Abstract:
The invention provides a zoom lens which comprises a first lens group G1 of positive refracting power and a second lens group G2 of negative refracting power, said first lens group comprising, in order from the object side, a first lens L 1  formed of plastics having not substantially refracting power, a second lens L 2  of negative refracting power and a third lens L 3  of positive refracting power. This zoom lens, albeit using a plastic lens, is less likely to undergo a variation in the focal position due to temperature and humidity changes, inexpensive, compact and of good performance.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a zoom lens system and, more particularly, to a compact and high-performance, two-group type of zoom lens system making use of a plastic lens, which is best fit for use on a lens shutter camera having a zoom ratio of about 2. 
     For achieving an inexpensive and compact zoom lens, it is of course effective to reduce the number of lenses used. Reducing the number of lens groups used should be preferable as well, because this makes it possible to simplify the construction of the lens frame, reduce performance degradation due to eccentric displacements of the groups and attain stable performance at low costs. In such a context, reducing the number of lenses used in a two-group type of zoom lens system is effective for achieving a compact and inexpensive zoom lens. 
     Even in the case of a zoom lens for a compact camera having a zoom ratio of 2 or thereabout, however, it is imperatively required to use an aspherical surface for gaining good-enough performance, if five or fewer lenses are used to this end. 
     One typical example of the prior art aiming at making a zoom lens compact by reducing the number of lenses used is set forth in Japanese Provisional Patent Publication No. 3-127008. However, the example of this publication teaches the use of an aspherical glass surface. Of course, the use of a glass lens places some limitation on cutting down the price of a zoom lens; to put it another way, the use of a plastic lens is inevitable for reducing that price further. For instance, this is typically disclosed in Japanese Provisional Patent Publication No. 2-181110. 
     When a plastic lens is employed, however, attention should be paid to large changes in its refractive index and lens configuration due to temperature and humidity changes. The above-mentioned publication states that any focal position displacement occurring by temperature changes can be set off by the use of positive and negative lenses. This procedure may be effective only in the context that the refractive index changes due to temperature changes are stabilized within a relatively short span of time. However, it is indeed impossible to neglect surface configuration changes which are affected by how lenses are held by the lens frame, and so it is very difficult to predict focal position displacements, if occurs. 
     When there are humidity changes, however, much time is needed for stabilizing lenses&#39; refractive indices and configurations. If the power of a single plastic lens is strong, as in the above-mentioned example of the prior art, the focal position will indeed be subject to some large displacement in the course of humidity absorption, although depending on to what degree each lens absorbs humidity. In addition, when varying plastic materials are used at the same time, as in the above-mentioned example, even larger focal position displacements will occur due to the fact that the degree of humidity absorption differs from time to time. 
     SUMMARY OF THE INVENTION 
     In view of such problems as mentioned above, an object of the invention is to provide a compact, inexpensive and high-performance zoom lens system making use of a plastic lens, in which variations in the focal position displacement due to temperature and humidity changes are reduced considerably and, more particularly, a zoom lens system that is chiefly suited for use on a lens shutter camera having a zoom ratio of 2 or therearound. 
     A zoom lens system of the invention designed for attaining the object mentioned above includes, in order from the object side, a first lens formed of a plastic lens in a powerless lens form, a second lens of negative power and a third lens of positive power, and comprises a first lens unit or group having positive power as a whole and a second lens group having a negative power. 
     Another zoom lens system of the invention includes, in order from the object side, a first lens formed of a plastic lens with the convex surface turning in the same direction so as to be made powerless, a second lens of negative power and a third lens of positive power, and comprises a first lens group having a positive power as a whole and a second lens group having negative power. 
     In each of the two aspects of the invention, the first lens may be a meniscus lens convex on the object side, a meniscus lens that is convex on the object side and includes an aspherical surface on at least one surface or a meniscus lens that is convex on the object side, includes an aspherical surface on at least one surface and conforms to the following condition (1): 
     
         -5&lt;f.sub.T.sup.2 /(f.sub.1 ·F)&lt;5                  (1) 
    
     where 
     f T  is the focal length of the total system at the telephoto end, 
     f 1  is the focal length of the first lens, and 
     F is the F-number at the telephoto end. 
     More preferably, the following condition (2) should be met: 
     
         -3&lt;f.sub.T.sup.2 /(f.sub.1 ·F)&lt;3                  (2) 
    
     The second lens group mentioned above may include a positive meniscus lens convex on the image side and a negative lens or at least one plastic lens, and conforms to the following conditions as well: 
     
         -3.5&lt;(f.sub.T.sup.2 -f.sub.W.sup.2)/(f.sub.1 F)&lt;2.5        (3) 
    
     
         -4&lt;f.sub.T.sup.2 /(f.sub.1 F)+f.sub.4 (β.sub.2T -β.sub.R).sup.2 /F&lt;4                                                      (4) 
    
     where 
     f T  is the focal length of the total system at the telephoto end, 
     f W  is the focal length of the total system at the wide angle end, 
     f 1  is the focal length of the first lens, 
     f 4  is the focal length of the plastic lens in the second lens group, 
     F is the F-number at the telephoto end, 
     β 2T  is the magnification of the second lens unit at the telephoto end, and 
     β R  is the magnification of an optical system located in the rear of said plastic lens. 
     More preferably, the following conditions (5) and (6) are met: 
     
         -2&lt;(f.sub.T.sup.2 -f.sub.W.sup.2)/(f.sub.1 ·F)&lt;1.5(5) 
    
     
         -2.5&lt;f.sub.T.sup.2 /(f.sub.1 ·F)+f.sub.4 (β.sub.2T -β.sub.R).sup.2 /F&lt;2.5                               (6) 
    
     In addition, the following condition (7) should preferably be satisfied: 
     
         f.sub.4 (β.sub.2T -β.sub.R).sup.2 /F&lt;3.5         (7) 
    
     For zooming, it is preferred that the first and second lens groups move toward the object side from the wide angle to telephoto end. It is then preferred that the first and second lens groups move toward each other during zooming; that is, the spacing between them is narrowing during zooming. 
     In the ensuing description, reference will be made to why and how the construction of the invention is adopted and acts. 
     If a plastic lens is used, it is possible to obtain a lens system at low costs, as already mentioned. However, the plastic lens is so large in terms of the refractive index and configuration changes due to temperature and humidity changes that there can be a large variation in the focal position. 
     As already stated, it is also indeed difficult to set off the focal position variation by means of positive and negative plastic lenses. 
     In order to reduce or limit the focal position variation, there is need of making the plastic lens used as close to a powerless lens as possible and reducing an adverse influence of the single plastic lens&#39;s changes in the refractive index and configuration upon a variation in the focal position of the total system. 
     For the reasons mentioned just above, the lens system of the invention comprises a first positive lens group G1 and a second negative lens group G2, and the first lens group G1 is constructed from, in order from the object side, a substantially powerless, plastic and aspherical meniscus lens L 1  that is convex on the object side, a negative lens L 2  and a positive lens L 3 . 
     Such a lens arrangement enables the lenses L 2  and L 3  to bring the principal point of the first lens group G1 close to the image side, while on-axial aberration and chromatic aberration of magnification are well corrected by the negative and positive lenses, so that the lens system can achieve compactness with a well-enough zoom ratio. Since the meniscus lens convex on the object side including an aspherical surface is used as the lens L 1 , spherical aberration and coma induced mainly through the lenses L 2  and L 3  can be well corrected regardless of the fact that the lens L 1  is nearly powerless. Now let f 1 , f T  and F denote the focal length of the lens L 1 , the focal length of the total system at the telephoto end and the F-number at the telephoto end. It is then desired that 
     
         -5&lt;f.sub.T.sup.2 /(f.sub.1 ·F)&lt;5                  (1) 
    
     so as to reduce the focal position variation of the total system due to temperature and humidity changes. 
     Referring now to the recent trend of compact cameras, they are mainly taking the form of cameras capable of making panoramic prints. To add to this, pseudo telephotography is envisaged, which is designed to use a part of film plane at the time of shooting and make a print of ordinary size at the time of printing. 
     With such cameras in which the magnification of enlargement becomes high at the time of printing, some difficulty is involved in obtaining clear photographs, unless the accuracy of focusing is higher than usual. 
     In addition, even higher accuracy is demanded not only for using a part of film and making prints on an ordinary enlarged scale but for making enlarged prints as well. 
     In order to obtain even higher accuracy, it is desired that the following condition (2) be satisfied in addition to the condition (1): 
     
         -3&lt;f.sub.T.sup.2 /(f.sub.1 ·F)&lt;3                  (2) 
    
     In terms of correcting the curvature of the field and distortion, it is also desired that the second lens group G2 be constructed from a positive meniscus lens L 4  convex on the image side and a negative lens L 5 . 
     Such a lens arrangement enables the plastic lens to be effectively used for making the zoom lens system compact, improving its performance and stabilizing its focal position with respect to temperature and humidity changes; that is, it is possible to achieve a compact, inexpensive and high-performance zoom lens system. 
     A zoom lens system of even higher performance is achievable at even lower costs by constructing the first lens group G1 from a substantially powerless, aspherical plastic lens L 1 , a negative lens L 2  and a positive lens L 3  and the second lens group G2 from a positive, aspherical and plastic lens L 4  and a negative lens L 5 , as shown in FIG. 3. 
     However, it is here noted that the use of the plastic lens in the second lens group G2 again gives rise to a focal position variation due to temperature and humidity changes. 
     For this reason, it is required that the focal lengths f 1  and f 4  of the lenses L 1  and L 4  conform to the following conditions (3) and (4): 
     
         -3.5&lt;(f.sub.T.sup.2 -f.sub.W.sup.2)/(f.sub.1 F)&lt;2.5        (3) 
    
     
         -4&lt;f.sub.T.sup.2 /(f.sub.1 F)+f.sub.4 (β.sub.2T -β.sub.R).sub.2 /F&lt;4                                                      (4) 
    
     where 
     f W  is the focal length of the total system at the wide angle end, 
     β 2T  is the magnification of the second lens group G2 at the telephoto end, and 
     β R  is the magnification of an optical system (the lens L 5  in FIG. 3) in the rear of the lens L 4  at the telephoto end. 
     The condition (3) is provided to limit a difference in the focal position displacement from the wide angle to telephoto conditions, which occurs through the lens L 1  due to temperature and humidity changes. Above and below the upper and lower limits, the positive or negative power of the lens L 1  becomes too strong to obtain stable performance, because there is a large focal position displacement. 
     The condition (4) is provided to limit the focal position displacements of the lenses L 1  and L 4  that occurs due to temperature and humidity changes. If the conditions (3) and (4) are satisfied at the same time, it is then possible to achieve a zoom lens system that is less subject to focal position displacement due to temperature and humidity changes regardless of using plastic lenses in the first and second lens groups G1 and G2. 
     When higher accuracy is needed for panoramic photography or enlargement, as is the case with the condition (2), it is desired to meet the following conditions (5) and (6): 
     
         -2&lt;(f.sub.T.sup.2 -f.sub.W.sup.2)/(f.sub.1 ·F)&lt;1.5(5) 
    
     
         -2.5&lt;f.sub.T.sup.2 /(f.sub.1 ·F)+f.sub.4 (β.sub.2T -β.sub.R).sup.2 /F&lt;2.5                               (6) 
    
     In this case, it is further desired to meet the following condition (7): 
     
         f.sub.4 (β.sub.2T -β.sub.R).sup.2 /F.sup.&lt; 3.5   (7) 
    
     The condition (7) is provided to limit some focal position displacement that occurs through the lens L 4  due to temperature and humidity changes. In order to further reduce the focal position displacement of the total system from the wide angle to telephoto conditions, it is desired to meet this condition (7) in addition the conditions (3) and (4) or (5) and (6). 
     Still other objects and advantages of the invention will be in part be obvious and will in part be apparent from the specification. 
     The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 represents a lens arrangement in section of Example 1 of the zoom lens system including a powerless lens according to the invention at the wide angle and telephoto ends. 
     FIG. 2 is a sectional view of Example 2 at the wide angle end. 
     FIG. 3 is a sectional view of Example 3 at the wide angle end. 
     FIG. 4 is a sectional view of Example 4 at the wide angle end. 
     FIG. 5 is a sectional view of Example 5 at the wide angle end. 
     FIG. 6 is a sectional view of Example 6 at the wide angle end. 
     FIG. 7 is a sectional view of Example 7 at the wide angle end. 
     FIG. 8 is aberration diagrams showing the spherical aberration, distortion, astigmatism and chromatic aberration of Example 1 in the (a) wide angle, (b) standard and (c) telephoto conditions. 
     FIG. 9 is aberration diagrams of Example 2 similar those of FIG. 8. 
     FIG. 10 is aberration diagrams of Example 3 similar to those of FIG. 8. 
     FIG. 11 is aberration diagrams of Example 4 similar those of FIG. 8. 
     FIG. 12 is aberration diagrams of Example 5 similar to those of FIG. 8. 
     FIG. 13 is aberration diagrams of Example 6 similar those of FIG. 8. 
     FIG. 14 is aberration diagrams of Example 7 similar to those of FIG. 8. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In what follows, the invention will now be explained with reference to Examples 1-7. The lens data of each example will be given later, and the lens arrangement of Example 1 at the wide angle and telephoto ends is shown in section in FIG. 1 while the lens arrangements of Examples 2-7 at the wide angles are shown in section in FIGS. 2-7. Throughout the drawings, FP stands for a focal plane. 
     Examples 1-7 are each directed to a two-group zoom lens system comprising the first and second lens groups G1 and G2. In the zoom lens system of Example 1, the first and second lens groups G1 and G2 move toward the object side from the wide angle to telephoto conditions while the space d 7  between them is narrowing for zooming. 
     The first lens group G1 comprises, in order from the object side, a plastic lens L 1  of weak power with the rear surface defined by an aspherical surface, a negative lens L 2  and a positive lens L 3 , said lens L 1  being a meniscus lens convex on the object side. The second lens group G2 comprises, in order from the object side, a positive lens L 4  and a negative lens L 5 . 
     In the zoom lens system of Example 2, the first and second lens groups G1 and G2 move toward the object side from the wide angle to telephoto conditions while the space d 6  between them is narrowing for zooming. 
     The first lens group G1 comprises, in order from the object side, a plastic lens L 1  of weak power with the rear surface defined by an aspherical surface, a negative lens L 2  and a positive lens L 3 , said lens L 1  being a meniscus lens convex on the object side. The negative and positive lenses L 2  and L 3  are so cemented together that the degradation of the performance of the zoom lens system by their eccentric displacements can be reduced while the chromatic aberration is well corrected. The second lens group G2 comprises, in order from the object side, a positive lens L 4  and a negative lens L 5 . 
     In the zoom lens system of Example 3, the first and second lens groups G1 and G2 move toward the object side from the wide angle to telephoto conditions while the space d 7  between them is narrowing for zooming. 
     The first lens group G1 comprises, in order from the object side, a plastic lens L 1  of weak power with the rear surface defined by an aspherical surface, a negative lens L 2  and a positive lens L 3 , said lens L 1  being a meniscus lens convex on the object side. The second lens group G2 comprises a plastic lens L 4  and a negative lens L 5 , the rear surface of said plastic lens L 4  being defined by an aspherical surface. 
     In the zoom lens system of Example 4 or 5, the first and second lens groups G1 and G2 move toward the object side from the wide angle to telephoto conditions while the space d 6  between them is narrowing for zooming. 
     The first lens group G1 comprises, in order from the object side, a plastic lens L 1  of weak power with the rear surface defined by an aspherical surface, a negative lens L 2  and a positive lens L 3 , said lens L 1  being a meniscus lens convex on the object side. The negative and positive lenses L 2  and L 3  are so cemented together that the degradation of the performance of the zoom lens system by their eccentric displacements can be reduced while the chromatic aberration is well corrected. The second lens group G2 comprises a plastic lens L 4  and a negative lens L 5 , both surfaces of said plastic lens L 4  being defined by aspherical surfaces. 
     In the zoom lens system of Example 6, the first and second lens groups G1 and G2 move toward the object side from the wide angle to telephoto conditions while the space d 7  between them is narrowing for zooming. 
     The first lens group G1 comprises, in order from the object side, a plastic lens L 1  of weak power with the rear surface defined by an aspherical surface, a negative lens L 2  and a positive lens L 3 , said lens L 1  being a meniscus lens convex on the object side. The negative and positive lenses L 2  and L 3  are cemented together. The second lens group G2 comprises a plastic lens L 4  and a negative lens L 5 , both surfaces of said plastic lens L 4  being defined by aspherical surfaces. 
     In the zoom lens system of Example 7, the first and second lens groups G1 and G2 move toward the object side from the wide angle to telephoto conditions while the space d 7  between them is narrowing for zooming. 
     The first lens group G1 comprises, in order from the object side, a plastic lens L 1  of weak power with the rear surface defined by an aspherical surface, a negative lens L 2  and a positive lens L 3 , said lens L 1  being a meniscus lens convex on the object side. The second lens group G2 comprises a plastic lens L 4  and a negative lens L 5 , both surfaces of said plastic lens L 4  being defined by aspherical surfaces. 
     It is noted that in all the examples PMMA (polymethyl methacrylate) is used as plastics. 
     In the lens data of Examples 1-7, symbols used but not used hereinbefore having the following means: f is the focal length of the total system, F NO  is the F-number, 2ω is the field angle, r 1 , r 2  . . . are the radii of curvature of the lens surfaces, d 1 , d 2  . . . are the spaces between the lens surfaces, n d1 , n d2  . . . the d-line refractive indices of the lenses and v 1 , v 2  . . . are the Abbe&#39;s number of the lenses. The aspherical configuration applied, when x and r stand for the axial direction and the direction normal thereto, respectively, is represented by 
     
         x=(y.sup.2 /r)/[1+{1-P(y.sup.2 /r.sup.2)}.sup.1/2 ]+A.sub.4y.sup.4 +A.sub.6y.sup.6 +A.sub.8y.sup.8 +A.sub.10y.sup.10 
    
     where 
     r is the paraxial radii of curvature, 
     P is the conical coefficient, and 
     A 4 , A 6 , A 8  and A 10  are the aspherical coefficients. 
     EXAMPLE 1 
     
         ______________________________________f = 39.0˜47.6˜58.1F.sub.NO = 5.4˜6.6˜8.02ω = 57.9°˜48.8°˜40.8°______________________________________r.sub.1 = 13.8017   d.sub.1 =                   3.000 n.sub.d1 = 1.49216                                  ν.sub.d1 = 57.50r.sub.2 = 13.6639   d.sub.2 =                   2.000 (Aspheric)r.sub.3 = ∞ (Stop)           d.sub.3 =                   1.800r.sub.4 = -28.0821  d.sub.4 =                   1.499 n.sub.d2 = 1.68893                                  ν.sub.d2 = 31.08r.sub.5 = 133.4203  d.sub.5 =                   0.869r.sub.6 = 52.4470   d.sub.6 =                   3.499 n.sub.d3 = 1.65160                                  ν.sub.d3 = 58.52r.sub.7 = -14.0091  d.sub.7 =                   (Vari-                   able)r.sub.8 = -23.6740  d.sub.8 =                   2.998 n.sub.d4 = 1.53256                                  ν.sub.d4 = 45.91r.sub.9 = -19.0810  d.sub.9 =                   6.820r.sub.10 = -16.7960  d.sub.10 =                   1.600 n.sub.d5 = 1.60729                                  ν.sub.d5 = 59.38r.sub.11 = -140.7120______________________________________Zooming Spaces______________________________________f       39.0          47.6    58.1d.sub.7 13.2998       7.6375  3.0022______________________________________Aspherical CoefficientsSecond surface______________________________________     P = 7.3423     A.sub.4 = -0.18567 × 10.sup.-3     A.sub.6 = -0.34568 × 10.sup.-5     A.sub.8 = -0.42417 × 10.sup.-6     A.sub.10 = 0.22258 × 10.sup.-8______________________________________ 
    
     EXAMPLE 2 
     
         ______________________________________f = 39.1˜47.7˜58.2F.sub.NO = 5.4˜6.6˜8.02ω = 57.8°˜48.7°˜40.7°______________________________________r.sub.1 = 16.4290   d.sub.1 =                   2.000 n.sub.d1 = 1.49216                                  ν.sub.d1 = 57.50r.sub.2 = 18.4496   d.sub.2 =                   2.200 (Aspheric)r.sub.3 = ∞ (Stop)           d.sub.3 =                   2.000r.sub.4 = -26.7392  d.sub.4 =                   2.137 n.sub.d2 = 1.62096                                  ν.sub.d2 = 35.88r.sub.5 = 33.3053   d.sub.5 =                   4.645 n.sub.d3 = 1.69680                                  ν.sub.d3 = 56.49r.sub.6 = -15.3801  d.sub.6 =                   (Vari-                   able)r.sub.7 = -58.4901  d.sub.7 =                   3.000 n.sub.d4 = 1.53256                                  ν.sub.d4 = 45.91r.sub.8 = -30.7889  d.sub.8 =                   6.584r.sub.9 = -21.4510  d.sub.9 1.640 n.sub.d5 = 1.62041                                  ν.sub.d5 = 60.06r.sub.10 = 366.8262______________________________________Zooming Spaces______________________________________f       39.1          47.7    58.2d.sub.6 15.1667       7.9333  2.0013______________________________________Aspherical CoefficientsSecond surface______________________________________     P = 7.2977     A.sub.4 = -0.37658 × 10.sup.-4     A.sub.6 = 0.40754 × 10.sup.-6     A.sub.8 = -0.12031 × 10.sup.-6     A.sub.10 = 0.32719 × 10.sup.-8______________________________________ 
    
     EXAMPLE 3 
     
         ______________________________________f = 39.1˜47.7˜58.2F.sub.NO = 5.4˜6.6˜8.02ω = 57.8°˜48.7°˜40.7______________________________________r.sub.1 = 37.5030   d.sub.1 =                   2.070 n.sub.d1 = 1.49216                                  ν.sub.d1 = 57.50r.sub.2 = 39.2314   d.sub.2 =                   3.732 (Aspheric)r.sub.3 = 938.2603  d.sub.3 =                   1.700 n.sub.d2 = 1.74077                                  ν.sub.d2 = 27.79r.sub.4 = 37.9237   d.sub.4 =                   1.157r.sub.5 = 49.3242   d.sub.5 =                   5.000 n.sub.d3 = 1.55232                                  ν.sub.d3 = 63.75r.sub.6 = -13.2806  d.sub.6 =                   1.000r.sub.7 = ∞ (Stop)           d.sub.7 =                   (Vari-                   able)r.sub.8 = -36.0739  d.sub.8 =                   3.299 n.sub.d4 = 1.49216                                  ν.sub.d4 = 57.50r.sub.9 = -33.9627  d.sub.9 =                   7.880 (Aspheric)r.sub.10 = -9.7157   d.sub.10 =                   1.640 n.sub.d5 = 1.48749                                  ν.sub.d5 = 70.20r.sub.11 = -31.4655______________________________________Zooming Spaces______________________________________f       39.1          47.7    58.2d.sub.7 10.0045       6.1596  3.0057______________________________________Aspherical Coefficients______________________________________Second surface     P = 7.0007     A.sub.4 = 0.10760 × 10.sup.-3     A.sub.6 = 0.12652 × 10.sup.-5     A.sub.8 = -0.14009 × 10.sup.-7     A.sub.10 = 0.52497 × 10.sup.-99th surface     P = 0.7323     A.sub.4 = -0.66357 × 10.sup.-4     A.sub.6 = 0.22125 × 10.sup.-7     A.sub.8 = -0.15111 × 10.sup.-7     A.sub.10 = 0.81772 × 10.sup.-10______________________________________ 
    
     EXAMPLE 4 
     
         ______________________________________f = 39.1˜47.7˜58.2F.sub.NO = 4.8˜5.8˜7.12ω = 57.8°˜48.7°˜40.7°______________________________________r.sub.1 = 54.7616   d.sub.1 =                   2.000 n.sub.d1 = 1.49216                                  ν.sub.d1 = 57.50r.sub.2 = 52.9242   d.sub.2 =                   1.999 (Aspheric)r.sub.3 = 72.9627   d.sub.3 =                   1.600 n.sub.d2 = 1.59270                                  ν.sub.d2 = 35.29r.sub.4 = 17.5841   d.sub.4 =                   9.104 n.sub.d3 = 1.48749                                  ν.sub.d3 = 70.20r.sub.5 = -12.4690  d.sub.5 =                   0.800r.sub.6 = ∞ (Stop)           d.sub.6 =                   (Vari-                   able)r.sub.7 = -30.5323  d.sub.7 =                   3.085 n.sub.d4 = 1.49216                                  ν.sub.d4 = 57.50 (Aspheric)r.sub.8 = -24.0804  d.sub.8 =                   4.922 (Aspheric)r.sub.9  = -8.0710   d.sub.9 =                   1.886 n.sub.d5 = 1.48749                                  ν.sub.d5 = 70.20r.sub.10 = -32.6435______________________________________Zooming Spaces______________________________________f       39.1          47.7    58.2d.sub.6 8.4864        5.5804  3.2000______________________________________Aspherical Coefficients______________________________________Second surface     P = -0.2414     A.sub.4 = 0.13474 × 10.sup.-3     A.sub.6 = 0.59240 × 10.sup.-6     A.sub.8 = 0.75852 × 10.sup.-9     A.sub.10 = 0.31983 × 10.sup.-97th surface     P = 1.0000     A.sub.4 = 0.80691 × 10.sup.-4     A.sub.6 = 0.74704 × 10.sup.-7     A.sub.8 = 0.13882 × 10.sup.-7     A.sub.10 = 08th surface     P = -1.1257     A.sub.4 = -0.66285 × 10.sup.-4     A.sub.6 = 0.65601 × 10.sup.-6     A.sub.8 = -0.45874 × 10.sup.-7     A.sub. 10 = 0.40598 × 10.sup.-9______________________________________ 
    
     EXAMPLE 5 
     
         ______________________________________f = 39.1˜51.6˜68.0F.sub.NO = 4.6˜6.0˜8.02ω = 57.8°˜45.4°˜35.2°______________________________________r.sub.1 = 83.5479   d.sub.1 =                   2.000 n.sub.d1 = 1.49216                                  ν.sub.d1 = 57.50r.sub.2 = 50.8906   d.sub.2 =                   6.643 (Aspheric)r.sub.3 = 60.5568   d.sub.3 =                   1.600 n.sub.d2 = 1.59270                                  ν.sub.d2 = 35.29r.sub.4 = 16.6254   d.sub.4 =                   5.783 n.sub.d3 = 1.49782                                  ν.sub.d3 = 66.83r.sub.5 = -15.1887  d.sub.5 =                   0.936r.sub.6 = ∞ (Stop)           d.sub.6 =                   (Vari-                   able)r.sub.7 = -36.3535  d.sub.7 =                   4.236 n.sub.d4 = 1.49216                                  ν.sub.d4 = 57.50 (Aspheric)r.sub.8 = -22.5844  d.sub.8 =                   5.557 (Aspheric)r.sub.9  = -11.0049  d.sub.9 =                   1.886 n.sub.d5 = 1.48749                                  ν.sub.d5 = 70.20r.sub.10 = -193.7938______________________________________Zooming Spaces______________________________________f       39.1          51.6    68.0d.sub.6 13.2916       7.5513  3.1994______________________________________Aspherical Coefficients______________________________________Second surface     P = 7.2967     A.sub.4 = 0.76367 × 10.sup.-4     A.sub.6 = 0.20592 × 10.sup.-6     A.sub.8 = 0.27824 × 10.sup.-8     A.sub.10 = 0.48829 × 10.sup.-107th surface     P = 1.0000     A.sub.4 = 0.93729 × 10.sup.-5     A.sub.6 = 0.12606 × 10.sup.-6     A.sub.8 = -0.30141 × 10.sup.-8     A.sub.10 = 08th surface     P = 1.0044     A.sub.4 = -0.43684 × 10.sup.-4     A.sub.6 = 0.64361 × 10.sup.-6     A.sub.8 = -0.15684 × 10.sup.-7     A.sub.10 = 0.61362 × 10.sup.-10______________________________________ 
    
     EXAMPLE 6 
     
         ______________________________________f = 39.2˜55.3˜78.0F.sub.NO = 4.0˜5.7˜8.02ω = 57.7°˜42.7°˜31.0°______________________________________r.sub.1 = 32.4820   d.sub.1 =                   2.000 n.sub.d1 = 1.49216                                  ν.sub.d1 = 57.50r.sub.2 = 27.3150   d.sub.2 =                   6.053 (Aspheric)r.sub.3 = 77.1868   d.sub.3 =                   1.600 n.sub.d2 = 1.64769                                  ν.sub.d2 = 33.80r.sub.4 = 21.3660   d.sub.4 =                   7.010 n.sub.d3 = 1.47069                                  ν.sub.d3 = 67.39r.sub.5 = -14.2345  d.sub.5 =                   1.000r.sub.6 = ∞ (Stop)           d.sub.6 =                   (Vari-                   able)r.sub.7 = -43.1827  d.sub.7 =                   3.500 n.sub.d4 = 1.49216                                  ν.sub.d4 = 57.50r.sub.8 = -23.4296  d.sub.8 =                   5.561 (Aspheric)r.sub.9 = - 11.7187 d.sub.9 =                   1.900 n.sub.d5 = 1.48749                                  ν.sub.d5 = 70.20r.sub.10 = -871.4262______________________________________Zooming Spaces______________________________________f       39.2          55.3    78.0d.sub.6 16.9499       8.7882  3.0002______________________________________Aspherical Coefficients______________________________________Second surface     P = 7.3085     A.sub.4 = 0.40364 × 10.sup.-4     A.sub.6 = -0.73843 × 10.sup.-7     A.sub.8 = 0.12057 × 10.sup.-7     A.sub.10 = -0.78879 × 10.sup.-108th surface     P = 0.9968     A.sub.4 = -0.33270 × 10.sup.-4     A.sub.6 = -0.99132 × 10.sup.-7     A.sub.8 = -0.24353 × 10.sup.-8     A.sub.10 = 0.77832 × 10.sup.-12______________________________________ 
    
     EXAMPLE 7 
     
         ______________________________________f = 39.3˜58.4˜86.9F.sub.NO = 3.6˜5.4˜8.02ω = 57.6°˜40.6°˜27.9°______________________________________r.sub.1 = 32.5411   d.sub.1 =                   2.000 n.sub.d1 = 1.49216                                  ν.sub.d1 = 57.50r.sub.2 = 28.4602   d.sub.2 =                   6.028 (Aspheric)r.sub.3 = 32.3365   d.sub.3 =                   1.600 n.sub.d2 = 1.69895                                  ν.sub.d2 = 30.12r.sub.4 = 17.8136   d.sub.4 =                   1.723r.sub.5 = 25.8956   d.sub.5 =                   7.277 n.sub.d3 = 1.46450                                  ν.sub.d3 = 65.94r.sub.6 = -14.7867  d.sub.6 =                   1.000r.sub.7 = ∞ (Stop)           d.sub.7 =                   (Vari-                   able)r.sub.8 = -45.0078  d.sub.8 =                   2.999 n.sub.d4 = 1.49216                                  ν.sub.d4 = 57.50 (Aspheric)r.sub.9 = - 28.5755 d.sub.9 =                   6.343 (Aspheric)r.sub.10 = -11.9582  d.sub.10 =                   1.000 n.sub.d5 = 1.51728                                  ν.sub.d5 = 69.56r.sub.11 = -151.5315______________________________________Zooming Spaces______________________________________f       39.3          58.4    86.9d.sub.7 16.9245       8.5875  2.9994______________________________________Aspherical Coefficients______________________________________Second surface     P = 7.2288     A.sub.4 = 0.25309 × 10.sup.-4     A.sub.6 = 0.32346 × 10.sup.-6     A.sub.8 = -0.22486 × 10.sup.-8     A.sub.10 = 0.29186 × 10.sup.-108th surface     P = 1.0000     A.sub.4 = 0.19680 × 10.sup.-4     A.sub.6 = 0.39318 × 10.sup.-6     A.sub.8 = -0.31302 × 10.sup.-8     A.sub.10 = 09th surface     P = 0.9970     A.sub.4 = -0.17618 × 10.sup.-4     A.sub.6 = 0.31893 × 10.sup.-6     A.sub. 8 = -0.54907 × 10.sup.-8     A.sub.10 = 0.12852 × 10.sup.-10______________________________________ 
    
     The spherical aberrations, distortions, astigmatisms and chromatic aberrations of magnification of the zoom lens systems of Example 1-7 at the (a) wide angle, (b) standard and (c) telephoto conditions are shown in the aberration diagrams of FIGS. 8(a), (b) and (c) through FIGS. 14(a), (b) and (c). 
     The values of the conditions (1) through (7) referred to in Examples 1 through 7 are tabulated below. 
     
         ______________________________________Cond.(1),(2) Cond.(3),(5)                   Cond.(4),(6)                               Cond. (7)______________________________________Ex. 10.94Ex. 21.84Ex. 3            0.19       0.35      0.01Ex. 4            -0.05      0.45      0.55Ex. 5            -1.43      0.06      2.2Ex. 6            -1.42      2         3.99Ex. 7            -1.37      1.58      3.3______________________________________ 
    
     As described above, the zoom lens system including a powerless lens according to the invention is an expensive, compact and high-performance zoom lens system, which is less likely to vary in terms of the focal position of the total system due to temperature and humidity changes. This zoom lens system lends itself well fit for use on a lens shutter camera having a zoom ratio of 2 or therearound.