Abstract:
A zoom lens system consists of sequentially from the object side a first lens unit having a negative refractive power, a second lens unit having a positive refractive power and a third lens unit having a negative refractive power. Zooming is performed by shifting the second lens unit and the third lens unit. The first lens unit keeps stationary during the zooming. Focusing is accomplished by shifting the second lens unit. The lengths of the first and the second lens unit along the optical axis are optimized so as to fulfill the conditions. The zoom lens system is suitable for particularly underwater cameras.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a compact zoom lens system, and more particularly to a compact zoom lens system suitable for lens shutter cameras, particularly underwater cameras. 
     2. Description of the Related Art 
     One type of camera reflecting the trend toward outdoor living in recent years is the underwater camera. An underwater camera must have an optical system suitable for underwater photo-taking. For example, if a zoom lens system suitable for underwater photo-taking is to be realized, the condition that the lens unit that comes in contact with water be motionless during zooming should be met. 
     One construction to meet this condition is a system in which a fixed non-powered barrier that does not move during zooming is located in front of a second zoom lens unit comprising positive and negative lenses. Conventionally, a system comprising a first lens unit having a negative refractive power, a second lens unit having a positive refractive power and a third lens unit having a negative refractive power, in that order from the object side, in which the second and third lens units are moved for zooming while the first lens unit is fixed during zooming, has been proposed as well (Japanese Laid-Open Patents Sho 64-74521, Hei 1-116615, etc.) 
     However, with the former construction, because the effective aperture of the barrier is large, the camera cannot be made compact in terms of its height and width. On the other hand, with the latter construction, because the length of the lens system along the optical axis is large, the camera cannot be made compact in terms of its depth. 
     SUMMARY OF THE INVENTION 
     The main object of the present invention is to provide a zoom lens system in which compactness is not lost even where the first lens unit is fixed during zooming, and which has a high level of optical performance. 
     This and other objects of the present invention are achieved by providing a zoom lens system containing a first lens unit having a negative refractive power, a second lens unit having a positive refractive power and a third lens unit having a negative refractive power, in that order from the object side, wherein zooming is performed by changing the distances between the lens units, and wherein the second lens unit contains at least one positive lens and one negative lens and the lengths of the first and second lens units are optimized. 
     These and other objects, advantages and features of the present invention will become apparent from the following descriptions thereof, taken in conjunction with the accompanying drawings which illustrate specific embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows the construction of the lenses in a first embodiment of the present invention. 
     FIG. 2 shows the construction of the lenses in a second embodiment of the present invention. 
     FIG. 3 shows the construction of the lenses in a third embodiment of the present invention. 
     FIG. 4 shows the construction of the lenses in a fourth embodiment of the present invention. 
     FIG. 5 shows the construction of the lenses in a fifth embodiment of the present invention. 
     FIG. 6 shows the construction of the lenses in a sixth embodiment of the present invention. 
     FIGS. 7A, 7B and 7C show aberration curves in the shortest focal length condition in the first embodiment. 
     FIGS. 8A, 8B and 8C show aberration curves in the middle focal length condition in the first embodiment. 
     FIGS. 9A, 9B and 9C show aberration curves in the longest focal length condition in the first embodiment. 
     FIGS. 10A, 10B and 10C show aberration curves in the shortest focal length condition in the second embodiment. 
     FIGS. 11A, 11B and 11C show aberration curves in the middle focal length condition in the second embodiment. 
     FIGS. 12A, 12B and 12C show aberration curves in the longest focal length condition in the second embodiment. 
     FIGS. 13A, 13B and 13C show aberration curves in the shortest focal length condition in the third embodiment. 
     FIGS. 14A, 14B and 14C show aberration curves in the middle focal length condition in the third embodiment. 
     FIGS. 15A, 15B and 15C show aberration curves in the longest focal length condition in the third embodiment. 
     FIGS. 16A, 16B and 16C show aberration curves in the shortest focal length condition in the fourth embodiment. 
     FIGS. 17A, 17B and 17C show aberration curves in the middle focal length condition in the fourth embodiment. 
     FIGS. 18A, 18B and 18C show aberration curves in the longest focal length condition in the fourth embodiment. 
     FIGS. 19A, 19B and 19C show aberration curves in the shortest focal length condition in the fifth embodiment. 
     FIGS. 20A, 20B and 20C show aberration curves in the middle focal length condition in the fifth embodiment. 
     FIGS. 21A, 21B and 21C show aberration curves in the longest focal length condition in the fifth embodiment. 
     FIGS. 22A, 22B and 22C show aberration curves in the shortest focal length condition in the sixth embodiment. 
     FIGS. 23A, 23B and 23C show aberration curves in the middle focal length condition in the sixth embodiment. 
     FIGS. 24A, 24B and 24C show aberration curves in the longest focal length condition in the sixth embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The embodiments of the present invention are explained below. The present invention is a zoom lens system containing a first lens unit having a negative refractive power, a second lens unit having a positive refractive power and a third lens unit having a negative refractive power, in that order from the object side, wherein zooming is carried out by changing the distances between the various lens units. In this zoom lens system, the second lens unit contains at least one positive lens and one negative lens. It also meets the following conditions (1) and (2). 
     
         0.08&lt;t1/y&#39;&lt;0.25                                            (1) 
    
     
         0.08&lt;t2/y&#39;&lt;0.28                                            (2) 
    
     In said conditions, t1 represents the length of the first lens unit along the optical axis, t2 represents the length of the second lens unit along the optical axis, and y&#39; represents the diagonal length of the film surface. 
     By means of the construction in which the lens units are aligned in the order of negative, positive and negative from the object side and in which the first lens unit is allowed to have a certain level of power, the effective aperture of the first lens unit can be reduced and the amount of movement of the second and third lens units can be reduced at the same time. In addition, by reducing the length of the first and second lens units along the optical axis, the camera&#39;s depth can be reduced as well. 
     If t1/y&#39; is below the lower limit in condition (1), the length of the entire first lens unit is small, which makes it difficult to compensate for the curvature of field and coma aberration caused by the difference in the height of the light rays. In addition, since the power of each lens of the first lens unit cannot be made as large as necessary because of the edge thickness of the positive lens of the first lens unit, coma aberration compensation becomes difficult. If t1/y&#39; exceeds the upper limit in condition (1), the length of the first lens unit becomes large, which makes it impossible to reduce the length of the optical system along the camera&#39;s depth. 
     If t2/y&#39; is below the lower limit in condition (2), the entire second lens unit becomes short, which makes it difficult to compensate for the spherical aberration and transverse chromatic aberration caused by the difference in the height of the light rays. In addition, since the power of each lens of the second lens unit cannot be made as large as necessary due to the edge thickness of the positive lens of the second lens unit, spherical aberration compensation becomes difficult. If t2/y&#39; exceeds the upper limit in condition (2), the length of the second lens unit becomes large, which makes it impossible to reduce the length of the optical system along the camera&#39;s depth. 
     The conventional models described above (Japanese Laid-Open Patents Sho 64-74521, Hei 1-116615, etc.) have a three-component construction comprising negative, positive and negative lens units aligned in that order from the object side, as in the case of the present invention. However, because the second lens unit is so thick that condition (2) cannot be met, they cannot realize compactness of the camera in terms of its depth. By contrast, the zoom lens system of the present invention has a construction which meets condition (2), as a result of which the second lens unit is compact along the optical axis, allowing compactness of the camera in terms of its depth. 
     As described above, to ensure performance the second lens unit having a positive refractive power must contain at least one positive lens and one negative lens. In an optical system of this type, it is necessary to compensate for monochromatic aberrations in particular independently in each lens unit. In order to do so, if a positive lens and negative lens are combined and located in the second lens unit, spherical aberration and coma aberration can be compensated for independently in the second lens unit. 
     Using said construction, if the following condition (3) is met, a zoom lens system which is further compact can be realized. 
     
         15&lt;|(fw/f1)×W0|&lt;40                 (3) 
    
     In said condition, fw represents the focal length of the entire system in the shortest focal length condition; f1 represents the focal length of the first lens unit; and W0 represents the distance between the tip of the lens system to the film surface in the shortest focal length condition. 
     If |(fw/f1)×W0| is below the lower limit in condition (3), the effective apertures of the first and third lens units become large in order to secure lighting contrast in a short focal length condition, which negates a reduction of the camera&#39;s size along its lens diameter. In addition, if |(fw/f1)×W0| exceeds the upper limit, the length of the lens system in the shortest focal length condition becomes large, which also negates a reduction in size. 
     In the present invention, it is desirable to use an aspherical surface in the second lens unit. If an aspherical surface is located in the second lens unit, aberrations can be compensated for more effectively and the number of lenses can be minimized. Having fewer lenses can help realize a lightweight and compact zoom lens system. Moreover, if an aspherical surface is placed in the second lens unit, spherical aberration and coma aberration can be compensated for more effectively while the number of lenses is reduced, as a result of which the length of the second lens unit can be reduced, achieving a reduction in the length of the entire lens system. 
     The conventional models described above (Japanese Laid-Open Patents Sho 64-74521, Hei 1-116615, etc.) have a three-component construction consisting of negative, positive and negative lens units aligned in that order from the object side, as in the case of the present invention. However, the large number of lenses in the second lens unit leads to increased cost and weight in addition to a long second lens unit. By contrast, if an aspherical surface is used in the second lens unit of the zoom lens system of the present invention, high optical performance can be secured by means of effective aberration compensation as described above while the number of lenses of the second lens unit can be reduced. Therefore, a zoom lens system which is lighter and more compact than the conventional models can be realized at a lower cost. 
     Further, if a negative lens having aspherical surfaces on both sides is used for the negative lens of the second lens unit, good aberration compensation can be achieved. Having aspherical surfaces on both sides offers an advantage over having separate aspherical surfaces (having the positive and negative lenses of the second lens unit have one aspherical surface each, for example) in that manufacturing of the aspherical surfaces is easier during the manufacturing process. 
     Aspherical surfaces should preferably be used in the first or third lens units. In a wide angle zoom lens system, in particular, having an aspherical surface in the first lens unit is effective in the compensation for distortion in a short focal length condition. On the other hand, if an aspherical surface is placed in the third lens unit, distortion and coma aberration can be compensated for effectively. 
     As described above, in order to realize a zoom lens system suitable as an underwater camera, it is desirable that the first lens unit keeps stationary during zooming. Due to the construction of the lens mount, it is almost an essential condition that the lens unit coming in contact with water not move during operation of the camera while it is used underwater. Using the present invention, even where the first lens unit is fixed during zooming, compactness is not lost as a result of said construction while a high level of optical performance is maintained. Moreover, in said conventional models equipped with a non-powered barrier that is fixed during zooming, the large effective aperture of the barrier prevents the camera from becoming compact in terms of height and width. With the present invention, these problems does not occur. 
     It is preferred that the construction be such that the positive second lens unit is zoomed out during focusing. This is because if the focusing is performed by zooming out the second lens unit, good performance can be secured at close range photo-taking. For example, if the construction were such that the first lens unit is zoomed out for focusing, it would be not suitable as a camera lens for underwater photo-taking, as described above. If rear focusing using the third lens unit were adopted, the reduction in lighting contrast in a short focal length condition would become marked, which is not desirable. 
     In a construction that meets condition (3), it is preferable that the following condition (4) also be met. By meeting condition (4), better performance in close range photo-taking can be secured. 
     
         15&lt;|(fw/f1)×W0|&lt;30                 (4) 
    
     In condition (4), if |(fw/f1)×W0| exceeds the upper limit, the power of the second lens unit must be relatively increased with the increase in power of the first lens unit, which increases the convergence in close-range photo-taking, making it difficult to accurately compensate for spherical aberration. 
     Embodiments of the present invention are shown below using specific numbers. In each embodiment, ri (i=1, 2, 3, . . . ) represents the radius of curvature of the ith lens surface from the object side; di (i=1, 2, 3, . . . ) represents the ith axial distance from the object side; and Ni (i=1, 2, 3, . . . ) and νi (i=1, 2, 3, . . . ) represent the refractive index and the Abbe number, to the d-line of the ith lens from the object side, respectively. f represents the focal length of the entire zoom lens system and F NO represents the F-number. 
     In each embodiment, the surfaces marked with asterisks in the radius of curvature column are aspherical, and are defined by the following equation which represents a surface configuration of an aspherical surface. ##EQU1## 
     In said equation, X represents the amount of displacement from the reference surface along the optical axis; Y represents height in a direction vertical to the optical axis; C represents a paraxial radius of curvature; e represents a quadric surface parameter; and Ai represents an ith-order aspherical coefficient. 
     
         ______________________________________Embodiment 1f = 29.0˜40.0˜56.0, F NO = 3.62˜4.72˜6.01Radius of Axial       Refractive  Abbecurvature distance    index       number______________________________________r1   -388.291         d1     1.000  N1   1.52584                                   ν1                                        52.06r2   22.872         d2     0.500r3   24.528         d3     3.500  N2   1.80500                                   ν2                                        40.97r4   41.358   d4   18.000˜11.696˜1.653r5*  19.296         d5     2.450  N3   1.79850                                   ν3                                        22.60r6*  13.855         d6     3.000r7   39.306         d7     3.800  N4   1.51728                                   ν4                                        69.43r8   -11.637   d8   12.829˜7.834˜4.911r9*  -65.280         d9     3.350  N5   1.52510                                   ν5                                        56.38r10  -23.492         d10    4.450r11  -12.977         d11    1.000  N6   1.63854                                   ν6                                        55.62r12  1784.089   Σd = 53.879˜42.579˜29.614______________________________________ 
    
     Aspherical Surface Coefficients 
     r5: e=0.10000×10 
     A4=-0.34056×10 -3   
     A6=-0.19147×10 -5   
     A8=-0.90488×10 -8   
     A10=-0.28594×10 -10   
     r6: e=0.10000×10 
     A4=-0.29546×10 -3   
     A6=-0.15539×10 -5   
     A8=0.12951×10 -7   
     A10=0.11530×10 -9   
     r9: e=0.10000×10 
     A3=0.55964×10 -4   
     A4=0.28487×10 -4   
     A5=-0.23841×10 -5   
     A6=-0.27640×10 -6   
     A7=-0.50073×10 -7   
     A8=0.26638×10 -7   
     A9=-0.23884×10 -8   
     A10=0.54324×10 -11   
     A11=-0.14502×10 -11   
     A12=0.78652×10 -12   
     
         ______________________________________Embodiment 2f = 28.7˜40.0˜56.0, F NO = 3.62˜4.68˜5.85Radius of Axial       Refractive  Abbecurvature distance    index       number______________________________________r1   -618.001         d1     1.000  N1   1.69350                                   ν1                                        50.29r2   17.058         d2     1.131r3   17.990         d3     4.800  N2   1.80700                                   ν2                                        39.79r4*  43.453   d4   17.500˜10.526˜1.012r5*  37.220         d5     2.819  N3   1.80518                                   ν3                                        25.43r6*  20.296         d6     0.627r7   29.694         d7     4.375  N4   1.48749                                   ν4                                        70.44r8   -10.290   d8   10.481˜6.382˜4.500r9*  -95.670         d9     3.350  N5   1.52510                                   ν5                                        56.38r10  -24.843         d10    4.576r11  -9.611         d11    1.000  N6   1.48749                                   ν6                                        70.44r12  1784.089   Σd = 51.660˜40.586˜29.191______________________________________ 
    
     Aspherical Surface Coefficients 
     r4: e=0.10000×10 
     A4=-0.21910×10 -5   
     A6=0.81146×10 -8   
     A8=-0.21505×10 -9   
     A10=0.76748×10 -12   
     r5: e=0.10000×10 
     A4=-0.41693×10 -3   
     A6=-0.22617×10 -5   
     A8=-0.18938×10 -7   
     A10=-0.10685×10 -9   
     r6: e=0.10000×10 
     A4=-0.31075×10 -3   
     A6=-0.51679×10 -6   
     A8=0.19094×10 -7   
     A10=0.12790×10 -9   
     r9: e=0.10000×10 
     A3=0.55964×10 -4   
     A4=0.39386×10 -4   
     A5=-0.20210×10 -6   
     A6=-0.42301×10 -7   
     A7=-0.30934×10 -7   
     A8=0.21613×10 -7   
     A9=-0.27773×10 -8   
     A10=0.72601×10 -10   
     A11=0.11466×10 -12   
     A12=0.10923×10 -11   
     
         ______________________________________Embodiment 3f = 28.7˜40.0˜56.0, F NO = 3.62˜4.78˜6.11Radius of Axial       Refractive  Abbecurvature distance    index       number______________________________________r1   -618.001         d1     1.000  N1   1.69680                                   ν1                                        55.43r2   19.043         d2     1.131r3   19.247         d3     4.800  N2   1.80700                                   ν2                                        39.79r4   51.488   d4   18.500˜11.756˜1.314r5*  30.537         d5     2.819  N3   1.70055                                   ν3                                        30.11r6*  12.493         d6     0.627r7   19.009         d7     4.501  N4   1.48749                                   ν4                                        70.44r8   -9.911   d8   12.481˜7.217˜4.200r9*  -89.543         d9     3.350  N5   1.58340                                   ν5                                        30.23r10  -26.690         d10    4.576r11  -10.309         d11    1.000  N6   1.51823                                   ν6                                        58.96r12  1784.089   Σd = 54.786˜42.777˜29.319______________________________________ 
    
     Aspherical Surface Coefficients 
     r5: e=0.10000×10 
     A4=-0.52004×10 -3   
     A6=-0.16282×10 -5   
     A8=-0.17228×10 -7   
     A10=-0.14369×10 -9   
     r6: e=0.10000×10 
     A4=-0.44161×10 -3   
     A6=0.76249×10 -6   
     A8=0.22617×10 -7   
     A10=0.12730×10 -9   
     r9: e=0.10000×10 
     A3=0.55964×10 -4   
     A4=0.27477×10 -4   
     A5=0.22326×10 -5   
     A6=-0.26809×10 -6   
     A7=-0.76841×10 -7   
     A8=0.26384×10 -7   
     A9=-0.22849×10 -8   
     A10=0.92719×10 -10   
     A11=-0.32029×10 -11   
     A12=0.30877×10 -12   
     
         ______________________________________Embodiment 4f = 28.7˜40.0˜56.0, F NO = 3.62˜4.62˜5.73Radius of Axial       Refractive  Abbecurvature distance    index       number______________________________________r1   -618.001         d1     1.000  N1   1.69680                                   ν1                                        55.43r2   16.252         d2     1.131r3   17.351         d3     4.800  N2   1.80700                                   ν2                                        39.79r4   37.507   d4   17.500˜10.509˜1.719r5   9.142         d5     5.021  N3   1.48749                                   ν3                                        70.44r6   -21.772         d6     0.627r7*  -10.701         d7     3.322  N4   1.70055                                   ν4                                        30.11r8*  -20.177   d8   9.121˜4.785˜3.600r9*  -30.594         d9     3.350  N5   1.58340                                   ν5                                        30.23r10  -19.086         d10    3.733r11  -10.127         d11    1.000  N6   1.51602                                   ν6                                        56.77r12  -40.844   Σd = 50.606˜39.279˜29.303______________________________________ 
    
     Aspherical Surface Coefficients 
     r7: e=0.10000×10 
     A4=0.36203×10 -3   
     A6=0.15224×10 -5   
     A8=-0.13740×10 -7   
     A10=-0.10641×10 -9   
     r8: e=0.10000×10 
     A4=0.40479×10 -3   
     A6=0.24775×10 -5   
     A8=0.18262×10 -7   
     A10=0.99273×10 -10   
     r9: e=0.10000×10 
     A3=0.55964×10 -4   
     A4=0.40187×10 -4   
     A5=-0.25207×10 -5   
     A6=-0.14776×10 -6   
     A7=-0.26686×10 -7   
     A8=0.31615×10 -7   
     A9=-0.26931×10-8 
     A10=-0.27530×10 -10   
     A11=-0.39628×10 -12   
     A12=0.10466×10 -11   
     
         ______________________________________Embodiment 5f = 28.7˜40.0˜56.0, F NO = 3.62˜4.68˜5.86Radius of Axial       Refractive  Abbecurvature distance    index       number______________________________________r1   -618.001         d1     1.000  N1   1.80100                                   ν1                                        46.54r2   17.744         d2     1.131r3   18.554         d3     5.000  N2   1.80750                                   ν2                                        35.43r4   59.284   d4   18.000˜11.058˜1.708r5   9.344         d5     5.000  N3   1.48749                                   ν3                                        70.44r6   -24.196         d6     0.627r7*  -11.287         d7     2.800  N4   1.75520                                   ν4                                        27.51r8*  -19.945   d8   10.424˜5.585˜3.600r9*  -21.636         d9     3.350  N5   1.58340                                   ν5                                        30.23r10  -14.695         d10    3.800r11  -9.176         d11    1.000  N6   1.51823                                   ν6                                        58.96r12  -36.766   Σd = 52.133˜40.351˜29.017______________________________________ 
    
     Aspherical Surface Coefficients 
     r7: e=0.10000×10 
     A4=0.37145×10 -3   
     A6=0.16937×10 -5   
     A8=-0.15777×10 -7   
     A10=-0.13227×10 -9   
     r8: e=0.10000×10 
     A4=0.39854×10 -3   
     A6=0.24461×10 -5   
     A8=0.19043×10 -7   
     A10=0.10309×10 -9   
     r9: e=0.10000×10 
     A3=0.55964×10 -4   
     A4=0.27673×10 -4   
     A5=-0.19534×10 -5   
     A6=-0.12079×10 -6   
     A7=-0.23385×10 -7   
     A8=0.34891×10 -7   
     A9=-0.26295×10 -8   
     A10=-0.99229×10 -11   
     A11=-0.37901×10 -12   
     A12=0.10410×10 -11   
     
         ______________________________________Embodiment 6f = 25.5˜39.6˜48.6, F NO = 3.62˜5.00˜5.77Radius of Axial       Refractive  Abbecurvature distance    index       number______________________________________r1   -549.094         d1     0.889  N1   1.69350                                   ν1                                        50.29r2   15.252         d2     1.005r3   16.318         d3     5.000  N2   1.80700                                   ν2                                        39.79r4*  40.091   d4   16.500˜7.000˜2.500r5*  33.286         d5     2.505  N3   1.84666                                   ν3                                        23.82r6*  19.280         d6     0.557r7   29.681         d7     3.740  N4   1.48749                                   ν4                                        70.44r8   -9.138   d8   9.270˜5.168˜3.998r9*  -83.576         d9     2.976  N5   1.54072                                   ν5                                        47.22r10  -22.287         d10    4.065r11  -8.508         d11    0.889  N6   1.48749                                   ν6                                        70.44r12  1585.162   Σd = 47.397˜33.794˜28.124______________________________________ 
    
     Aspherical Surface Coefficients 
     r4: e=0.10000×10 
     A4=-0.58622×10 -5   
     A6=0.55777×10 -7   
     A8=-0.78973×10 -9   
     A10=0.23640×10 -11   
     r5: e=0.10000×10 
     A4=-0.59880×10 -3   
     A6=-0.40485×10 -5   
     A8=-0.43447×10 -7   
     A10=-0.31201×10 -9   
     r6: e=0.10000×10 
     A4=-0.45665×10 -3   
     A6=-0.10128×10 -5   
     A8=0.42417×10 -7   
     A10=0.36281×10 -9   
     r9: e=0.10000×10 
     A3=0.70892×10 -4   
     A4=0.60642×10 -4   
     A5=-0.18549×10 -5   
     A6=-0.12151×10 -6   
     A7=-0.59251×10 -7   
     A8=0.54404×10 -7   
     A9=-0.71229×10 -8   
     A10=0.15165×10 -9   
     A11=0.25685×10 -13   
     A12=0.39647×10 -11   
     FIGS. 1 through 6 show the constructions of the lens units of embodiments 1 through 6 described above. They show the positions of the lenses in the shortest focal length condition. Loci m1, m2 and m3 in the drawings show the movements of first lens unit Gr1, second lens unit Gr2 and third lens unit Gr3 during zooming from their positions in the shortest focal length condition to their positions in the longest focal length condition, respectively. Each embodiment has a three-component construction consisting of negative, positive and negative lens units, wherein each lens unit consists of two lenses, and wherein the first lens unit keeps stationary during zooming. Second lens unit Gr2 of each embodiment consists of a positive lens and a negative lens. 
     Embodiments 1 through 3 and 6 have, from the object side, positive first lens unit Gr1 consisting of a negative lens having concave surfaces on both sides and a positive meniscus lens having a convex surface on the object side, second lens unit Gr2 consisting of a negative meniscus lens having a concave surface on the image side and a positive lens having convex surfaces on both sides, and third lens unit Gr3 consisting of a positive meniscus lens having a convex surface on the image side and a negative lens having concave surfaces on both sides. Embodiments 4 and 5 have, from the object side, first lens unit Gr1 consisting of a negative lens having concave surfaces on both sides and a positive meniscus lens having a convex surface on the object side, second lens unit Gr2 consisting of a positive lens having convex surfaces on both sides and a negative meniscus lens having a concave surface on the object side, and third lens unit Gr3 consisting of a positive meniscus lens having a convex surface on the image side and a negative meniscus lens having a concave surface on the object side. 
     In embodiments 1 and 3, both sides of the negative meniscus lens of second lens unit Gr2, which has a concave surface on the image side, and the object side surface of the positive meniscus lens of third lens unit Gr3, which has a convex surface on the image side, are aspherical. In embodiments 2 and 6, the image side surface of the positive meniscus lens of first lens unit Gr1, which has a convex surface on the object side, both sides of the negative meniscus lens of second lens unit Gr2, which has a concave surface on the image side, and the object side surface of the positive meniscus lens of third lens unit Gr3, which has a convex surface on the image side, are aspherical. In embodiments 4 and 5, both sides of the positive lens of second lens unit Gr2, which has convex surfaces on both sides, and the object side surface of the positive meniscus lens of third lens unit Gr3, which has a convex surface on the image side, are aspherical. 
     FIGS. 7A-7C, 10A-10C, 13A-13C, 16A-16C, 19A-19C and 22A-22C show the aberrations of embodiments 1 through 6 in the shortest focal length condition. FIGS. 8A-8C, 11A-11C, 14A-14C, 17A-17C, 20A-20C and 23A-23C show the aberrations of embodiments 1 through 6 in the middle focal length condition. FIGS. 9A-9C, 12A-12C, 15A-15C, 18A-18C, 21A-21C and 24A-24C show the aberrations of embodiments 1 through 6 in the longest focal length condition. 
     In the drawings indicating spherical aberrations, solid line d represents the spherical aberration with regard to the d-line, and dashed line g represents the spherical aberration with regard to the g-line, while the dotted line SC represents the sine condition. In the drawings indicating astigmatism, dotted line DM and solid line DS represent the astigmatism on the meridional surface and the sagittal surface, respectively. 
     Table 1 shows the numbers that meet conditions (1) through (3) in embodiments 1 through 6, as well as the construction of the lenses of second lens unit Gr2. 
     
                       TABLE 1______________________________________  Second lens unit  construction             t1/y   t2/y   |(fw/f1) × W0|______________________________________Embodiment    -+           0.12   0.21 19.2Embodiment    -+           0.16   0.18 26.12Embodiment    -+           0.16   0.18 18.83Embodiment    +-           0.16   0.21 31.14Embodiment    +-           0.16   0.19 26.75Embodiment    -+           0.16   0.16 23.86______________________________________ 
    
     Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the invention, they should be construed as being included therein.