Patent Publication Number: US-5530592-A

Title: Zoom lens of rear focus type

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to zoom lenses of the rear focus type and, more particularly, to large relative aperture, high range zoom lenses of the rear focus type whose zoom ratio is 8 to 12 and F-number is 1.6 to 2 with a long back focal distance for use in photographic cameras, video cameras, or broadcasting cameras. 
     2. Description of the Related Art 
     For the zoom lenses in a photographic camera or a video camera, there have been previous proposals for focusing by moving other than the front or first lens unit. The so-called rear focus type has thus found its use in a variety of zoom lenses. 
     As compared with the zoom lens of the type in which focusing is performed by moving the first lens unit, the zoom lenses of the rear focus type generally have the advantages that the first lens unit gets a smaller effective diameter so as to facilitate minimization of the bulk and size of the entire lens system, and also that close-up photography, particularly photomacrography, becomes easy to do, and further that, since focusing movement is imparted to a lens unit of relatively small size and light weight, a smaller incentive suffices for driving it, making it possible to adjust the focus rapidly. 
     In application of such a rear focus type to the so-called 4-unit zoom lens, an example is shown in Japanese Laid-Open Patent Application No. Sho 63-44614 where a zoom lens comprises, from front to rear, a first lens unit of positive refractive power, a second lens unit of negative refractive power for varying the focal length, a third lens unit of negative refractive power for compensating for the image shift with zooming and a fourth lens unit of positive refractive power, totaling four lens units, whereby the third lens unit is used for focusing. However, this zoom lens has to secure a space in which the third lens unit moves. So, there is a tendency to increase the physical length. 
     In Japanese Laid-Open Patent Application No. Sho 58-136012, the zooming section is constructed with three or more lens units, one of which is moved for focusing purposes. 
     In Japanese Laid-Open Patent Applications Nos. Sho 63-247316 and Sho 62-24213, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power are arranged in this order from the object side, wherein the second lens unit is moved to vary the focal length and the fourth lens unit is moved to compensate for the image shift with zooming, whereby focusing is performed by moving the fourth lens unit. 
     In Japanese Laid-Open Patent Application No. Sho 58-160913, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power are arranged in this order from the object side, wherein the first and second lens units are moved to vary the focal length, while simultaneously moving the fourth lens unit to compensate for the image shift. Of these lens units, one or two or more lens units are moved for focusing purposes. 
     In Japanese Laid-Open Patent Application No. Sho 63-278013, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of negative refractive power and a fourth lens unit of positive refractive power are arranged in this order from the object side, wherein the second lens unit is moved to vary the focal length and the fourth lens unit is moved to compensate for the image shift with zooming and also to effect focusing. 
     Another form of the 4-unit zoom lens is proposed in Japanese Laid-Open Patent Application No. Sho 57-111507, comprising, from front to rear, a first lens unit of positive refractive power which is stationary during zooming and movable for focusing, a second lens unit of negative refractive power and a third lens unit of positive refractive power, wherein the second and third lens units move in opposite directions to each other during zooming, whereby the third lens unit has two positive lens units arranged to move in differential relation to each other. With such a 4-unit zoom lens of plus-minus-plus-plus refractive power arrangement, according to this document, location of an aperture stop is made within the third lens unit. 
     Since, in this configuration, however, the second and third lens units move always in opposite directions, there is need to make wider room for the separation between the second and third lens units at the wide-angle end. Also, since the stop lies within the third lens unit, as this implies that at the wide-angle end, the position of the entrance pupil is nearest to the image side, such a zoom lens is not suited to a use for minimizing the diameter of the front lens members and the size of the entire lens system. 
     Then, because focusing is performed by the first lens unit, the diameter of the front lens members must be increased in order to admit a light beam going to the corners of the image frame at the minimum object distance. If attempts are made to minimize the bulk and size by adapting this zooming configuration to the rear focus type, however, a problem arises in that the refractive power arrangement is not found optimum, or the variation of aberrations with focusing is not corrected sufficiently well, as the rear focus type is used. 
     Also, even in Japanese Laid-Open Patent Application No. Hei 3-200113, a zoom lens of similar form is shown comprising, from front to rear, a positive first lens unit which is stationary during zooming, a negative second lens unit which moves back and forth to vary the focal length from one end to another, a positive third lens unit which moves in associative relation to the movement of the second lens unit, and a positive fourth lens unit which moves in part or in whole to compensate for the shift of the image plane with zooming. 
     According to this document, what is meant by the associative movement of the positive third lens unit with the movement of the second lens unit is done in order to reduce that amount of compensation for the image shift which is required for the fourth lens unit, in other words, to let the third lens unit share part of the compensating function. Concretely speaking, it is taken as desired that, when zooming in a region of from a middle focal length position to the telephoto end, the third lens unit moves from the image side to the object side. 
     With this arrangement, however, because the second and third lens units move in opposite directions, there is need to make wider room for the separation between the second and third lens units at the wide-angle end. As this implies that at the wide-angle end, the entrance pupil takes its place nearest to the image plane, there is a difficult problem of minimizing the diameter of the front lens member and the size of the entire lens system. 
     Again, Japanese Laid-Open Patent Application No. Hei 3-158813, too, discloses a similar zoom lens as comprising, from front to rear, a positive first lens unit, a negative second lens unit, a positive third lens unit and a positive fourth lens unit, the second and third lens units being axially moved to vary the focal length, and the aperture stop being moved in unison with the third lens unit. 
     According to this document, during zooming from the wide-angle end to the telephoto end, the separation between the second and third lens units decreases. The third lens unit carrying the aperture stop also takes the rearmost possible position when in the wide-angle end. So, that neighborhood of the third lens unit which contains the stop approaches nearest to the image plane in the wide-angle end, or in a slightly zoomed position therefrom, where the front lens members necessarily have a largest diameter. As this means that the entrance pupil goes up deep, a disadvantage is produced at reducing the diameter of the front members. Moreover, distortion gets large in the wide-angle end. With this distortion removed for good optical performance, therefore, a problem arises in that it is very difficult to shorten the diameter of the front members and reduce the size of the entire lens system. 
     In another Japanese Laid-Open Patent Application No. Hei 3-215810, a zoom lens of the rear focus type is proposed comprising, from front to rear, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a stop, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, totaling four lens units, wherein during zooming from the wide-angle end to the telephoto end, the second lens unit moves toward the image side, while simultaneously moving the stop, the third lens unit and the fourth lens unit independently of each other so that each depicts a locus convex toward the object side, and wherein focusing is performed by moving the fourth lens unit. 
     In general, the use of the rear focus type in zoom lenses gives the advantages that, as has been described before, the entire lens system is minimized in bulk and size, and rapid focusing can be carried out, and further close-up photography becomes easy to do. 
     For the counterpart, however, the variation of aberrations with focusing increases largely, giving rise to a problem in that it becomes very difficult to obtain high optical performance throughout the entire focusing range in such a manner that the bulk and size of the entire lens system are limited to a minimum. 
     At the present time, for the video camera for public use, the image sensor is in the form of a single chip. In this case, the color separation prism is not needed that is commonly used in the multiple chip type cameras for professional use. The zoom lens for the single chip type camera has a relatively short back focal distance. 
     In the case of the multiple chip type, the color separation prism and its associated parts are arranged behind the photographic lens (zoom lens). For this reason, the zoom lens for the multiple chip type camera necessitates a relatively long back focal distance compared with the zoom lens for the single chip type video camera for public use. 
     SUMMARY OF THE INVENTION 
     The present invention employs the rear focus type to make a good compromise between the requirements of increasing the aperture ratio and zoom ratio and of minimizing the bulk and size of the entire lens system. It is, therefore, an object of the invention to provide a zoom lens of the rear focus type which maintains good stability of optical performance throughout the entire zooming range and throughout the entire focusing range, and whose back focal distance is made as long as needed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of the paraxial refractive power arrangement of a zoom lens according to the invent ion. 
     FIG. 2 is a longitudinal section view of a numerical example 1 of a zoom lens of the invention. 
     FIG. 3 is a longitudinal section view of a numerical example 2 of a zoom lens of the invention. 
     FIG. 4 is a longitudinal section view of a numerical example 3 of a zoom lens of the invention. 
     FIG. 5 is a longitudinal section view of a numerical example 4 of a zoom lens of the invention. 
     FIG. 6 is a longitudinal section view of a numerical example 5 of a zoom lens of the invention. 
     FIG. 7 is a longitudinal section view of a numerical example 6 of a zoom lens of the invention. 
     FIG. 8 is a longitudinal section view of a numerical example 7 of a zoom lens of the invention. 
     FIG. 9 is a longitudinal section view of a numerical example 8 of a zoom lens of the invention. 
     FIGS. 10(A) and 10(B) are graphic representations of the aberrations of the numerical example 1 of the zoom lens of the invention. 
     FIGS. 11(A) and 11(B) are graphic representations of the aberrations of the numerical example 2 of the zoom lens of the invention. 
     FIGS. 12(A) and 12(B) are graphic representations of the aberrations of the numerical example 3 of the zoom lens of the invention. 
     FIGS. 13 (A) and 13(B) are graphic representations of the aberrations of the numerical example 4 of the zoom lens of the invention. 
     FIGS. 14(A) and 14(B) are graphic representations of the aberrations of the numerical example 5 of the zoom lens of the invention. 
     FIGS. 15(A) and 15(B) are graphic representations of the aberrations of the numerical example 6 of the zoom lens of the invention. 
     FIGS. 16(A) and 16(B) are graphic representations of the aberrations of the numerical example 7 of the zoom lens of the invention. 
     FIGS. 17(A) and 17(B) are graphic representations of the aberrations of the numerical example 8 of the zoom lens of the invention. 
     FIG. 18 is a longitudinal section view of a numerical example 9 of a zoom lens of the invention. 
     FIG. 19 is a longitudinal section view of a numerical example 10 of a zoom lens of the invention. 
     FIG. 20 is a longitudinal section view of a numerical example 11 of a zoom lens of the invention. 
     FIG. 21 is a longitudinal section view of a numerical example 12 of a zoom lens of the invention. 
     FIG. 22 is a longitudinal section view of a numerical example 13 of a zoom lens of the invention. 
     FIGS. 23(A) and 23(B) are graphic representations of the aberrations of the numerical example 9 of the zoom lens of the invention. 
     FIGS. 24(A) and 24(B) are graphic representations of the aberrations of the numerical example 10 of the zoom lens of the invention. 
     FIGS. 25(A) and 25(B) are graphic representations of the aberrations of the numerical example 11 of the zoom lens of the invention. 
     FIGS. 26(A) and 26(B) are graphic representations of the aberrations of the numerical example 12 of the zoom lens of the invention. 
     FIGS. 27(A) and 27(B) are graphic representations of the aberrations of the numerical example 13 of the invention. 
     FIG. 28 is a longitudinal section view of a numerical example 14 of a zoom lens of the invention. 
     FIG. 29 is a longitudinal section view of a numerical example 15 of a zoom lens of the invention. 
     FIG. 30 is a longitudinal section view of a numerical example 16 of a zoom lens of the invention. 
     FIG. 31 is a longitudinal section view of a numerical example 17 of a zoom lens of the invention. 
     FIGS. 32(A) and 32(B) are graphic representations of the aberrations of the numerical example 14 of the zoom lens of the invention. 
     FIGS. 33(A) and 33(B) are graphic representations of the aberrations of the numerical example 15 of the zoom lens of the invention. 
     FIGS. 34(A) and 34(B) are graphic representations of the aberrations of the numerical example 16 of the zoom lens of the invention. 
     FIGS. 35(A) and 35(B) are graphic representations of the aberrations of the numerical example 17 of the zoom lens of the invention. 
     ΔS: Sagittal Image Focus 
     ΔM: Meridionai Image Focus 
     d: Spectral d-line 
     g: Spectral g-line 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic diagram of an embodiment of a zoom lens of the rear focus type according to the invention with the paraxial refractive power arrangement shown. FIGS. 2 to 9 are lens block diagrams of the numerical examples 1 to 8 to be described later. FIGS. 10(A) and 10(B) to FIGS. 17(A) and 17(B) show the aberrations of the numerical examples 1 to 8 of the invention. 
     In the drawings, reference character L1 denotes a first lens unit of positive refractive power, reference character L2 denotes a second lens unit of negative refractive power, reference character L3 denotes a third lens unit of positive refractive power, reference character L4 denotes a fourth lens unit of positive refractive power. Reference character SP denotes an aperture stop arranged in front of the third lens unit. Reference character G denotes a glass block such as a color separation prism or filter. Reference character IP is an image plane. 
     During zooming from the wide-angle end to the telephoto end, as indicated by the arrows, the second lens unit is moved toward the image side and the shift of the image plane resulting from the variation of the focal length is compensated for by moving the fourth lens unit. 
     The fourth lens unit is made axially movable also for focusing, as the rear focus type is employed. Such a combination of the zooming and focusing movements of the fourth lens unit is shown by the loci of the solid line curve 4a and the dashed line curve 4b in FIG. 1 when focused on an infinitely distant object and an object at the minimum distance, respectively, as the image shift with zooming from the wide-angle end to the telephoto end is compensated for. It is to be noted that the first and third lens units remain stationary during zooming and focusing. 
     In the present embodiment, as the function of compensating for the image shift with zooming and the focusing function are provided in the fourth lens unit, particular feature is made such that, as shown by the curves 4a and 4b in FIG. 1, the locus of zooming movement from the wide-angle end to the telephoto end is convex toward the object side. This affords to efficiently utilizing the space between the third and fourth lens units. With the help of this feature, a shortening of the physical length of the complete lens is achieved advantageously. 
     In the present embodiment, it is in the forward direction that the fourth lens unit moves to effect focusing to suit from an infinitely distant object to an object at the minimum distance as shown by, for example, a straight line 4c of FIG. 1 with the setting in the telephoto end. 
     In the present embodiment, as compared with the conventional 4-unit zoom lens in which the first lens unit moves forward to produce the same focusing effect, the effective diameter of the first lens unit is prevented from unduly increasing by employing such a form of the rear focus type as described above. 
     Another feature is that the aperture stop is positioned just in front of the third lens unit to thereby decrease the variation of aberrations due to the movable lens units. Also, the separations between the lens units that precede the stop are shortened to thereby achieve the shortening of the diameter of the front lens members with ease. 
     Yet another feature is, in the specific embodiment of the invention, that the refractive powers of the lens units are determined with respect to the following condition: 
     
         3.5&lt;F3/F4                                                  . . . (1) 
    
     where F3 and F4 are the focal lengths of the third and fourth lens units, respectively. 
     With the help of these features, a high range zoom lens is obtained which has fulfilled the three requirements at once of minimizing the bulk and size of the entire system, of securing the predetermined back focal distance and nonetheless of maintaining a good optical performance throughout the entire zooming range and throughout the entire focusing range. 
     The technical significance of the condition (1) described above is explained below. 
     The inequality of condition (1) is concerned with the ratio of the focal lengths of the third and fourth lens units and has an aim to maintain good optical performance in such a manner that the back focal distance is made sufficiently long, while still permitting an improvement of the compact form of that part of the system which follows the stop to be achieved. 
     When the focal length of the third lens unit is shorter than the lower limit of the condition (1), it becomes difficult to correct the variation of spherical aberration with zooming or focusing. Additional problems are produced that it also becomes difficult to secure the sufficient back focal distance and that the total movement of the fourth lens unit is increased largely, causing large variation of aberrations with zooming or focusing. 
     To achieve further improvements of the compromise between the minimization of the bulk and size of the entire lens system and the maintenance of good stability of optical performance throughout the entire zooming range and throughout the entire focusing range, the following conditions are set forth: 
     
         3&lt;Bfw/Fw&lt;5                                                 . . . (2) 
    
     
         -0.8&lt;Fw/F2&lt;-0.4                                            . . . (3) 
    
     
         5&lt;F1/Fw&lt;12                                                 . . . (4) 
    
     where F1 and F2 are the focal lengths of the first and second lens units, respectively, Fw is the shortest focal length of the entire lens system, and Bfw is the back focal distance in the wide-angle end. 
     The inequalities of condition (2) are concerned with the ratio of the back focal distance Bfw to the focal length Fw for the wide-angle end. When the back focal distance is shorter than the lower limit of the condition (2), it becomes impossible in some cases to arrange the color separation prism. In other cases, the telecentric system fails to form so that the light beam is incident on the color separation prism at a tight angle, causing color shading to arise objectionably. 
     Conversely when the back focal distance is too long as exceeding the upper limit, the fourth lens unit gets a large effective diameter and a heavy weight. This is an obstacle to smooth focusing, or like problems arise. 
     The inequalities of condition (3) are concerned with the ratio of the focal length for the wide-angle end to the focal length of the second lens unit. When the focal length of the second lens unit is too short as exceeding the upper limit of the condition (3), the Petzval sum increases in the under direction, causing inclination of the image plane. This and other aberrations become difficult to correct well. 
     Conversely when the focal length of the second lens unit is too long as exceeding the lower limit, the total zooming movement of the second lens unit increases largely, giving rise to a problem in that the diameter of the front lens members becomes too large. 
     The inequalities of condition (4) are concerned with the object point relative to the second lens unit, or its magnification. To set the entire lens system to a minimum size, it is preferable that the second lens unit zooms across the unity of magnification. If this occurs in a middle region, the fourth lens unit reciprocates almost perfectly when zooming from one terminal end to the other. Thus, it becomes possible to obtain a high zoom ratio with a most advantageous space efficiency. 
     Concretely speaking, when the upper limit of this condition (4) is exceeded, the object point for the second lens unit goes far away, so that the second lens unit is given a lower magnification, thus producing an adverse effect of minimizing the size of the lens system. In addition, the separation between the first and second lens units increases largely. Therefore, it becomes difficult to achieve the minimization of the size of the entire lens system. 
     When the lower limit is exceeded, the magnification of the second lens unit becomes too large to achieve the increase of the zoom ratio with ease. 
     Another feature is that the second lens unit is preferably constructed as comprising, from front to rear, a negative first lens whose rear surface is concave and has a stronger refracting power than that of the front surface thereof, an air space, a negative second lens whose surfaces both are concave, an air space and a positive third lens whose front surface is convex and has a stronger refracting power than that of the rear surface thereof. 
     By this, the front principal point of the second lens unit is set to a more frontal position so as to shorten its principal point interval from the first lens unit. In turn, the height of incidence from the optical axis of the off-axial light beam on the first lens unit is lowered to reduce the diameter of the first lens unit. 
     Also, from front to rear, the first lens unit is constructed from a negative first lens, a positive second lens and a positive third lens; the second lens unit from a negative first lens, a negative second lens and a positive third lens; the third lens unit either from a positive first lens and a negative second lens or from a negative first lens and a positive second lens; and the fourth lens unit either from a positive first lens, a negative second lens and a positive third lens or from a positive first lens, a negative second lens, a positive third lens and a positive fourth lens. 
     By this, a high optical performance is obtained throughout the entire zooming range and over the entire area of the image frame. 
     A further feature is to satisfy the following conditions: 
     
         1&lt;(1/R.sub.22,F -1/R.sub.21,R).F.sub.2 &lt;2                  . . . (5) 
    
     
         -0.55&lt;FT/FT.sub.1,3 &lt;0                                     . . . (6) 
    
     where R 21 ,R is the radius of curvature of the rear surface of the first lens in the second lens unit; R 22 ,F is the radius of curvature of the front lens surface of the second lens in the second lens unit; FT is the longest focal length of the entire lens system; FT 1 ,3 is the overall focal length of the first to third lens units in the telephoto end. 
     Here, the inequalities of condition (5) are concerned with the air lens and the focal length of the second lens unit. When the lower limit is exceeded, distortion tends to become barrel type in the wide-angle end. 
     Meanwhile, when the upper limit is exceeded, distortion tends to become pincushion type in the telephoto end. 
     The inequalities of condition (6) are concerned with the axial light beam emerging from the third lens unit in the telephoto end. When the lower limit is exceeded, the axial beam emerging from the third lens unit gets a strong divergence. Therefore, the diameter of the fourth lens unit has to be increased. Another problem arises in that spherical aberration varies to a large extent with focusing. 
     Meanwhile, when the upper limit is exceeded, the axial beam emerging from the third lens unit gets a strong convergence, giving rise to problems in that the sufficient back focal distance and exit pupil can no longer be obtained. 
     Next, numerical examples 1 to 8 of the invention are shown. In the numerical data for the examples 1 to 8, Ri is the radius of curvature of the i-th lens surface, when counted from the object side; Di is the i-th lens thickness or air separation, when counted from the object side; and Ni and νi are respectively the refractive index and Abbe number of the glass of the i-th lens element, when counted from the object side. The last three lens surfaces in the numerical examples 1 to 8 are glass blocks such as a color separation prism and a filter or the like. The values of the factors in the above-described conditions (1) to (6) for the numerical examples 1 to 8 are also listed in Table-1. 
     The shape of an aspheric surface is expressed in the coordinates with an X axis in the axial direction and an H axis in the direction perpendicular to the optical axis, the direction in which light advances being taken as positive, by the following equation: ##EQU1## where R is the radius of the osculating sphere, and K, A, B, C, D and E are the aspheric coefficients. Also, the notation &#34;e-0X&#34; means &#34;10 -X  &#34;. 
     
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Numerical Example 1:                                                      
f = 1-8.0  Fno = 1:1.65  2ω = 56.4°-7.3°              
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R1 = 9.760 D1 = 0.305  N1 = 1.80518                                       
                                   ν1 = 25.4                           
R2 = 5.255 D2 = 0.915  N2 = 1.60311                                       
                                   ν2 = 60.7                           
R3 = 57.424                                                               
           D3 = 0.033                                                     
R4 = 5.401 D4 = 0.559  N3 = 1.69680                                       
                                   ν3 = 55.5                           
R5 = 14.080                                                               
           D5 = Variable                                                  
R6 = 6.644 D6 = 0.152  N4 = 1.88300                                       
                                   ν4 = 40.8                           
R7 = 1.421 D7 = 0.593                                                     
R8 = -7.409                                                               
           D8 = 0.135  N5 = 1.69680                                       
                                   ν5 = 55.5                           
R9 = 3.188 D9 = 0.169                                                     
R10 = 2.673                                                               
           D10 = 0.423 N6 = 1.84666                                       
                                   ν6 = 23.8                           
R11 = 19.594                                                              
           D11 = Variable                                                 
R12 = (Stop)                                                              
           D12 = 0.254                                                    
R13 = 16.349                                                              
           D13 = 0.152 N7 = 1.80400                                       
                                    ν7 = 46.6                          
R14 = 1.863                                                               
           D14 = 0.644 N8 = 1.60342                                       
                                   ν8 = 38.0                           
R15 = -5.937                                                              
           D15 = Variable                                                 
R16 = -90.322                                                             
           D16 = 0.423 N9 = 1.48749                                       
                                   ν9 = 70.2                           
R17 = -4.286                                                              
           D17 = 0.025                                                    
R18 = 8.585                                                               
           D18 = 0.169 N10 = 1.80518                                      
                                   ν10 = 25.4                          
R19 = 2.706                                                               
           D19 = 0.576 N11 = 1.48749                                      
                                   ν11 = 70.2                          
R20 = -19.124                                                             
           D20 = 0.025                                                    
R21 = 3.892                                                               
           D21 = 0.542 N12 = 1.48749                                      
                                   ν12 = 70.2                          
R22 = -5.134                                                              
           D22 = 0.678                                                    
R23 = ∞                                                             
           D23 = 0.423 N13 = 1.51633                                      
                                   ν13 = 64.2                          
R24 = ∞                                                             
           D24 = 3.389 N14 = 1.60342                                      
                                   ν14 = 38.0                          
R25 = ∞                                                             
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            Focal Length                                                  
Variable Separation                                                       
              1.00        2.80   8.00                                     
______________________________________                                    
D5            0.17        3.19   5.20                                     
D11           5.29        2.27   0.25                                     
D15           2.25        1.93   2.25                                     
______________________________________                                    
R15: Aspheric Surface:                                                    
K = -4.801 e-01                                                           
            A = 1.002 e-03                                                
                          B = -9.996 e-04                                 
C = -1.090 e-04                                                           
            D = 1.947 e-04                                                
______________________________________                                    
Numerical Example 2:                                                      
f = 1-12.65  Fno = 1:1.65-2  2ω = 60.0°-5.2°          
______________________________________                                    
R1 = 15.833                                                               
           D1 = 0.346  N1 = 1.80518                                       
                                   ν1 = 25.4                           
R2 = 6.746 D2 = 1.500  N2 = 1.60311                                       
                                   ν2 = 60.7                           
R3 = -38.377                                                              
           D3 = 0.038                                                     
R4 = 6.039 D4 = 0.730  N3 = 1.69680                                       
                                   ν3 = 55.5                           
R5 = 15.975                                                               
           D5 = Variable                                                  
R6 = 7.024 D6 = 0.173  N4 = 1.88300                                       
                                   ν4 = 40.8                           
R7 = 1.692 D7 = 0.786                                                     
R8 = -3.073                                                               
           D8 = 0.153  N5 = 1.69680                                       
                                   ν5 = 55.5                           
R9 = 2.791 D9 = 0.192                                                     
R10 = 3.248                                                               
           D10 =  0.423                                                   
                       N6 = 1.84666                                       
                                   ν6 = 23.8                           
R11 = -36.211                                                             
           D11 = Variable                                                 
R12 = (Stop)                                                              
           D12 = 0.288                                                    
R13 = 7.387                                                               
           D13 = 0.807 N7 = 1.62588                                       
                                   ν7 = 35.7                           
R14 = -2.053                                                              
           D14 = 0.173 N8 = 1.77250                                       
                                   ν8 = 49.6                           
R15 = -13.939                                                             
           D15 = Variable                                                 
R16 = 84.254                                                              
           D16 = 0.480 N9 = 1.51633                                       
                                   ν9 = 64.2                           
R17 = -4.349                                                              
           D17 = 0.028                                                    
R18 = 8.629                                                               
           D18 = 0.192 N10 = 1.80518                                      
                                   ν10 = 25.4                          
R19 = 2.482                                                               
           D19 = 0.769 N11 = 1.48749                                      
                                   ν11 = 70.2                          
R20 = -22.864                                                             
           D20 = 0.028                                                    
R21 = 2.808                                                               
           D21 = 0.538 N12 = 1.48749                                      
                                   ν12 = 70.2                          
R22 = 47.595                                                              
           D22 = 0.769                                                    
R23 = ∞                                                             
           D23 = 0.480 N13 = 1.51633                                      
                                   ν13 = 64.2                          
R24 = ∞                                                             
           D24 = 3.461 N14 = 1.60342                                      
                                   ν14 = 38.0                          
R25 = ∞                                                             
______________________________________                                    
            Focal Length                                                  
Variable Separation                                                       
              1.00        3.24   12.65                                    
______________________________________                                    
D5            0.21        3.61   5.88                                     
D11           5.96        2.56   0.29                                     
D15           2.91        2.28   2.91                                     
______________________________________                                    
Numerical Example 3:                                                      
f = 1-10.0  Fno = 1:1.65-1.85  2ω = 56.4°-6.1°        
______________________________________                                    
R1 = 13.935                                                               
           D1 = 0.321  N1 = 1.80518                                       
                                   ν1 = 25.4                           
R2 = 6.199 D2 = 1.160  N2 = 1.60311                                       
                                   ν2 = 60.7                           
R3 = -46.242                                                              
           D3 = 0.035                                                     
R4 = 6.062 D4 = 0.642  N3 = 1.69680                                       
                                   ν3 = 55.5                           
R5 = 18.709                                                               
           D5 = Variable                                                  
R6 = 5.215 D6 = 0.142  N4 = 1.88300                                       
                                   ν4 = 40.8                           
R7 = 1.640 D7 = 0.653                                                     
R8 = -2.610                                                               
           D8 = 0.142  N5 = 1.69680                                       
                                   ν5 = 55.5                           
R9 = 2.588 D9 = 0.178                                                     
R10 =  3.060                                                              
           D10 = 0.392 N6 = 1.84666                                       
                                   ν6 = 23.8                           
R11 = -19.876                                                             
           D11 = Variable                                                 
R12 = (Stop)                                                              
           D12 = 0.267                                                    
R13 = 8.618                                                               
           D13 = 0.714 N7 = 1.60342                                       
                                   ν7 = 38.0                           
R14 = -1.616                                                              
           D14 = 0.142 N8 = 1.77250                                       
                                   ν8 = 49.6                           
R15 = -19.460                                                             
           D15 = Variable                                                 
R16 = -104.168                                                            
           D16 = 0.446 N9 = 1.51633                                       
                                   ν9 = 64.2                           
R17 = -4.034                                                              
           D17 = 0.026                                                    
R18 = 8.573                                                               
           D18 = 0.178 N10 = 1.84666                                      
                                   ν10 = 23.8                          
R19 = 2.919                                                               
           D19 = 0.714 N11 = 1.48749                                      
                                   ν11 = 70.2                          
R20 = -11.679                                                             
           D20 = 0.026                                                    
R21 = 3.431                                                               
           D21 = 0.500 N12 = 1.51633                                      
                                   ν12 = 64.2                          
R22 = -22.410                                                             
           D22 = 0.714                                                    
R23 = ∞                                                             
           D23 = 0.446 N13 = 1.51633                                      
                                   ν13 =  64.2                         
R24 = ∞                                                             
           D24 = 3.571 N14 = 1.60342                                      
                                   ν14 = 38.0                          
R25 = ∞                                                             
______________________________________                                    
            Focal Length                                                  
Variable Separation                                                       
              1.00        3.17   10.00                                    
______________________________________                                    
D5            0.28        3.33   5.36                                     
D11           5.35        2.30   0.27                                     
D15           2.46        1.89   2.46                                     
______________________________________                                    
R13: Aspheric Surface:                                                    
K = 9.750 e+00                                                            
            A = -4.632 e-04                                               
                          B = 6.703 e-04                                  
C = 7.613 e-05                                                            
            D = 6.047 e-05                                                
______________________________________                                    
Numerical Example 4:                                                      
f = 1-10.0  Fno = 1:1.65-1.85  2ω = 56.4°-7.3°        
______________________________________                                    
R1 = 10.876                                                               
           D1 = 0.321  N1 = 1.80518                                       
                                   ν1 = 25.4                           
R2 = 5.631 D2 = 1.160  N2 = 1.60311                                       
                                   ν2 = 60.7                           
R3 = -32.694                                                              
           D3 = 0.035                                                     
R4 = 6.601 D4 = 0.571  N3 = 1.69680                                       
                                   ν3 = 55.5                           
R5 = 15.120                                                               
           D5 = Variable                                                  
R6 = 6.050 D6 = 0.142  N4 = 1.88300                                       
                                   ν4 = 40.8                           
R7 = 1.679 D7 = 0.703                                                     
R8 = -2.707                                                               
           D8 = 0.142  N5 = 1.69680                                       
                                   ν5 = 55.5                           
R9 = 2.918 D9 = 0.178                                                     
R10 = 3.235                                                               
           D10 = 0.392 N6 = 1.84666                                       
                                   ν6 = 23.8                           
R11 = -23.215                                                             
           D11 = Variable                                                 
R12 = (Stop)                                                              
           D12 = 0.267                                                    
R13 = 112.150                                                             
           D13 = 0.714 N7 = 1.60342                                       
                                   ν7 = 38.0                           
R14 = -1.888                                                              
           D14 = 0.142 N8 = 1.80400                                       
                                   ν8 = 46.6                           
R15 = -6.647                                                              
           D15 = Variable                                                 
R16 = 3.724                                                               
           D16 = 0.642 N9 = 1.48749                                       
                                   ν9 = 70.2                           
R17 = -7.678                                                              
           D17 = 0.026                                                    
R18 = 5.683                                                               
           D18 = 0.178 N10 = 1.84666                                      
                                   ν10 = 23.8                          
R19 = 2.652                                                               
           D19 = 0.821 N11 = 1.48749                                      
                                   ν11 = 70.2                          
R20 = -4.896                                                              
           D20 =  0.714                                                   
R21 = ∞                                                             
           D21 = 0.446 N12 = 1.51633                                      
                                   ν12 = 64.2                          
R22 = ∞                                                             
           D22 = 3.571 N13 = 1.60342                                      
                                   ν13 = 38.0                          
R23 = ∞                                                             
______________________________________                                    
            Focal Length                                                  
Variable Separation                                                       
              1.00        3.10   10.00                                    
______________________________________                                    
D5            0.19        3.24   5.27                                     
D11           5.35        2.30   0.27                                     
D15           2.88        2.36   2.88                                     
______________________________________                                    
R13: Aspheric Surface:                                                    
K = 9.793 e+00                                                            
            A = -3.707 e-03                                               
                          B = 3.379 e-03                                  
C = -1.152 e-03                                                           
            D = -4.489 e-04                                               
R20: Aspheric Surface:                                                    
K = -1.430 e-01                                                           
            A = 1.004 e-02                                                
                          B = 9.089 e-04                                  
C = -1.559 e-04                                                           
            D = -2.526 e-05                                               
______________________________________                                    
Numerical Example 5:                                                      
f = 1-10.0  Fno = 1:1.65-1.8  2ω = 56.4°-7.3°         
______________________________________                                    
R1 = 9.706 D1 = 0.321  N1 = 1.80518                                       
                                   ν1 =  25.4                          
R2 = 5.409 D2 = 1.160  N2 = 1.51633                                       
                                   ν2 = 64.2                           
R3 = 1713.793                                                             
           D3 = 0.035                                                     
R4 = 6.342 D4 = 0.625  N3 = 1.69680                                       
                                   ν3 = 55.5                           
R5 = 31.759                                                               
           D5 = Variable                                                  
R6 = 4.624 D6 = 0.142  N4 = 1.88300                                       
                                   ν4 = 40.8                           
R7 = 1.649 D7 = 0.722                                                     
R8 = -2.569                                                               
           D8 = 0.142  N5 = 1.69680                                       
                                   ν5 = 55.5                           
R9 = 2.896 D9 = 0.178                                                     
R10 = 3.254                                                               
           D10 = 0.392 N6 = 1.84666                                       
                                   ν6 = 23.8                           
R11 = -34.333                                                             
           D11 = Variable                                                 
R12 = (Stop)                                                              
           D12 = 0.267                                                    
R13 = 121.593                                                             
           D13 = 0.714 N7 = 1.61293                                       
                                   ν7 = 37.0                           
R14 = -1.968                                                              
           D14 = 0.142 N8 = 1.77250                                       
                                   ν8 = 49.6                           
R15 = -6.912                                                              
           D15 = Variable                                                 
R16 = 3.821                                                               
           D16 = 0.642 N9 = 1.51633                                       
                                   ν9 = 64.2                           
R17 = -5.790                                                              
           D17 = 0.026                                                    
R18 = 5.124                                                               
           D18 = 0.178 N10 = 1.84666                                      
                                   ν10 = 23.8                          
R19 = 2.187                                                               
           D19 = 0.535                                                    
R20 = 2.335                                                               
           D20 = 0.821 N11 = 1.48749                                      
                                   ν11 = 70.2                          
R21 = -4.801                                                              
           D21 = 0.714                                                    
R22 = ∞                                                             
           D22 = 0.446 N12 = 1.51633                                      
                                   ν12 = 64.2                          
R23 = ∞                                                             
           D23 = 3.571 N13 = 1.60342                                      
                                   ν13 = 38.0                          
R24 = ∞                                                             
______________________________________                                    
            Focal Length                                                  
Variable Separation                                                       
              1.00        3.08   10.00                                    
______________________________________                                    
D5            0.19        3.24   5.27                                     
D11           5.35        2.30   0.27                                     
D15           2.25        1.75   2.25                                     
______________________________________                                    
R13: Aspheric Surface:                                                    
K = -4.352 e+03                                                           
            A = -5.040 e-03                                               
                          B = 1.880 e-03                                  
C = -1.175 e-03                                                           
            D = -2.798 e-04                                               
R21: Aspheric Surface:                                                    
K = -5.510 e-00                                                           
            A = 3.988 e- 03                                               
                          B = 5.564 e-04                                  
C = 6.141 e-04                                                            
            D = -2.209 e-04                                               
______________________________________                                    
Numerical Example 6:                                                      
f = 1-10.0  Fno = 1:1.65-1.95  2ω = 56.4°-7.3°        
______________________________________                                    
R1 = 12.257                                                               
           D1 = 0.285  N1 = 1.80518                                       
                                   ν1 = 25.4                           
R2 = 5.953 D2 = 0.267                                                     
R3 = 7.776 D3 = 0.803  N2 = 1.49700                                       
                                   ν2 = 81.6                           
R4 = -246.325                                                             
           D4 = 0.035                                                     
R5 = 5.666 D5 = 0.928  N3 = 1.69680                                       
                                   ν3 = 55.5                           
R6 = 226.941                                                              
           D6 = Variable                                                  
R7 = 5.144 D7 = 0.142  N4 = 1.88300                                       
                                   ν4 = 40.8                           
R8 = 1.664 D8 = 0.667                                                     
R9 = -2.600                                                               
           D9 = 0.142  N5 = 1.69680                                       
                                   ν5 = 55.5                           
R10 = 2.617                                                               
           D10 = 0.178                                                    
R11 = 3.115                                                               
           D11 = 0.392 N6 = 1.84666                                       
                                   ν6 = 23.8                           
R12 =  -21.490                                                            
           D12 = Variable                                                 
R13 = (Stop)                                                              
           D13 = 0.267                                                    
R14 = 8.690                                                               
           D14 = 0.714 N7 = 1.61293                                       
                                   ν7 = 37.0                           
R15 = -1.618                                                              
           D15 = 0.142 N8 = 1.77250                                       
                                   ν8 = 49.6                           
R16 = -17.470                                                             
           D16 = Variable                                                 
R17 = -80.048                                                             
           D17 = 0.446 N9 = 1.48749                                       
                                   ν9 = 70.2                           
R18 = -4.119                                                              
           D18 = 0.026                                                    
R19 = 8.509                                                               
           D19 = 0.178 N10 = 1.84666                                      
                                   ν10 = 23.8                          
R20 = 2.945                                                               
           D20 = 0.714 N11 = 1.48749                                      
                                   ν11 = 70.2                          
R21 = -11.983                                                             
           D21 = 0.026                                                    
R22 = 3.435                                                               
           D22 = 0.500 N12 = 1.51633                                      
                                   ν12 = 64.2                          
R23 = -25.034                                                             
           D23 = 0.714                                                    
R24 = ∞                                                             
           D24 = 0.446 N13 = 1.51633                                      
                                   ν13 = 64.2                          
R25 = ∞                                                             
           D25 = 3.571 N14 = 1.60342                                      
                                   ν14 =  38.0                         
R26 = ∞                                                             
______________________________________                                    
            Focal Length                                                  
Variable Separation                                                       
              1.00        3.15   10.00                                    
______________________________________                                    
D6            0.25        3.29   5.33                                     
D12           5.35        2.30   0.27                                     
D16           2.61        2.02   2.61                                     
______________________________________                                    
R14: Aspheric Surface:                                                    
K = 9.753 e+00                                                            
            A = -2.024 e-05                                               
                          B = -1.009 e-03                                 
C = -2.656 e-05                                                           
            D = 3.290 e-05                                                
______________________________________                                    
Numerical Example 7:                                                      
f = 1-10.0  Fno = 1:1.65-1.96  2ω = 56.4°-7.3°        
______________________________________                                    
R1 = 12.989                                                               
           D1 = 0.285  N1 = 1.80518                                       
                                   ν1 = 25.4                           
R2 = 6.175 D2 = 0.257                                                     
R3 = 7.871 D3 = 0.803  N2 = 1.49700                                       
                                   ν2 = 81.6                           
R4 = -80.049                                                              
           D4 = 0.089                                                     
R5 = 5.647 D5 = 0.857  N3 = 1.69680                                       
                                   ν3 = 55.5                           
R6 = 85.989                                                               
           D6 = Variable                                                  
R7 = 4.788 D7 = 0.142  N4 = 1.88300                                       
                                   ν4 = 40.8                           
R8 = 1.628 D8 = 0.691                                                     
R9 = -2.589                                                               
           D9 = 0.142  N5 = 1.69680                                       
                                   ν5 = 55.5                           
R10 = 2.635                                                               
           D10 = 0.178                                                    
R11 = 3.131                                                               
           D11 = 0.357 N6 = 1.84666                                       
                                   ν6 = 23.8                           
R12 = -21.153                                                             
           D12 = Variable                                                 
R13 = (Stop)                                                              
           D13 = 0.267                                                    
R14 = 8.729                                                               
           D14 = 0.678 N7 = 1.61293                                       
                                   ν7 = 37.0                           
R15 = -1.896                                                              
           D15 = 0.046                                                    
R16 = -1.765                                                              
           D16 = 0.142 N8 = 1.77250                                       
                                   ν8 = 49.6                           
R17 = -12.326                                                             
           D17 = Variable                                                 
R18 = -28.412                                                             
           D18 = 0.464 N9 = 1.48749                                       
                                   ν9 = 70.2                           
R19 = -4.142                                                              
           D19 = 0.026                                                    
R20 = 8.004                                                               
           D20 = 0.160 N10 = 1.84666                                      
                                   ν10 = 23.8                          
R21 = 2.872                                                               
           D21 = 0.678 N11 = 1.48749                                      
                                   ν11 = 70.2                          
R22 = -12.333                                                             
           D22 = 0.026                                                    
R23 = 3.713                                                               
           D23 = 0.535 N12 = 1.51633                                      
                                   ν12 = 64.2                          
R24 = -11.186                                                             
           D24 = 0.714                                                    
R25 = ∞                                                             
           D25 = 0.446 N13 = 1.51633                                      
                                   ν13 = 64.2                          
R26 = ∞                                                             
           D26 = 3.571 N14 = 1.60342                                      
                                   ν14 = 38.0                          
R27 = ∞                                                             
______________________________________                                    
            Focal Length                                                  
Variable Separation                                                       
              1.00        3.14   10.00                                    
______________________________________                                    
D6            0.25        3.30   5.33                                     
D12           5.35        2.30   0.27                                     
D17           2.49        1.92   2.49                                     
______________________________________                                    
R14: Aspheric Surface:                                                    
K = 9.785 e+00                                                            
            A = 8.820 e-04                                                
                          B = 2.131 e-03                                  
C = -2.658 e-05                                                           
            D = 2.341 e-04                                                
______________________________________                                    
Numerical Example 8:                                                      
f = 1-8.0  Fno = 1:1.65-2  2ω = 53.1°-7.2°            
______________________________________                                    
R1 = 14.585                                                               
           D1 = 0.300  N1 = 1.80518                                       
                                   ν1 = 25.4                           
R2 = 4.950 D2 = 1.000  N2 = 1.60311                                       
                                   ν2 = 60.7                           
R3 = -12.965                                                              
           D3 = 0.033                                                     
R4 = 3.950 D4 = 0.500  N3 = 1.69680                                       
                                   ν3 = 55.5                           
R5 = 9.147 D5 = Variable                                                  
R6 = 7.777 D6 = 0.150  N4 = 1.88300                                       
                                   ν4 = 40.8                           
R7 = 1.459 D7 = 0.538                                                     
R8 = -1.730                                                               
           D8 = 0.133  N5 = 1.69680                                       
                                   ν5 = 55.5                           
R9 = 2.114 D9 = 0.383  N6 = 1.84666                                       
                                   ν6 = 23.8                           
R10 = -8.785                                                              
           D10 = Variable                                                 
R11 = (Stop)                                                              
           D11 = 0.250                                                    
R12 = 7.249                                                               
           D12 = 0.666 N7 = 1.62588                                       
                                   ν7 = 35.7                           
R13 = -1.513                                                              
           D13 = 0.150 N8 = 1.77250                                       
                                   ν8 = 49.6                           
R14 = -8.358                                                              
           D14 = Variable                                                 
R15 = -107.336                                                            
           D15 = 0.416 N9 = 1.51633                                       
                                   ν9 = 64.2                           
R16 = -3.600                                                              
           D16 = 0.025                                                    
R17 =  7.512                                                              
           D17 = 0.166 N10 = 1.80518                                      
                                   ν10 = 25.4                          
R18 = 2.098                                                               
           D18 = 0.666 N11 = 1.48749                                      
                                   ν11 = 70.2                          
R19 = -12.762                                                             
           D19 = 0.025                                                    
R20 = 3.046                                                               
           D20 = 0.550 N12 = 1.48749                                      
                                   ν12 = 70.2                          
R21 = -7.730                                                              
           D21 = 0.666                                                    
R22 = ∞                                                             
           D22 = 0.416 N13 = 1.51633                                      
                                   ν13 = 64.2                          
R23 = ∞                                                             
           D23 = 3.000 N14 = 1.60342                                      
                                   ν14 = 38.0                          
R24 = ∞                                                             
______________________________________                                    
            Focal Length                                                  
Variable Separation                                                       
              1.00        2.87   8.00                                     
______________________________________                                    
D5            0.25        2.23   3.55                                     
D10           3.55        1.57   0.25                                     
D14           2.00        1.53   2.00                                     
______________________________________                                    
R12: Aspheric Surface:                                                    
K = 2.994 e+02                                                            
            A = -1.033 e-03                                               
                          B = -1.860 e-04                                 
C = -6.948 e-04                                                           
            D = -1.434 e-04                                               
R21: Aspheric Surface:                                                    
K = -1.144 e-02                                                           
            A = 7.630 e-04                                                
                          B = -1.593 e-03                                 
C = -3.781 e-05                                                           
            D = 1.670 e-04                                                
______________________________________                                    
 
    
     
                                           TABLE 1                                 
__________________________________________________________________________
       Numerical Example                                                  
Condition                                                                 
       1    2    3    4    5    6    7    8                               
__________________________________________________________________________
(1)    6.827                                                              
            3.808                                                         
                 41.224                                                   
                      13.620                                              
                           7.945                                          
                                16.038                                    
                                     10.869                               
                                          4.068                           
(2)    3.308                                                              
            3.597                                                         
                 3.963                                                    
                      3.772                                               
                           3.491                                          
                                4.053                                     
                                     3.597                                
                                          3.127                           
(3)    -0.492                                                             
            -0.578                                                        
                 -0.560                                                   
                      -0.560                                              
                           -0.560                                         
                                -0.560                                    
                                     -0.560                               
                                          -0.75                           
(4)    8.821                                                              
            9.194                                                         
                 8.647                                                    
                      8.647                                               
                           8.647                                          
                                8.361                                     
                                     8.361                                
                                          6.105                           
(5)    1.705                                                              
            1.586                                                         
                 1.773                                                    
                      1.723                                               
                           1.778                                          
                                1.760                                     
                                     1.786                                
                                          1.684                           
(6)    -0.169                                                             
            -0.128                                                        
                 -0.351                                                   
                      -0.290                                              
                           -0.237                                         
                                -0.325                                    
                                     -0.298                               
                                          -0.221                          
__________________________________________________________________________
 
    
     According to the present embodiment, by using the four constituent lens units as a whole, giving the ranges for the refractive powers of these lens units and employing a focusing configuration of moving the fourth lens unit, as has been described above, it is made possible to achieve good correction of aberrations throughout the entire zooming range in such a manner that the bulk and size of the entire system are minimized. Hence, a zoom lens of the rear focus type which has a large relative aperture, whose back focal distance is sufficiently long, and which maintains good stability of aberration correction during focusing can be realized. 
     Another embodiment of the invention where improvements are made over the zoom lens described above is described below. 
     FIGS. 18 to 22 in lens block diagram show the numerical examples 9 to 13 of zoom lenses of the invention. 
     In these figures, reference character L1 denotes a first lens unit of positive refractive power, reference character L2 denotes a second lens unit of negative refractive power, reference character L3 denotes a third lens unit of positive refractive power and reference character L4 denotes a fourth lens unit of positive refractive power. Reference character SP denotes an aperture stop arranged in front of the third lens unit. Reference character G denotes a glass block such as a color separation prism, face plate, or filter, and reference character IP denotes an image plane. 
     This zoom lens of the rear focus type comprises, from front to rear, the first lens unit of positive refractive power, the second lens unit of negative refractive power, the third lens unit of positive refractive power having the stop and the fourth lens unit of positive refractive power, totaling four lens units, wherein during zooming from the wide-angle end to the telephoto end, the second lens unit is moved toward the image side, while simultaneously moving the stop and the third lens unit in unison so as to have a locus convex toward the object side, and the fourth lens unit, too, so as to have a locus convex toward the object side, and during focusing, the fourth lens unit is moved, and wherein letting the focal length of the i-th lens unit be denoted by Fi, the following condition is satisfied: 
     
         2.5&lt;F3/F4                                                  . . . (7) 
    
     The inequality of condition (7) is concerned with the ratio of the focal lengths of the third and fourth lens units and has an aim to achieve improvements of the compact form of the section that follows the stop, while still permitting good optical performance to be sustained with the back focal distance being made sufficiently long. When the focal length of the third lens unit is shorter than the lower limit of the condition (7), it becomes difficult to correct the variation of spherical aberration with zooming or focusing. Another problem arises in that the sufficient back focal distance can hardly be secured, or that the amount of movement for zooming and focusing of the fourth lens unit increases greatly. Therefore, the variation of aberrations with zooming or focusing is increased objectionably. 
     In the present embodiment, to further improve the compromise between the requirements of minimizing the bulk and size of the entire lens system and of assuring high optical performance over the entire area of the image frame, it is preferred to satisfy at least one of the following conditions or features. 
     The following condition is satisfied: 
     
         -0.70&lt;Fw/F2&lt;-0.4                                           . . . (8) 
    
     where Fw is the shortest focal length of the entire lens system. 
     The inequalities of condition (8) are concerned with the ratio of the focal length for the wide-angle end of the entire system to the focal length of the second lens unit. When the focal length of the second lens unit is shorter than the upper limit of the condition (8), the Petzval sum increases in the under direction and the resultant inclination of the image focus and other aberrations become difficult to correct well. Conversely when the focal length of the second lens unit is longer than the lower limit, the total zooming movement of the second lens unit increases, giving rise to a problem in that the front lens members get too large a diameter. 
     Another preferred feature is that the axial light beam, when emerging from the third lens unit, is made nearly afocal or slightly divergent. With the help of this feature, the back focal distance and the exit pupil distance can be increased with an advantage of as far suppressing the variation of aberrations with zooming or focusing as possible. 
     By the way, suppose the zoom type used in the present embodiment is combined with the arrangement of the stop in between, for example, the third and fourth lens units, then because the entrance pupil lies deep in from the first lens unit, it would result that the height of incidence of the off-axial light beam on the first lens unit is highest in the middle zooming region at a nearer position to the wide-angle end. 
     Yet another feature in the present embodiment is, therefore, that the stop is put either on the object side or in the interior of the third lens unit, and is made movable in unison with the third lens unit, while depicting a locus convex toward the object side, as zooming from the wide-angle end to the telephoto end. Thus, the entrance pupil is brought to a nearer position to the first lens unit so that the zooming position at which the height of incidence is highest comes nearer to the wide-angle end. By this, a valuable decrease of the effective diameter of the first lens unit is attained. 
     It should be noted in the present embodiment that the stop and the third lens unit not only move in the locus convex toward the object side, but also are made to reciprocate almost perfectly, thus assisting in achieving minimization of the diameter of the front lens members and a great increase of the maximum image angle at once with ease. 
     A further feature in the present embodiment is that with regard to the minimization of the size of the entire lens system, it is preferred to satisfy the following condition: 
     
         3.5&lt;Bfw/Fw&lt;6.0                                             . . . (9) 
    
     where Bfw is the back focal distance (as measured without the glass block, filter or like part denoted by &#34;G&#34; in the practical examples) in the wide-angle end with an object at infinity. 
     The inequalities (9) are the necessary ones to advantageously minimize the size of the entire lens system. When the lower limit is exceeded, it results not only that the filter, the color separation prism or the like block is hardly inserted, but also that the exit pupil is rather short. In application of the invention to, for example, a video camera, therefore, the formation of an image onto the image sensor deviates from the telecentric system. When the upper limit is exceeded, the size comes to unduly increase. 
     With further regard to minimization of the diameter of the front lens members, it is preferred to satisfy the following condition: 
     
         6.0&lt;F1/Fw&lt;15.0                                             . . . (10) 
    
     These inequalities are concerned with the object point for the second lens unit, or its magnification. To set the entire lens system to a minimum, it is preferred that the second lens unit moves across the unity of magnification. If the unity of magnification occurs at a middle point in the zooming range, the fourth lens unit when zooming moves back and forth in almost equal distances. Therefore, a great increase of the zooming range becomes possible with a highest space efficiency. 
     In more detail, when the upper limit of the condition (10) is exceeded, the second lens unit gets a far object point with the result of a low magnification of the second lens unit. Thus, it becomes difficult to produce advantages when achieving the minimization. Further, the separation between the first and second lens units increases to increase the difficulty of achieving the minimization of the size. When the lower limit is exceeded, the magnification of the second lens unit increases to increase the difficulty of achieving the great increase of the zooming range. 
     Now, how much small the interval el between the principal points of the first and second lens units can be made is one of the important points when the maximum image angle is increased. To this purpose, for the first lens unit in a specific example, the following construction and arrangement of its parts are preferable: 
     The first lens unit comprises, from front to rear, a negative lens of the meniscus form convex toward the object side followed after an air space by a positive lens whose front surface is convex, and another positive lens whose front surface is convex, wherein the air lens defined by the negative and positive lenses has a negative refractive power. 
     With the use of such constituent parts, the rear principal point of the first lens unit is set nearer to the second lens unit. Thus, the principal point interval e1 between the first and second lens units can be taken short. This is advantageous at increasing the maximum image angle. 
     The second lens unit is even made to assist in shortening the principal point interval e1 in the wide-angle end. For this purpose, the second lens unit is formed with its front principal point set in a more front position. This is desired for increasing the maximum image angle. 
     In a specific example, the second lens unit comprises, from front to rear, a negative meniscus lens convex toward the object side and a bi-concave negative lens followed after an air space by a positive lens. By this air separation, the front principal point of the second lens unit is brought to a nearer position to the first lens unit. So, it is easier to take the short principal point interval el on the wide-angle side. This is advantageous at increasing the maximum image angle. 
     Now, as the total movement of the stop and the third lens unit becomes rather longer, spherical aberration varies with zooming at a rapidly higher rate, particularly in the middle region of the zooming range. So, much over-correction of spherical aberration results. To remove this, it is preferred to introduce at least one aspheric surface into the third lens unit. In a specific example, the aspheric surface is preferably formed to such shape that the positive refractive power gets progressively weaker toward the margin of the lens, or that the negative refractive power gets progressively stronger. 
     It is also preferred to apply aspheric sphere to at least one lens surface in the fourth lens unit, so that coma is well corrected. 
     Next, numerical examples 9 to 13 of the invention are shown. The values of the factors in the above-described conditions (7) to (10) for the numerical examples 9 to 13 are listed in Table-2. The shape of an aspheric surface is expressed in the coordinates with an X axis in the axial direction and an H axis in the direction perpendicular to the optical axis, the direction in which light advances being taken as positive, by the following equation: ##EQU2## where R is the radius of the osculating sphere, and K, A, B, C, D and E are the aspheric coefficients. 
     
         ______________________________________                                    
Numerical Example 9:                                                      
f = 1-12.65  Fno = 1:1.6-1.9  2ω = 66.2°-5.9°         
______________________________________                                    
R1 = 35.931                                                               
           D1 = 0.391  N1 = 1.80518                                       
                                   ν1 = 25.4                           
R2 = 8.393 D2 = 0.342                                                     
R3 = 12.050                                                               
           D3 = 1.239  N2 = 1.69680                                       
                                   ν2 = 55.5                           
R4 = -36.035                                                              
           D4 = 0.043                                                     
R5 = 6.370 D5 = 1.043  N3 = 1.69680                                       
                                   ν3 = 55.5                           
R6 = 58.643                                                               
           D6 = Variable                                                  
R7 = 9.634 D7 = 0.173  N4 = 1.88300                                       
                                   ν4 = 40.8                           
R8 = 1.857 D8 = 0.844                                                     
R9 = -3.185                                                               
           D9 = 0.152  N5 = 1.69680                                       
                                   ν5 = 55.5                           
R10 = 3.140                                                               
           D10 = 0.217                                                    
R11 = 3.696                                                               
           D11 = 0.500 N6 = 1.84666                                       
                                   ν6 = 23.8                           
R12 = -21.979                                                             
           D12 = Variable                                                 
R13 = (Stop)                                                              
           D13 = 0.326                                                    
R14 = - 31.104                                                            
           D14 = 0.152 N7 = 1.60311                                       
                                   ν7 = 60.7                           
R15 = 4.653                                                               
           D15 = 0.264                                                    
R16 = -9.342                                                              
           D16 = 0.434 N8 = 1.60342                                       
                                   ν8 = 38.0                           
R17 = -4.592                                                              
           D17 = 0.652                                                    
R18 = 4.592                                                               
           D18 = 0.239 N9 = 1.60342                                       
                                   ν9 = 38.0                           
R19 = -2.721                                                              
           D19 = 0.195 N10 = 1.78590                                      
                                   ν10 = 44.2                          
R20 = -51.472                                                             
           D20 = Variable                                                 
R21 = 47.709                                                              
           D21 = 0.652 N11 = 1.51633                                      
                                   ν11 = 64.2                          
R22 = -4.925                                                              
           D22 = 0.032                                                    
R23 = 11.157                                                              
           D23 = 0.195 N12 = 1.80518                                      
                                   ν12 = 25.4                          
R24 = 2.984                                                               
           D24 = 1.130 N13 = 1.48749                                      
                                   ν13 = 70.2                          
R25 = -23.396                                                             
           D25 = 0.032                                                    
R26 = 5.008                                                               
           D26 = 0.739 N14 = 1.48749                                      
                                   ν14 = 70.2                          
R27 = -15.870                                                             
           D27 = 0.869                                                    
R28 =  ∞                                                            
           D28 = 0.543 N15 = 1.51633                                      
                                   ν15 = 64.2                          
R29 = ∞                                                             
           D29 = 4.347 N16 = 1.60342                                      
                                   ν16 = 38.0                          
R30 = ∞                                                             
______________________________________                                    
            Focal Length                                                  
Variable Separation                                                       
              1.00        3.45   12.65                                    
______________________________________                                    
D6            0.31        3.95   6.37                                     
D12           6.38        1.96   0.33                                     
D20           1.84        1.77   1.84                                     
______________________________________                                    
Numerical Example 10:                                                     
f = 1-12.65  Fno = 1:1.8-1.9  2ω = 66.2°-5.9°         
______________________________________                                    
R1 = 35.975                                                               
           D1 = 0.391  N1 = 1.80518                                       
                                   ν1 = 25.4                           
R2 = 8.396 D2 = 0.272                                                     
R3 = 11.644                                                               
           D3 = 1.130  N2 = 1.69680                                       
                                   ν2 = 55.5                           
R4 = -37.605                                                              
           D4 = 0.043                                                     
R5 = 6.400 D5 = 1.195  N3 = 1.69680                                       
                                   ν3 = 55.5                           
R6 = 60.254                                                               
           D6 = Variable                                                  
R7 = 8.615 D7 = 0.173  N4 = 1.88300                                       
                                   ν4 = 40.8                           
R8 = 1.785 D8 = 0.823                                                     
R9 =  -3.019                                                              
           D9 = 0.152  N5 = 1.69680                                       
                                   ν5 = 55.5                           
R10 = 3.112                                                               
           D10 = 0.217                                                    
R11 = 3.676                                                               
           D11 = 0.500 N6 = 1.84666                                       
                                   ν6 = 23.8                           
R12 = -16.603                                                             
           D12 = Variable                                                 
R13 = (Stop)                                                              
           D13 = 1.413                                                    
R14 = 5.119                                                               
           D14 = 1.043 N7 = 1.60342                                       
                                   ν7 = 38.0                           
R15 = -3.506                                                              
           D15 = 0.195 N8 = 1.88300                                       
                                   ν8 = 40.8                           
R16 = -46.563                                                             
           D16 = Variable                                                 
R17 = 58.011                                                              
           D17 = 0.434 N9 = 1.51633                                       
                                   ν9 = 64.2                           
R18 = -5.449                                                              
           D18 = 0.032                                                    
R19 = 7.785                                                               
           D19 = 0.195 N10 = 1.80518                                      
                                   ν10 = 25.4                          
R20 = 2.794                                                               
           D20 = 0.869 N11 = 1.48749                                      
                                   ν11 = 70.2                          
R21 = -14.358                                                             
           D21 = 0.032                                                    
R22 = 3.399                                                               
           D22 = 0.434 N12 = 1.48749                                      
                                   ν12 = 70.2                          
R23 = 23.023                                                              
           D23 = 0.869                                                    
R24 = ∞                                                             
           D24 = 0.543 N13 = 1.51633                                      
                                   ν13 = 64.2                          
R25 = ∞                                                             
           D25 = 3.260 N14 = 1.60342                                      
                                   ν14 = 38.0                          
R26 = ∞                                                             
______________________________________                                    
            Focal Length                                                  
Variable Separation                                                       
              1.00        3.43   12.65                                    
______________________________________                                    
D6            0.17        3.81   6.23                                     
D12           6.38        1.96   0.33                                     
D16           2.11        2.05   2.11                                     
______________________________________                                    
Aspheric Coefficients: Surface R14                                        
                    K = 2.597 × 10.sup.-2                           
                    A = -9.137 × 10.sup.-4                          
                    B = 1.986 × 10.sup.-4                           
                    C = -1.129 × 10.sup.-4                          
                    D = -1.168 × 10.sup.-5                          
______________________________________                                    
Numerical Example 11:                                                     
f = 1-12.65  Fno = 1:1.8-1.9  2ω = 66.2°-5.9°         
______________________________________                                    
R1 = 36.928                                                               
           D1 = 0.391  N1 = 1.80518                                       
                                   ν1 = 25.4                           
R2 = 8.368 D2 = 0.312                                                     
R3 = 11.877                                                               
           D3 = 1.130  N2 = 1.69680                                       
                                   ν2 = 55.5                           
R4 = -36.615                                                              
           D4 = 0.043                                                     
R5 = 6.411 D5 = 1.195  N3 = 1.69680                                       
                                   ν3 = 55.5                           
R6 = 66.685                                                               
           D6 = Variable                                                  
R7 = 9.469 D7 = 0.173  N4 = 1.88300                                       
                                   ν4 = 40.8                           
R8 = 1.788 D8 = 0.820                                                     
R9 = -3.019                                                               
           D9 = 0.152  N5 = 1.69680                                       
                                   ν5 = 55.5                           
R10 = 3.123                                                               
           D10 = 0.217                                                    
R11 = 3.695                                                               
           D11 = 0.500 N6 = 1.84666                                       
                                   ν6 = 23.8                           
R12 = -14.962                                                             
           D12 = Variable                                                 
R13 = (Stop)                                                              
           D13 = 1.413                                                    
R14 = 5.247                                                               
           D14 = 0.652 N7 = 1.60342                                       
                                   ν7 = 38.0                           
R15 = 17.260                                                              
           D15 = Variable                                                 
R16 = -13.719                                                             
           D16 = 0.434 N8 = 1.51633                                       
                                   ν8 = 64.2                           
R17 = -5.243                                                              
           D17 = 0.032                                                    
R18 = 7.556                                                               
           D18 = 0.195 N9 = 1.80518                                       
                                   ν9 = 25.4                           
R19 = 2.779                                                               
           D19 = 0.869 N10 = 1.48749                                      
                                   ν10 = 70.2                          
R20 = -23.427                                                             
           D20 = 0.032                                                    
R21 = 3.644                                                               
           D21 = 0.434 N11 = 1.48749                                      
                                   ν11 = 70.2                          
R22 = -16.329                                                             
           D22 = 0.869                                                    
R23 = ∞                                                             
           D23 = 0.543 N12 = 1.51633                                      
                                   ν12 = 64.2                          
R24 = ∞                                                             
           D24 = 3.260 N13 = 1.60342                                      
                                   ν13 = 38.0                          
R25 = ∞                                                             
______________________________________                                    
            Focal Length                                                  
Variable Separation                                                       
              1.00        3.42   12.65                                    
______________________________________                                    
D6            0.24        3.87   6.29                                     
D12           6.38        1.96   0.33                                     
D15           2.37        2.31   2.37                                     
______________________________________                                    
Aspheric Coefficients: Surface R14                                        
                    K = -2.612 × 10.sup.-1                          
                    A = -2.388 × 10.sup.-3                          
                    B = 2.364 × 10.sup.-4                           
                    C = -6.451 × 10.sup.-5                          
                    D = -3.051 × 10.sup.-5                          
______________________________________                                    
Numerical Example 12:                                                     
f = 1- 12.65  Fno = 1:1.8-1.9  2ω = 66.2°-5.9°        
______________________________________                                    
R1 = 25.907                                                               
           D1 = 0.391  N1 = 1.84666                                       
                                   ν1 = 23.8                           
R2 = 8.416 D2 = 0.312                                                     
R3 = 12.008                                                               
           D3 = 1.130  N2 = 1.69680                                       
                                   ν2 = 55.5                           
R4 = -45.235                                                              
           D4 = 0.043                                                     
R5 = 6.531 D5 = 1.195  N3 = 1.69680                                       
                                   ν3 = 55.5                           
R6 = 73.064                                                               
           D6 = Variable                                                  
R7 = 8.118 D7 = 0.173  N4 = 1.88300                                       
                                   ν4 = 40.8                           
R8 = 1.730 D8 = 0.861                                                     
R9 = -2.748                                                               
           D9 = 0.152  N5 = 1.69680                                       
                                   ν5 = 55.5                           
R10 = 3.014                                                               
           D10 = 0.217                                                    
R11 = 3.614                                                               
           D11 = 0.434 N6 = 1.84666                                       
                                   ν6 = 23.8                           
R12 = -11.227                                                             
           D12 = Variable                                                 
R13 = (Stop)                                                              
           D13 = 1.413                                                    
R14 = 5.282                                                               
           D14 = 0.434 N7 = 1.60311                                       
                                    ν7 = 60.7                          
R15 = 15.536                                                              
           D15 = Variable                                                 
R16 = -5.179                                                              
           D16 = 0.434 N8 = 1.51742                                       
                                   ν8 = 52.4                           
R17 = -4.751                                                              
           D17 = 0.032                                                    
R18 = 6.988                                                               
           D18 = 0.195 N9 = 1.80518                                       
                                   ν9 = 25.4                           
R19 = 3.039                                                               
           D19 = 0.760 N10 = 1.48749                                      
                                   ν10 = 70.2                          
R20 = -5.586                                                              
           D20 = 0.032                                                    
R21 = 3.239                                                               
           D21 = 0.543 N11 = 1.48749                                      
                                   ν11 = 70.2                          
R22 = 13.677                                                              
           D22 = 0.869                                                    
R23 = ∞                                                             
           D23 = 0.543 N12 = 1.51633                                      
                                   ν12 = 64.2                          
R24 = ∞                                                             
           D24 = 3.260 N13 = 1.60342                                      
                                   ν13 = 38.0                          
R25 = ∞                                                             
______________________________________                                    
            Focal Length                                                  
Variable Separation                                                       
              1.00        3.42   12.65                                    
______________________________________                                    
D6            0.17        3.80   6.22                                     
D12           6.38        1.96   0.33                                     
D15           2.54        2.49   2.54                                     
______________________________________                                    
Aspheric Coefficients: Surface R14                                        
                    K = 3.610 × 10.sup.-3                           
                    A = -1.746 × 10.sup.-3                          
                    B = 4.681 × 10.sup.-5                           
                    C = 4.104 × 10.sup.-5                           
                    D = -3.320 × 10.sup.-5                          
______________________________________                                    
Numerical Example 13:                                                     
f = 1-10  Fno = 1:1.6-1.7  2ω = 60.0°-6.6°            
______________________________________                                    
R1 = 12.083                                                               
           D1 = 0.346  N1 = 1.80518                                       
                                   ν1 = 25.4                           
R2 = 5.692 D2 = 1.538  N2 = 1.60311                                       
                                   ν2 = 60.7                           
R3 = -26.824                                                              
           D3 = 0.038                                                     
R4 = 5.854 D4 = 0.653  N3 = 1.71300                                       
                                   ν3 = 53.8                           
R5 = 15.975                                                               
           D5 = Variable                                                  
R6 = 9.280 D6 = 0.173  N4 = 1.88300                                       
                                   ν4 = 40.8                           
R7 = 1.826 D7 = 0.685                                                     
R8 = -2.629                                                               
           D8 = 0.158  N5 = 1.69680                                       
                                   ν5 = 55.5                           
R9 = 2.658 D9 = 0.192                                                     
R10 = 3.057                                                               
           D10 = 0.403 N6 = 1.84666                                       
                                   ν6 = 23.8                           
R11 = 145.551                                                             
           D11 = Variable                                                 
R12 = -13.497                                                             
           D12 = 0.134 N7 = 1.60311                                       
                                   ν7 = 60.7                           
R13 = 4.849                                                               
           D13 = 0.192                                                    
R14 = -10.238                                                             
           D14 = 0.384 N8 = 1.60342                                       
                                   ν8 = 38.0                           
R15 = -4.381                                                              
           D15 = 0.288                                                    
R16 = (Stop)                                                              
           D16 = 0.288                                                    
R17 = 4.709                                                               
           D17 = 0.961 N9 = 1.60342                                       
                                   ν9 = 38.0                           
R18 = -2.543                                                              
           D18 = 0.173 N10 = 1.78590                                      
                                   ν10 = 44.2                          
R19 = -52.529                                                             
           D19 = Variable                                                 
R20 = 24.257                                                              
           D20 = 0.576 N11 = 1.51633                                      
                                   ν11 = 64.2                          
R21 = -5.630                                                              
           D21 = 0.028                                                    
R22 = 10.997                                                              
           D22 = 0.211 N12 = 1.80518                                      
                                   ν12 = 25.4                          
R23 = 3.332                                                               
           D23 = 0.961 N13 = 1.48749                                      
                                   ν13 = 70.2                          
R24 = -8.681                                                              
           D24 =  0.028                                                   
R25 = 4.292                                                               
           D25 = 0.576 N14 = 1.48749                                      
                                   ν14 = 70.2                          
R26 = 128.098                                                             
           D26 = 0.769                                                    
R27 = ∞                                                             
           D27 = 0.384 N15 = 1.51633                                      
                                   ν15 = 64.2                          
R28 = ∞                                                             
           D28 = 3.846 N16 = 1.60342                                      
                                   ν16 = 38.0                          
R29 = ∞                                                             
______________________________________                                    
            Focal Length                                                  
Variable Separation                                                       
              1.00        3.05   10.00                                    
______________________________________                                    
D5            0.19        2.82   4.57                                     
D11           4.67        1.67   0.29                                     
D19           3.14        2.89   3.14                                     
______________________________________                                    
 
    
     
                       TABLE 2                                                     
______________________________________                                    
        Numerical Example                                                 
Condition 9        10      11     12    13                                
______________________________________                                    
 (7) F3/F4                                                                
          3.889    2.823   3.003  76.718                                  
                                        9.645                             
 (8) Fw/F2                                                                
          -0.541   -0.541  -0.541 -0.541                                  
                                        -0.650                            
 (9) Bfw/Fw                                                               
          4.911    4.002   3.995  4.124 4.812                             
(10) F1/Fw                                                                
          9.333    9.333   9.333  9.333 7.712                             
______________________________________                                    
 
    
     Another embodiment of the invention where further improvements are made is described below. 
     FIGS. 28 to 31 in lens block diagrams show numerical examples 14 to 17 of zoom lenses of the invention. FIGS. 32(A) and 32(B) to FIGS. 35(A) and 35(B) show the aberration curves. 
     In these figures, reference character L1 denotes a first lens unit of positive refractive power, reference character L2 denotes a second lens unit of negative refractive power, reference character L3 denotes a third lens unit of positive refractive power, reference character L4 denotes a fourth lens unit of positive refractive power and reference character L5 denotes a fifth lens unit of positive refractive power. Reference character SP denotes an aperture stop arranged within the third lens unit or in front of the third lens unit. Reference character G denotes a glass block such as color separation prism, face plate or filter. Reference character IP denotes an image plane. 
     In the present embodiment, a zoom lens of the rear focus type comprises, from front to rear, the first lens unit of positive refractive power, the second lens unit of negative refractive power, the third lens unit of positive refractive power having the stop, the fourth lens unit of positive refractive power and the fifth lens unit of positive refractive power, wherein during zooming from the wide-angle end to the telephoto end, the second lens unit is moved toward the image side and, at the same time, the stop and the third lens unit are moved in unison, while simultaneously moving the fourth lens unit to compensate for the image shift, and during focusing, the fourth lens unit is moved. 
     In such a manner, focusing in the present embodiment is performed by using the fourth lens unit. As compared with the use of the first lens unit for focusing purposes, or the so-called front focus type, it is, therefore, made easier to secure the light beam going to the corners of the image frame when shooting an object at the minimum distance with the setting in the wide-angle side. With the help of this, the effective diameter of the front or first lens unit is shortened. 
     Another feature is that the aperture stop is positioned either within the third lens unit or in between the second and third lens units. When zooming, the stop is moved in unison with the third lens unit, as described before. By this, the variation of aberrations with zooming is lessened. By shortening the separations between the stop and the preceding lens units, it is made easier to achieve the reduction of the effective diameter of the first lens unit. 
     If, in this zoom type, the stop lies in between, for example, the third and fourth lens units, as this means that the entrance pupil lies deep in from the first lens unit, the maximum possible height of incidence of the off-axial light beam on the first lens unit would occur in the middle zooming region at a nearer position to the wide-angle end. 
     So, in the invention, the stop is arranged either in the interior of the third lens unit or on the object side of the third lens unit. By this, when zooming from the wide-angle end to the telephoto end, the stop is moved in unison with the third lens unit so as to have a locus convex toward the object side. Thus, the entrance pupil takes its place close to the first lens unit. In turn, the maximum possible height of incidence occurs at or near the wide-angle end. By this, the effective diameter of the first lens unit is reduced advantageously. 
     In connection with their convex locus of motion toward the object side, it should be noted that, in the invention, the stop and the third lens unit are made to reciprocate almost perfectly. This assists in easily achieving the minimization of the diameter of the front lens members and the great increase of the maximum image angle. 
     To achieve further improvements of the compromise between the requirements of minimizing the bulk and size of the entire lens system and of securing high optical performance over the entire area of the image frame, it is preferred to satisfy at least one of the following conditions or features. 
     Letting the focal length of the i-th lens unit be denoted by Fi, the following condition is satisfied: 
     
         2.5&lt;F3/F4                                                  . . . (11) 
    
     The inequality of condition (11) has an aim to prevent the entire lens system from unduly increasing in the bulk and size, while still permitting good optical performance to be maintained throughout the entire zooming range, thereby obtaining a zoom lens of the rear focus type which has a long back focal distance and is of simple form. 
     That is, the condition (11) is concerned with the focal lengths of the third and fourth lens units and has an aim to sustain the good optical performance when to make a sufficiently long back focal distance, while still permitting improvements of the compact form of that section which follows the stop to be achieved. When the lower limit of the condition (11) is exceeded, as this means that the focal length of the third lens unit is too short, it becomes difficult to correct the variation of spherical aberration with zooming or focusing. Another problem arises in that the sufficient back focal distance becomes difficult to secure, or in that the amount of movement of the fourth lens unit increases to increase the variation of aberrations with zooming or focusing. 
     Another feature is that, letting the shortest focal length of the entire lens system be denoted by Fw, the condition 
     
         -0.8&lt;Fw/F2&lt;-0.4                                            . . . (12) 
    
     
         0.1&lt;F4/F5&lt;1.2                                              . . . (13) 
    
     are satisfied. 
     The inequalities of condition (12) are concerned with the ratio of the focal length for the wide-angle end of the entire lens system to the focal length of the second lens unit. When the upper limit of the condition (12) is exceeded, as this means that the focal length of the second lens unit is too short, the Petzval sum increases in the under direction. The resultant inclination of the image focus and other aberrations become difficult to correct well. Conversely when the lower limit is exceeded, as this means that the focal length of the second lens unit is too long, the total zooming movement of the second lens unit increases largely, giving rise to a problem in that the front lens members get too large a diameter. 
     The inequalities of condition (13) are concerned with the ratio of the focal lengths of the fourth and fifth lens units and have an aim to make a sufficiently long back focal distance while still permitting improvements of the compact form of that section which follows the stop to be achieved, and nonetheless to sustain the good optical performance. When the lower limit of the condition (13) is exceeded, as this means that the focal length of the fifth lens unit is too long and the focal length of the fourth lens unit is too short, it becomes difficult to correct well spherical aberration and others in such a manner that the sufficient back focal distance is secured. Conversely when the upper limit is exceeded, as this means that the focal length of the fourth lens unit is too long, a problem arises in that the amount of movement of the fourth lens unit during zooming and during focusing increases greatly. This is accompanied with another problem that the variation of aberrations with zooming or focusing increases greatly. 
     Also, in the present embodiment, the axial light beam when emerging from the third lens unit is made almost afocal or slightly divergent, thereby making it possible to advantageously increase the back focal distance and the exit pupil, while as far suppressing the variation of aberrations with zooming or focusing as possible. 
     With regard to a valuable minimization of the diameter of the front lens members, it is preferred that the first lens unit is constructed as comprising, from front to rear, a negative first lens of meniscus form convex toward the object side followed after an air separation by a positive second lens having a convex surface facing the object side and further a positive third lens having a convex surface facing the object side, wherein the air lens defined by the first and second lenses has a negative refractive power. 
     Further with regard to the minimization of the size of the entire lens system, it is preferred to satisfy the following condition: 
     
         3.5&lt;Bfw/Fw&lt;6.0                                             . . . (14) 
    
     where Bfw is the back focal distance in the wide-angle end with an object at infinity (without the &#34;G&#34; such as glass block or filter in the specific examples). 
     These inequalities of condition (14) are the necessary one to advantageously minimize the size of the entire lens system. When the lower limit is exceeded, the filter or like block not only can hardly be inserted, but also the exit pupil becomes rather short. In application of the invent ion to, for example, a video camera, therefore, the formation of an image onto the image sensor comes to deviate from the telecentric system. When the upper limit is exceeded, a large size results. 
     To minimize the diameter of the front lens members, it is preferred also to satisfy the following condition: 
     
         6.0&lt;F1/Fw&lt;15.0                                             . . . (15) 
    
     These inequalities are concerned with the object point for the second lens unit, that is, its magnification. To set the entire lens system to a minimum, it is preferred that the second lens unit when zooming moves across the unity of magnification. If the unity of magnification occurs at the middle region, the locus of zooming movement of the fourth lens unit takes an almost doubled form, making it possible to obtain the high zoom ratio with the maximum space efficiency. 
     In more detail, when the upper limit of this condition (15) is exceeded, the second lens unit is given a far object point, causing the magnification of the second lens unit to become low. Therefore, it becomes difficult to obtain the valuable minimization. Further, the separation between the first and second lens units increases to increase the difficulty of achieving the minimization of the bulk and size. When the lower limit is exceeded, the magnification of the second lens unit increases to increase the difficulty of achieving the valuable increase of the zoom ratio. 
     Also, the zoom lens of the present embodiment is made included with an aspherical lens. By this, it is made possible to further improve the performance, increase the speed in F-number, and reduce the number of constituent lens elements. 
     Particularly when the aspherical lens is used in the fourth lens unit, a valuable decrease of the number of constituent lens elements becomes possible. At the same time, it becomes possible to advantageously correct spherical aberration by the aspheric surface. 
     Further, by using another aspherical lens in the fifth lens unit, it becomes possible to advantageously correct coma and others. 
     For the second lens unit, it is preferred, from the standpoint of the minimum variation of aberrations with zooming, that it comprises, from front to rear, a negative first lens of meniscus form convex toward the object side, a negative second lens of bi-convex form and a positive third lens. 
     Next, numerical examples 14 to 17 of the invention are shown. The values of the factors in the above-described conditions (11) to (15) for the numerical examples 14 to 17 are also listed in a Table-3. The expression of the aspheric surface is the same as described before. 
     
         ______________________________________                                    
Numerical Example 14:                                                     
f = 1-10.0  Fno = 1:1.65  2ω = 60.0°-6.6°             
______________________________________                                    
R1 = 16.764                                                               
           D1 = 0.365  N1 = 1.80518                                       
                                   ν1 = 25.4                           
R2 = 6.729 D2 = 1.500  N2 = 1.60311                                       
                                   ν2 = 60.7                           
R3 = -27.650                                                              
           D3 = 0.038                                                     
R4 = 6.196 D4 = 0.730  N3 = 1.71300                                       
                                   ν3 = 53.8                           
R5 = 18.529                                                               
           D5 = Variable                                                  
R6 = 8.884 D6 = 0.173  N4 = 1.88300                                       
                                   ν4 = 40.8                           
R7 = 1.979 D7 = 0.923                                                     
R8 = -2.438                                                               
           D8 = 0.173  N5 = 1.69680                                       
                                   ν5 = 55.5                           
R9 = 2.653 D9 = 0.576  N6 = 1.84666                                       
                                   ν6 = 23.8                           
R10 = -22.105                                                             
           D10 = Variable                                                 
R11 = 100.867                                                             
           D11 = 0.134 N7 = 1.60311                                       
                                   ν7 = 60.7                           
R12 = 2.619                                                               
           D12 = 0.769                                                    
R13 = (Stop)                                                              
           D13 = 0.288                                                    
R14 = 5.736                                                               
           D14 = 0.769 N8 = 1.60342                                       
                                   ν8 = 38.0                           
R15 = -7.212                                                              
           D15 = Variable                                                 
R16 = 14.834                                                              
           D16 = 0.653 N9 = 1.51633                                       
                                   ν9 = 64.2                           
R17 = -7.247                                                              
           D17 = 0.028                                                    
R18 = 8.312                                                               
           D18 = 0.173 N10 = 1.80518                                      
                                   ν10 = 25.4                          
R19 = 3.021                                                               
           D19 = 1.153 N11 = 1.48749                                      
                                   ν11 = 70.2                          
R20 = -6.831                                                              
           D20 = Variable                                                 
R21 = 4.127                                                               
           D21 = 0.500 N12 = 1.48749                                      
                                   ν12 = 70.2                          
R22 = 8.802                                                               
           D22 = 0.769                                                    
R23 = ∞                                                             
           D23 = 0.480 N13 = 1.51633                                      
                                   ν13 = 64.2                          
R24 = ∞                                                             
           D24 = 3.846 N14 = 1.60342                                      
                                   ν14 = 38.0                          
R25 = ∞                                                             
______________________________________                                    
Aspheric Coefficients: Surface R14                                        
                    K = -6.822 × 10.sup.-2                          
                    A =  1.348 × 10.sup.-4                          
                    B = 1.975 × 10.sup.-5                           
                    C = 7.452 × 10.sup.-5                           
                    D = -4.393 × 10.sup.-5                          
______________________________________                                    
            Focal Length                                                  
Variable Separation                                                       
              1.00        3.39   10.00                                    
______________________________________                                    
D5            0.22        3.17   5.14                                     
D10           5.22        2.03   0.29                                     
D15           3.89        2.73   3.89                                     
D20           0.77        2.16   0.77                                     
______________________________________                                    
Numerical Example 15:                                                     
f = 1-8  Fno = 1:1.65  2ω = 58.1°-7.9°                
______________________________________                                    
R1 = 12.184                                                               
           D1 = 0.296  N1 = 1.84666                                       
                                   ν1 = 23.8                           
R2 = 5.537 D2 = 1.277  N2 = 1.60311                                       
                                   ν2 = 60.7                           
R3 = -19.188                                                              
           D3 = 0.037                                                     
R4 = 4.609 D4 = 0.629  N3 = 1.69680                                       
                                   ν3 = 55.5                           
R5 = 12.642                                                               
           D5 = Variable                                                  
R6 = 13.075                                                               
           D6 = 0.166  N4 = 1.88300                                       
                                   ν4 = 40.8                           
R7 = 1.692 D7 = 0.720                                                     
R8 = -2.032                                                               
           D8 =  0.166 N5 = 1.69680                                       
                                   ν5 = 55.5                           
R9 = 2.173 D9 = 0.444  N6 = 1.84666                                       
                                   ν6 = 23.8                           
R10 = -15.701                                                             
           D10 = Variable                                                 
R11 = -274.990                                                            
           D11 = 0.129 N7 = 1.60311                                       
                                   ν7 = 60.7                           
R12 = 2.469                                                               
           D12 = 0.740                                                    
R13 = (Stop)                                                              
           D13 = 0.277                                                    
R14 = 5.816                                                               
           D14 = 0.648 N8 = 1.60342                                       
                                   ν8 = 38.0                           
R15 = -6.169                                                              
           D15 = Variable                                                 
R16 = 12.953                                                              
           D16 = 0.629 N9 = 1.51633                                       
                                   ν9 = 64.2                           
R17 = -6.978                                                              
           D17 = 0.027                                                    
R18 = 5.893                                                               
           D18 = 0.166 N10 = 1.84666                                      
                                   ν10 = 23.8                          
R19 = 2.746                                                               
           D19 = 1.111 N11 = 1.48749                                      
                                   ν11 = 70.2                          
R20 = -7.051                                                              
           D20 = Variable                                                 
R21 = 3.844                                                               
           D21 = 0.407 N12 = 1.48749                                      
                                   ν12 =  70.2                         
R22 = 6.512                                                               
           D22 = 0.740                                                    
R23 = ∞                                                             
           D23 = 0.493 N13 = 1.51633                                      
                                   ν13 = 64.2                          
R24 = ∞                                                             
           D24 = 3.703 N14 = 1.60342                                      
                                   ν14 = 38.0                          
R25 = ∞                                                             
______________________________________                                    
Aspheric Coefficients: Surface R14                                        
                    K = -3.954 × 10.sup.-1                          
                    A = 1.510 × 10.sup.-4                           
                    B = 1.041 × 10.sup.-4                           
                    C = 2.429 × 10.sup.-4                           
                    D = 1.345 × 10.sup.-4                           
______________________________________                                    
            Focal Length                                                  
Variable Separation                                                       
              1.00        3.08   8.00                                     
______________________________________                                    
D5            0.22        2.28   3.65                                     
D10           3.70        1.41   0.26                                     
D15           3.71        2.85   3.71                                     
D20           0.74        1.83   0.74                                     
______________________________________                                    
Numerical Example 16:                                                     
f = 1-8  Fno = 1:1.65  2ω = 58.1°-7.9°                
______________________________________                                    
R1 = 10.019                                                               
           D1 = 0.277  N1 = 1.80518                                       
                                   ν1 = 25.4                           
R2 = 4.876 D2 = 1.259  N2 = 1.60311                                       
                                   ν2 = 60.7                           
R3 = -39.080                                                              
           D3 = 0.037                                                     
R4 = 4.790 D4 = 0.703  N3 = 1.69680                                       
                                   ν3 = 55.5                           
R5 = 15.516                                                               
           D5 = Variable                                                  
R6 = 12.553                                                               
           D6 = 0.166  N4 = 1.88300                                       
                                   ν4 = 40.8                           
R7 = 1.538 D7 = 0.635                                                     
R8 = -2.510                                                               
           D8 = 0.129  N5 = 1.69680                                       
                                   ν5 = 55.5                           
R9 = 2.733 D9 = 0.185                                                     
R10 = 3.114                                                               
           D10 = 0.370 N6 = 1.84666                                       
                                   ν6 = 23.8                           
R11 = -25.032                                                             
           D11 = Variable                                                 
R12 = 43.470                                                              
           D12 = 0.129 N7 = 1.60311                                       
                                   ν7 = 60.7                           
R13 = 2.717                                                               
           D13 = 0.740                                                    
R14 = (Stop)                                                              
           D14 = 0.277                                                    
R15 = 5.891                                                               
           D15 = 0.648 N8 = 1.60342                                       
                                   ν8 = 38.0                           
R16 = -5.939                                                              
           D16 = Variable                                                 
R17 = 10.288                                                              
           D17 = 0.629 N9 = 1.51633                                       
                                   ν9 =  64.2                          
R18 = -6.932                                                              
           D18 = 0.027                                                    
R19 = 7.417                                                               
           D19 = 0.166 N10 = 1.80518                                      
                                   ν10 = 25.4                          
R20 = 2.617                                                               
           D20 = 1.111 N11 = 1.48749                                      
                                   ν11 = 70.2                          
R21 = -6.833                                                              
           D21 = Variable                                                 
R22 = 3.163                                                               
           D22 = 0.370 N12 = 1.58313                                      
                                   ν12 = 59.4                          
R23 = 4.993                                                               
           D23 = 0.740                                                    
R24 = ∞                                                             
           D24 = 0.463 N13 = 1.51633                                      
                                   ν13 = 64.2                          
R25 = ∞                                                             
           D25 = 3.703 N14 = 1.60342                                      
                                   ν14 = 38.0                          
R26 = ∞                                                             
______________________________________                                    
Aspheric Coefficients: Surface R15                                        
                    K = -2.530 × 10.sup.-1                          
                    A = -1.968 × 10.sup.-4                          
                    B = 1.898 × 10.sup.-4                           
                    C = 1.111 × 10.sup.-4                           
                    D = -9.399 × 10.sup.-5                          
Aspheric Coefficients: Surface R22                                        
                    K = 1.392 × 10.sup.-1                           
                    A = -3.106 × 10.sup.-5                          
                    B = -3.892 × 10.sup.-4                          
                    C = 1.106 × 10.sup.-4                           
______________________________________                                    
            Focal Length                                                  
Variable Separation                                                       
              1.00        3.05   8.00                                     
______________________________________                                    
D5            0.19        2.26   3.63                                     
D11           3.70        1.41   0.26                                     
D16           3.55        2.62   3.55                                     
D21           0.74        1.89   0.74                                     
______________________________________                                    
Numerical Example 17:                                                     
f = 1-12.65  Fno = 1:1.7-2.4  2ω = 64.0°-5.7°         
______________________________________                                    
R1 = 21.532                                                               
           D1 = 0.312  N1 = 1.84666                                       
                                   ν1 = 23.8                           
R2 = 8.061 D2 = 0.091                                                     
R3 = 9.418 D3 = 0.916  N2 = 1.69680                                       
                                   ν2 = 55.5                           
R4 = -49.597                                                              
           D4 = 0.041                                                     
R5 = 6.245 D5 = 0.645  N3 = 1.69680                                       
                                   ν3 = 55.5                           
R6 = 31.447                                                               
           D6 = Variable                                                  
R7 = 9.270 D7 = 0.166  N4 = 1.88300                                       
                                   ν4 = 40.8                           
R8 = 1.615 D8 = 0.713                                                     
R9 = -2.851                                                               
           D9 = 0.145  N5 = 1.69680                                       
                                   ν5 = 55.5                           
R10 = 4.253                                                               
           D10 = 0.208                                                    
R11 = 4.042                                                               
           D11 = 0.416 N6 = 1.84666                                       
                                   ν6 = 23.8                           
R12 = -15.249                                                             
           D12 = Variable                                                 
R13 = (Stop)                                                              
           D13 = 0.937                                                    
R14 = 5.515                                                               
           D14 = 1.000 N7 = 1.60342                                       
                                   ν7 = 38.0                           
R15 = -5.139                                                              
           D15 = 0.187 N8 = 1.88300                                       
                                   ν8 = 40.8                           
R16 = -70.309                                                             
           D16 = Variable                                                 
R17 = 14.435                                                              
           D17 = 0.479 N9 = 1.51633                                       
                                   ν9 = 64.2                           
R18 = -6.403                                                              
           D18 = 0.031                                                    
R19 = 6.120                                                               
           D19 = 0.187 N10 = 1.80518                                      
                                   ν10 = 25.4                          
R20 = 2.617                                                               
           D20 = 0.937 N11 = 1.48749                                      
                                   ν11 = 70.2                          
R21 = -6.615                                                              
           D21 = Variable                                                 
R22 = 3.194                                                               
           D22 = 0.416 N12 = 1.48749                                      
                                   ν12 =  70.2                         
R23 = 4.564                                                               
           D23 = 0.833                                                    
R24 = ∞                                                             
           D24 = 0.520 N13 = 1.51633                                      
                                   ν13 = 64.2                          
R25 = ∞                                                             
           D25 = 3.125 N14 = 1.60342                                      
                                   ν14 = 38.0                          
R26 = ∞                                                             
______________________________________                                    
Aspheric Coefficients: Surface R14                                        
                    K = -1.049 × 10.sup.-0                          
                    A = -2.648 × 10.sup.-3                          
                    B = 5.360 × 10.sup.-4                           
                    C = -3.779 × 10.sup.-4                          
                    D = 4.930 × 10.sup.-5                           
______________________________________                                    
            Focal Length                                                  
Variable Separation                                                       
              1.00        3.98   12.65                                    
______________________________________                                    
D6            0.17        3.65   5.97                                     
D12           6.11        1.88   0.31                                     
D16           2.75        1.71   2.75                                     
D21           0.21        1.99   0.21                                     
______________________________________                                    
          Numerical Example                                               
Condition   14      15         16    17                                   
______________________________________                                    
(11) F3/F4  12.323  18.157     4.111 2.749                                
(12) Fw/F2  -0.578  -0.720     -0.720                                     
                                     -0.656                               
(13) F4/F5  0.401   0.290      0.414 0.261                                
(14) Bfw/Fw 4.273   4.094      3.730 3.563                                
(15) F1/Fw  8.860   8.588      6.520 6.502                                
______________________________________                                    
 
    
     According to the invention, by setting forth the rules of design as described above, it is made possible to realize a zoom lens of the rear focus type having a long back focal distance, which, while being minimized in the bulk and size of the entire lens system, has as wide an image angle as 60 degrees in the wide-angle end and a high zoom ratio with maintenance of good optical performance throughout the entire zooming range and throughout the entire focusing range.