Abstract:
An improved zoom lens system is disclosed which includes at least three lens groups which are arranged, in order from the object side, as a first lens group having a positive focal length, a second lens group having a positive focal length and a third lens group having a negative focal length. When zooming is carried out from the wide-angle to the narrow-angle end, the first, second and third lens groups are all moved towards the object so that the distance between the first and second lens groups is increased whereas the distance between the second and third lens groups is decreased. The system is characterized in that the second lens group has at least two aspheric surfaces.

Description:
BACKGROUND OF THE INVENTION 
     The present application is based upon Japanese Patent Application No. Hei. 3-315607 filed Sep. 24, 1991, a disclosure of which is incorporated herein by reference. 
     The present invention relates generally to a zoom lens system that is suitable for use with compact cameras which have a smaller constraint on back focus than single-lens reflex cameras. More specifically, the present invention relates to a zoom lens system that features a high zoom ratio of 2.5 and more. 
     Various types of zoom lens systems have heretofore been known for use with compact cameras. Zoom lenses consisting of three lens groups or more with a zoom ratio exceeding 2 are categorized as follows: 
     (i) Four-group zoom lens system comprising four lens groups (positive, negative, positive and negative groups), with a stop diaphragm being provided between the second and third groups, all lens groups being moved independently of each other towards the object (this type includes a system where some of the four lens groups are moved in unison). Examples of such zoom lens systems are disclosed in Japanese Patent Laid-Open Publications No. SHO 63-43115, No. SHO 63-159818 and No. SHO 63-157120. 
     (ii) Three-group zoom lens system comprising three lens groups (positive, positive and negative groups), with a stop diaphragm being provided in the second group, all lens groups being moved independently of each other towards the object. Examples of this zoom lens system are disclosed in Japanese Patent Laid-Open Publications No. SHO 63-153511 and No. SHO 63-161423. 
     (iii) Three-group zoom lens system comprising three lens groups (positive, positive and negative groups), with a stop diaphragm being provided between the second and third groups, all lens groups being moved towards the object (see, for example, commonly assigned Japanese Patent Application Laid-Open No. HEI 2-73211). 
     (iv) Practically four-group zoom lens system that has the same composition as the system (iii) except that the second group is divided into a front and a rear group that are movable independently of each other (see Example 3 in the specification of commonly assigned Japanese Patent Application Laid-Open No. HEI. 2-73211, supra). 
     The above-described conventional zoom lens systems have their own problems. In the system (i), all of the four lens groups have to be moved independently of each other, so a large number of cams must be used; however, it is mechanistically difficult to accommodate those cams in the small space available for lenses for use with a compact camera. 
     The systems (i) and (ii) require that a shutter block also serving as a stop diaphragm be disposed either between the second and third lens groups (which are subject to substantial deterioration in performance due to manufacturing errors) or within the second group. Under the circumstances, high precision is required for the position of the shutter block while, at the same time, it is difficult to assure consistent optical performance since the imaging performance will be deteriorated greatly in the presence of slight errors. 
     Further, all systems (i) to (iv) have one problem in common; that is, the overall compactness of those systems is insufficient for using them with a compact camera and in each case, the overall system is composed of as many as 10 elements and more, with at least 5 elements being used in the second group. 
     The present invention has been accomplished under these circumstances of the prior art and has as an object providing a zoom lens system that features a high zoom ratio of 2.5 or more, that is short in the overall lens length, that is composed of a smaller number of lens elements and which hence is suitable for use with a compact camera. 
     SUMMARY OF THE INVENTION 
     The above-stated object of the present invention can be attained by a zoom lens system that includes at least three lens groups which are arranged, in order from the object side, as a first lens group having a positive focal length, a second lens group having a positive focal length and a third lens group having a negative focal length, wherein when zooming is carried out from the wide-angle to the narrow-angle end, the first, second and third lens groups are all moved towards the object so that the distance between the first second lens groups is increased whereas the distance between the second and third lens groups is decreased, which system is characterized in that the second lens group has at least two aspheric surfaces. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanied drawings: 
     FIG. 1 is a simplified cross-sectional view of the zoom lens system according to Example 1 are the wide-angle end; 
     FIG. 2 is a set of graphs plotting the aberration curves obtained with the zoom lens system of Example 1; 
     FIG. 3 is a simplified cross-sectional view of the zoom lens system according to Example 2 at the wide-angle end; 
     FIG. 4 is a set of graphs plotting the aberration curves obtained with the zoom lens system of Example 2; 
     FIG. 5 is a simplified cross-sectional view of the zoom lens system according to Example 3 at the wide-angle end; 
     FIG. 6 is a set of graphs plotting the aberration curves obtained with the zoom lens system of Example 3; 
     FIG. 7 is a simplified cross-sectional view of the zoom lens system according to Example 4 at the wide-angle end; 
     FIG. 8 is a set of graphs plotting the aberration curves obtained with the zoom lens system of Example 4; 
     FIG. 9 is a simplified cross-sectional view of the zoom lens system according to Example 5 at the wide-angle end; 
     FIG. 10 is a set of graphs plotting the aberration curves obtained with the zoom lens system of Example 5; 
     FIG. 11 is a simplified cross-sectional view of the zoom lens system according to Example 6 at the wide-angle end; 
     FIG. 12 is a set of graphs plotting the aberration curves obtained with the zoom lens system of Example 6; 
     FIG. 13 is a simplified cross-sectional view of the zoom lens system according to Example 7 at the wide-angle end; 
     FIG. 14 is a set of graphs plotting the aberration curves obtained with the zoom lens system of Example 7; 
     FIG. 15 is simplified cross-sectional view of the zoom lens system according to Example 8 at the wide-angle end; 
     FIG. 16 is a set of graphs plotting the aberration curves obtained with the zoom lens system of Example 8; 
     FIG. 17 is a simplified cross-sectional view of the zoom lens system according to Example 9 at the wide-angle end; and 
     FIG. 18 is a set of graphs plotting the aberration curves obtained with the zoom lens system of Example 9. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Examples of the present invention are described below. 
     Each of the zoom lens systems according to the examples of the present invention which are described hereinafter is one of a telephoto type that comprise, in order from the object side, a first lens group having a positive focal length, a second lens group of a great power having a positive focal length, and a third lens group having a negative focal length. 
     These zoom lens systems employ aspheric surfaces in the second lens group and successfully reduce the number of constituent lens elements and the overall lens length. Since the second lens group has a small lens diameter, aspheric surfaces can be easily fabricated. 
     The second lens group has such a great positive power that it is difficult to attain balance between spherical aberration and other aberrations solely by means of spherical surfaces. Thus, the second lens group has the following problems: if the lens diameter of the first lens group is reduced, the spherical aberration that develops in it tends to be undercorrected. On the other hand, the second lens group has a very great positive power, so it is difficult to attain balance between spherical and other aberrations that occur in the second lens group. It is therefore preferred that the spherical aberration that occurs in the second lens group be corrected by the divergent aspheric surface whereas all other aberrations be corrected by the aspheric surfaces, thereby attaining balance between all the aberrations that occur in the second lens group. 
     If the number of constituent lens elements is reduced, astigmatism is prone to be undercorrected and, hence, it is preferably corrected by aspheric surfaces. 
     If aspheric surfaces are also employed in the third lens group, the conventional lens arrangement which comprises, in order from the object side, a positive, a negative and a negative element can be simplified to a two-element composition that comprises a positive and a negative element or two negative elements. 
     The following is a supplemental discussion of the amount of variation in the coefficient of the third-order aberration due to an aspheric surface. The shape of an aspheric surface can generally be expressed by equation [1]: ##EQU1## where x is the distance by which the coordinates at the point on the aspheric surface where the height from the optical axis is y are departed from the plane tangent to the vertex of the aspheric surface; c is the curvature (1/r) of the vertex of the aspheric surface; K is the conic constant; and α4, α6, α8 and α10 are the aspheric coefficients of the fourth, sixth, eighth and tenth orders, respectively. 
     By calculating the focal length as f=1.0, namely, by substituting K=x/f, Y=y/f, C=fc, A4=f 3  α4, A6=f 5  α6, A8=f 7  α8 and A10=f 9  α10 into equation (1), the following equation (2) is obtained: ##EQU2## 
     The second and subsequent terms of equation (2) define the amount of asphericity and the relationship between the coefficient A4 of the second term and the third-order aspheric coefficient φ is expressed by: 
     
         φ=8(N&#39;-N)A4 
    
     where N is the refractive index of the medium before the aspheric surface and N&#39; is the refractive index of the medium after the aspheric surface. 
     The aspheric coefficient φ provides the following amounts of variation in third-order aberration coefficients as one may call in the theory of aberrations: 
     ΔI=h 4  φ 
     ΔII=h 3  Hφ 
     ΔIII=h 2  H 2  φ 
     ΔIV=h 2  H 2  φ 
     ΔV=hH 3  φ 
     where 
     I: spherical aberration coefficient; 
     II: coma coefficient; 
     III: astigmatism coefficient 
     IV: saqittal field curvature coefficient; 
     V: distortion coefficient; 
     h: the height of paraxial on-axis rays at which they pass through each lens surface; and 
     H: the height of paraxial off-axis rays passing through the center of the pupil, at which height they pass through each lens surface. 
     When aspheric surfaces are to be provided in the second lens group, the use of only one aspheric surface is insufficient to achieve a substantial reduction in the number of constituent lens elements. Therefore, it is preferred to provide at least two aspheric surfaces that satisfy the following conditions (a) and (b): 
     (a) -40&lt;ΔI2&lt;0 
     (b) -4&lt;ΔIII2&lt;0 
     where 
     ΔI2: the sum of variations in the coefficient of the third order spherical aberration due to all aspheric surfaces in the second lens group (the aberration coefficient is such as is calculated with the focal length of the overall system at the wide-angle end being taken as 1.0); and 
     ΔIII2: the sum of variation in the coefficient of astigmatism due to all aspheric surfaces in the second lens group. 
     Condition (a) must be satisfied in order to correct spherical aberration effectively by aspheric surfaces. If the upper limit of this condition is exceeded, the aspheric surfaces are no longer effective in correcting spherical aberration. If the lower limit of condition (a) is not reached, overcorrection of spherical aberration occurs. 
     Condition (b) must be satisfied in order to correct astigmatism effectively by aspheric surfaces. If the upper limit of this condition is exceeded, the aspheric surfaces are no longer effective in correcting astigmatism. If the lower limit of condition (b) is not reached, overcorrection of astigmatism occurs. 
     In order to satisfy both conditions (a) and (b), at least two aspheric surfaces must be provided in the second lens group. If one aspheric surface is provided for each of the two lens elements that compose the second lens group, the respective aspheric surfaces can be designed to perform different functions. In addition, the individual lens element can be fabricated easily since they have an aspheric surface on only one side. On the other hand, it is difficult to fabricate a lens element having two aspheric surfaces since it is a bispheric lens having a large amount of asphericity; however, this is advantageous from an economic viewpoint. 
     A discussion is made below concerning the molding of aspheric lenses. When fabricating aspheric lenses by molding optical glass, the optical glass that can be used is limited since SF-glass is not suitable for use because of the difficulty involved in molding. Under the circumstances, the second lens group is preferably designed to comprise, in order from the object side, a sub-group 2a having a negative focal length and a sub-group 2b having a positive focal length, said sub-group 2a having an aspheric lens that satisfies the following conditions (c) and (d): 
     (c) 1.68&lt;N2a 
     (d) 32&lt;ν2a 
     where 
     N2a: the refractive index at the d-line of the aspheric lens in the sub-group 2a; and 
     ν2a: the Abbe number at the d-line of the aspheric lens in the sub-group 2a. 
     Condition (c) specifies the refractive index of the aspheric lens in the sub-group 2a. Effective correction of aberrations can be assured by composing the aspheric lens in the sub-group 2a of a high-index glass that satisfies this condition. 
     Condition (d) specifies the Abbe number of the aspheric lens in the sub-group 2a. If the aspheric lens in the sub-group 2a is made of an optical material the Abbe number of which satisfies this condition, the lens can be molded easily, which is more preferred for the purposes of the present invention. 
     In the examples, all lens systems are described as falling within the category of &#34;three-group&#34; type; it should, however be noted that the second lens group may be considered to consist of two sub-groups and, in this respect, the applicability of the preset invention will extend to the four-group zoom lens system that is described in the background part of this specification. Likewise, a three-group zoom lens system in which the last lens group is followed by a rear lens group having a smaller power is also included within the scope of the present invention. 
     The stop diaphragm may be positioned either within the second lens group or behind it. In the former case, the diameter of the front group can be reduced but, on the other hand, difficulty is involved in designing an effective lens composition. In the latter case (where the stop diaphragm is positioned between the second and third lens groups), the lens block can be separated from the shutter block, contributing to the realization of a simple mechanistic structure. 
     Examples 1 to 9 of the zoom lens system of the present invention are described below with reference to data sheets, in which f denotes the focal length, fB the back focus, r the radius of curvature of an individual lens surface (or the curvature radius of the vertex in the case of an aspheric surface), d the lens thickness or the airspace between lenses (the foregoing parameters are in millimeters), FNO the F number, ω the half view angle (in degrees), n the refractive index of an individual lens at the d-line, and ν the Abbe number of an individual lens at the d-line. In each data sheet, aspheric surfaces are distinguished from spherical surfaces by putting an asterisk after surface number, and A4, A6 and A8 denote the aspheric coefficients of the fourth, sixth and eighth orders, respectively. 
     EXAMPLE 1 
     FIG. 1 is a simplified cross-sectional view of the zoom lens system according to Example 1 at the wide-angle end. Specific data for example are as shown in Table 1. The aberration curves obtained with this lens system are plotted in FIGS. 2(a), 2(b) and 2(c). 
     
                       TABLE 1______________________________________FNO. = 1:3.6, f = 29.00, ω = 36.9, fB = 8.30SurfaceNo.       r         d          n     ν______________________________________1         -48.126   1.50       1.83400                                37.22         -827.145  2.023         86.943    4.00       1.69680                                55.54         -35.944   variable 5*       -21.729   1.50       1.73077                                40.56         21.987    2.43       1.80518                                25.47         109.603   3.858         16.557    2.30       1.80518                                25.49         11.098    6.38       1.58913                                61.210*       -16.145   variable11*       -44.354   3.42       1.68893                                31.112        -19.176   2.5013        -11.580   1.40       1.77250                                49.614        228.880______________________________________Fifth surface: aspheric              Tenth surface: aspheric______________________________________K = 0              K = 0A.sub.4 = 0.40328626 × 10.sup.-4              A.sub.4 = 0.76751422 × 10.sup.-4A.sub.6 = 0.30242012 × 10.sup.-6              A.sub.6 = 0.92777629 × 10.sup.-7A.sub.8 = 0.14154205 × 10.sup.-8              A.sub.8 = 0.14241736 × 10.sup.-8______________________________________    Eleventh surface: aspheric______________________________________    K = 0    A.sub.4 = 0.44758114 × 10.sup.-4    A.sub.6 = 0.22807597 × 10.sup.-6    A.sub.8 = 0.64325486 × 10.sup.-9______________________________________ 
    
     The values of Fno., f, fB, ω, d4 and d10 vary with zooming as shown in Table 2. 
     
                       TABLE 2______________________________________FNo.    3.6            5.9    8.5f       29.00          50.02  78.08fB      8.30           28.17  53.94ω 36.9           23.0   15.3d4      3.36           6.90   9.69d10     12.25          6.00   2.75______________________________________ 
    
     EXAMPLE 2 
     FIG. 3 is a simplified cross-sectional view of the zoom lens system according to Example 2 at the wide-angle end. Specific data for this example are as shown in Table 3. The aberration carvers obtained with this lens system are plotted in FIGS. 4(a), 4(b) and 4(c). 
     
                       TABLE 3______________________________________FNO. = 1:3.6, f = 29.00, ω = 36.6, fB = 8.10SurfaceNo.       r         d          n     ν______________________________________1         -36.990   1.50       1.80400                                46.62         -202.084  3.603         165.024   3.80       1.69680                                55.54         -31.924   variable 5*       -19.914   3.49       1.80400                                46.66         -207.063  2.847         16.820    2.30       1.80518                                25.48         11.942    6.67       1.58913                                61.2 9*       -16.085   variable10        -34.131   2.78       1.80518                                25.411        -18.175   0.1012        -54.094   1.30       1.77250                                49.613        1968.945  3.6114        -13.182   1.40       1.83481                                42.715        -185.635______________________________________Fifth surface: aspheric              Ninth surface: aspheric______________________________________K = 0              K = 0A.sub.4 = -0.40328626 × 10.sup.-4              A.sub.4 = 0.76751422 × 10.sup.-4A.sub.6 = -0.30242012 × 10.sup.-6              A.sub.6 = 0.92777629 × 10.sup.-7A.sub.8 = 0.14154205 × 10.sup.-8              A.sub.8 = 0.14241736 × 10.sup.-8______________________________________ 
    
     The values of Fno., f, fB, ν, d4 and d9 vary with zooming as shown in Table 4 below. 
     
                       TABLE 4______________________________________FNo.    3.6            6.0    8.5f       29.00          50.00  77.30fB      8.10           27.19  51.18ω 36.6           23.1   15.5d4      3.79           6.98   9.74d9      10.63          5.15   2.35______________________________________ 
    
     EXAMPLE 3 
     FIG. 5 is a simplified cross-sectional view of the zoom lens system according to Example 3 at the wide-angle end. Specific data for this example are as shown in Table 5. The aberration curves obtained with this lens system are plotted in FIGS. 6(a), 6(b) and 6(c). 
     
                       TABLE 5______________________________________FNO. = 1:3.6, f = 29.00, ω = 36.9, fB = 8.30SurfaceNo.       r         d          n     ν______________________________________1         -44.091   1.50       1.83400                                37.22         -158.262  2.113         172.771   4.12       1.69680                                55.54         -36.480   variable 5*       -20.537   5.26       1.78590                                44.26         -928.676  2.337         17.816    2.30       1.80518                                25.48         13.815    7.00       1.58913                                61.2 9*       -15.870   variable10*       -252.722  2.95       1.68893                                31.111        -24.052   3.1812        -11.747   1.40       1.77250                                49.613        329.855______________________________________Fifth surface: aspheric              Ninth surface: aspheric______________________________________K = 0              K = 0A.sub.4 = -0.40328626 × 10.sup.-4              A.sub.4 = 0.76751422 × 10.sup.-4A.sub.6 = -0.30242012 × 10.sup.-6              A.sub.6 = 0.92777629 × 10.sup.-7A.sub.8 = 0.14154205 × 10.sup.-8              A.sub.8 = 0.14241736 × 10.sup.-8______________________________________    Tenth surface: aspheric______________________________________    K =  0    A.sub.4 = 0.44758114 × 10.sup.-4    A.sub.6 = 0.22807597 × 10.sup.-6    A.sub.8 = 0.64325486 × 10.sup.-9______________________________________ 
    
     The values of Fnc., f, f3, ν, d4 and d9 vary with zooming as shown in Table 6below. 
     
                       TABLE 6______________________________________FNo.    3.6            5.9    8.5f       29.00          50.01  77.32fB      8.30           27.13  50.91ω 36.9           22.9   15.4d4      3.30           7.14   10.00d9      11.13          5.34   2.35______________________________________ 
    
     EXAMPLE 4 
     FIG. 7 is a simplified cross-sectional view of the zoom lens system according to Example 4 at the wide-angle end. Specific data for this example are as shown in Table 7. The aberration curves obtained with this lens system are plotted in FIGS. 8(a), 8(b) and 8(c). 
     
                       TABLE 7______________________________________FNO. = 1:3.6, f = 29.00, ω = 36.7, fB = 8.30SurfaceNo.       r         d          n     ν______________________________________1         -48.781   1.50       1.80400                                46.62         150.487   5.003         41.744    4.12       1.69680                                55.54         -42.680   variable 5*       -21.729   2.00       1.72298                                33.06         48.552    2.70       1.80518                                25.47         270.592   0.358         17.157    10.00      1.51728                                69.6 9*       -14.701   variable10*       -60.442   2.93       1.68893                                31.111        -25.695   2.7012        -12.357   1.40       1.77250                                49.613        173.629______________________________________Fifth surface: aspheric              Ninth surface: aspheric______________________________________K = 0              K = 0A.sub.4 = -0.40328626 × 10.sup.-4              A.sub.4 = 0.76751422 × 10.sup.-4A.sub.6 = -0.30242012 × 10.sup.-6              A.sub.6 = 0.92777629 × 10.sup.-7A.sub.8 = 0.14154205 × 10.sup.-8              A.sub.8 = 0.14241736 × 10.sup.-8______________________________________    Tenth surface: aspheric______________________________________    K =  0    A.sub.4 = 0.44758114 × 10.sup.-4    A.sub.6 = 0.22307597 × 10.sup.-6    A.sub.8 = 0.64325486 × 10.sup.-9______________________________________ 
    
     The values of Fno., fB, ω, d4 and d9 vary with zooming as shown in Table 8 below. 
     
                       TABLE 8______________________________________FNo.    3.6            5.8    8.5f       29.00          50.02  77.32fB      8.30           26.99  50.60ω 36.7           22.9   15.4d4      3.79           7.94   10.81d9      11.33          5.42   2.33______________________________________ 
    
     EXAMPLE 5 
     FIG. 9 is a simplified cross-sectional view of the zoom lens system according to Example 5 at the wide-angle end. Specific data for this example are as shown in Table 9. The aberration curves obtained with this lens system are plotted in FIGS. 10(a), 10(b) and 10(c). 
     
                       TABLE 9______________________________________FNO. = 1:3.6, f = 29.00, ω = 36.6, fB = 8.30SurfaceNo.       r         d          n     ν______________________________________1         -38.147   1.50       1.80400                                46.62         -130.204  2.333         79.694    4.12       1.65160                                58.54         -38.742   variable 5*       -18.567   7.00       1.71300                                53.8 6*       -339.540  1.167         19.209    2.30       1.80518                                25.48         12.728    7.25       1.58913                                61.29         -14.803   variable10*       -31.616   3.20       1.68893                                31.111        -16.654   2.9712        -10.224   1.40       1.77250                                49.613        -334.112______________________________________Fifth surface: aspheric              Sixth surface: aspheric______________________________________K = 0              K = 0A.sub.4 = 0.53933790 × 10.sup.-4              A.sub.4 = 0.11851729 × 10.sup.-3A.sub.6 = 0.43256456 × 10.sup.-7              A.sub.6 = 0.57572280 × 10.sup.-6A.sub.8 = -0.80134834 × 10.sup.-9              A.sub.8 = 0.73147250 × 10.sup.-8______________________________________    Tenth surface: aspheric______________________________________    K =  0    A.sub.4 = 0.44863517 × 10.sup.-4    A.sub.6 = 0.97674423 × 10.sup.-8    A.sub.8 = -0.49338025 × 10.sup.-8______________________________________ 
    
     The values of Fno., f, fB, ω, d4 and d9 vary with zooming as shown in Table 10 below. 
     
                       TABLE 10______________________________________FNo.    3.6            5.9    8.5f       29.00          50.01  77.32fB      8.30           27.40  51.58ω 36.6           23.2   15.6d4      3.90           6.75   9.47d9      11.67          5.78   2.73______________________________________ 
    
     EXAMPLE 6 
     FIG. 11 is a simplified cross-sectional view of the zoom lens system according to Example 6 at the wide-angle end. Specific data for this example are as shown in Table 11. The aberration curves obtained with this lens system are plotted in FIGS. 12(a), 12(b) and 12(c). 
     
                       TABLE 11______________________________________FNO. = 1:3.6, f = 29.00, ω = 37.1, fB = 8.30SurfaceNo.       r         d          n     ν______________________________________1         -47.137   1.50       1.80400                                46.62         102.949   4.273         34.599    4.10       1.69680                                55.54         -50.062   variable 5*       -18.894   6.00       1.73077                                40.5 6*       589.080   0.40 7*       14.793    10.15      1.51278                                69.6 8*       -14.849   variable 9*       -35.539   variable10        -22.691   3.5211        -12.643   1.40       1.77250                                49.612        584.859______________________________________Fifth surface: aspheric              Sixth surface: aspheric______________________________________K = 0              K = 0A.sub.4 = 0.44264905 × 10.sup.-4              A.sub.4 = 0.21044890 × 10.sup.-3A.sub.6 = -0.29857214 × 10.sup.-6              A.sub.8 = -0.92688416 × 10.sup.-6A.sub.8 = -0.10413189 × 10.sup.-8              A.sub.8 = 0.17837263 × 10.sup.-7______________________________________Seventh surface: aspheric              Eighth surface: aspheric______________________________________K =  0             K = 0A.sub.4 = 0.183945889 × 10.sup.-3              A.sub.4 = 0.10352965 × 10.sup.-3A.sub.6 = -0.17263808 × 10.sup.-5              A.sub.8 = 0.28528680 × 10.sup.-6A.sub.8 = 0.32134510 × 10.sup.-7              A.sub.8 = 0.30704568 × 10.sup.-7______________________________________    Ninth surface: aspheric______________________________________    K = 0    A.sub.4 = 0.43905689 × 10.sup.-4    A.sub.6 = 0.27532515 × 10.sup.-6    A.sub.8 = 0.12788380 × 10.sup.-8______________________________________ 
    
     The values of Fno., f, fB, ω, d4 and d8 vary with zooming as shown in Table 12 below. 
     
                       TABLE 12______________________________________FNo.    3.6            5.8    8.5f       29.00          50.01  77.48fB      8.30           26.35  49.52ω 37.1           23.0   15.4d4      3.46           8.06   10.84d8      10.71          5.21   2.34______________________________________ 
    
     EXAMPLE 7 
     FIG. 13 is a simplified cross-sectional view of the zoom lens system according to Example 7 at the wide-angle end. Specific data for this example are as shown in Table 13. The aberration curves obtained with this lens system are plotted in FIGS. 14(a), 14(b) and 14(c). 
     
                       TABLE 13______________________________________FNO. = 1:3.8, f = 37.99, ω = 29.2, fB = 8.50SurfaceNo.       r         d          n     ν______________________________________1         -46.849   1.50       1.83400                                37.22         -120.146  0.023         22.585    4.62       1.48749                                70.24         -126.251  variable 5*       -27.245   2.5        1.83400                                37.26         12.806    1.41 7*       213.136   6.88       1.62299                                58.18         -12.198   variable 9*       -22.397   3.03       1.48749                                70.210        -21.089   variable11        -12.899   1.25       1.56907                                71.312        -468.504Fifth surface: aspheric              Seventh surface: aspheric______________________________________K = 0              K = 0A.sub.4 = 0.20748329 × 10.sup.-4              A.sub.4 = 0.22180924 × 10.sup.-4A.sub.6 = 0.26813269 × 10.sup.-6              A.sub.6 = 0.31810508 × 10.sup.-7A.sub.8 = 0        A.sub.8 = 0______________________________________    Ninth surface: aspheric______________________________________    K = 0    A.sub.4 = 0.58355359 × 10.sup.-4    A.sub.6 =  0.10192653 × 10.sup.-6    A.sub.8 = 0.58310163 × 10.sup.-9______________________________________ 
    
     The values of Fno., f, fB, ω, d4 and d8 vary with zooming as shown in Table 14. 
     
                       TABLE 14______________________________________FNo.    3.8            5.4    8.2f       37.99          60.00  102.00fB      8.50           24.29  53.68ω 29.2           19.4   11.8d4      2.00           8.36   13.60d8      14.60          8.24   3.00______________________________________ 
    
     EXAMPLE 8 
     FIG. 15 is a simplified cross-sectional view of the zoom lens system according to Example 8 at the wide-angle end. Specific data for this example are as shown in Table 15. The aberration curves obtained with this lens system are plotted in FIGS. 16(a), 16(b) and 16(c). 
     
                       TABLE 15______________________________________FNO. = 1:3.6, f = 38.00, ω = 29.1, fB = 8.59SurfaceNo.       r         d          n     ν______________________________________1         -40.624   1.50       2.83400                                37.22         -131.101  0.203         24.074    4.12       1.48749                                70.24         -74.799   variable 5*       24.844    2.50       1.73077                                40.56         13.447    1.64 7*       661.131   6.68       1.58913                                61.28         -12.420   variable 9*       -23.000   3.16       1.49176                                57.410        -20.448   3.0311        -12.060   1.40       1.64000                                60.112        -101.186______________________________________Fifth surface: aspheric              Seventh surface: aspheric______________________________________K = 0              K = 0A.sub.4 = -0.17966510 × 10.sup.-3              A.sub.4 = 0.15361650 × 10.sup.-3A.sub.6 = -0.19844449 × 10.sup.-5              A.sub.6 = 0.26760264 × 10.sup.-5A.sub.8 = -0.12125023 × 10.sup.-7              A.sub.8 = 0______________________________________    Ninth surface: aspheric______________________________________    K = 0    A.sub.4 = 0.78007555 × 10.sup.-4    A.sub.6 = 0.17348425 × 10.sup.-6    A.sub.8 = 0.15729598 × 10.sup.-8______________________________________ 
    
     The values of Fno., f, fB, ω, d4 and d8 vary with zooming as shown in Table 16 below. 
     
                       TABLE 16______________________________________FNo.    3.6            5.2    8.2f       38.00          60.00  102.00fB      8.50           24.58  54.67ω 29.1           19.4   11.9d4      2.00           7.78   12.39d8      23.89          8.11   3.50______________________________________ 
    
     EXAMPLE 5 
     FIG. 17 is a simplified cross-sectional view of the zoom lens system according to Example 9 at the wide-angle end. Specific data for this example are as shown in Table 17. The aberration curves obtained with this lens system are plotted in FIGS. 18(a), 18(b) and 18(c). 
     
                       TABLE 17______________________________________FNO. = 1:3.6, f = 38.00, ω = 29.1, fB = 8.50SurfaceNo.       r         d          n     ν______________________________________1         -47.431   1.50       1.83400                                37.22         -656.486  0.203         22.259    3.92       1.58913                                61.24         -193.075  variable 5*       31.683    2.50       1.73077                                40.5 6*       12.063    1.007         36.333    7.50       1.58913                                61.28         -13.056   variable 9*       -23.000   2.97       1.48749                                70.210        -30.680   2.9411        -14.596   1.40       1.56907                                71.312        -329.137______________________________________Fifth surface: aspheric              Sixth surface: aspheric______________________________________K = 0              K = 0A.sub.4 = -0.42114860 × 10.sup.-3              A.sub.4 = -0.45457599 × 10.sup.-3A.sub.6 = 0.90193505 × 10.sup.-6              A.sub.6 = 0.22145289 × 10.sup.-5A.sub.8 = 0        A.sub.8 = 0______________________________________    Ninth surface: aspheric    K = 0    A.sub.4 = 0.38145979 × 10.sup.-4    A.sub.6 = -0.20084770 × 10.sup.-7    A.sub.8 = 0.27896596 × 10.sup.-8______________________________________ 
    
     The values of Fno., f, fB, ω, d4 and d8 vary with zooming as shown in Table 18 below. 
     
                       TABLE 18______________________________________FNo.    3.6            5.2    8.1f       38.00          60.00  102.00fB      8.50           24.62  54.90ω 29.1           19.5   11.9d4      2.00           7.81   12.47d8      13.97          8.16   3.50______________________________________ 
    
     Table 19 shows values that satisfy the conditions (a) to (d) in Examples 1 to 9. 
     
                       TABLE 19______________________________________Condition   ΔI2               ΔIII2 N2a  ν2a______________________________________Ex. 1       -21.0   -0.72       1.731                                40.5Ex. 2       -22.1   -0.55       1.804                                46.6Ex. 3       -20.3   -0.32       1.786                                44.2Ex. 4       -22.3   -0.61       1.723                                33.0Ex. 5       -20.0   -0.38       1.713                                53.8Ex. 6       -28.1   -0.25       1.731                                40.5Ex. 7       -20.3   -2.71       1.834                                37.2Ex. 8       -24.0   -2.59       1.731                                40.5Ex. 9       -15.7   -2.89       1.731                                40.5______________________________________ 
    
     ADVANTAGES OF THE INVENTION 
     As described on the foregoing pages, the zoom lens system of the present invention adopts an arrangement as simple as a three-group composition and yet, by introducing special features in the arrangement of lens elements in the second lens group and their shape, it achieves a high zoom ratio of 2.5 and more while featuring a wider view angle and reducing the lens diameter and the overall lens length. At the same time, it experiences less aberrational variations during zooming from the wide-angle to the narrow-angle end or from infinity to near distance. Further, it successfully reduces the number of lens elements used in the overall system. Consequently, the present invention offers a zoom lens system of a telephoto type that consists of a total of six elements in three groups and which is suitable for use with a compact camera