Abstract:
A zoom lens having at least three components, or, from front to rear, a first component of positive refractive power, a second component for variation of the image magnification, and a third component for image shift compensation, wherein said first lens component is constructed from a front group of lens elements l A  of positive refractive power and a rear group of lens elements l B  of negative refractive power, thereby letting f A  and f B  denote the focal lengths of said front and rear lens groups respectively, the following condition is satisfied: 
     
       1.5f.sub.A &lt;|f.sub.B |&lt;4f.sub.A 
     
     and said rear lens group l B  is made movable for focusing.

Description:
This is a continuation of application Ser. No. 511,669, filed July 7, 1983 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to zoom lenses, and more particularly to zoom lenses having the focusing provision in part of lens components which remain stationary during zooming. 
     2. Description of the Prior Art 
     In conventional zoom lens having a variator preceded by a focusing component, the characteristics that the once-established in-focus condition is not broken by any subsequent zooming operation, and that, in any zooming position, focusing is effected by a predetermined amount of forward movement for a certain object distance have been enjoyed. 
     As compared with this, other focusing methods by moving one of the components that follows the variator have to differentiate the amount of forward movement of the focusing component depending upon the zooming position despite a constant object distance. Therefore the focusing control or cam mechanism tended to be complicated in structure. 
     For this reason, many zoom lenses have the focusing provision made at a component in front of the variator, or, for example, the first component counting from front. 
     This focusing arrangement is, however, accompanied with the following disadvantages: 
     (1) Since the diameter of the first component is generally large, and its weight is also heavy, the operativity when in focusing becomes bad. 
     (2) As the first component moves forward to effect focusing down to shorter object distances, the total length of the entire lens system becomes longer, and the weight balance gets worse. (3) When the first component is driven to move by automatic focusing mechanism, the driving system must be of high torque, making it difficult to operate the focusing system quickly with high accuracy. 
     To avoid these, a proposal has been made that the front or first component is divided into two parts of which the rear part only is moved to effect focusing, namely, the so-called inner focusing method, as, for example, disclosed in Japanese Laid-Open patent applications Sho Nos. 52-109952, 55-57815 and 55-117119 and U.S. Pat. Nos. 3,598,476, 4,110,006 and 4,099,845. 
     In those lens system employing the inner focusing method, from the standpoint of aberration correction, the first component is made constructed in a considerably complicated form, and has a considerably heavy weight. 
     In Japanese Pat. Publication Sho No. 52-41068, it is proposed that the first lens component is constructed with two lens groups of negative and positive powers, and the rear or positive lens group is made movable for focusing. 
     This focusing method when applied to the positive lead type of zoom lenses where the front lens group is of positive refractive power, produces the following drawbacks: 
     (4) Since the first component is of positive refractive power, letting f I  denote the focal length of the first component, and f IA  and f IB  the focal lengths of the front and rear lens groups of negative and positive powers respectively, we have the following approximate equation: 
     
         f.sub.I =f.sub.Ia =f.sub.IB 
    
     At this time, f I  &gt;f IA  and |f IB  |&gt;f IA  are given. Therefore, the focal length f IB  of the focusing lens group becomes small, making it difficult to assure good correction of aberrations throughout the focusing range. Also from the aspect of production, high precision techniques are required. 
     (5) In order that the rear lens group of positive power admits of the diverged light pencil by the front or negative lens group, the diameter of the rear lens group is necessarily increased. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a zoom lens having its total length maintained unchanged against focusing with achievement of simplifying the movable lens group for focusing, while still permitting good correction of aberrations. 
     To achieve this, the feature of the invention is the design of the zoom lens in such a way that the lens system has at least three components, or, from front to rear, a first component of positive refractive power, a second component for variation of image magnification, and a third component for compensating for the image shift resulting from the change of the image magnification, 
     Wherein said first lens component is constructed from a 1 A  -th lens group of positive refractive power and a 1 B  -th lens group of negative refractive power, 
     whereby letting f A  and f B  denote the focal lengths of said 1 A  -th and 1 B  -th lens groups respectively, the following condition is satisfied: 
     
         1.5f.sub.A &lt;|f.sub.B |&lt;4f.sub.A          ( 1) 
    
     and said 1 B  -th lens group is made movable for focusing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1, 4 and 7 are lens block diagrams of embodiments 1, 2 and 3 of the invention respectively. 
     FIGS. 2-1 through -12 and FIGS. 3-1 through -12 are graphic representations of the various aberrations of the lens of FIG. 1 with an object at infinity and at a distance of 3 meters respectively. 
     FIGS. 5-1 through -12 and FIGS. 6-1 through -12 are graphic representations of the various aberrations of the lens FIG. 4 with an object at infinity and at a distance of 3 meters respectively. 
     FIGS. 8-1 through -12 and FIGS. 9-1 through -12 are graphic representations of the various aberrations of the lens of FIG. 7 with an object at infinity and at a distance of 3 meters respectively. 
     FIG. 10 is a diagram of geometry considered to explain the function of the lens groups constituting part of the zoom lens of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the zoom lens of the invention, the second and third lens counting from front components movable for zooming are preceded by the first lens component which is divided into two parts of positive and negative powers respectively, of which the rear part or 1 B  -th lens group of negative power is imparted with independent movement for focusing. The employment of such focusing method provides the features that the amount of forward movement of the focusing lens group required to focus on the same object as measured from the position for an infinitely distant object is constant in any zooming position, further that the diameters of the components that follow the 1 B  -th lens group can be reduced, giving an advantage at a minimization of the bulk and size of the complete zoom lens, and that without recouse to an unduly large increase in the negative refractive power, the required focusing range can be formed by a lessened amount of forward movement of the focusing group. 
     And since the refractive power of the first component is positive, it results that f A  &lt;|f B  |. At this time, as illustrated in FIG. 10, the 1 B  -th lens group contributes an enlarging system to the imaging relationship. Therefore, focusing for the required range of variation of the object distance can be effected by a reduced amount of forward movement. 
     In other words, if the focusing movement is allowed, the refractive power of the 1 b  -th lens group for focusing may be weakened. 
     Arising from this there are characteristics such as: 
     (a) The construction and arrangment and form of the elements of the focusing lens group can be simplified. 
     (b) It becomes easy to reduce the weight of the focusing lens group. 
     (c) The range of variation of aberrations with focusing can be lessened. 
     (d) Manufacturing becomes easy. 
     And, by satisfying the above-stated condition (1), good stability aberration correction throughout the focusing range is achieved. 
     When the upper limit of the inequalities of condition (1) is exceeded, the total movement of the 1 B  -th lens group is objectionably increased. This calls for an increase in the physical length of the lens and also for an increase in the diameter of the 1 A  -th lens group. 
     When the lower limit is exceeded, variation of the aberrations with focusing is increased objectionably particularly regarding spherical aberration and astigmatism. 
     Further, to facilitate a better accomplishment of the object of the invention, it is preferred to satisfy the following conditions: 
     The 1 A  -th lens group is provided with at least one cemented lens surface or air lens of negative refractive power and has an overall refractive power denoted by φ AX . The 1 A  -th lens group and the 1 B  -th lens group each have at least one positive lens and at least negative lens. 
     Letting f I  denote the focal length of the first component, 
     ν AP  the mean value of the Abbe numbers of the glasses of the positive lenses in the 1 A  -th lens group, 
     ν AN  the mean value of the Abbe numbers of the glasses of the negative lenses in the 1 A  -th lens group, and 
     r Bf  and r Br  the radii of curvature of the front and rear lens surfaces of the 1 B  -th lens group respectively, the following conditions are satisfied: ##EQU1## 
     By giving the 1 A  -th lens group the prerequisite of the negative refractive power, good correction of spherical aberration, astigmatism and longitudinal chromatic aberration is maintained in the focusing range. 
     The inequalities of condition (2) are to assist in achievement of good correction of these aberrations. When the lower limit is exceeded, the residual aberrations of the 1 A  -th lens group are increased. When the upper limit is exceeded, the possibility of producing higher order aberrations is increased, and insufficient correction of spherical aberration in the telephoto positions results. 
     The inequality of condition (3) is to well correct chromatic aberrations. When this is violated, it becomes difficult to stabilize the chromatic aberrations throughout the focusing range. 
     The inequalities of condition (4) are to define a proper lens form of the 1 A  -th lens gorup by which the aberrations are stabilized throughout the focusing range. When the upper limit is exceeded, variation with focusing of the spherical aberration and coma is increased. When the lower limit is exceeded, though the range of variation with focusing of the aberrations is lessened, it is objectionable that the spherical aberration is not well corrected for the telephoto positions. 
     It is to be noted that the zoom section of the second and third components of the zoom lens according to the present invention constitutes an afocal system. With the afocal system of a magnification, r, when followed by an image forming system or fourth component, letting f T  denote the longest focal length of the entire system and f R  the focal length of the fourth component, we have f T  =rf R . To achieve a reduction in the total length of the lens system, it is recommended that the focal length f R  is taken at a small value, and the afocal magnification, r, at a large value. 
     The afocal magnification in terms of the image magnification, β II , of the second component and the focal length f III  of the third component, is expressed by r=(f T  /f III ) β II . That the f T  is increased while leaving the image magnification, β II , of the second component at a constant value, is advantageous in increasing the r, but leads to create a wasteful space between the first and second components, thus hindering the total length of the lens system to be shortened. To minimize this wasteful space, it is preferred that the first component is constructed with positive and negative lens groups as in the present invention, by which the principal point of the first component is shifted toward the front. 
     As in the above, according to the present invention, a zoom lens of shortened total length while still permitting good stability of aberration correction throughout the focusing range can be realized. 
     Next, numerical examples of embodiments of the invention are shown. In the numberical examples, Ri is the radius of curvature of the i-th lens surface counting from front, Di is the i-th lens thickness or air separation counting from front, and Ni and νi are the refractive index and Abbe number of the glass of the i-th lens element counting from front respectively. 
     In the numerical examples, the Ni and νi are described consecutively from the above. 
     
         ______________________________________Numerical Example 1______________________________________Ri           Di          Ni        νi______________________________________ 1     133.12    7.11        1.51633 64.1 2     -906.29   0.30        1. 3     77.24     3.04        1.80518 25.4 4     53.65     10.41       1.48749 70.1 5     646.02    0.49        1. 6     430.30    2.53        1.51633 64.1 7     82.53     3.73        1.59270 35.3 8     115.69    Variable    1. 9     -388.37   1.57        1.69680 55.510     53.83     3.10        1.11     -59.40    1.47        1.69680 55.512     54.26     3.43        1.80518 25.413     -722.70   Variable    1.14     154.30    3.73        1.51633 64.115     -77.84    0.19        1.16     -824.64   4.42        1.51633 64.117     -50.51    1.67        1.80610 40.918     -189.02   Variable    1.19     38.72     3.51        1.61375 56.420     74.60     1.27        1.21     50.37     1.92        1.80518 25.422     26.78     3.89        1.62606 39.223     65.44     41.63       1.24     79.68     2.92        1.51633 64.125     -141.74   2.10        1.26     -36.61    1.42        1.77250 49.627     69.94     4.37        1.67270 32.128     -89.73______________________________________f: 100 - 383  F No: 5.6       d8    d13       d18     f______________________________________Wide Angle Side         12.677  58.264    29.021                                 100Intermediate  59.776  35.229     4.957                                 214Telephoto Side         78.003   1.087    20.872                                 383______________________________________ 
    
     
         ______________________________________Numerical Example 2______________________________________Ri             Di         Ni        νi______________________________________ 1    154.80       2.50       1.80518 25.4 2    72.83        9.00       1.61720 54.0 3    -3676.48     0.12       1. 4    94.74        6.50       1.62280 57.0 5    0.15E + 0    0.48       1. 6    418.43       2.50       1.51633 64.1 7    97.16        3.75       1.59270 35.3 8    120.21       Variable   1. 9    335.34       1.50       1.71300 53.810    49.20        9.95       1.11    -56.14       1.50       1.71300 53.812    59.42        3.87       1.84666 23.913    -2089.67     Variable   1.14    149.66       6.75       1.51633 64.115    -38.62       1.87       1.72825 28.516    -68.83       Variable   1.17    44.06        5.07       1.61272 58.718    1081.22      2.75       1.19    -318.37      2.25       1.80518 25.420    439.23       56.24      1.21    -24.98       2.25       1.78300 36.122    -47.79       0.12       1.23    148.29       5.62       1.60323 42.324    -100.86______________________________________f: 100 - 250  F No: 4.5     d8    d13       d16     f______________________________________Wide Angle Side       13.649  37.782    16.212                               100Intermediate       43.271  20.702    3.67    168.8Telephoto Side       57.288   0.499     9.856                               250______________________________________ 
    
     
         ______________________________________Numerical Example 3______________________________________Ri           Di          Ni        νi______________________________________ 1     139.09    7.07        1.51633 64.1 2     -903.63   0.19        1. 3     75.05     1.96        1.80518 25.4 4     52.73     0.25        1. 5     52.65     11.78       1.48749 70.1 6     973.29    1.58        1. 7     470.33    2.16        1.51633 64.1 8     74.28     3.24        1.59270 35.3 9     109.87    Variable    1.10     -389.55   1.57        1.69680 55.511     53.95     3.46        1.12     -60.06    1.47        1.69680 55.513     54.48     3.43        1.80518 25.414     -796.30   Variable    1.15     155.04    3.73        1.51633 64.116     -77.37    0.19        1.17     -697.13   4.42        1.51633 64.118     -50.21    1.67        1.80610 40.919     -183.55   Variable    1.20     39.58     3.51        1.61375 56.421     77.30     1.27        1.22     49.80     1.92        1.80518 25.423     26.47     3.89        1.62606 39.224     67.58     43.47       1.25     82.22     2.92        1.51633 64.126     -149.39   2.13        1.27     -36.02    1.42        1.77250 49.628     67.04     4.37        1.67270 32.129     -91.14    69.49       1.______________________________________f: 100 - 383  F No: 5.6       d9    d14       d19     f______________________________________Wide Angle Side         12.028  58.005    29.022                                 100Intermediate  59.129  34.969     4.957                                 214Telephoto Side         77.356   0.826    20.873                                 383______________________________________