Abstract:
A zoom lens has, in succession from the object side, a first lens unit having positive refractive power, a second lens unit having negative refractive power, and a third lens unit having positive refractive power. The second and third lens units are moved to thereby effect zooming. When variations in the magnifications of the second and third lens units from the wide angle end to the telephoto end are Z 2  and Z 3 , respectively, the zoom lens satisfies the condition that Z 3  /Z 2  &gt;1.4. The zoom lens is compact and yet covers a wide angle range of an angle of view of 60°-80°.

Description:
This application is a continuation of application Ser. No. 08/086,854 filed Jul. 7, 1993, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a zoom lens suitable for a camera for photographic film, a video camera and a still video camera, and particularly to a compact zoom lens of three-unit construction. 
     2. Related Background Art 
     In recent years, with the tendency of home video cameras toward compactness and light weight, compactness and lighter weight and particularly, a shorter full length and reduced diameter of the front lens have been required of not only camera bodies, but also zoom lenses. 
     On the other hand, with the compactness of zoom lenses, demand is increasing for zoom lenses in which the angle of view at the wide angle end covers a wide angle range of the order of 60°-80°. 
     Zoom lenses which have, in succession from the object side, a first lens unit having positive refractive power, a second lens unit having negative refractive power and a third lens unit having positive refractive power and in which the second lens unit and third lens unit are moved to thereby effect zooming are disclosed, for example, in Japanese Laid-Open Patent Application No. 51-56645, Japanese Laid-Open Patent Application No. 51-68244 and Japanese Laid-Open Patent Application No. 63-304218. 
     These zoom lenses, however, could hardly be said to satisfy the aforementioned requirement, i.e., the requirement for being compact and yet securing a sufficiently wide angle range. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a zoom lens which is compact and yet covers a wide angle range, and the feature of the present invention is that the zoom lens has, in succession from an object side, a first lens unit having positive power, a second lens unit having negative refractive power and a third lens unit having positive refractive power, the second lens unit and the third lens unit being moved to thereby effect zooming, and when variations in the magnifications of the second lens unit and the third lens unit from the wide angle end to the telephoto end are Z 2  and Z 3 , respectively, the zoom lens satisfies the following condition: ##EQU1## 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a zoom lens according to a first embodiment of the present invention. 
     FIGS. 2A-1, 2A-2, 2A-3, 2B-1, 2B-2 and 2B-3 show aberrations in the zoom lens according to the first embodiment. 
     FIG. 3 is a cross-sectional view of a zoom lens according to a second embodiment of the present invention. 
     FIGS. 4A-1, 4A-2, 4A-3, 4B-1, 4B-2 and 4B-3 show aberrations in the zoom lens according to the second embodiment. 
     FIG. 5 is a cross-sectional view of a zoom lens according to a third embodiment of the present invention. 
     FIGS. 6A-1, 6A-2, 6A-3, 6B-1, 6B-2 and 6B-3 show aberrations in the zoom lens according to the third embodiment. 
     FIG. 7 is a cross-sectional view of a zoom lens according to a fourth embodiment of the present invention. 
     FIGS. 8A-1, 8A-2, 8A-3, 8B-1, 8B-2 and 8B-3 show aberrations in the zoom lens according to the fourth embodiment. 
     FIG. 9 is a cross-sectional view of a zoom lens according to a fifth embodiment of the present invention. 
     FIGS. 10A-1, 10A-2, 10A-3, 10B-1, 10B-2 and 10B-3 show aberrations in the zoom lens according to the fifth embodiment. 
     FIG. 11 is a cross-sectional view of a zoom lens according to a sixth embodiment of the present invention. 
     FIGS. 12A-1, 12A-2, 12A-3, 12B-1, 12B-2 and 12B-3 show aberrations in the zoom lens according to the sixth embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1, 3, 5, 7, 9 and 11 are cross-sectional views of zoom lenses according to the present invention. The reference character I designates a first lens unit having positive refractive power, the reference character II denotes a second lens unit having negative refractive power, the reference character III designates a third lens unit having positive refractive power, and the reference character SP denotes a stop. During zooming from the wide angle end to the telephoto end, the second lens unit and the third lens unit are moved along zoom loci as indicated by the arrow. During the zooming, the first lens unit remains stationary. The reference character G designates a filter member such as an infrared cut filter or an optical low-pass filter. 
     When variations in the magnifications of the second lens unit and the third lens unit from the wide angle end to the telephoto end are Z 2  and Z 3 , respectively, the following condition is satisfied: ##EQU2## 
     The conditional formulation (1) prescribes the ratio of the magnification variation to the zooming of the second lens unit and the third lens unit, and is a condition for achieving the wide angle and compactness of a zoom lens of three-unit construction, and yet correcting aberrations, particularly distortion and curvature of image field, well. If this range is departed from, the amount of movement of the second lens unit will tend to become great in order to obtain a desired zoom ratio, and if moreover, an attempt is made to shorten the focal length at the wide angle end, i.e., to achieve a wide angle, the diameter of the front lens as the first lens unit will tend to become large. Further, the negative refractive power of the second lens unit will become strong, and at the wide angle end, barrel-shaped distortion and Petzval sum will be in the negative direction, and the image plane will tend to become over. 
     Also, to secure the angle of view widely in a wide angle state, it is desirable that the following conditional expression be satisfied: 
     
         1.40&lt;|F.sub.2 /F.sub.W |&lt;2.60,           (2) 
    
     where F 2  is the focal length of the second lens unit, and F W  is the focal length of the whole system at the wide angle end. If this range is departed from, it will become difficult to draw near the wide angle side and yet maintain a good performance. If the upper limit of this conditional expression is exceeded, the diameter of the front lens will become large or the amount of movement of the second lens unit will become great, and this is not suitable for compactness. Also, if the lower limit of this conditional expression is exceeded, the refractive power of the second lens unit will become too strong and Petzval sum will become great in the negative direction and thus, the fluctuation of the image plane will become unable to be eliminated. 
     Further, to optimize the sharing of magnification change by the lens units, it is preferable that the amounts of movement of the lens units satisfy the following conditional expressions: 
     
         0.5&lt;|M.sub.2 |/(F.sub.T -F.sub.W)&lt;1.6    (3) 
    
     
         0.5&lt;|M.sub.3 |/(F.sub.T -F.sub.W)&lt;1.0,   (4) 
    
     where M 2  and M 3  are respectively the amounts of movement of the second lens unit and the third lens unit during the zooming from the wide angle end to the telephoto end, and F T  is the focal length of the whole system at the telephoto end. 
     If these ranges are departed from, the sharing of magnification change by the lens units will not only become ill-balanced, but also the size of the zoom lens and aberrations created in each lens unit will be affected. If the upper limits of these expressions (3) and (4) are exceeded, the whole system will become bulky, and if the lower limits of these expressions are exceeded, the amounts of movement for obtaining a desired magnification ratio will become small and it will become necessary to strengthen the refractive power of each lens unit and the correction of aberrations will become difficult. 
     Further, it is desirable that the amounts of movement of the lens units be within the following range: 
     
         0.7&lt;|M.sub.2 /M.sub.3 |&lt;2.5              (5) 
    
     If this range is departed from, the whole system will become large and the balance of the sharing of magnification change by the lens units will be aggravated, and this is not suitable. 
     Also, to set the focal length at the wide angle side shortly, it is desirable that the following conditional expressions be further satisfied: 
     
         5&lt;F.sub.1 /F.sub.W &lt;53                                     (6) 
    
     
         1&lt;F.sub.3 /F.sub.W &lt;3                                      (7) 
    
     
         0.4&lt;E.sub.1W /F.sub.W &lt;1.8,                                (8) 
    
     where F 1  and F 3  are respectively the focal lengths of the first lens unit and the third lens unit and E 1W  is the principal point spacing between the first lens unit and the second lens unit at the wide angle end. 
     Conditional expression (6) is concerned in the refractive power of the first lens unit, and if the upper limit of this conditional expression is exceeded, it will become difficult to correct barrel-shaped distortion. Also, if the lower limit of this conditional expression is exceeded, the refractive power of the first lens unit will become strong and Petzval sum will tend to assume a positive value and the image plane will tend to become under. 
     Also, it is preferable that the first lens unit be comprised of a positive lens, and particularly, to correct distortion as described above, it is desirable that the first lens unit be a positive lens having its convex surface facing the object side. 
     Conditional expression (7) is concerned in the refractive power of the third lens unit, and if the upper limit of this conditional expression is exceeded, the amount of movement for obtaining a desired magnification change ratio will become great and the zoom lens will become bulky, and this is not preferable. Also, if the lower limit of this conditional expression is exceeded, the refractive power of the third lens unit will become strong and the image plane will tend to become under or the back focal length at the wide angle end will become short, and there will be no room for containing a glass black such as a filter. Further, the principal ray will not perpendicularly (telecentrically) enter an imaging element such as a CCD and it will cause so-called shading, and this is not preferable. 
     Conditional expression (8) prescribes the ratio of the principal point spacing between the first lens unit and the second lens unit to the focal length at the wide angle end, and is a condition for achieving the compactness of the zoom lens system and yet maintaining good aberrations. 
     Further, when the focal length of the whole system at the telephoto end is F T , it is desirable in the relation to conditional expression (6) to provide such a degree of focal length that satisfies the following conditional expression: 
     
         F.sub.1 &lt;30F.sub.T                                         (9) 
    
     If the upper limit of this conditional expression is exceeded, the full length of the zoom lens will increase, and this is not preferable. If the lower limit of this conditional expression is exceeded, the correction of coma will become particularly difficult. 
     Finally, to reduce the number of lenses in the zoom lens according to the present invention and yet maintain good aberrations, it is desirable that the first lens unit be comprised of a single positive lens having its convex surface facing the object side in order to correct effectively correct distortion by the first lens unit, and for the same purpose, a fixed stop be disposed between the second lens unit and the third lens unit. 
     The focusing of the zoom lens of the present invention can be accomplished by moving any of the first, second and third lens units. 
     The numerical value embodiments of the present invention will be shown below. In the numerical value embodiments below, ri represents the radius of curvature of the ith lens surface from the object side, di represents the thickness and air gap of the ith lens from the object side, and ni and μi represent the refractive index and Abbe number, respectively of the glass of the ith lens from the object side. R 28  and R 29  are face plates, filters or the like. 
     Also, the relations between the aforementioned conditional expressions and the numerical values in the numerical value embodiments will be shown in Table 1 below. 
     When the direction of the optical axis is X-axis and the direction perpendicular to the optical axis is H-axis and the direction of travel of light is positive and paraxial radii of curvature B, C, D and E are aspherical surface coefficients, the aspherical surface shape is indicated by the following equation: 
     
         ______________________________________ ##STR1## 
    
     
         ______________________________________Embodiment 1f = 1 to 3.0  fno = 1:(2.49 to 3.6)  2ω = 74.5° to28.5°______________________________________     r1 = 5.935 d1 = 0.72 n1 = 1.51633                                   υ1 = 64.2     r2 = 114.032                d2 = changeable     r3 = 3.497 d3 = 0.24 n2 = 1.77250                                   υ2 = 49.6     r4 = 1.147 d4 = 0.58     r5 = -4.799                d5 = 0.19 n3 = 1.80400                                   υ3 = 46.6     r6 = 2.491 d6 = 0.25     r7 = 2.433 d7 = 0.53 n4 = 1.69895                                   υ4 = 30.l     r8 = -51.970                d8 = changeable     r9 = (stop)                d9 = changeable     r10 = 6.674                d10 = 0.33                          n5 = 1.83481                                   υ5 = 42.7     r11 = -6.674                d11 = 0.04     r12 = 1.540                d12 = 0.58                          n6 = 1.72000                                   υ6 = 50.3     r13 = 6.308                d13 = 0.07     r14 = -7.120                d14 = 0.33                          n7 = 1.84666                                   υ7 = 23.8     r15 = 1.423                d15 = 0.12     r16 = 5.516                d16 = 0.62                          n8 = 1.80400                                   υ8 = 46.6     r17 = -2.394                d17 = changeable      r18 = ∞                 d18 = 1.19                           n9 = 1.51633                                    υ9 = 64.2     r19 = ∞______________________________________      FOCAL LENGTHCHANGEABLE   W           M      TINTERVAL     1.00        2.39   2.99______________________________________d2           0.24        1.80   1.79d8           1.93        0.36   0.38d9           1.93        0.88   0.33 d17         0.71        1.77   2.31______________________________________Embodiment 2f = 1 to 2.92  fno = 1:(2.30 to 3.30)  2ω = 64.2° to24.2°______________________________________     r1 = 3.578 d1 = 0.47 n1 = 1.51633                                   υ1 = 64.2     r2 = 21.337                d2 = changeable     r3 = 3.688 d3 = 0.20 n2 = 1.83481                                   υ2 = 42.7     r4 = 1.006 d4 = 0.34     r5 = -4.683                d5 = 0.16 n3 = 1.83481                                   υ3 = 42.7     r6 = 2.112 d6 = 0.18     r7 = 2.046 d7 = 0.39 n4 = 1.69895                                   υ4 = 30.1     r8 = -7.857                d8 = changeable     r9 = (stop)                d9 = changeable     r10 = 6.793                d10 = 0.29                          n5 = 1.83400                                   υ5 = 37.2     r11 = -6.793                d11 = 0.03     r12 = 1.171                d12 = 0.41                          n6 = 1.72000                                   υ6 = 50.3     r13 = 13.242                d13 = 0.04     r14 = -9.959                d14 = 0.43                          n7 = 1.84666                                   υ7 = 23.8     r15 = 0.976                d15 = 0.11     r16 = 3.088                d16 = 0.33                          n8 = 1.83481                                   υ8 = 42.7     r17 = -3.088                d17 = changeable      r18 = ∞                 d18 = 0.98                           n9 = 1.51633                                    υ9 = 64.2     r19 = ∞______________________________________      FOCAL LENGTHCHANGEABLE   W           M      TINTERVAL     1.00        2.34   2.92______________________________________d2           0.20        1.46   1.50d8           1.59        0.33   0.30d9           1.44        0.67   0.28 d17         0.59        1.36   1.75______________________________________Embodiment 3f = 1 to 1.61  fno = 1:(2.09 to 2.91)  2ω = 63.7° to29.6°______________________________________     r1 = 17.554                d1 = 0.31 n1 = 1.51633                                   υ1 = 64.2     r2 = 263.217                d2 = changeable     r3 = 2.789 d3 = 0.19 n2 = 1.80400                                   υ2 = 46.6     r4 = 1.233 d4 = 0.44     r5 = -42.527                d5 = 0.16 n3 = 1.80400                                   υ3 = 46.6     r6 = 1.952 d6 = 0.17     r7 = 1.886 d7 = 0.41 n4 = 1.68893                                   υ4 = 31.1     r8 = 17.734                d8 = changeable     r9 = (stop)                d9 = changeable     r10 = 5.212                d10 = 0.31                          n5 = 1.83481                                   υ5 = 42.7     r11 = -5.212                d11 = 0.03     r12 = 1.251                d12 = 0.45                          n6 = 1.71300                                   υ6 = 53.8     r13 = 12.044                d13 = 0.03     r14 = -14.574                d14 = 0.56                          n7 = 1.84666                                   υ7 = 23.8     r15 = 0.930                d15 = 0.14     r16 = 3.101                d16 = 0.41                          n8 = 1.80400                                   υ8 = 46.6     r17 = -3.100                d17 = changeable      r18 = ∞                 d18 = 0.97                           n9 = 1.51633                                    υ9 = 64.2     r19 = ∞______________________________________      FOCAL LENGTHCHANGEABLE   W           M      TINTERVAL     1.00        2.36   1.61______________________________________d2           0.17        1.95   1.48d8           2.03        0.26   0.73d9           1.15        0.27   0.76 d17         0.19        1.07   0.58______________________________________Embodiment 4f = 1 to 3.98  fno = 1:(2.3 to 3.6)  2ω = 74.4° to21.6°______________________________________     r1 = 5.381 d1 = 0.99 n1 = 1.51633                                   υ1 = 64.2     r2 = 39.933                d2 = changeable     r3 = 3.995 d3 = 0.24 n2 = 1.77250                                   υ2 = 49.6     r4 = 1.317 d4 = 0.73     r5 = -6.892                d5 = 0.19 n3 = 1.80400                                   υ3 = 46.6     r6 = 2.434 d6 = 0.23     r7 = 2.412 d7 = 0.53 n4 = 1.69895                                   υ4 = 30.1     r8 = 47.272                d8 = changeable     r9 = (stop)                d9 = changeable     r10 = 7.011                d10 = 0.32                          n5 = 1.80400                                   υ5 = 46.6     r11 = -7.011                d11 = 0.04     r12 = 1.440                d12 = 0.62                          n6 = 1.69680                                   υ6 = 55.5     r13 = 5.617                d13 = 0.08     r14 = -9.201                d14 = 0.24                          n7 = 1.84666                                   υ7 = 23.8     r15 = 1.364                d15 = 0.14     r16 = 5.547                d16 = 0.55                          n8 = 1.83400                                   υ8 = 37.2     r17 = -2.806                d17 = changeable      r18 = ∞                 d18 = 1.19                           n9 = 1.51633                                    υ9 = 64.2     r19 = ∞______________________________________      FOCAL LENGTHCHANGEABLE   W           M      TINTERVAL     1.00        3.11   3.98______________________________________d2           0.24        2.58   2.54d8           2.68        0.34   0.38d9           2.20        0.95   0.31 d17         0.71        1.96   2.61______________________________________Embodiment 5f = 5.15 to 12.12  FNO = 2 to 2.8  2ω = 60.4° to______________________________________27.8°r1 = 93.483      d1 = 1.60     n1 = 1.51633                               υ1 = 64.2r2 = 279.352      d2 = changeable*r3 = -202.847      d3 = 1.00     n2 = 1.80400                               υ2 = 46.6r4 = 6.116 d4 = 2.09r5 = 9.395 d5 = 1.90     n3 = 1.67270                               υ3 = 32.1r6 = 28.200      d6 = changeabler7 = (stop)      d7 = changeabler8 = 26.334      d8 = 1.60     n4 = 1.83481                               υ4 = 42.7r9 = -26.636      d9 = 0.15r10 = 6.569      d10 = 2.30    n5 = 1.71300                               υ5 = 53.8r11 = 72.494      d11 = 0.15r12 = -75.741      d12 = 2.87    n6 = 1.84666                               υ6 = 23.8r13 = 4.789      d13 = 0.72r14 = 14.872      d14 = 2.10    n7 = 1.80400                               υ7 = 46.6r15 = -17.513      d15 = changeabler16 = ∞      d16 = 5.00    n8 = 1.51633                               υ8 = 64.2r17 = ∞______________________________________           FOCAL LENGTHCHANGEABLE        W      TINTERVAL          5.15   12.12______________________________________d2                3.18   13.45d8                11.56  1.29d9                5.92   1.55 d17              1.00   5.36______________________________________Third Surface: Aspherical SurfaceR.sub.0 = -202.847, B = 2.010 × 10.sup.-4, C = -6.716 ×10.sup.-7,D = -5.130 × 10.sup.-10, E = -1.396 × 10.sup.-10Embodiment 6f = 5.15 to 12.39  FNO = 1:(2 to 2.8)  2ω = 60.4° to27.2°______________________________________r1 = 55.506      d1 = 1.60     n1 = 1.51633                               υ1 = 64.2r2 = -171.365      d2 = changeabler3 = 14.530      d3 = 1.00     n2 = 1.80400                               υ2 = 46.6r4 = 6.071 d4 = 2.28r5 = -443.467      d5 = 0.80     n3 = 1.80400                               υ3 = 46.6r6 = 9.031 d6 = 0.88r7 = 8.921 d7 = 2.10     n4 = 1.68893                               υ4 = 31.1r8 = 59.057      d8 = changeabler9 = (stop)      d9 = changeable*r10 = 7.139      d10 = 2.30    n5 = 1.80400                               υ5 = 46.6r11 = -42.369      d11 = 0.15r12 = -93.663      d12 = 2.87    n6 = 1.84666                               υ6 = 23.8r13 = 6.292      d13 = 0.72r14 = 18.851      d14 = 2.10    n7 = 1.80400                               υ7 = 46.6r15 = -11.000      d15 = changeabler16 = ∞      d16 = 5.00    n8 = 1.51633                               υ8 = 64.2r17 = ∞______________________________________           FOCAL LENGTHCHANGEABLE        W      MINTERVAL          5.15   12.39______________________________________d2                2.89   12.17d8                10.66  1.38d9                5.92   1.80 d17              1.00   5.12______________________________________Tenth Surface: Aspherical SurfaceR.sub.0 = 7.139, B = -3.828 × 10.sup.-4, C = -6.742 ×10.sup.-7,D = 1.339 × 10.sup.-7, E = -1.787 × 10.sup.-8______________________________________ 
    
     
                                           TABLE 1__________________________________________________________________________       Embodiment             Embodiment                   Embodiment                         Embodiment                               Embodiment                                     Embodiment       1     2     3     4     5     6__________________________________________________________________________Cond. Ex. (1): Z.sub.3 /Z.sub.2       2.089 1.804 2.117 2.173 2.168 1.793Cond. Ex. (2): |F.sub.2 /F.sub.W |       1.710 1.580 2.370 1.854 2.574 2.081Cond. Ex. (3): |M.sub.2 |/(F.sub.T -F.sub.W)       0.779 0.698 1.310 0.771 1.474 1.281Cond. Ex. (4): |M.sub.3 |/(F.sub.T -F.sub.W)       0.806 0.625 0.648 0.635 0.625 0.568Cond. Ex. (5): |M.sub.2 /M.sub.3 |       0.967 1.117 2.023 1.214 2.357 2.256Cond. Ex. (6): F.sub.1 /F.sub.W       12.09 8.237 36.41 11.93 52.73 15.81Cond. Ex. (7): F.sub.3 /F.sub.W       2.347 1.935 1.850 2.449 1.851 1.9297Cond. Ex. (8): E.sub.1W /F.sub.W       0.940 0.596 0.593 1.404 0.830 0.915Cond. Ex. (9): F.sub.1 /F.sub.T       4.03  2.826 15.46 2.995 22.39 6.56__________________________________________________________________________