Abstract:
The present invention relates to a photographic lens capable of zooming by independently moving the front lens group and the rear lens group. The front lens group presents a negative meniscus lens, a biconcave lens and a positive lens, while the rear lens group presents two positive lenses, a positive meniscus lens, a biconcave lens and a positive lens. Let the focal length of the front lens group be Fl, the longest focal length of the total system be Ft and the distance between the secondary principal point of the front lens group and the primary principal point of the rear lens group when the focal length is Et, the zoom lens satisfies the following relations. ##EQU1##

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a photographic lens consisting of a divergent front lens group and a convergent rear lens group, more particularly to a zoom lens capable of altering the focal length by changing the distance between the front lens group and the rear lens group. 
     The zoom lens of this type has long been known, while it is only quite recently that a lens with an image quality capable of being brought into practice has been realized. 
     In accordance with this system, the zoom lens is always of inverted telephoto arrangement regardless of the relative position of the front lens group to the rear lens group, being arranged in such a manner that the distortion which is the most remarkable of the aberrations taking place at the wide angle side, while the arrangement is not profitable for the compensation of the spherical aberration and the coma at the telephoto side. Consequently, the spherical aberration and the coma have so far been compensated only by prolonging the total length of the lens system or increasing the number of the lenses composing the system. 
     The U.S. Pat. No. 3,848,969 discloses a zoom lens of two lens groups, whereby the front lens group and the rear lens group respectively consist of five single lenses. Further in case of this zoom lens, the front lens group moves linearly during the zooming operation, while the rear lens group moves non-linearly for compensation operation. Further, one of the lenses in the front lens group consists of lanthanum crown glass. 
     SUMMARY OF THE INVENTION 
     The first purpose of the present invention is to offer a zoom lens whose front lens group consists of a small number of lenses, while the superior image quality is maintained. 
     The second purpose of the present invention is to shorten the total length of the lens system. 
     The third purpose of the present invention is to decrease the number of the large diameter lenses in the front lens group so as to cut the weight of the system. 
     The fourth purpose of the present invention is to obtain a satisfactory compensation for various aberrations without using any lenses consisting of special glass such as lanthanum crown glass. 
     Hereby, the zoom lens in accordance with the present invention is the one so designed that the zooming is carried out by changing the distance between the divergent front lens group and the convergent rear lens group. Hereby, the front lens group consists of a negative meniscus lens with a plane concave toward the image, a negative lens and a positive lens in sequence from the object side, whereby the air lens formed between the second negative lens and the positive lens assumes a meniscus shape, while the rear lens group consists of three positive lenses, a negative lens and a positive lens from the object, whereby the planes of the three positive lenses are convex toward the object. Hereby, the following relations are established. ##EQU2## whereby fl: Focal length of the front lens group 
     Ft: Focal length of the total system at the end of the telephoto side 
     Et: Distance between the principal point of the front lens group and that of the rear lens group at the end of the telephoto side. 
     Further, the following relation is established. ##EQU3## whereby fa: Synthesized focal length of the first two positive lenses out of the three positive lenses at the object side in the rear lens group. 
     fb: Focal length of the third positive lens 
     In case of the embodiments to be explained later, the front lens group consists of a negative meniscus lens with a plane convex toward the forward direction, a biconcave lens the radius of curvature of whose front plane is larger than that of the rear plane and a positive lens with a plane convex along the forward direction. Hereby the axial distance between the negative meniscus lens and the biconcave lens is larger than that between the biconcave lens and the positive lens. 
     Further, the rear lens group consists of two positive lens, a positive meniscus lens with a plane convex along the forward direction, a biconcave lens and a positive lens. Hereby, in case of the first and the third embodiment, during the zooming operation the front lens group moves backwards as the focal length of the total system becomes longer, and returns a little forwards. Further, in case of the second embodiment, the front lens group moves backwards and stops at the inflection point. On the other hand, the rear lens group moves forwards substantially linearly as the focal length becomes longer. 
     Namely, the trace of the front lens group include an inflection point so that the zooming effect can remarkably increased as compared with the conventional zoom lens whose front lens group stops far before the inflection point. In consequently, in case of the same zooming ratio, the aberrations can be compensated much more easily. Hereby, in the present specification &#34;along the forward direction&#34; is used in the meaning of &#34;object side&#34;, while &#34;along the backward direction&#34; is used in the meaning of &#34;image side&#34;. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows the first embodiment of a lens in accordance with the present invention in section. 
     FIGS. 2A-2C respectively show various aberrations at the end of the wide angle side of the first embodiment. 
     FIGS. 2D-2F respectively show various aberrations in the middle of the first embodiment. 
     FIGS. 2G-2I respectively show various aberrations at the end of the telephoto side of the first embodiment. 
     FIG. 3 shows the second embodiment of a lens in accordance with the present invention in section. 
     FIGS. 4A-4C respectively show various aberrations at the end of the wide angle side of the second embodiment. 
     FIGS. 4D-4F respectively show various aberrations in the middle of the second embodiment. 
     FIGS. 4G-4I respectively show various aberrations at the end of the telephoto side of the second embodiment. 
     FIG. 5 shows the third embodiment of a lens in accordance with the present invention in section. 
     FIGS. 6A-6C respectively show various aberrations at the end of the wide angle lens of the third embodiment. 
     FIGS. 6D-6F respectively show various aberrations in the middle of the third embodiment. 
     FIGS. 6G-6I respectively show various aberrations at the end of the telephoto side of the third embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The afore mentioned relations (1) and (2) are the conditions for realizing a compact system with superior efficiency. 
     Beyond the upper limit value in accordance with relation (1), it is convenient for the compensation of the aberrations at the telephoto side, while the divergency of the front lens group becomes so weak that the distance between the front lens group and the rear lens group at the end of the wide angle side becomes large in such a manner that the diameter of the front lens becomes large, whereby the whole system becomes remarkably large. 
     On the other hand, beyond the lower limit value the divergency of the front lens group becomes so large that more varrel distortion takes place at the wide angle side beyond tolerance. Further, the distance between the front lens group and the rear lens group becomes small so that the total length of the lens system becomes small, whereby the back focal length becomes unnecessarily long so that the distance between the first lens plane and the image plane becomes large contrary to the expectation. Beyond the upper limit value in accordance with the relation (2) the largest diameter of the effective light beam of the rear lens group becomes too large to be applied for the large aperture, while the back focal length at the wide angle side becomes large beyond necessity and the total system becomes large. Beyond the lower limit value the distance between the front lens group and the rear lens group becomes negative and in accordance with the lens arrangement to be explained later the lenses are substantially in collision with each other, which means that the system is unrealizable. 
     Namely, the front lens group consists of a negative meniscus lens with a concave plane at the object side, a negative lens and a positive lens in sequence from the object side to the image side, whereby an air lens between the second negative lens and the following positive lens present a meniscus shape convex toward the object side, and the radius of curvature of the image side plane of the negative lens is larger than that of the object side plane, while the radius of curvature of the object side plane of the positive lens is larger than that of the image side. 
     Hereby, the shape of the negative meniscus lens is effective for reducing the barrel distorsion taking place at the wide angle side remarkably and compensating the astigmatism profitably. Further, the air lens formed between the negative lens and the positive lens serves to reduce the too much spherical aberration taking place mostly due to the front lens group particularly at the telephoto side. Further, the three lenses are shaped in such a manner that the second principal plane of the front lens group is as close to the image plane in order to prevent the collision of the front lens group with the rear lens group even when the both lens groups are closest to each other. 
     The rear lens group consists of three positive lenses, a negative lens and a positive lens in sequence from the object side, whereby the radius of curvature of the object side plane of each of the three positive lenses is larger than that of the image side plane, while the lenses are convex toward the object. 
     Hereby, the fact that all of the object side planes of the three positive lenses at the side of the object are convex toward the object, each having a large radius of curvature is effective for bringing the first principal plane of the rear lens group as close to the object as possible in order to prevent the collision of the lenses with each other in combination with the afore mentioned effect even when the distance between the principal plane of the front lens group and that of the rear lens group is small to some extent at the end of the telephoto side so as to realize a compact lens system as a whole. 
     Further, all over the zooming range, an under astigmatism is produced by means of the three positive lens and the closer to the telephoto side the more under spherical aberration takes place, whereby these aberrations are compensated by means of a negative lens. 
     In order to compensate the astigmatism and the spherical aberration properly at the same time at this time, the image side plane of the negative lens has to be at least a concave plane with a large radius of curvature. 
     The last positive lens in the rear lens group serves to keep a necessary amount of the back focal length at the end of the wide angle lens as well as a proper balance amoung the aberrations as a whole. 
     The above is the basic arrangement, whereby the further details are explained in accordance with the embodiment. In FIG. 1 and FIG. 5, I is a divergent front lens group, while II is a convergent rear lens group. During the zooming operation from the wide angle side toward the telephoto side the rear lens group (II) travels linearly from the image side to the object side, while the front lens group (I) travels from the object side to the image side and again along the direction toward the object at the telephoto side in order to keep the image plane at a certain determined position. The first lens group consists of a negative meniscus lens (R1, R2) with a concave plane toward the image side, an at both sides concave lens (R3, R4) the radius of curvature of whose object side plane is larger than that of the image side plane and a positive meniscus lens (R5, R6) convex toward the object side, the radius of whose image side plane is larger or an a both sides convex lens (R5, R6) in sequence from the object side. Hereby, R3 can be plane or slightly convex toward the object side. 
     Further, the rear lens group consists of an at both sides convex lens (R7, R8), two at object side convex positive meniscus lenses (R9, R10) (R11, R12), a plane-concave lens (R13, R14) the radius of curvature of whose object side plane is larger or a both side concave lens (R13, R14) and a both side convex lens (R15, R16) in sequence from the object side. 
     Hereby, even if the planes R8, R13 and R15 are otherwise shaped than in case of the embodiment, the aberration are hardly deteriorated. 
     Hereby, what is important as to the rear lens group is to strengthen the power of the first block consisting of the first and the second positive lens more than that of the third lens and to widen the distance between the block and the lens. Namely, let the synthesized focal length of the first and the second positive lens out of the rear lens group be fa and the focal length of the third positive lens be fb, the following relation is obtained. ##EQU4## 
     Namely, the lens system can be made compact while the efficiency can be raised by means of increasing the convergency of the lens closest to the object to some extent. 
     Beyond the above mentioned upper limit value, the first principal plane of the rear lens group can not assume the desired position in such a manner that the rear lens comes in collision with the front lens group at the end of the telephoto side. Further, beyond the lower limit value, an under astigmatism is produced by means of the two positive lenses in sequence from the object all over the zooming lens while closer to the telephoto side the more the under spherical aberration is produced, whereby in order to compensate these aberrations at the same time it is necessary to increase the divergency of the negative lens in the rear lens and the radius of curvature at the image side remarkably. As the result, many aberrations of higher harmonies are produced in such a manner that it becomes difficult to realize a large aperture and compensate the aberrations properly. Hereby, the diaphragm is provided between R10 and R11. Further as to the chromatic aberration it is better to improve that of the front lens group and that of the rear lens group independently. This is in order to prevent the alteration of the chromatic aberration due to the alteration of the focal length or the alteration of the object distance when the front lens group is advanced independently. 
     Consequently due to the convergent effect of the front lens group the first and the second negative lenses are composed of low dispersion glass while the positive lens is composed of high dispersion glass. Further, as to the rear lens group, due to the convergent effect it is desirable that the three positive lenses at the object side are composed of low dispersion glass, while the negative lens is composed of high dispersion glass. Namely, it is one of the solution to choose the Abbe&#39;s number of the two negative lenses in the front lens group and that of the three positive lenses at the object side to be larger than 45 and the Abbe&#39;s number of the positive lens in the front lens group and that of the negative lens in the rear lens group to be smaller than 40. 
     Further, when the afore mentioned conditions are all satisfied, it is possible to compose all of the lens system only with glass not including any rare element, still keeping good efficiency. Namely, as is applied in the embodiment to be explained later, the two negative lenses in the front lens group and the three positive lenses of the rear lens group at the object side consist of dense crown [dense barium crown] or extra dense crown [extra dense barium crown ] glass, the positive lens in the front lens group and the rear lens group consist of flint or dense flint glass and the last positive lens in the rear lens group consists of flint glass. 
     Being composed in a compact and a simple way, consisting of only glass stable in quality, low in price, not including any rare element, the whole lens system including the lens barrel becomes remarkably compact, superior in portability, low in price and convenient in mass production. Hereby, it goes without saying that the lens system can also be realized with glass including rare element. 
     In this way, as is in case of the embodiment, remarkably compact zoom lenses with high efficiency have been realized by means of eight lens composition not including any rare element in such a manner that the picture angle covers 62°-34.8° (respectively 35.5°), the F-number covers 3.5-4.5 and the total length (between the first plane and the picture plane) at the end of the wide angle side is only 8.3 times as long as the focal length at the end of the wide angle side. 
     EMBODIMENT 1 
     
         ______________________________________Focal length f = 100 - 190.3 FNo. = 4Picture angle FA = 62° - 35.05°                              AbbeRadius of   Lens thickness                    Refractive                              numbercurvature   &amp; Lens distance                    index (Nd)                              (Vd)______________________________________R1   101.799    D1     5.      N1 1.65844                                  V1 50.9R2   58.7639    D2     20.95R3   -2567.561  D3     3.61    N2 1.65844                                  V2 50.9R4   109.0964   D4     12.06R5   90.9147    D5     9.36    N3 1.7552                                  V3 27.5R6   185.0424   lR7   107.2556   D6     8.17    N4 1.60311                                  V4 60.7R8   -1095.825  D7     0.42R9   66.0308    D8     8.51    N5 1.60311                                  V5 60.7R10  180.8917   D9     6.67R11  83.1661    D10    12.61   N6 1.60311                                  V6 60.7R12  139.4522   D11    2.68R13  -463.8195  D12    5.12    N7 1.80518                                  V7 25.4R14  50.2922    D13    13.26R15  219.199    D14    8.83    N8 1.63636                                  V8 35.4R16  -100.9856______________________________________    l = 84.69 - 26.12 - 1.32    F1 = -161.11    Et = 40.27       (Distance between the first plane    Lw = 324.        and the image plane at the end                of the wide angle side)______________________________________ 
    
     EMBODIMENT 2 
     
         ______________________________________Focal length f = 100 - 191.7 FNo. = 3.5Picture angle FA = 62° - 34.8°                              AbbeRadius of   Lens thickness                    Refractive                              numbercurvature   &amp; Lens distance                    index (Nd)                              (Vd)______________________________________R1   103.566    D1     5.56    N1 1.65844                                  V1 50.9R2   62.8298    D2     29.11R3   -1473.732  D3     3.61    N2 1.63854                                  V2 55.4R4   171.7279   D4     5.71R5   89.5863    D5     10.56   N3 1.68893                                  V3 31.1R6   141.3738   lR7   113.3528   D6     7.5     N4 1.65844                                  V4 50.9R8   -1863.216  D7     0.42R9   60.8953    D8     9.17    N5 1.60311                                  V5 60.7R10  185.7648   D9     6.94R11  86.7785    D10    12.03   N6 1.60311                                  V6 60.7R12  124.0226   D11    2.92R13  -1419.585  D12    7.55    N7 1.80518                                  V7 25.4R14  45.2119    D13    15.67R15  144.819    D14    7.5     N8 1.63636                                  V8 35.4R16  -144.127______________________________________ l = 103.43 - 33.33 - 2.85 F1 = -191.7 Et = 27.78 Lw = 322 
    
     EMBODIMENT 3 
     
         ______________________________________Focal length f = 100 - 191.7 FNo. = 4.5Picture angle FA = 62° - 34.8°Radius of   Lens thickness                    Refractory                              Abbecurvature   &amp; Lens distance                    index (Nd)                              number______________________________________R1   164.8231   D1     5.56    N1 1.65844                                  V1 50.9R2   57.3466    D2     16.R3   -315.5245  D3     3.61    N2 1.65844                                  V2 50.9R4   119.4922   D4     12.34R5   102.9216   D5     9.72    N3 1.60342                                  V3 38.R6   -923.974   lR7   124.2773   D6     7.78    N4 1.65844                                  V4 50.9R8   861.739    D7     0.42R9   82.2475    D8     9.44    N5 1.60311                                  V5 60.7R10  197.0104   D9     7.22R11  73.5102    D10    8.55    N6 1.60311                                  V6 60.7R12  132.9075   D11    3.79R13  -437.221   D12    10.34   N7 1.80518                                  V7 25.4R14  57.8737    D13    8.57R15  618.1303   D14    7.5     N8 1.63636                                  V8 35.4R16  -85.2843______________________________________ l = 80.75 - 28.56 - 5.86 Fl = -138.9 Et = 55.56 Lw = 334 
    
     
         ______________________________________Lens Embodiment 1  Embodiment 2                          Embodiment 3______________________________________1    SSK           SSK         SSK2    SSK           SK          SSK3    SF            SF          F4    SK            SSK         SSK5    SK            SK          SK6    SK            SK          SK7    SF            SF          SF8    F             F           F______________________________________ SK dense crown SSK extra dense crown F flint SF dense flint 
    
     
         ______________________________________Embodiment 1    Embodiment 2                       Embodiment 3______________________________________|Fl|/Ft  0.847        1           0.725Et/Ft  0.212        0.145       0.29fb/fa  3.79         5.52        2.7______________________________________