2  which results in the value of |f 2 /fA| than the upper limit of the conditional expression (3) is preferable in correcting aberration, but is not preferable since the amount of movement of the second lens unit L 2  must be increased in order to provide a desired zoom ratio, leading to an increased size of the entire lens system. On the other hand, if the lower limit is not reached, the Petzval sum becomes a negative large number to incline the image plane, thereby making it difficult to maintain satisfactory optical performance. 
   The conditional expression (4) relates to optimal distribution of refractive powers to reduce the size of the third lens unit L 3  and the fourth lens unit L 4  which form an image-forming system. Especially, the conditional expression (4) is provided for causing a luminous flux emerging from the third lens unit L 3  to be incident on the fourth lens unit L 4  substantially in an afocal manner and for ensuring an optimal back focal distance when an optimal spacing is set between the third lens unit L 3  and the fourth lens unit L 4 . 
   If a value of f 3 /f 4  exceeds the upper limit of the conditional expression (4), the luminous flux emerging from the third lens unit L 3  deviates from the afocal state to increase the size of the fourth lens unit L 4 . In addition, variations in aberration are disadvantageously increased in association with the movement of the fourth lens unit L 4 . On the other hand, if the lower limit is not reached, the refractive power of the fourth lens unit L 4  is low to increase the amount of movement of the fourth lens unit L 4  for focusing to result in an increased overall length of the zoom lens system. 
   The conditional expression (5) is provided for reducing the distances from the third lens unit L 3  to the fourth lens unit L 4 , which form the image-forming system, and the image plane. If a value of |β 4 t| exceeds the upper limit of the conditional expression (5), the back focal distance is extremely small to interfere with a filter member such as an optical low pass filter and an infrared cut filter or the solid-state image-pickup element disposed on the image plane. On the other hand, a small value of |β 4   t | less than the lower limit of the conditional expression (5) is not preferable since the back focal distance is extremely large to increase the overall length of the zoom lens system. 
   More preferably, the numerical ranges of the conditional expressions (2) to (5) may be set as follows:
 
0.36 &lt;f 1 /ft &lt;0.50  (2a)
 
0.24 &lt;|f 2 /fA |&lt;0.32  (3a)
 
0.80 &lt;f 3 /f 4   &lt;1.0  (4a)
 
0.485&lt;|β4 t |&lt;0.65  (5a)
 
   The second lens subunit L 3  b has a lens surface closest to the image side which is a convex surface toward the image side (a convex shape), and the following conditional expression is satisfied:
 
0.99 &lt;|R 3 bL/f 3|&lt;12.5  (6)
 
where R 3 bL represents the radius of curvature of the convex surface of the second lens subunit L 3   b , and f 3  represents a focal length of the third lens unit L 3 .
 
   The conditional expression (6) is provided for limiting the shape of the lens surface closest to the image side of the second lens subunit L 3   b , in which the lens surface is formed in the convex shape toward the image side and the radius of curvature thereof is limited. When the zoom lens of each of Embodiments 1 to 3 is applied to an image-taking apparatus using a solid-state image-pickup element on an image plane, the reflectivity is relatively high on the surface of the solid-state image-pickup element and this often causes ghosts or flare. Forming the lens surface in the convex shape can diverge reflected light from the solid-state image-pickup element to reduce the amount of light incident on the image plane. In addition, spherical aberration on the wide-angle end side can be more favorably corrected by satisfying the conditional expression (6). 
   More preferably, the numerical range of the conditional expression (6) may be set as follows:
 
1.2 &lt;|R 3 bL/f 3|&lt;9.2  (6a)
 
   The second lens unit L 2  is formed of three or more negative lenses and one or more positive lenses. The third lens unit L 3  is formed of the first lens subunit L 3   a  having the positive refractive power, the aperture stop, and the second lens subunit L 3   b  having one or more negative lenses and one or more positive lenses, in order from the object side. 
   In each of Embodiments 1 to 3, since the structure of the lenses of the second lens unit L 2  which largely contributes to variable magnification is set as described above, the Petzval sum can be maintained at a satisfactory value even when the power is increased (the focal length is reduced), thereby achieving excellent optical performance. 
   In addition, an aspheric surface provided in the second lens unit L 2  can increase the power to reduce the size of the entire lens system while favorable performance is maintained. 
   The first lens subunit L 3   a  has a positive lens having an aspheric surface, and the second lens subunit L 3   b  has a negative lens having a concave surface toward the image side and a positive lens having convex lens surfaces on both sides. This favorably prevents undercorrection of curvature of field at the wide-angle end by using the aspheric surface. 
   The fourth lens unit L 4  is moved to achieve focusing. 
   In each of Embodiments 1 to 3, the rear focusing scheme with the fourth lens unit L 4  is employed to reduce the size of the entire lens system, allow quick focusing, and facilitate taking close-ups. 
   All or some lenses of the third lens unit L 3  are moved to have a component perpendicular to the optical axis to move an image, thereby correcting an image blur caused when the zoom lens vibrates. When vibration isolation is performed in the zoom lens by parallel or rotationally decentering some lenses of the image-taking system to have a component perpendicular to the optical axis, an extra optical system is not required for preventing a displacement of a taken image and the vibration isolation is easily performed. 
   According to each of Embodiments 1 to 3 as described above, a zoom lens system can be achieved with a smaller size of the entire lens system and higher optical performance even at a high zoom ratio. 
   In addition, it is possible to realize a zoom lens which has excellent optical performance over the entire zoom range from the wide-angle end to the telephoto end even at a high zoom ratio of 14 or more and over the entire object distance from an object at infinity to an object at an extremely short distance, and which has a small number of constituent lenses even with a large aperture ratio at an F number of approximately 1.8. 
   Next, numeric data of Numerical Examples 1 to 3 corresponding to Embodiments 1 to 3 is shown. In each of Numerical Examples 1 to 3, i represents the order of an optical surface from the object side, Ri represents the radius of curvature of an i-th optical surface (an i-th surface), Di represents a distance between an i-th surface and an i+1-th surface, Ni and νi represent the refractive index and the Abbe number of the material of an i-th optical member for the d-line. Two planes closest to the image side are the surfaces of the glass block G. In addition, f represents a focal length, Fno represents an F number, and ω represents half of the field angle. 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     +     Fh   12             
where k represents the conic constant, B, C, D, E, and F represent aspheric coefficients of the order 4, 6, 8, 10, and 12, respectively, x represents a displacement in the optical axis direction at a height h from the optical axis relative to the surface vertex, and R represents a radius of curvature. Furthermore, “e-0X” means “x10 31 X ”. Table 1 shows numerical values calculated with the aforementioned conditional expressions in the respective Numerical Examples.
 
   NUMERICAL EXAMPLE 1 
   
     
       
             
           
             
             
             
             
           
         
             
                 
             
             
               f = 1~15.81 Fno = 1.87~3.08 2ω = 44.1~2.9 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               R1 = 10.150 
               D1 = 0.28 
               N1 = 1.846660 
               ν1 = 23.9 
             
             
               R2 = 4.857 
               D2 = 1.16 
               N2 = 1.696797 
               ν2 = 55.5 
             
             
               R3 = 112.565 
               D3 = 0.04 
             
             
               R4 = 4.836 
               D4 = 0.65 
               N3 = 1.772499 
               ν3 = 49.6 
             
             
               R5 = 12.649 
               D5 = Variable 
             
             
               R6 = 7.425 
               D6 = 0.14 
               N4 = 1.882997 
               ν4 = 40.8 
             
             
               R7 = 1.250 
               D7 = 0.72 
             
             
               R8 = −4.043 
               D8 = 0.14 
               N5 = 1.882997 
               ν5 = 40.8 
             
             
               R9 = 17.697 
               D9 = 0.06 
             
             
               R10 = 2.258 
               D10 = 0.70 
               N6 = 1.846660 
               ν6 = 23.9 
             
             
               R11 = −4.055 
               D11 = 0.05 
             
             
               R12 = −2.965 
               D12 = 0.14 
               N7 = 1.785896 
               ν7 = 44.2 
             
             
               R13 = 3.650 
               D13 = Variable 
             
             
               R14‡ = 3.370 
               D14 = 0.46 
               N8 = 1.740130 
               ν8 = 49.2 
             
             
               R15 = −48.641 
               D15 = 0.34 
             
             
               R16 = Aperture Stop 
               D16 = 0.46 
             
             
               R17 = 7.898 
               D17 = 0.14 
               N9 = 1.846660 
               ν9 = 23.9 
             
             
               R18 = 2.350 
               D18 = 0.65 
               N10 = 1.516330 
               ν10 = 64.1 
             
             
               R19 = −5.433 
               D19 = Variable 
             
             
               R20 = 3.400 
               D20 = 0.63 
               N11 = 1.696797 
               ν11 = 55.5 
             
             
               R21 = −2.112 
               D21 = 0.14 
               N12 = 1.834000 
               ν12 = 37.2 
             
             
               R22 = −9.719 
               D22 = Variable 
             
             
               R23 = ∞ 
               D23 = 0.65 
               N13 = 1.516330 
               ν13 = 64.1 
             
             
               R24 = ∞ 
             
             
                 
             
           
        
       
     
   
   
     
       
             
             
           
             
             
             
             
             
           
         
             
                 
                 
             
             
                 
               Focal Length 
             
           
        
         
             
                 
               Variable Spacing 
               1.00 
               7.18 
               15.81 
             
             
                 
                 
             
             
                 
               D5 
               0.12 
               3.60 
               4.26 
             
             
                 
               D13 
               4.34 
               0.86 
               0.20 
             
             
                 
               D19 
               1.84 
               0.69 
               2.24 
             
             
                 
               D22 
               0.60 
               1.75 
               0.20 
             
             
                 
                 
             
           
        
       
     
       
       
         
           *denotes an aspheric surface. 
           Aspheric Coefficient 
           R 14  k=−3.46987e+00 B=4.52418e−03 C=−1.16451e−03 D=4.19744e−03 E=−5.48293e−03 F=2.10828e−03 
         
       
     
  
   NUMERICAL EXAMPLE 2 
   
     
       
             
           
             
             
             
             
           
         
             
                 
             
             
               f = 1~15.79 Fno = 1.85~3.49 2ω = 43.3~2.9 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               R1 = 10.171 
               D1 = 0.28 
               N1 = 1.846660 
               ν1 = 23.9 
             
             
               R2 = 4.985 
               D2 = 1.00 
               N2 = 1.603112 
               ν2 = 60.6 
             
             
               R3 = −82.248 
               D3 = 0.05 
             
             
               R4 = 4.772 
               D4 = 0.69 
               N3 = 1.772499 
               ν3 = 49.6 
             
             
               R5 = 13.859 
               D5 = Variable 
             
             
               R6 = 9.431 
               D6 = 0.14 
               N4 = 1.834807 
               ν4 = 42.7 
             
             
               R7 = 1.159 
               D7 = 0.69 
             
             
               R8 = −3.129 
               D8 = 0.14 
               N5 = 1.804000 
               ν5 = 46.6 
             
             
               R9 = 7.280 
               D9 = 0.10 
             
             
               R10 = 2.761 
               D10 = 0.47 
               N6 = 1.846660 
               ν6 = 23.9 
             
             
               R11 = −4.744 
               D11 = 0.00 
               N7 = 1.846660 
               ν7 = 23.9 
             
             
               R12 = −4.744 
               D12 = 0.13 
               N8 = 1.834807 
               ν8 = 42.7 
             
             
               R13 = 8.175 
               D13 = Variable 
             
             
               R14‡ = 2.435 
               D14 = 0.46 
               N9 = 1.730770 
               ν9 = 40.5 
             
             
               R15 = 13.215 
               D15 = 0.39 
             
             
               R16 = Aperture 
               D16 = 0.51 
             
             
               Stop 
             
             
               R17 = 6.152 
               D17 = 0.13 
               N10 = 1.846660 
               ν10 = 23.9 
             
             
               R18 = 1.731 
               D18 = 0.75 
               N11 = 1.516330 
               ν11 = 58.3 
             
             
               R19 = −9.116 
               D19 = Variable 
             
             
               R20‡ = 2.939 
               D20 = 0.40 
               N12 = 1.693500 
               ν12 = 53.2 
             
             
               R21 = −6.942 
               D21 = 0.13 
               N13 = 1.846660 
               ν13 = 23.9 
             
             
               R22 = −34.816 
               D22 = Variable 
             
             
               R23 = ∞ 
               D23 = 0.76 
               N14 = 1.516330 
               ν14 = 64.1 
             
             
               R24 = ∞ 
             
             
                 
             
           
        
       
     
   
   
     
       
             
             
           
             
             
             
             
             
           
         
             
                 
                 
             
             
                 
               Focal Length 
             
           
        
         
             
                 
               Variable Spacing 
               1.00 
               7.47 
               15.79 
             
             
                 
                 
             
             
                 
               D5 
               0.12 
               3.65 
               4.32 
             
             
                 
               D13 
               4.36 
               0.84 
               0.16 
             
             
                 
               D19 
               1.57 
               0.39 
               1.95 
             
             
                 
               D22 
               0.59 
               1.77 
               0.21 
             
             
                 
                 
             
           
        
       
     
       
       
         
           *denotes an aspheric surface. 
           Aspheric Coefficient 
           R 14  k=−1.57942e+00 B=5.38617e−03 C=9.16238e−04 D=2.66834e−04 E=−1.30429e−03 F=6.04124e−04 R 20  k=−4.09278e+00 B=1.65647e−02 C=−9.09464e−03 D=6.17476e−03 E=1.41630e−02 F=−2.18714e−02 
         
       
     
  
   NUMERICAL EXAMPLE 3 
   
     
       
             
           
             
             
             
             
           
         
             
                 
             
             
               f = 1~17.80 Fno = 1.86~3.47 2ω = 46.4~2.8 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               R1 = 11.168 
               D1 = 0.30 
               N1 = 1.846660 
               ν1 = 23.9 
             
             
               R2 = 5.083 
               D2 = 1.34 
               N2 = 1.696797 
               ν2 = 55.5 
             
             
               R3 = −119.329 
               D3 = 0.04 
             
             
               R4 = 4.577 
               D4 = 0.68 
               N3 = 1.772499 
               ν3 = 49.6 
             
             
               R5 = 10.200 
               D5 = Variable 
             
             
               R6 = 4.705 
               D6 = 0.16 
               N4 = 1.882997 
               ν4 = 40.8 
             
             
               R7 = 1.181 
               D7 = 0.83 
             
             
               R8 = −3.965 
               D8 = 0.16 
               N5 = 1.882997 
               ν5 = 40.8 
             
             
               R9 = 10.242 
               D9 = 0.05 
             
             
               R10 = 2.045 
               D10 = 0.77 
               N6 = 1.846660 
               ν6 = 23.9 
             
             
               R11 = −4.085 
               D11 = 0.05 
             
             
               R12 = −2.707 
               D12 = 0.16 
               N7 = 1.785896 
               ν7 = 44.2 
             
             
               R13 = 3.168 
               D13 = Variable 
             
             
               R14‡ = 3.657 
               D14 = 0.50 
               N8 = 1.740130 
               ν8 = 49.3 
             
             
               R15 = −19.481 
               D15 = 0.38 
             
             
               R16 = Aperture 
               D16 = 0.51 
             
             
               Stop 
             
             
               R17 = 7.706 
               D17 = 0.16 
               N9 = 1.846660 
               ν9 = 23.9 
             
             
               R18 = 2.276 
               D18 = 0.66 
               N10 = 1.516330 
               ν10 = 64.1 
             
             
               R19 = −6.557 
               D19 = Variable 
             
             
               R20 = 3.339 
               D20 = 0.62 
               N11 = 1.696797 
               ν11 = 55.5 
             
             
               R21 = −2.138 
               D21 = 0.16 
               N12 = 1.834000 
               ν12 = 37.2 
             
             
               R22 = −8.824 
               D22 = Variable 
             
             
               R23 = ∞ 
               D23 = 0.62 
               N13 = 1.516330 
               ν13 = 64.1 
             
             
               R24 = ∞ 
             
             
                 
             
           
        
       
     
   
   
     
       
             
             
           
             
             
             
             
             
           
         
             
                 
                 
             
             
                 
               Focal Length 
             
           
        
         
             
                 
               Variable Spacing 
               1.00 
               7.48 
               17.80 
             
             
                 
                 
             
             
                 
               D5 
               0.13 
               3.50 
               4.14 
             
             
                 
               D13 
               4.28 
               0.91 
               0.27 
             
             
                 
               D19 
               1.94 
               0.71 
               2.42 
             
             
                 
               D22 
               0.57 
               1.81 
               0.09 
             
             
                 
                 
             
           
        
       
     
       
       
         
           * denotes an aspheric surface. 
           Aspheric Surface 
           R 14  k=−3.59369e+00 B=3.72365e−03 C=−1.53095e−03 D=6.01276e−03 E=−8.20186e−03 F=3.34618e−03
 
(table 1)
 
         
       
     
  
   
     
       
             
             
             
             
           
             
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
               Conditinal 
               Numerical 
                 
                 
             
             
               Expression 
               Example 1 
               Numerica Example 2 
               Numerical Example 3 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               (1) 
               0.074 
               0.075 
               0.060 
             
             
               (2) 
               0.436 
               0.426 
               0.383 
             
             
               (3) 
               0.295 
               0.298 
               0.255 
             
             
               (4) 
               0.817 
               0.956 
               0.891 
             
             
               (5) 
               0.595 
               0.490 
               0.570 
             
             
               (6) 
               1.48 
               2.27 
               1.73 
             
             
                 
             
           
        
       
     
   
   Next, description is made for an embodiment of a video camera which uses the zoom lens of the present invention as an image-taking optical system with reference to  FIG. 13 . 
   In  FIG. 13 , reference numeral  10  shows a video camera body or a digital still camera body,  11  shows an image-taking optical system realized by the zoom lens of the present invention,  12  shows a solid-state image-pickup element (a photoelectrical conversion element) such as a CCD sensor or a CMOS sensor which receives an object image formed by the image-taking optical system  11 ,  13  shows a recording means for recording the object image received by the image-pickup element  12 , and  14  shows a finder for observing an object image displayed on a display element, not shown. The display element is realized by a liquid crystal panel or the like, on which the object image formed on the image-pickup element  12  is displayed. 
   The zoom lens of the present invention can be applied to an image-taking apparatus such as a video camera to realize an image-taking apparatus which has a small size and excellent optical performance. 
   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.