Abstract:
In a zoom lens comprising three lens groups of positive, positive, and negative in which, upon changing power from its wide angle end to telephoto end, all the three lens groups are moved toward the object side while changing distances between the lens groups, so as to attain variable power; each lens form of the first lens group is specified, and predetermined conditional expressions are satisfied, so as to yield optical performances which are satisfactory to professionals and high-standard amateurs. Upon changing the power from the wide angle end to telephoto end, the air gap between the first lens group G 1  and the second lens group G 2  increases, while the air gap between the second lens group G 2  and the third lens group G 3  decreases. Further, this zoom lens satisfies the following conditional expressions (1) to (3): 
     
       2.2≦f.sub.2 /f.sub.1 ≦6.6                    (1) 
     
     
       2.2≦f.sub.2 /f.sub.w ≦5.0                    (2) 
     
     
       -1.1≦R.sub.3 /f.sub.w ≦0.5                   (3)

Description:
RELATED APPLICATIONS 
     This application claims the priority of Japanese Patent Application No. 9-90103 filed on Mar. 24, 1997, which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a zoom lens composed of three groups suitable for a lens-shutter camera or the like and, in particular, to a three-group zoom lens incorporated in a camera used by professionals or high-standard amateurs for which a high image quality is required. 
     2. Description of the Prior Art 
     Recently, as a lens system used in a lens-shutter camera, the one equipped with a zoom lens has been constituting the mainstream. Since it is required for such a zoom lens to have a compact size to begin with, a two-group zoom lens of a telephoto type refracting power arrangement comprising a first lens group having a positive refracting power and a second lens group having a negative refracting power has been put into practical use. 
     Meanwhile, since lens-shutter cameras used by professionals or high-standard amateurs require a high image quality in general, known as a zoom lens incorporated in such a camera is a three-group zoom lens in which the above-mentioned first lens group is divided into two positive lens groups, so as to yield a refracting power group arrangement of positive, positive, and negative as a whole, thereby securing a favorable image quality. 
     Known as a typical technique for achieving a high image quality of a zoom lens while reducing its size is the one in which the number of lens sheets is increased. When simply increasing the number of lens sheets, however, the lens as a whole becomes larger in size. In particular, when a wider angle and a higher image quality are to be obtained, the front lens enhances its diameter so that it becomes difficult for the lens to be built into the main body of the camera. 
     SUMMARY OF THE INVENTION 
     In view of such circumstances, it is an object of the present invention to provide a compact three-group zoom lens exhibiting favorable optical performances in its whole variable power range, while securing a predetermined angle of view at its wide angle end. 
     The present invention provides a three-group zoom lens comprising, successively from an object side, a first lens group having a positive refracting power, a second lens group having a positive refracting power, and a third lens group having a negative refracting power, in which, upon changing power from a wide angle end to a telephoto end, the three lens groups are moved toward the object side while changing a distance between the lens groups, so as to change the power; 
     wherein the first lens group comprises, successively from the object side, a positive first lens having a convex surface directed onto the object side, a negative second lens having a concave surface directed onto the object side, a positive third lens, and a fourth lens composed of at least one sheet each of positive and negative lenses; and 
     wherein the following conditional expressions: 
     
         2.2≦f.sub.2 /f.sub.1 ≦6.6                    (1) 
    
     
         2.2≦f.sub.2 /f.sub.w ≦5.0                    (2) 
    
     
         -1.1≦R.sub.3 /f.sub.w ≦-0.5                  (3) 
    
     wherein 
     f 1  is a focal length of the first lens group; 
     f 2  is a focal length of the second lens group; 
     f w  is a focal length of the whole system at the wide angle end; and 
     R 3  is a radius of curvature of a surface of the second lens on the object side are satisfied. 
     Preferably, the zoom lens is configured such that, upon changing the power from the wide angle end to the telephoto end, an air gap between the first lens group and the second lens group increases, while an air gap between the second lens group and the third lens group decreases. 
     Preferably, the third lens group is constituted by four sheets of lenses. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view showing a basic lens configuration of a three-group lens in accordance with an embodiment of the present invention at its wide angle end; 
     FIG. 2 is a schematic view showing loci of movement of the respective lens groups in the three-group lens in accordance with the embodiment of the present invention from its wide angle end to telephoto end; 
     FIGS. 3A-1 to 3B-3 are aberration charts showing spherical aberration, astigmatism, and distortion of the three-group zoom lens in accordance with Example 1 of the present invention at its wide angle end and telephoto end; 
     FIG. 4 is an aberration chart showing coma of the three-group zoom lens in accordance with Example 1 of the present invention at its wide angle end; 
     FIG. 5 is an aberration chart showing coma of the three-group zoom lens in accordance with Example 1 of the present invention at its telephoto end; 
     FIGS. 6A-1 to 6B-3 are aberration charts showing spherical aberration, astigmatism, and distortion of the three-group zoom lens in accordance with Example 2 of the present invention at its wide angle end and telephoto end; 
     FIG. 7 is an aberration chart showing coma of the three-group zoom lens in accordance with Example 2 of the present invention at its wide angle end; 
     FIG. 8 is an aberration chart showing coma of the three-group zoom lens in accordance with Example 2 of the present invention at its telephoto end; 
     FIGS. 9A-1 to 9B-3 are aberration charts showing spherical aberration, astigmatism, and distortion of the three-group zoom lens in accordance with Example 3 of the present invention at its wide angle end and telephoto end; 
     FIG. 10 is an aberration chart showing coma of the three-group zoom lens in accordance with Example 3 of the present invention at its wide angle end; 
     FIG. 11 is an aberration chart showing coma of the three-group zoom lens in accordance with Example 3 of the present invention at its telephoto end; 
     FIGS. 12A-1 to 12B-3 are aberration charts showing spherical aberration, astigmatism, and distortion of the three-group zoom lens in accordance with Example 4 of the present invention at its wide angle end and telephoto end; 
     FIG. 13 is an aberration chart showing coma of the three-group zoom lens in accordance with Example 4 of the present invention at its wide angle end; 
     FIG. 14 is an aberration chart showing coma of the three-group zoom lens in accordance with Example 4 of the present invention at its telephoto end; 
     FIGS. 15A-1 to 15B-3 are aberration charts showing spherical aberration, astigmatism, and distortion of the three-group zoom lens in accordance with Example 5 of the present invention at its wide angle end and telephoto end; 
     FIG. 16 is an aberration chart showing coma of the three-group zoom lens in accordance with Example 5 of the present invention at its wide angle end; and 
     FIG. 17 is an aberration chart showing coma of the three-group zoom lens in accordance with Example 5 of the present invention at its telephoto end. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, embodiments of the present invention will be explained with reference to the accompanying drawings. 
     A three-group lens in accordance with an embodiment shown in FIG. 1 comprises, successively from an object side, a first lens group G 1 , constituted by five sheets of lenses L 1  to L 5 , having a positive refracting power as a whole; a second lens group G 2 , constituted by a single sheet of a lens L 6 , having a positive refracting power; and a third lens group G 3 , constituted by four sheets of lenses L 7  to L 10 , having a negative refracting power as a whole. The first lens group G 1 , the second lens group G 2 , and the third lens group G 3  are configured such that, upon zooming, when the power is changed from the wide angle end to telephoto end, as shown in FIG. 2, the air gap between the first lens group G 1  and the second lens group G 2  increases while the air gap between the second lens group G 2  and the third lens group G 3  decreases. In this zoom lens, as the lens groups move along its optical axis X while changing the distances between the lens groups, a focal length f of the whole system changes, and a luminous flux is efficiently converged onto an imaging position P on an imaging surface. In the second lens group G 2 , a stop 1 is disposed in front of the lens L 6 . 
     The first lens group G 1  comprises, successively from the object side, the first lens L 1  made of a positive meniscus lens having a convex surface directed onto the object side; the second lens L 2  made of a biconcave lens having a surface with a stronger curvature directed onto the object side; the third lens L 3  made of a positive meniscus lens having a convex surface directed onto the imaging surface side; the fourth lens L 4  made of a negative meniscus lens having a convex surface directed onto the object side; and the fifth lens L 5  made of a biconvex lens having a surface with a stronger curvature directed onto the object side. The second lens group G 2  comprises the sixth lens L 6  made of a biconvex lens. The third lens group G 3  comprises, successively from the object side, the seventh lens L 7  made of a positive meniscus lens having a convex surface directed onto the imaging surface side; the eighth lens L 8  made of a negative meniscus lens having a convex surface directed onto the imaging surface side; the ninth lens L 9  made of a negative meniscus lens having a convex surface directed onto the imaging surface side; and the tenth lens L 10  made of a positive meniscus lens having a convex surface directed onto the imaging surface side. 
     Further, this zoom lens satisfies the following conditional expressions: 
     
         2.2≦f.sub.2 /f.sub.1 ≦6.6                    (1) 
    
     
         2.2≦f.sub.2 /f.sub.w ≦5.0                    (2) 
    
     
         -1.1≦R.sub.3 /f.sub.w ≦0.5                   (3) 
    
     wherein 
     f 1  is a focal length of the first lens group; 
     f 2  is a focal length of the second lens group; 
     f w  is a focal length of the whole system at the wide angle end; and 
     R 3  is a radius of curvature of a surface of the second lens on the object side. 
     When the form of each lens is specified as mentioned above, and the above-mentioned conditional expressions are satisfied, various kinds of aberration in a wide range of photographing from long shot to close-up shot can be corrected favorably, whereby high optical performances can be obtained throughout the object distance. Also, as the first lens group is configured as mentioned above, the ratio of lens corner illumination at the wide angle end can be ameliorated. 
     In the following, the technical meaning of each conditional expression will be explained. 
     Above the upper limit of the above-mentioned conditional expression (1), when focusing with the first lens group, spherical aberration increases so much that image surface curvature fluctuates greatly upon focusing at close range, thus making it hard to correct. 
     Below the lower limit of conditional expression (1), by contrast, the amount of focusing at close range becomes so large that the lens system increases its size, thereby making the main body of the camera bulky. Also, distortion increases so much that it becomes hard to correct. 
     Also, above the upper limit of conditional expression (2), when focusing with the first lens group, image surface curvature fluctuates greatly upon focusing at close range, thereby making it hard to correct. 
     Below the lower limit of conditional expression (2), by contrast, the second lens group has a higher sensitivity to eccentricity, whereby its manufacture becomes difficult. 
     Above the upper limit of conditional expression (3), image surface curvature increases so much that it becomes hard to correct. 
     Below the lower limit of conditional expression (3), by contrast, coma increases so much that it becomes hard to correct. 
     When the third lens group is constituted by the above-mentioned four sheets of lenses, chromatic aberration in magnification can be corrected favorably. 
     In the following, the above-mentioned three-group zoom lens will be explained with reference to specific examples. 
     EXAMPLE 1 
     Table 1 (follows) shows radius of curvature R (mm) of each lens surface, center thickness of each lens and air gap between neighboring lenses (hereinafter collectively referred to as axial surface spacing) d (mm), and values of refractive index n and Abbe number υ of each lens at d-line in the zoom lens in accordance with Example 1. 
     The numbers in this table successively increase from the object side. 
     In this example, focal length f&#39; and Fno of the whole lens system, and values of f 2  /f 1 , f 2  /f w , and R 3  /f w  are set as shown in the lower part of Table 1. 
     EXAMPLE 2 
     Table 2 (follows) shows radius of curvature R (mm) of each lens surface, axial surface spacing d (mm), and values of refractive index n and Abbe number υ of each lens at d-line in the zoom lens in accordance with Example 2. 
     The numbers in this table successively increase from the object side. 
     In this example, focal length f&#39; and Fno of the whole lens system, and values of f 2  /f 1 , f 2  /f w , and R 3  /f w  are set as shown in the lower part of Table 2. 
     EXAMPLE 3 
     Table 3 (follows) shows radius of curvature R (mm) of each lens surface, axial surface spacing d (mm), and values of refractive index n and Abbe number υ of each lens at d-line in the zoom lens in accordance with Example 3. 
     The numbers in this table successively increase from the object side. 
     In this example, focal length f&#39; and Fno of the whole lens system, and values of f 2  /f 1 , f 2  /f w , and R 3  /f w  are set as shown in the lower part of Table 3. 
     EXAMPLE 4 
     Table 4 (follows) shows radius of curvature R (mm) of each lens surface, axial surface spacing d (mm), and values of refractive index n and Abbe number υ of each lens at d-line in the zoom lens in accordance with Example 4. 
     The numbers in this table successively increase from the object side. 
     In this example, focal length f and Fno of the whole lens system, and values of f 2  /f 1 , f 2  /f w , and R 3  /f w  are set as shown in the lower part of Table 4. 
     EXAMPLE 5 
     Table 5 (follows) shows radius of curvature R (mm) of each lens surface, axial surface spacing d (mm), and values of refractive index n and Abbe number υ of each lens at d-line in the zoom lens in accordance with Example 5. 
     The numbers in this table successively increase from the object side. 
     In this example, focal length f&#39; and Fno of the whole lens system, and values of f 2  /f 1 , f 2  /f w , and R 3  /f w  are set as shown in the lower part of Table 5. 
     FIGS. 3(3A-1 to 3B-3), 6(6A-1 to 6B-3), 9(9A-1 to 9B-3), 12(12A-1 to 12B-3), and 15(15A-1 to 15B-3) are aberration charts showing spherical aberration, astigmatism, and distortion of the three-group zoom lenses of the respective examples at both wide angle end and telephoto end. FIGS. 4, 7, 10, 13, and 16 are aberration charts showing coma of the three-group zoom lenses of the respective examples at the wide angle end. FIGS. 5, 8, 11, 14, and 17 are aberration charts showing coma of the three-group zoom lenses of the respective examples at the telephoto end. Each spherical aberration chart shows a sine condition in addition to spherical aberration. Each astigmatism chart shows respective aberrations with respect to sagittal (S) and tangential (T) image surfaces. As can be seen from these aberration charts, each kind of aberration can be favorably corrected in the three-group zoom lens of each embodiment mentioned above. 
     The compact three-group zoom lens of the present invention should not be restricted to the above-mentioned examples. For example, the forms and number of sheets of lenses constituting each lens group can be selected appropriately. Also, for example, the fourth lens subgroup is at least composed of at least one positive refracting lens and at least one negative refracting lens constituting the first lens group, any of the positive and negative lenses can be disposed on the object side. 
     The three-group zoom lens in accordance with the present invention has an F number of about 4.6 and an angle of view of about 63 degrees at its wide angle end, and an F number of about 7.0 at its telephoto end, thus yielding a variable power ratio of about 1.5, whereby various kinds of aberration can be favorably corrected in a wide range of photographing from long shot to close-up shot. Accordingly, a high resolution and a high contrast can be obtained throughout the object distance to such an extent that professionals and high-standard amateurs can be satisfied. 
     
                       TABLE 1______________________________________Surface  R         d           n     ν______________________________________1        51.7343   4.17        1.75281                                52.72        94.0155   4.733        -64.4647  2.66        1.70465                                47.44        283.7263  9.515        -280.7411 4.07        1.51047                                64.06        -109.3292 0.187        55.5030   1.98        1.64465                                34.78        32.1745   0.299        35.5499   7.79        1.48951                                64.910       -66.8000  3.10 to 5.3111       0.0000    5.7012       177.0088  9.31        1.64796                                55.513       -1922.5486              22.26 to 2.1914       -111.1019 9.20        1.49383                                63.315       -36.7569  4.6816       -38.2637  3.10        1.49835                                65.017       -162.2179 8.3218       -38.5418  4.16        1.80501                                47.519       -155.5066 0.2220       -147.5118 4.71        1.80144                                24.921       -110.6366______________________________________ 
    
     f&#39;=100.00 to 153.80 Fno.=4.63 to 7.13 
     f 2  /f 1  =2.51 
     f 2  /f w  =2.51 
     R 3  /f w  =-0.64 
     
                       TABLE 2______________________________________Surface  R        d            n     ν______________________________________1        48.9087  4.33         1.73150                                51.72        93.3683  4.413        -69.0702 2.66         1.80501                                47.44        236.1169 10.735        -509.0070             4.07         1.48500                                65.46        -100.6244             0.187        49.6916  1.95         1.69285                                31.98        30.7467  0.389        34.8866  7.35         1.57556                                59.910       -80.6320 2.66 to 5.3111       0.0000   5.1412       410.5979 9.45         1.49556                                65.113       -486.1689             20.34 to 2.0614       -105.8801             8.62         1.66550                                52.715       -35.9059 4.2016       -34.1217 3.10         1.80501                                47.417       -139.2947             7.9018       -37.5228 4.16         1.80317                                47.619       -111.2683             0.1820       -142.8805             5.94         1.80500                                24.721       -92.3095______________________________________ 
    
     f&#39;=100.00 to 153.81 Fno.=4.63 to 7.13 
     f 2  /f 1  =5.74 
     f 2  /f w  =4.51 
     R 3  /f w  =-0.69 
     
                       TABLE 3______________________________________Surface  R        d            n     ν______________________________________1        46.2086  3.74         1.76715                                43.52        74.8365  4.523        -54.8957 2.63         1.80501                                47.44        356.2013 6.825        -315.7873             4.03         1.55371                                61.76        -90.1703 0.187        51.6415  1.93         1.80500                                31.38        30.9629  0.279        33.9580  8.04         1.57392                                58.310       -62.9161 3.51 to 5.2611       0.0000   5.0812       175.2599 9.35         1.51408                                64.413       -433.2712             16.98 to 2.0314       -104.9397             9.36         1.52586                                60.015       -34.8728 5.1616       -34.0584 4.34         1.72489                                53.017       -176.5921             9.0418       -37.0690 4.12         1.76936                                51.019       -116.5340             0.2620       -145.8484             5.94         1.80501                                24.721       -92.0436______________________________________ 
    
     f&#39;=100.00 to 153.85 Fno.=4.63 to 7.13 
     f 2  /f 1  =2.84 
     f 2  /f w  =2.44 
     R 3  /f w  =-0.55 
     
                       TABLE 4______________________________________Surface  R        d            n     ν______________________________________1        50.7484  5.73         1.71725                                55.62        173.9600 3.443        -99.7978 2.65         1.80501                                45.24        104.6960 12.755        -280.9465             4.07         1.48500                                65.16        -143.2412             0.557        51.8286  1.94         1.61868                                36.18        31.2293  0.369        36.1151  6.42         1.58776                                61.510       -94.9104 3.53 to 5.3011       0.0000   5.1312       946.9778 9.69         1.62208                                60.313       -188.4805             18.04 to 1.7914       -97.5346 8.47         1.61775                                60.115       -34.2705 4.2916       -33.9980 3.09         1.80502                                47.417       -123.0461             8.0218       -35.0322 4.15         1.80500                                47.519       -105.8040             0.1820       -147.2813             6.09         1.80500                                24.721       -91.7896______________________________________ 
    
     f&#39;=100.00 to 153.76 Fno.=4.63 to 7.13 
     f 2  /f 1  =2.97 
     f 2  /f w  =2.54 
     R 3  /f w  =-1.00 
     
                       TABLE 5______________________________________Surface  R        d            n     ν______________________________________1        48.8739  4.69         1.71300                                53.92        93.4123  5.063        -70.3083 2.63         1.80236                                46.74        246.3973 10.315        -487.5404             4.07         1.48749                                70.46        -106.1721             0.187        49.1490  1.90         1.64769                                33.98        30.2817  0.429        34.9311  7.81         1.56384                                60.810       -82.5127 4.76 to 5.2911       0.0000   5.1112       290.1405 9.49         1.48749                                70.413       -290.1405             17.46 to 1.9714       -98.7423 8.15         1.64850                                53.015       -35.0651 4.4816       -33.1503 3.05         1.80236                                46.717       -163.3521             8.4118       -37.3695 4.11         1.80236                                46.719       -111.0930             0.1820       -147.2812             5.96         1.80517                                25.521       -85.4218______________________________________ 
    
     f&#39;=100.00 to 153.80 Fno.=4.63 to 7.13 
     f 2  /f 1  =3.66 
     f 2  /f w  =2.99 
     R 3  /f w  =-0.70