Abstract:
A telephoto zoom lens comprising, in succession from the object side, a first lens group having a positive refractive power, a second lens group having a negative refractive power, and a third lens group having a positive refractive power, the lens groups being moved independently of each other during zooming. The telephoto zoom lens meets certain conditions, including a condition that prescribes the amount of change in spacing between the first and second lens groups by zooming, so that the spacing increases at the telephoto end. The telephoto ratio is large, and the lens is relatively compact at the wide angle end. The burden borne by each lens group regarding correction of spherical aberration is reduced, and correction of spherical aberration is relatively easy, so that the degree of freedom in lens design is enhanced.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a compact zoom lens, and in particular to improvements in a compact telephoto zoom lens which comprises, in succession from the object side, three positive, negative and positive groups. 
     2. Related Background Art 
     A telephoto zoom lens comprising, in succession from the object side, three groups having positive, negative and positive refractive powers is a developed form of the well-known telephoto zoom lens comprising four positive, negative, positive and positive groups. In a telephoto lens of the three-group construction of this type, the positive third group in the conventional four-group construction for keeping the position of the image surface constant and the positive fourth group which is an imaging group are made integral with each other to constitute a positive third lens group and further, the third lens group including the imaging group is moved along the optic axis with the other lens groups during zooming, whereby a magnification changing operation may be accomplished. Accordingly, the magnification changing operation is borne in common by the second lens group and the third lens group and therefore, the amount of magnification change by the negative second lens group which served as the conventional magnification changing group, that is, the variation in magnification caused by the positional relation between the image point of the first lens group and the second lens group during zooming, could be made smaller in this zoom lens of three-group construction than in the conventional telephoto zoom lens of four-group construction. 
     In the conventional zoom lens of three-group construction of this type, however, if the refractive power of the first lens group is strengthened to make the magnification changing system more compact, the imaging magnification of the second lens group will become great and the image point of the second lens group, namely, the object point of the third lens group, will move greatly toward the object side. Therefore, it becomes difficult to sufficiently secure the spacing between the second lens group and the third lens group with the focal length of the zoom lens being maximum at the telephoto end. Accordingly, to solve this, it becomes necessary that the third lens group be made into a telephoto type in which the telephoto ratio (the ratio of the full length to the focal length) is extremely small. However, making the third lens group which is the imaging group into a telephoto type in which the telephoto ratio is extremely small has led to the disadvantage that it becomes difficult to correct spherical aberration, particularly the spherical aberration at the telephoto side on which the focal length is long. 
     Further, if the refractive power of the second lens group having a negative refractive power is strengthened to provide a compact construction, the positive refractive power component in the second lens group will be relatively weakened and it will become difficult to secure the degree of freedom of the aberration correction of the lens group. Therefore, it will become difficult to correct the fluctuation of various aberrations caused by zooming. If in order to overcome this, an attempt is made to make the curvature of the joined surface of the negative lens and the positive lens in the second lens group sharper and strengthen the positive refractive power component, it has led to the disadvantage that fluctuation of chromatic spherical aberration is caused by zooming. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to overcome the above-noted disadvantages peculiar to the conventional telephoto zoom lens of three-group construction and to provide a very compact telephoto zoom lens having a good imaging performance over the entire range of magnification change. 
     The present invention is a telephoto zoom lens which comprises, in succession from the object side, a first lens group having a positive refractive power, a second lens group having a negative refractive power and a third lens group having a positive refractive power, said lens groups being moved independently of each other for a magnification change and focusing, and satisfying the following conditions: ##EQU1## where f w  : the focal length of the entire system at the wide end (the shortest focal length of the entire system) 
     f 1  : the focal length of the first lens group 
     f 2  : the focal length of the second lens group 
     f 3  : the focal length of the third lens group 
     β 2W  : the imaging magnification at the wide end (the shortest focal length state of the entire system) of the second lens group 
     β 2T  : the imaging magnification at the telephoto end (the longest focal length state of the entire system) of the second lens group 
     β 3W  : the imaging magnification at the wide end of the third lens group 
     β 3T  : the imaging magnification at the telephoto end of the third lens group. 
     In the present invention, the first lens group is of relatively weak refractive power. Thus, the object point of the third lens group can be moved toward the image side and the imaging magnification of the second lens group can be made small. If the imaging magnification of the second lens group becomes small, the load of the second lens group for aberration correction can be mitigated and the refractive power of the second lens group can be more strengthened. Thereby, the magnification changing system can be constructed compactly in spite of the refractive power of the first lens group having been weakened, and the object point of the third lens group can be moved more toward the image side. In such case, if the refractive power of the second lens group is strengthened, the imaging magnification of the third lens group becomes great, thus resulting in bulkiness of the imaging system. However, the object point of the third lens group moves greatly toward the image side and it becomes unnecessary to make the third lens group into a telephoto type in which the telephoto ratio is extremely great and therefore, the load of the third lens group for aberration correction is mitigated and the refractive power of the third lens group can be strengthened. Accordingly, the imaging system can be constructed compactly. In this manner, success has been attained in making the lens system compact without making the third lens group into a telephoto type in which the telephoto ratio is extremely great. 
     Expressions (1), (2) and (3) prescribe appropriate ranges of refractive power of the respective lens groups. If the upper limit of expression (1) is exceeded, the refractive power of the first lens group will become too weak and it will become difficult to construct the magnification changing system compactly. If the lower limit of expression (1) is exceeded, the imaging magnification of the second lens group will become great and further, the object point of the third lens group will move toward the object side and it will become necessary to make the third lens group into a telephoto type in which the telephoto ratio is extremely great, and aberration correction will become difficult. If the upper limit of expression (2) is exceeded, it will become difficult to make the magnification changing system compact and further, the object point of the third lens group will move toward the object side with a result that it will become necessary to make the third lens group into a telephoto type in which the telephoto ratio is extremely great, and this is not preferable. If the lower limit of expression (2) is exceeded, Petzval sum will become excessively negative and not only it will become difficult to correct astigmatism and curvature of image field, but also the imaging magnification of the third lens group will become excessively great, thus resulting in bulkiness of the imaging system. If the upper limit of expression (3) is exceeded, the imaging system will become bulky, and if the lower limit of expression (3) is exceeded, the refractive power of the third lens group will become excessively strong and correction of spherical aberration will become difficult. 
     The inverse number of the imaging magnification of the second lens group is indicative of the distance between the image point of the first lens group and the second lens group. Accordingly, expressions (4) and (5) prescribe the amount of magnification change borne by the second lens group due to the difference and ratio by zooming in the distance between the image point of the first lens group and the second lens group. If the upper limits of expressions (4) and (5) are exceeded, the amount of magnification change borne by the second lens group will become great and therefore, the fluctuation of aberrations by zooming caused by the magnification changing action of the second lens group will also become greater and the aberration correction by the second lens group will become difficult. To solve this, the structure of the second lens group must be made more complex and therefore, it will become impossible to strengthen the refractive power of the second lens group and it will become difficult to make the lens system compact. 
     The inverse number of the imaging magnification of the third lens group is indicative of the distance between the image point of the second lens group and the third lens group. Accordingly, expressions (6) and (7) prescribe the amount of magnification change borne by the third lens group due to the difference and ratio by zooming in the distance between the image point of the second lens group and the third lens group. If the upper limits of expressions (6) and (7) are exceeded, the amount of magnification change borne by the third lens group will become excessively great, and this is not preferable in aberration correction. If the lower limits of expressions (6) and (7) are exceeded, the amount of magnification change borne by the third lens group will become excessively small and therefore, the amount of magnification change borne by the second lens group must be made excessively great, and this is not preferable. 
     As described above, according to the present invention, an appropriate refractive power of each lens group and the imaging magnification during zooming are provided as shown in the above-mentioned conditions (1) -(7), whereby there can be achieved a compact zoom lens having an excellent imaging performance over the entire range of magnification change. 
     To balance the amount of magnification change borne by the second lens group with the amount of magnification change borne by the third lens group, it is desirable to provide the lower limits of conditions (4) and (5) as follows: ##EQU2## 
     Also, in the present invention as described above, it is further desirable to make the third lens group into a construction as shown below. The third lens group is divided, in succession from the object side, a forward positive lens group and a rearward negative lens group. Further, the forward positive lens group comprises, in succession from the object side, three positive lens components, i.e., a positive single lens, a cemented lens consisting of a positive lens and a negative lens having its surface of sharper curvature facing the object side cemented together and having a positive refractive power, and a positive meniscus lens having its convex surface facing the object side, and it is desirable to satisfy the following conditions: 
     
         0.6&lt;f.sub.3F /f.sub.3 &lt;1.3                                 (8) 
    
     
         1.5&lt;|f.sub.3R /f.sub.3 |&lt;8.5             (9) 
    
     
         0.35&lt;f.sub.3F /f.sub.31 &lt;0.65                              (10) 
    
     where 
     f 3F  : the focal length of the forward positive lens group in the third lens group 
     f 3R  : the focal length of the rearward negative lens group in the third lens group 
     f 31  : the focal length of the first lens in the forward positive lens group in the third lens group. 
     Expressions (8), (9) and (10) prescribe the structure of the third lens group. 
     If the upper limit of expression (8) is exceeded, it will become difficult to make the third lens group compact. If the lower limit of expression (8) is exceeded, the relative aperture of the third lens group will become great and correction of spherical aberration will become difficult. 
     If the upper limit of expression (9) is exceeded, the full length of the third lens group will become great and it will become difficult to make the third lens group compact. If the lower limit of expression (9) is exceeded, Petzval sum will become excessively negative and correction of astigmatism and curvature of image field will become difficult. 
     If the upper limit of expression (10) is exceeded, extroversive coma will occur and further, spherical aberration, particularly the spherical aberration of the wide side, will become negative and the fluctuation of spherical aberration by zooming will become excessively great, and this is not preferable. 
     Also, where in the three-group telephoto zoom lens of the present invention which satisfies the above-mentioned conditions (1), (2) and (3), the second lens group is constructed of three lenses, in succession from the object side, namely, a negative first lens having its surface of sharper curvature facing the image side, a positive meniscus second lens joined thereto and having its convex surface facing the object side, and a negative third lens, it is desirable to satisfy the following conditions: ##EQU3## where n 22  : the refractive index of the second lens in the second lens group 
     f 2  : the focal length of the second lens group 
     r 22  : the radius of curvature of the joined surface of the first lens and the second lens in the second lens group 
     r 24  : the radius of curvature of that surface of the third lens in the second lens group which is adjacent to the object side 
     r 25  : the radius of curvature of that surface of the third lens in the second lens group which is adjacent to the image side. 
     In this case, it becomes possible to make great the difference in refractive index between the negative first lens and the positive meniscus second lens constituting the cemented negative lens in the second lens group, thereby strengthening the positive refractive power and securing the degree of freedom of aberration correction of the lens group. More specifically, it becomes possible to make the refractive index of the positive second lens higher and strengthen the positive refractive power component in the second lens group and at the same time, weaken the curvature of the joined surface of the first lens and the second lens. Accordingly, it is possible to strengthen the positive refractive power component in the second lens group and secure the degree of freedom of aberration correction of the second lens group while suppressing the fluctuation of chromatic spherical aberration caused by zooming, and it becomes easy to correct various aberrations well while constructing the magnification changing system compactly. 
     The above-mentioned expression (11) prescribes an appropriate range of refractive index of the second lens in the second lens group. If the upper limit of expression (11) is exceeded, the range of refractive index of the ordinary glass material will be exceeded, and this is not preferable. Also, if the positive refractive power component in the second lens group is weakened, the degree of freedom of aberration correction of the second lens group will decrease and therefore, it is not easy to strengthen the refractive power of the second lens group and thereby construct the magnification changing system compactly, and for this reason, it is desirable that the lower limit of expression (11) be not exceeded. 
     Expression (12) prescribes the appropriate curvature of the joined surface of the first lens and the second lens in the second lens group, and to suppress the fluctuation of chromatic spherical aberration caused by zooming, it is desirable that the upper limit of expression (12) be not exceeded. If the lower limit of expression (12) is exceeded, achromatism will become difficult within the range of the existing glass material. 
     Expression (13) prescribes the shape of the third lens in the second lens group. If the upper limit of expression (13) is exceeded, astigmatism will become excessively positive and introversive coma will occur, and this is not preferable. If the lower limit of expression (13) is exceeded, astigmatism will become excessively negative and extroversive coma will occur, and this is not preferable. 
     Further objects, features and effects of the present invention will become fully apparent from the following detailed description taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a lens construction according to a first embodiment of the present invention. 
     FIG. 2 shows a lens construction used for the description of second, third and fourth embodiments of the present invention. 
     FIG. 3 shows a lens construction according to a fifth embodiment of the present invention. 
     FIG. 4 shows a lens construction according to a sixth embodiment of the present invention. 
     FIG. 5 shows a lens construction according to a seventh embodiment of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In a first embodiment of the present invention which is shown in FIG. 1, a first lens group G1 having a positive refractive power comprises, in succession from the object side, a negative meniscus lens L11 having its convex surface facing the object side, a biconvex positive lens L12 joined thereto and a biconvex positive lens L13, a second lens group G2 having a negative refractive power comprises, in succession from the object side, a negative first lens L21 having its surface of sharper curvature facing the image side, a positive meniscus second lens L22 joined thereto and having its convex surface facing the object side, and a negative third lens L23 having its surface of sharper curvature facing the object side, a forward group G3F in a third lens group G3 having a positive refractive power comprises a positive lens L31 having its surface of sharper curvature facing the image side, a biconvex positive lens L32, a negative meniscus lens L33 joined thereto and having its convex surface facing the image side, and a positive meniscus lens L34 having its convex surface facing the object side, and a rearward group G3R in the third lens group G3 comprises a negative lens L35 having its surface of sharper curvature facing the image side, and a biconvex positive lens L36. 
     In the first embodiment, as shown in FIG. 1, a stop S is disposed between the forward group G3F and the rearward group G3R in the third lens group G3. 
     Also shown in FIG. 1 is the movement locus of each lens group when zooming is effected from the wide angle side to the telephoto side. The first lens group G1 is linearly moved toward the object side, the second lens group G2 is moved toward the image side while depicting a convex non-linear locus, and the third lens group G3 is non-linearly moved toward the object side. 
     The numerical data of the first embodiment of the present invention will be shown below. In the table below, the numbers at the left end represent the order from the object side, f represents the focal length of the entire system, and Bf represents the back focal length. 
     
         __________________________________________________________________________First Embodimentf = 71.500-205.000F-number 4.1-5.5__________________________________________________________________________   Radius of       Center thickness               Refractive                     Abbe   curvature       and space               index numberNo.   r        d       n     ν__________________________________________________________________________ 1 r.sub.11153.845       d.sub.1         2.300 n.sub.11                 1.80458                     25.5 L.sub.11 2 r.sub.1271.737 d.sub.2         5.800 n.sub.12                 1.51680                     64.1 L.sub.12 3 r.sub.13-449.462       d.sub.3         0.100                 G.sub.1 4 r.sub.14107.481       d.sub.4         4.600 n.sub.13                 1.51680                     64.1 L.sub.13 5 r.sub.15-278.364       d.sub.5         (variable) 6 r.sub.21-228.633       d.sub.6         1.200 n.sub.21                 1.69350                     53.8 L.sub.21 7 r.sub.2218.350 d.sub.7         4.000 n.sub.22                 1.80458                     25.5 L.sub.22 8 r.sub.2345.502 d.sub.8         3.800                 G.sub.2 9 r.sub.24-33.847       d.sub.9         1.200 n.sub.23                 1.69350                     53.8 L.sub.2310 r.sub.25550.868       d.sub.10         (variable)11 r.sub.31104.039       d.sub.11         4.200 n.sub.31                 1.49782                     82.6 L.sub.3112 r.sub.32-38.434       d.sub.12         0.20013 r.sub.3352.401 d.sub.13         5.500 n.sub.32                 1.48749                     70.2 L.sub.3214 r.sub.34-31.880       d.sub.14         1.400 n.sub.33                 1.74000                     28.3 L.sub.3315 r.sub.35-126.224       d.sub.15         0.80016 r.sub.3632.945 d.sub.16         4.600 n.sub.34                 1.62041                     60.3 L.sub.34                               G.sub.317 r.sub.37182.768       d.sub.17         13.10018 r.sub.38-181.312       d.sub.18         1.600 n.sub.35                 1.76684                     46.8 L.sub.3519 r.sub.3924.638 d.sub.19         9.80020 r.sub.40113.747       d.sub.20         3.200 n.sub.36                 1.62588                     35.6 L.sub.3621 r.sub.41-66.352__________________________________________________________________________f      71.500        115.000                    205.000__________________________________________________________________________d 5    1.782         24.276                    38.607d 10   16.005        9.748                    0.640B f    55.650        63.190                    83.289__________________________________________________________________________        f.sub.1 /f.sub.W = 1.566        |f.sub.2 /f.sub.W | = 0.373        f.sub.3 /f.sub.W = 0.483         ##STR1##        β.sub.2W /β.sub.2T = 0.505         ##STR2##        β.sub.3W /β.sub.3T = 0.690        f.sub.3F /f.sub.3 = 0.757        |f.sub.3R /f.sub.3 | = 1.963        f.sub.3F /f.sub.31 = 0.459__________________________________________________________________________ 
    
     The above values satisfy the conditions of expressions (1)-(10), (4&#39;) and (5&#39;). 
     The values of conditions (11)-(13) about the second lens group G2 are as follows: ##EQU4## n 22  slightly deviates from the lower limit of condition (11), and r 22  /f 2  slightly deviates from the upper limit of condition (12). When extremely precise depiction is required as when a photograph is greatly enlarged and used, it is desirable to set the values of n 22  and r 22  within the ranges of conditions (11) and (12) in order to suppress the fluctuation of chromatic spherical aberration during zooming. 
     FIG. 2 shows the lens construction according to second, third and fourth embodiments of the present invention. In FIG. 2, a first lens group G1 comprises, in succession from the object side, a positive lens L11 having its surface of sharper curvature facing the object side, a negative meniscus lens L12 having its convex surface facing the object side and a positive lens L13 joined thereto and having its convex surface of sharper curvature facing the object side, a second lens group G2 comprises, in succession from the object side, a negative first lens L21 having its surface of sharper curvature facing the image side, a positive meniscus second lens L22 joined thereto and having its convex surface facing the object side and a negative lens L23 having its surface of sharper curvature facing the object side, a forward group G3F in a third lens group G3 comprises, in succession from the object side, a positive lens L31 having its convex surface of sharper curvature facing the image side, a biconvex positive lens L32, a negative lens L33 joined thereto and having its surface of sharper curvature facing the object side and a positive meniscus lens L34 having its convex surface facing the object side, and a rearward group G3R in the third lens group comprises a negative meniscus lens L35 having its convex surface facing the image side and a positive lens L36 having its surface of sharper curvature facing the image side. 
     A variable stop S1 and a fixed stop S2 are disposed between the forward group G3F and the rearward group G3R in the third lens group G3. 
     Also shown in FIG. 2 is the movement locus of each lens group when zooming is effected from the wide angle side to the telephoto side. The first lens group is linearly moved toward the object side, the second lens group is moved toward the image side while depicting a convex non-linear locus, and the third lens group is non-linearly moved toward the object side. The numerical data of the second, third and fourth embodiments of the present invention will be shown below. 
     
         __________________________________________________________________________Second Embodimentf = 71.500-205.000F-number 4.1-5.5__________________________________________________________________________   Radius of       Center thickness               Refractive                     Abbe   curvature       and space               index numberNo.   r        d       n     ν__________________________________________________________________________ 1 r.sub.11120.465       d.sub.1         3.700 n.sub.11                 1.51680                     64.1 L.sub.11 2 r.sub.121885.001       d.sub.2         0.100 3 r.sub.1387.589 d.sub.3         2.000 n.sub.12                 1.80458                     25.5 L.sub.12                               G.sub.1 4 r.sub.1452.453 d.sub.4         7.600 n.sub.13                 1.51680                     64.1 L.sub.13 5 r.sub.15-413.504       d.sub.5         (variable) 6 r.sub.21-150.683       d.sub.6         1.200 n.sub.21                 1.65160                     58.5 L.sub.21 7 r.sub.2220.543 d.sub.7         3.400 n.sub.22                 1.86074                     23.0 L.sub.22 8 r.sub.2336.361 d.sub.8         4.200                 G.sub.2 9 r.sub.24-35.217       d.sub.9         1.200 n.sub.23                 1.65160                     58.5 L.sub.2310 r.sub.255931.968       d.sub.10         (variable)11 r.sub.31106.024       d.sub.11         4.500 n.sub.31                 1.50137                     56.5 L.sub. 3112 r.sub.32-37.807       d.sub.12         0.20013 r.sub.3365.312 d.sub.13         5.300 n.sub.32                 1.51860                     70.1 L.sub.3214 r.sub.34-31.533       d.sub.14         1.400 n.sub.33                 1.75520                     27.6 L.sub.3315 r.sub.35-393.056       d.sub.15         0.80016 r.sub.3629.359 d.sub.16         3.600 n.sub.34                 1.69350                     53.8 L.sub.34                               G.sub.317 r.sub.3746.171 d.sub.17         40.20018 r.sub.38-18.228       d.sub.18         2.100 n.sub.35                 1.77279                     49.4 L.sub.3519 r.sub.39-29.161       d.sub.19         0.20020 r.sub.401024.571       d.sub.20         2.800 n.sub.36                 1.69895                     30.1 L.sub.3621 r.sub.41-73.471__________________________________________________________________________f      71.500        115.000                    205.000__________________________________________________________________________d 5    1.400         24.501                    36.417d 10   16.800        10.418                    0.946B f    41.267        46.675                    69.107__________________________________________________________________________        f.sub.1 /f.sub.W = 1.567        |f.sub.2 /f.sub.W|  = 0.373        f.sub.3 /f.sub.W = 0.483         ##STR3##        β.sub.2W /β.sub.2T = 0.513         ##STR4##        β.sub.3W / β.sub.3T = 0.680        f.sub.3F /f.sub.3 = 0.989        |f.sub.3R /f.sub.3 | = 8.028        f.sub.3F /f.sub.31 = 0.607__________________________________________________________________________ 
    
     The above values satisfy conditions (1)-(10), (4&#39;) and (5&#39;). 
     Also, 
     n 22  =1.86074 
     r 2  /f 2  =-0.770 
     (r 25  +r 24 )/(r.sub. -r 24 )=0.9867 
     The above values satisfy conditions (11)-(13). 
     
         __________________________________________________________________________Third Embodimentf = 71.500-205.000F-number 4.1-5.5__________________________________________________________________________   Radius of       Center thickness               Refractive                     Abbe   curvature       and space               index numberNo.   r        d       n     ν__________________________________________________________________________ 1 r.sub.11124.745       d.sub.1         3.700 n.sub.11                 1.51680                     64.1 L.sub.11 2 r.sub.124325.365       d.sub.2         0.100 3 r.sub.1387.610 d.sub.3         2.000 n.sub.12                 1.80458                     25.5 L.sub.12                               G.sub.1 4 r.sub.1452.453 d.sub.4         7.700 n.sub.13                 1.51680                     64.1 L.sub.13 5 r.sub.15-416.542       d.sub.5         (variable) 6 r.sub.21-150.751       d.sub.6         1.200 n.sub.21                 1.65160                     58.5 L.sub.21 7 r.sub.2220.543 d.sub.7         3.400 n.sub.22                 1.86074                     23.0 L.sub.22 8 r.sub.2336.370 d.sub.8         4.200                 G.sub.2 9 r.sub.24-35.484       d.sub.9         1.200 n.sub.23                 1.65160                     58.5 L.sub.2310 r.sub.252543.972       d.sub.10         (variable)11 r.sub.31103.308       d.sub.11         4.500 n.sub.31                 1.50137                     56.5 L.sub.3112 r.sub.32-38.149       d.sub.12         0.20013 r.sub.3364.331 d.sub.13         5.300 n.sub.32                 1.51860                     70.1 L.sub.3214 r.sub.34-31.533       d.sub.14         1.400 n.sub.33                 1.75520                     27.6 L.sub.3315 r.sub.35-368.870       d.sub.15         0.80016 r.sub.3629.048 d.sub.16         3.600 n.sub.34                 1.69350                     53.8 L.sub.34                               G.sub.317 r.sub.3744.423 d.sub.17         40.20018 r.sub.38-18.138       d.sub.18         2.100 n.sub.35                 1.76684                     46.8 L.sub.3519 r.sub.39-28.876       d.sub.19         0.20020 r.sub.40-4032.012       d.sub.20         2.800 n.sub.36                 1.72825                     28.3 L.sub.3621 r.sub.41-71.662__________________________________________________________________________f      71.500        135.000                    205.006__________________________________________________________________________d 5    1.741         28.503                    36.546d 10   16.985        7.923                    1.065B f    40.922        51.636                    68.755__________________________________________________________________________        f.sub.1 /f.sub.W = 1.567        |f.sub.2 /f.sub.W | = 0.373        f.sub.3 /f.sub.W = 0.483         ##STR5##        β.sub.2W /β.sub.2T = 0.513         ##STR6##        β.sub.3W /β.sub.3T = 0.679        f.sub.3F /f.sub.2 = 0.989        |f.sub.3R /f.sub.3 | = 7.957        f.sub.3F /f.sub.31 = 0.608__________________________________________________________________________ 
    
     The above values satisfy conditions (1)-(10), (4&#39;) and (5&#39;). 
     Also, 
     n 22  =1.86074 
     r 2  /f 2  =-0.770 
     (r 25  +r 24 )/(r 25  -r 24 )=0.9725 
     The above values satisfy conditions (11)-(13). 
     
         __________________________________________________________________________Fourth Embodimentf = 71.500-204.999F-number 4.1-5.5__________________________________________________________________________   Radius of       Center thickness               Refractive                     Abbe   curvature       and space               index numberNo.   r        d       n     ν__________________________________________________________________________ 1 r.sub.1198.438 d.sub.1         3.500 n.sub.11                 1.46450                     65.8 L.sub.11 2 r.sub.12802.616       d.sub.2         0.100 3 r.sub.1376.116 d.sub.3         2.000 n.sub.12                 1.80458                     25.5 L.sub.12                               G.sub.1 4 r.sub.1444.695 d.sub.4         7.300 n.sub.13                 1.51835                     60.3 L.sub.13 5 r.sub.15-279.389       d.sub.5         (variable) 6 r.sub.21-128.684       d.sub.6         1.200 n.sub.21                 1.65160                     58.5 L.sub.21 7 r.sub.2220.994 d.sub.7         3.200 n.sub.22                 1.86074                     23.0 L.sub.22 8 r.sub.2337.184 d.sub.8         4.100                 G.sub.2 9 r.sub.24-37.845       d.sub.9         1.200 n.sub.23                 1.65160                     58.5 L.sub.2310 r.sub.256107.025       d.sub.10         (variable)11 r.sub.31119.160       d.sub.11         4.300 n.sub.31                 1.50137                     56.5 L.sub.3112 r.sub.32-37.088       d.sub.12         0.20013 r.sub.3371.445 d.sub.13         5.300 n.sub.32                 1.51860                     70.1 L.sub.3214 r.sub.34-29.228       d.sub.14         1.400 n.sub.33                 1.75520                     27.6 L.sub.3315 r.sub.35-333.136       d.sub.15         0.80016 r.sub.3622.903 d.sub.16         3.600 n.sub.34                 1.54814                     45.9 L.sub.34                               G.sub.317 r.sub.3738.213 d.sub.17         34.80018 r.sub.38-15.897       d.sub.18         2.100 n.sub.35                 1.77279                     49.4 L.sub.3519 r.sub.39-24.699       d.sub.19         0.20020 r.sub.40-306.837       d.sub.20         2.800 n.sub.36                 1.86074                     23.0 L.sub.3621 r.sub.41-82.419__________________________________________________________________________f      71.500        135.000                    204.999__________________________________________________________________________d 5    2.250         22.806                    29.375d 10   19.898        8.849                    0.957B f    41.337        53.103                    69.563__________________________________________________________________________        f.sub.1 /f.sub.W = 1.385        |f.sub.2 /f.sub.W | = 0.381        f.sub.3 /f.sub.W = 0.483         ##STR7##        β.sub.2W /β.sub.2T = 0.534         ##STR8##        β.sub.3W /β.sub.3T = 0.653        f.sub.3F /f.sub.3 = 0.996        |f.sub.3R /f.sub.3 | = 4.065        f.sub.3F /f.sub.31 = 0.603        n.sub.22 = 1.86074        r.sub.22 /f.sub.2 = -0.769         ##STR9##__________________________________________________________________________ 
    
     The above values satisfy conditions (1)-(13). 
     In a fifth embodiment of the present invention, as shown in FIG. 3, a first lens group G1 comprises, in succession from the object side, a negative meniscus lens L11 having its convex surface facing the object side, a positive lens L12 joined thereto and having its surface of sharper curvature facing the object side, and a biconvex positive lens L13, a second lens group G2 comprises, in succession from the object side, a negative first lens L21 having its surface of sharper curvature facing the image side, a positive meniscus second lens L22 joined thereto and having its convex surface facing the object side, and a negative third lens L23 having its surface of sharper curvature facing the object side, and a third lens group G3 comprises a forward group G3F comprising, in succession from the object side, a biconvex positive lens L31, a biconvex positive lens L32, a biconvex positive lens L33 and a biconcave negative lens L34 joined thereto, and a rearward group G3R comprising a negative meniscus lens L35 having its surface of sharper curvature facing the object side and a positive lens L36 having its surface of sharper curvature facing the image side. 
     In the fifth embodiment, as shown in FIG. 3, a variable stop S1 and a fixed stop S2 are provided between the forward group G3F and the rearward group G3R in the third lens group G3. 
     Also shown in FIG. 3 is the movement locus of each lens group when zooming is effected from the wide angle side to the telephoto side. The first lens group is linearly moved toward the object side, the second lens group is moved toward the image side while depicting a convex non-linear locus, and the third lens group is non-linearly moved toward the object side. 
     The numerical data of the fifth embodiment of the present invention will be shown below. In the table below, the numbers at the left end represent the order from the object side, f represents the focal length of the entire system, and Bf represents the back focal length. 
     
         __________________________________________________________________________Fifth Embodiment__________________________________________________________________________f = 71.500-205.000F-number 4.1-5.5__________________________________________________________________________   Radius of      Center thickness               Refractive                      Abbe   curvature      and space               index  numberNo.   r       d        n      ν__________________________________________________________________________ 1 r.sub.11 118.310      d.sub.1         2.300 n.sub.11                  1.80458                      25.5 L.sub.11 2 r.sub.12 63.790      d.sub.2         6.000 n.sub.12                  1.51680                      64.1 L.sub.12 3 r.sub.13 1325.679      d.sub.3         0.100                  G.sub.1 4 r.sub.14 95.278      d.sub.4         5.400 n.sub.13                  1.51680                      64.1 L.sub.13 5 r.sub.15 -289.179      d.sub.5         (variable) 6 r.sub.21 -155.511      d.sub.6         1.200 n.sub.21                  1.65160                      58.5 L.sub.21 7 r.sub.22 20.550      d.sub.7         3.400 n.sub.22                  1.86074                      23.0 L.sub.22 8 r.sub.23 36.637      d.sub.8         4.200                  G.sub.2 9 r.sub.24 -34.912      d.sub.9         1.200 n.sub.23                  1.65160                      58.5 L.sub.2310 r.sub.25 2426.367      d.sub.10         (variable)11 r.sub.31 115.890      d.sub.11         3.500 n.sub. 31                  1.51860                      70.1 L.sub.3112 r.sub.32 -84.561      d.sub.12         0.20013 r.sub.33 50.935      d.sub.13         4.300 n.sub.32                  1.48749                      70.2 L.sub.3214 r.sub.34 -88.680      d.sub.14         0.80015 r.sub.35 38.863      d.sub.15         5.200 n.sub.33                  1.51680                      64.1 L.sub.33                                G.sub.316 r.sub.36 -54.825      d.sub.16         1.400 n.sub.34                  1.78470                      26.1 L.sub.3417 r.sub.37 145.469      d.sub.17         39.80018 r.sub.38 -19.312      d.sub.18         2.100 n.sub.35                  1.77279                      49.4 L.sub.3519 r.sub.39 -32.084      d.sub.19         0.20020 r.sub.40 -701.705      d.sub.20         2.800 n.sub.36                  1.74077                      27.6 L.sub.3621 r.sub.41 -64.629__________________________________________________________________________f            71.500   115.000  205.000d 5          1.586    25.921   37.783d 10         17.300   11.136   1.853B f          42.725   47.511   70.890__________________________________________________________________________       f.sub.1 /f.sub.W = 1.566       |f.sub.2 /f.sub.W | = 0.373       f.sub.3 /f.sub.W = 0.483        ##STR10##       β.sub.2W /β.sub.2T = 0.509        ##STR11##       β.sub.3W /β.sub.3T = 0.684       f.sub.3F /f.sub.3 = 0.989       |f.sub.3R /f.sub.3 | = 8.131       f.sub.3F f.sub.31 = 0.360       n.sub.22 = 1.86074       r.sub.22 /f.sub.2 = -0.770        ##STR12##__________________________________________________________________________ 
    
     The above values satisfy conditions (1)-(13), (4&#39;) and (5&#39;). 
     FIG. 4 shows a lens construction according to a sixth embodiment of the present invention in which the second lens group G2 in the first embodiment shown in FIG. 1 is improved to satisfy conditions (11)-(13). 
     In FIG. 4, a first lens group G1 comprises, in succession from the object side, a negative meniscus lens L11 having its convex surface facing the object side, a positive lens L12 joined thereto and having its surface of sharper curvature facing the object side, and a biconvex positive lens L13, a second lens group G2 comprises, in succession from the object side, a negative first lens L21 having its surface of sharper curvature facing the image side, a positive meniscus second lens L22 joined thereto and having its convex surface facing the object side, and a negative third lens L23 having its surface of sharper curvature facing the image side, and a third lens group G3 comprises a forward group G3F comprising, in succession from the object side, a positive lens L31 having its surface of sharper curvature facing the image side, a biconvex positive lens L32, a negative meniscus lens L33 joined thereto and having its convex surface facing the image side, and a positive meniscus lens L34 having its convex surface facing the object side, and a rearward group G3R comprising a negative lens L35 having its surface of sharper curvature facing the image side and a positive lens L36 having its surface of sharper curvature facing the image side. 
     In the sixth embodiment, as shown in FIG. 4, a stop S is disposed between the forward group G3F and the rearward group G3R in the third lens group G3. 
     Also shown in FIG. 4 is the movement locus of each lens group when zooming is effected from the wide angle side to the telephoto side. The first lens group is linearly moved toward the object side, the second lens group is moved toward the image side while depicting a convex non-linear locus, and the third lens group is non-linearly moved toward the object side. 
     The numerical data of the sixth embodiment of the present invention will be shown below. 
     
         __________________________________________________________________________Sixth Embodiment__________________________________________________________________________          f = 71.500-205.000          F-number 4.1-5.5__________________________________________________________________________   Radius of      Center thickness               Refractive                      Abbe   curvature      and space               index  numberNo.   r       d        n      ν__________________________________________________________________________ 1 r.sub.11 153.844      d.sub.1         2.300 n.sub.11                  1.80458                      25.5 L.sub.11 2 r.sub.12 71.737      d.sub.2         5.800 n.sub.12                  1.51680                      64.1 L.sub.12 3 r.sub.13 -449.461      d.sub.3         0.100                  G.sub.1 4 r.sub.14 107.481      d.sub.4         4.600 n.sub.13                  1.51680                      64.1 L.sub.13 5 r.sub.15 -278.364      d.sub.5         (variable) 6 r.sub.21 -207.164      d.sub.6         1.200 n.sub.21                  1.69350                      53.8 L.sub.21 7 r.sub.22 20.395      d.sub.7         4.000 n.sub.22                  1.86074                      23.0 L.sub.22 8 r.sub.23 40.805      d.sub.8         4.000                  G.sub.2 9 r.sub.24 -32.559      d.sub.9         1.200 n.sub.23                  1.69350                      53.8 L.sub.2310 r.sub.25 -687.887      d.sub.10         (variable)11 r.sub.31 104.039      d.sub.11         4.200 n.sub.31                  1.49782                      82.6 L.sub.3112 r.sub.32 -38.434      d.sub.12         0.20013 r.sub.33 52.401      d.sub.13         5.500 n.sub.32                  1.48749                      70.2 L.sub.3214 r.sub.34 -31.880      d.sub.14         1.400 n.sub.33                  1.74000                      28.3 L.sub.3315 r.sub.35 -126.223      d.sub.15         0.800                  G.sub.316 r.sub.36 32.945      d.sub.16         4.600 n.sub.34                  1.62041                      60.3 L.sub.3417 r.sub.37 182.768      d.sub.17         13.10018 r.sub.38 -181.312      d.sub.18         1.600 n.sub.35                  1.76684                      46.8 L.sub.3519 r.sub.39 24.638      d.sub.19         9.80020 r.sub.40 113.747      d.sub.20         3.200 n.sub.36                  1.62588                      35.6 L.sub.3621 r.sub.41 -66.352__________________________________________________________________________f            71.500   115.000  205.000d 5          1.825    24.320   38.650d 10         15.816   9.559    0.452B f          55.650   63.190   83.289__________________________________________________________________________       f.sub.1 /f.sub.W = 1.566       |f.sub.2 /f.sub.W | = 0.373       f.sub.3 /f.sub.W = 0.483        ##STR13##       β.sub.2W /β.sub.2T = 0.505        ##STR14##       β.sub.3W /β.sub.3T = 0.690       f.sub.3F /f.sub.3 = 0.757       |f.sub.3R /f.sub.3 | = 1.963       f.sub.3F /f.sub.31 = 0.459       n.sub.22 = 1.86074       r.sub.22 /f.sub.2 = -0.764        ##STR15##__________________________________________________________________________ 
    
     The above values all satisfy conditions (1)-(13), (4&#39;) and (5&#39;). 
     FIG. 5 shows a lens construction according to a seventh embodiment of the present invention in which the construction of the forward group G3F in the third lens group shown in FIG. 3 is changed. 
     In FIG. 5, a first lens group G1 comprises, in succession from the object side, a positive lens L11 having its surface of sharper curvature facing the object side, a negative meniscus lens L12 having its convex surface facing the object side, and a positive lens L13 joined thereto and having its surface of sharper curvature facing the object side, a second lens group G2 comprises, in succession from the object side, a negative first lens L21 having its surface of sharper curvature facing the image side, a positive meniscus second lens L22 joined thereto and having its convex surface facing the object side, and a negative third lens L23 having its surface of sharper curvature facing the object side, and a third lens group G3 comprises a forward group G3F comprising, in succession from the object side, a positive lens L31 having its surface of sharper curvature facing the image side, a negative meniscus lens L32 joined thereto and having its convex surface facing the image side, a biconvex positive lens L33 having its surface of sharper curvature facing the object side, and a negative lens L34 having its surface of sharper curvature facing the object side, and a rearward group G3R comprising a negative meniscus lens L35 having its convex surface facing the image side and a positive lens L36 having its surface of sharper curvature facing the image side. A variable stop S1 and a fixed stop S2 are provided between the forward group G3F and the rearward group G3R. 
     As described above, in the seventh embodiment, the forward group G3F in the third lens group G3 entirely differs from the constructions shown in FIGS. 1 to 4. Therefore, condition (10) is not satisfied. However, coma and spherical aberration based on condition (10) are corrected well by the unique construction of the forward group G3F. 
     Also shown in FIG. 5 is the movement locus of each lens group when zooming is effected from the wide angle side to the telephoto side. The first lens group is linearly moved toward the object side, the second lens group is moved toward the image side while depicting a convex non-linear locus, and the third lens group is non-linearly moved toward the object side. 
     The numerical data of the seventh embodiment of the present invention will be shown below. 
     
         __________________________________________________________________________Seventh Embodimentf = 72.318-205.565F-number 4.5-5.0__________________________________________________________________________   Radius of       Center thickness               Refractive                     Abbe   curvature       and space               index numberNo.   r        d       n     ν__________________________________________________________________________ 1 r.sub.11107.200       d.sub.1         3.850 n.sub.11                 1.51680                     64.1 L.sub.11 2 r.sub.12-1450.001       d.sub.2         0.200 3 r.sub.1379.420 d.sub.3         1.950 n.sub.12                 1.80518                     25.4 L.sub.12                               G.sub.1 4 r.sub.1447.810 d.sub.4         6.300 n.sub.13                 1.51680                     64.1 L.sub.13 5 r.sub.15-1449.991       d.sub.5         (variable) 6 r.sub.21-203.083       d.sub.6         1.000 n.sub.21                 1.69680                     55.6 L.sub.21 7 r.sub.2220.547 d.sub.7         3.860 n.sub.22                 1.86074                     23.0 L.sub.22 8 r.sub.2337.163 d.sub.8         3.200                 G.sub.2 9 r.sub.24-38.171       d.sub.9         1.000 n.sub.23                 1.69680                     55.6 L.sub.2310 r.sub.25-497.113       d.sub.10         (variable)11 r.sub.3185.720 d.sub.11         4.550 n.sub.31                 1.61272                     58.6 L.sub. 3112 r.sub.32-31.160       d.sub.12         1.000 n.sub.32                 1.71736                     29.5 L.sub.3213 r.sub.33-54.350       d.sub.13         0.20014 r.sub.3427.615 d.sub.14         5.160 n.sub.33                 1.48749                     70.2 L.sub.3315 r.sub.35-88.000       d.sub.15         2.94016 r.sub.36-55.000       d.sub.16         2.000 n.sub.34                 1.75520                     27.6 L.sub.34                               G.sub.317 r.sub.370.000  d.sub.17         36.10018 r.sub.38-17.300       d.sub.18         1.270 n.sub.35                 1.69680                     55.6 L.sub.3519 r.sub.39-30.500       d.sub.19         0.20020 r.sub.40700.000       d.sub.20         2.480 n.sub.36                 1.71736                     29.5 L.sub.3621 r.sub.41-68.001__________________________________________________________________________f      72.818        133.831                    205.565__________________________________________________________________________d 5    .939          25.106                    32.939d 10   20.091        10.046                    1.650B f    42.052        49.813                    64.511__________________________________________________________________________        f.sub.1 /f.sub.W = 1.422        |f.sub.2 /f.sub.W | = 0.380        f.sub.3 /f.sub.W = 0.478         ##STR16##        β.sub.2W /β.sub.2T = 0.496         ##STR17##        β.sub.3W /β.sub.3T = 0.714        f.sub.3F /f.sub.3 = 1.008        |f.sub.3R /f.sub.3 | = 6.325        n.sub.22 = 1.86074        r.sub.22 /f.sub.2 = -0.742         ##STR18##__________________________________________________________________________ 
    
     The above values satisfy conditions (1)-(9) and (11)-(13) except condition (10).