BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to zoom lenses and, more particularly, to zoom lenses of increased relative aperture with good optical performance over the entire zooming range, while still permitting the physical size to me minimized with the total length shortened to be suited to photographic cameras, video cameras, etc. 
     2. Description of the Related Art 
     To photographic cameras and video cameras there has been a demand for a zoom lens having a good compromise between the requirement of increasing the relative aperture with a high range and the requirement of reducing the bulk and size in such a manner that high grade optical performance is preserved. 
     Of these, for the video camera, because of its image pickup element being relatively low in sensitivity, the zoom lens has to get as high a relative aperture as possible. 
     At present, the 2/3 in. image pickup tube is widely used in the video camera from the two points of view of compactness and image quality. Also, from the standpoint of good manageability and high facility for further minimization of the size, 8 m/m video cameras are coming to be used in gradually increasing numbers. The image pickup tube to be used in this camera is required to be furthermore reduced in size while preserving high grade imagery. Recently the 1/2 in. image pickup tube or plate has found its use in 8 m/m video cameras. 
     If the zoom lens is of the so-called 4-unit type in principle, it is in general case that an increase of the relative aperture to as high as 1.4-1.6 or thereabout in F-number can be achieved when proper rules of lens design are set forth particularly for the fourth lens unit that is arranged on the image side of the zoom section to be stationary during zooming, and the image forming section or the fifth lens unit to well correct the residual aberrations of the zoom section. 
     Also, in order to shorten the total length of the entire lens system, the effective method is to reduce the bulk and size of the front or first lens unit. To allow for this to be achieved, the F-number may be increased. But, to avoid the F-number from being so much increased, it becomes important to set forth proper rules of design for the image forming section. 
     In addition thereto, if the reduction of the physical size of the entire lens system and the increase of the relative aperture are attempted by relying merely on strengthening of the refractive power of each lens unit, then the spherical aberration in the paraxial region, the coma from the zonal to the marginal region, and higher order aberrations such as sagittal halo are increased largely. So, it becomes difficult to get high optical performance. 
     Suppose, for example, the front or first lens unit is selected for shortening the total length by the method of increasing the refractive power, then the overall magnifying power of the zoom section up to the image forming section has to be increased. As a result, the first lens unit produces many aberrations, and the tolerances for the lens design parameters becomes severer. Thus, it becomes difficult to assure the prescribed optical performance. 
     Also, in the case of using the 2/3 in. image pickup element, according to the prior art, the total length L of the entire lens system in terms of the diagonal φ A  of the effective picture frame falls in a range of 10φ A  to 12φ A , as disclosed in Japanese Laid-Open Patent Application No. Sho 60-260912. This implies that the total length of the entire lens system is caused to become comparatively long. Like this, it has been very difficult to achieve a reduction of the total length L to shorter than 10φ A  in such a manner that good optical performance is preserved throughout the zooming range. As other concomitant techniques mention may be made of U.S. Pat. Nos. 4,518,228, 4,525,036, 4,618,219, 4,621,905, 4,653,874, and 4,659,187. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a zoom lens having a small F-number, a high zoom ratio and the standard image angle at the wide angle end with the size of the entire lens system being reduced while still permitting high grade optical performance to be preserved throughout the entire zooming range. 
     The zoom lens of the invention comprises, from front to rear, a first lens unit of positive power for focusing, a second lens unit of negative power having the magnification varying function, a third lens unit of negative power for compensating for the shift of an image plane resulting from the variation of the magnification, a fourth lens unit having a positive lens for making the diverging light beam from the third lens unit an almost parallel light beam, and a fifth lens unit having an image forming function and having lenses of positive, negative, positive, negative, positive and positive powers in this order, wherein letting the focal length of the j-th lens in the i-th lens unit be denoted by f i ,j, the radius of curvatyre of the j-th lens surface in the i-th lens unit by R i ,j, the Abbe number of the glass of the j-th lens in the i-th lens unit by ν i ,j, and the focal length of the i-th lens unit by Fi, the following conditions are satisfied: 
     
         0.7&lt;|R.sub.4,2 /F.sub.4 |&lt;0.85           (1) 
    
     
         1.05&lt;|f.sub.5,2 /F.sub.5 |&lt;1.5           (2) 
    
     
         0.6&lt;|f.sub.5,4 /f.sub.5,5 |&lt;1.5          (3) 
    
     
         50&lt;(ν.sub.5,1 +ν.sub.5,6)/2&lt;59                       (4) 
    
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a longitudinal section view of a typical one of examples of specific zoom lenses of the invention. 
     FIGS. 2(A) and 2(B) to FIGS. 5(A) and 5(B) are graphic representations of the aberrations of numerical examples 1 to 4 of the invention respectively, with FIGS. 2(A), 3(A), 4(A) and 5(A) in the wide angle end, and FIGS. 2(B), 3(B), 4(B) and 5(B) ij tge telephoto end. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows the form of a zoom lens of the invention in correspondence to numerical examples thereof. In the figure, I is a first lens unit of positive refractive power axially movable for focusing. II is a second lens unit of negative refractive power axially movable for varying the image magnification. III is a third lens unit of negative refractive power for compensating for the shift of an image plane as it occurs when the image magnification varies. IV is a fourth lens unit of positive refractive power for making the diverging light beam incident thereon from the third lens unit III almost afocal in emerging therefrom. V is a fixed fifth lens unit having the image forming function and comprising six lens elements of positive, negative, positive, negative, positive and positive powers in this order. SP is a fixed, aperture size-variable diaphragm. 
     In the embodiments of the invention, in such a zoom type, by satisfying the above-mentioned inequalities of conditions (1)-(4) for the construction and arrangement of the elements of the fourth and fifth lens units, despite a great increase in each of the relative aperture and the range of variation of the magnification, good correction of aberrations is achieved for high grade optical performance throughout the entire zooming range. 
     The technical signifcances of the above-defined conditions each are explained below. 
     The inequalities of condition (1) represent the range of refracting power for the rear surface of the first lens in the fourth lens unit. The use of a lens form of strong rearward curvature in the first lens of the fourth lens unit leads to production of various aberrations, particularly spherical aberration and coma, when the light beam diverging in passing through the first to the third lens units travels across that lens. In order that the light beam leaves as an almost afocal one for the fifth lens unit without causing such aberrations to be increased as far as possible, the condition (1) must be satisfied. 
     Also, the requirement of making up the almost afocal beam from the diverging light beam of the third lens unit, when fulfilled, unequivocally determines the refractive power for the fourth lens. For this reason, when the refracting power of the rear lens surface becomes too weak as exceeding beyond the upper limit of the inequalities of condition (1), the refracting power of the front lens surface must be so much increased. As a result, the tendency toward under-correction of spherical aberration is intensified. When the refracting power of the rear lens surface becomes too strong beyond the lower limit, on the other hand, the coma is increased largely. 
     The inequalities of condition (2) give a range of refractive power ratio for the second lens in the fifth lens unit to the whole of the fifth lens unit to well correct particularly spherical aberration. When the upper limit is exceeded, it is over-corrected. When the lower limit is exceeded, under-correction comes to result. 
     The inequalities of condition (3) give a range of refractive power ratio for the fourth and fifth lenses in the fifth lens unit to well correct astigmatism without causing other aberrations, mainly coma, to be produced as far as possible. When the upper limit is exceeded, the coma is increased largely. When the lower limit is exceeded, the astigmatism becomes difficult to well correct. 
     The inequalities of condition (4) give a range of the Abbe numbers of the media of the first and sixth lenses in the fifth lens unit to correct longitudinal and lateral chromatic aberrations in good balance. When the upper limit is exceeded, over-correction of longitudinal chromatic aberration results. When the lower limit is exceeded, the lateral chromatic aberration is under-corrected objectionably. 
     In order to achieve a reduction of the physical length of the entire lens system while minimizing the variation with zooming of the aberrations, it is preferred to satisfy the following condition: 
     
         0.75&lt;|F.sub.2 /Fw|&lt;0.85                  (5) 
    
     where Fw is the shortest focal length of the entire system. 
     The factor in the inequalities of condition (5) represents the refractive power of the second lens unit. When the lower limit is exceeded, the refractive power of the second lens unit becomes too strong, causing the range of variation of aberrations with zooming to increase. When the refractive power of the second lens unit becomes weak beyond the upper limit, the physical length is increased objectionably, because it must be compensated for by increasing the total zooming movement of the second lens unit to obtain the equivalent zoom ratio. 
     The objects of the invention are accomplished when all the conditions set forth above are satisfied. Yet, to achieve a further improvement of the aberration correction, it is preferred that the fourth and fifth lens units are constructed in such forms as described below. 
     The fourth lens unit is a bi-convex lens with the rear surface of strong curvature toward the rear. The fifth lens unit comprises, from front to rear, a bi-convex first lens with the front surface having a stronger curvature than the rear surface, a negative meniscus-shaped second lens of forward convexity, a positive third lens with the front surface of strong curvature toward the front, a negative meniscus-shaped fourth lens of forward convexity, a bi-convex fifth lens with the rear surface of strong curvature, and a positive sixth lens with the front surface of strong curvature toward the front. The air separation between the third and fourth lenses is longest in this lens unit. 
     It should be noted that the term &#34;rear surface of strong curvature&#34; means that it is compared with the curvature of the other or front surface. This applies to the term &#34;front surface of strong curvature&#34; as well. By designing the fourth and fifth lens units in such a way, the residual aberrations, for example, spherical aberration and inward coma from the zonal to the marginal region of the picture frame, of the zoom section are corrected entirely in good balance. 
     Four examples of specific zoom lenses of the invention can be constructed in accordance with the numerical data given in the following tables for the radii of curvature, R, the axial thicknesses or air separations, D, and the refractive indices, N, and Abbe numbers, ν, of the glasses of the various lenses with the subscriptions numbered consecutively from front to rear. A block defined between flat surfaces R28 and R29 represents a face plate, or a filter. 
     The values of the factors in the above-cited conditions for the numerical examples are given in Table 1. 
     
         ______________________________________Numerical Example 1 (FIGS. 2(A) and 2(B))F = 1-5.56 FNo = 1:1.6-2.4 2ω = 47.6°-9.0°______________________________________R1 = 4.9342      D1 = 0.0945 N1 = 1.80518                              ν1 = 25.4R2 = 2.3937      D2 = 0.4961 N2 = 1.51633                              ν2 = 64.1R3 = -10.2228      D3 = 0.0118R4 = 2.1189      D4 = 0.3622 N3 = 1.60311                              ν3 = 60.7R5 = 11.8103      D5 = VariableR6 = 5.1504      D6 = 0.0630 N4 = 1.69680                              ν4 = 55.5R7 = 0.9067      D7 = 0.2717R8 = -1.1671      D8 = 0.0630 N5 = 1.69680                              ν5 = 55.5R9 = 1.1671      D9 = 0.2441 N6 = 1.84666                              ν6 = 23.9R10 = 11.5367      D10 = VariableR11 = -2.2007      D11 = 0.0630                  N7 = 1.71300                              ν7 = 53.8R12 = -50.7152       D12 = VariableR13 = 9.2253      D13 = 0.3465                  N8 = 1.69680                              ν8 = 55.5R14 = -1.7144      D14 = 0.0787R15 = Stop D15 = 0.1575R16 = 2.7254      D16 = 0.2913                  N9 = 1.65844                              ν9 = 50.9R17 = -11.0099      D17 = 0.1417R18 = -1.8215      D18 = 0.0866                  N10 = 1.84666                              ν10 = 23.9R19 = -7.3111      D19 = 0.0118R20 = 1.6340      D20 = 0.2520                  N11 = 1.56384                              ν11 = 60.7R21 = 6.1695      D21 = 0.8189R22 = 2.5700      D22 = 0.0630                  N12 = 1.83400                              ν12 = 37.2R23 = 1.2021      D23 = 0.1024R24 = 1.8982      D24 = 0.2362                  N13 = 1.51633                              ν13 = 64.1R25 = -2.9332      D25 = 0.0118R26 = 3.6846      D26 = 0.2283                  N14 = 1.51742                              ν14 = 52.4R27 = -15.6427      D27 = 0.3150R28 = ∞      D28 = 0.4331                  N15 = 1.51633                              ν15 = 64.1R29 = ∞______________________________________f              1             5.56______________________________________D5             0.0910       1.5119D10            1.5019       0.2231D12            0.2359       0.0938Total Length = 8.109 (= 9.19 · φ.sub.EA)______________________________________ 
    
     
         ______________________________________Numerical Example 2 (FIGS. 3(A) and 3(B))F = 1-5.56 FNo = 1:1.6-2.4 2ω = 47.6°-9.0°______________________________________R1 = 4.9314      D1 = 0.0945 N1 = 1.80518                              ν1 = 25.4R2 = 2.3926      D2 = 0.5042 N2 = 1.51633                              ν2 = 64.1R3 = -10.2219      D3 = 0.0118R4 = 2.1255      D4 = 0.3624 N3 = 1.60311                              ν3 = 60.7R5 = 11.9694      D5 = VariableR6 = 5.5784      D6 = 0.0630 N4 = 1.69680                              ν4 = 55.5R7 = 0.9197      D7 = 0.2721R8 = -1.1850      D8 = 0.0630 N5 = 1.69680                              ν5 = 55.5R9 = 1.1855      D9 = 0.2285 N6 = 1.84666                              ν6 = 23.9R10 = 10.6989      D10 = VariableR11 = -2.1763      D11 = 0.0788                  N7 = 1.71300                              ν7 = 53.8R12 = -40.3230       D12 = VariableR13 = 9.2309      D13 = 0.3545                  N8 = 1.69680                              ν8 = 55.5R14 = -1.7155      D14 = 0.0788R15 = Stop D15 = 0.1800R16 = 3.0328      D16 = 0.2758                  N9 = 1.65844                              ν9 = 50.9R17 = -7.4073      D17 = 0.1534R18 = -1.7877      D18 = 0.0867                  N10 = 1.84666                              ν10 = 23.9R19 = -6.5984      D19 = 0.0118R20 = 1.5956      D20 = 0.2994                  N11 = 1.56384                              ν11 = 60.7R21 = 7.4734      D21 = 0.8036R22 = 4.0283      D22 = 0.0630                  N12 = 1.83400                              ν12 = 37.2R23 = 1.1962      D23 = 0.0561R24 = 1.6708      D24 = 0.3230                  N13 = 1.51823                              ν13 = 59.0R25 = -2.3852      D25 = 0.0118R26 = 2.9824      D26 = 0.1418                  N14 = 1.51742                              ν14 = 52.4R27 = ∞      D27 = 0.3151R28 =  ∞      D28 = 0.3151                  N15 = 1.51633                              ν15 = 64.1R29 = ∞______________________________________f              1            5.56______________________________________D5             0.0907       1.5128D10            1.5412       0.2586D12            0.2329       0.0934Total Length = 8.0734 (= 9.15 · φ.sub.EA)______________________________________ 
    
     
         ______________________________________Numerical Example 3 (FIGS. 4(A) and 4(B))F = 1-5.56 FNo = 1:1.6-2.4 2ω = 47.3°-9.18°______________________________________R1 = 4.9866      D1 = 0.0956 N1 = 1.80518                              ν1 = 25.4R2 = 2.4194      D2 = 0.5099 N2 = 1.51633                              ν2 = 64.1R3 = -10.3364      D3 = 0.0120R4 = 2.1493      D4 = 0.3665 N3 = 1.60311                              ν3 = 60.7R5 = 12.1034      D5 = VariableR6 = 5.6408      D6 = 0.0637 N4 = 1.69680                              ν4 = 55.5R7 = 0.9300      D7 = 0.2752R8 = -1.1983      D8 = 0.0637 N5 = 1.69680                              ν5 = 55.5R9 = 1.1988      D9 = 0.2310 N6 = 1.84666                              ν6 = 23.9R10 = 10.8187      D10 = VariableR11 = -2.2007      D11 = 0.797 N7 = 1.71300                              ν7 = 53.8R12 = -40.7745       D12 = VariableR13 = 9.4928      D13 = 0.3585                  N8 = 1.69680                              ν8 = 55.5R14 = -1.7292      D14 = 0.0797R15 = Stop D15 = 0.1593R16 = 3.1390      D16 = 0.2788                  N9 = 1.65844                              ν9 = 50.9R17 = -7.0677      D17 = 0.1502R18 = -1.8030      D18 = 0.0876                  N10 = 1.84666                              ν10 = 23.9R19 = -6.3846      D19 = 0.0120R20 = 1.5990      D20 = 0.3027                  N11 = 1.56384                              ν11 = 60.7R21 = 7.3765      D21 = 0.8126R22 = 4.2344      D22 = 0.0637                  N12 = 1.83400                              ν12 = 37.2R23 = 1.2054      D23 = 0.0700R24 = 1.7071      D24 = 0.3266                  N13 = 1.51823                              ν13 = 59.0R25 = -2.3530      D25 = 0.0120R26 = 3.0106      D26 = 0.1434                  N14 = 1.51742                              ν14 = 52.4R27 = -95.6033      D27 = 0.3187R28 =  ∞      D28 = 0.3187                  N15 = 1.51633                              ν15 = 64.1R29 = ∞______________________________________f              1            5.56______________________________________D5             0.0918       1.5297D10            1.5584       0.2615D12            0.2350       0.0939Total Length = 8.1461 (= 9.13 · φ.sub.EA)______________________________________ 
    
     
         ______________________________________Numerical Example 4 (FIGS. 5(A) and 5(B))F = 1-5.56 FNo = 1:1.6-2.4 2ω = 47.6°-9.0°______________________________________R1 = 4.9254      D1 = 0.1102 N1 = 1.80518                              ν1 = 25.4R2 = 2.3897      D2 = 0.4934 N2 = 1.51633                              ν2 = 64.1R3 = -10.2094      D3 = 0.0118R4 = 2.1229      D4 = 0.3620 N3 = 1.60311                              ν3 = 60.7R5 = 11.9548      D5 = VariableR6 = 5.5715      D6 = 0.0630 N4 = 1.69680                              ν4 = 5.5R7 = 0.9186      D7 = 0.2718R8 = -1.1836      D8 = 0.0630 N5 = 1.69680                              ν5 = 55.5R9 = 1.1841      D9 = 0.2282 N6 = 1.84666                              ν6 = 23.9R10 = 10.6858      D10 = VariableR11 = -2.1737      D11 = 0.0787                  N7 = 1.71300                              ν7 = 53.8R12 = -40.2737       D12 = VariableR13 = 9.2196      D13 = 0.3541                  N8 = 1.69680                              ν8 = 55.5R14 = -1.7134      D14 = 0.0787R15 = Stop D15 = 0.1479R16 = 3.4482      D16 = 0.2675                  N9 = 1.51633                              ν9 = 64.1R17 = -5.6826      D17 = 0.1841R18 = -1.5079      D18 = 0.0866                  N10 = 1.84666                              ν10 = 23.9R19 = -2.8542      D19 = 0.0118R20 = 1.5567      D20 = 0.3620                  N11 = 1.60311                              ν11 = 60.7R21 = -43.2099      D21 = 0.6372R22 = -5.5552      D22 = 0.0630                  N12 = 1.83400                              ν12 = 37.2R23 = 1.3759      D23 = 0.1036R24 = 4.4717      D24 = 0.3305                  N13 = 1.51633                              ν13 = 64.1R25 = -1.3545      D25 = 0.0118R26 = 2.0246      D26 = 0.2203                  N14 = 1.51742                              ν14 = 52.4R27 = ∞      D27 = 0.3148R28 = ∞      D28 = 0.3148                  N15 = 1.51633                              ν15 = 64.1R29 = ∞______________________________________f              1            5.56______________________________________D5             0.0885       1.5088D10            1.5393       0.2583D12            0.2326       0.0933Total Length = 8.0698 (= 9.16 · φ.sub.EA)______________________________________ 
    
     
                       TABLE 1______________________________________                          Numeri-                                 Numeri-      Numerical Numerical cal Ex-                                 cal Ex-Conditions Example 1 Example 2 ample 3                                 ample 4______________________________________(1) |R.sub.4,2 /F.sub.4 |          0.8156    0.8153  0.8129 0.8153(2) |f.sub.5,2 /F.sub.5 |          1.1123    1.1068  1.1329 1.4736(3) |f.sub.5,4 /f.sub.5,5 |          1.2187    1.0575  1.0392 0.6413(4) (ν.sub.5,1 + ν.sub.5,6)/2          51.65     51.65   51.65  58.25(5) |F.sub.2 /Fw|          0.7874    0.7880  0.7968 0.7870    Total Length          9.19φ.sub.EA                    9.15φ.sub.EA                            9.13φ.sub.EA                                   9.16φ.sub.EA    of Lens______________________________________ 
    
     It will be appreciated from the foregoing that according to the present invention, it is made possible to realize a large relative aperture, high range zoom lens of reduced size, while still preserving high grade optical performance, suited to photographic camera or video camera. 
     In particular, the present invention has achieved a great advance in reduction of the size of the zoom lens in terms of the total length to as short as L=9.13φ A  to 9.19φ A .
A zoom lens comprising a positive first lens unit for focusing, a negative second lens unit as the variator, a negative third lens unit as the compensator, a positive fourth lens unit for making afocal the diverging light beam from the third unit in travelling thereacross, and an image forming or fifth lens unit having six lenses, satisfying the following conditions: 0.7<|R.sub.4,2 /F.sub.4 |<0.85 1.05<|f.sub.5,2 /F.sub.5 |<1.5 0.6<|f.sub.5,4 /f.sub.5,5 |<1.5 50<(n.sub.5,1 +n.sub.5,6)/2 59 where R 4,2 is the radius of curvature of the second surface counting from front of the fourth lens unit, F 4 and F 5 are the focal lengths of the fourth and fifth lens units respectively; f 5,2, f 5,4, and f 5,5 are the focal lengths of the second, fourth and fifth lenses in the fifth lens unit, and n 5,1 and n 5,6 are the Abbe numbers of the glasses of the first and sixth lenses in the fifth lens unit, respectively.