Abstract:
The disclosed two-group wide-image-angle zoom lens is zoomed by changing the distance between the front and the rear group. The front group exhibits a negative refracting power and includes a first positive lens, a second negative lens concave toward the image end, a third positive lens, a fourth negative lens concave toward the image end, and a fifth positive image lens convex toward the object end, all in sequence; the rear group is closer to the image end than the front group, exhibits a positive refracting power, and includes a first positive sub-group, a second positive sub-group, a third negative sub-group, and a fourth positive sub-group.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a photographing lens, particularly a two group type zoom lens whose front lens group has a negative refractive power. 
     2. Description of the Prior Art 
     Zoom lenses whose front lens groups have a negative refractive power may be divided into two classes. In one class, the frontmost lens is positive as disclosed in U.S. Pat. Nos. 4,142,779, 4,147,410, and 4,169,660, and in the other class, the foremost lens is negative as disclosed in U.S. Pat. No. 3,848,969. 
     In such zoom lenses, the frontmost positive lens of the front group is intended to correct distortion. However, when the zoom ratio is large, and when the image angle in the wide angle position is wide, difficulties tend to arise in correcting distortion and astigmatic aberrations. This arises because in this type of lens, the incident height of the off-axis principal ray striking the positive lens is large, while the power of the air lens formed by the rear surface of the first lens and the front surface of the second lens is also large. Hence, when the distortion of the largest image angle in the wide angle position is adequately corrected, substantial barrel distortion occurs for a middle image angle. On the other hand, when the distortion for a middle image angle is adequately corrected, the astigmatic aberration at the largest image angle becomes too large to be corrected. 
     Furthermore, in such a zoom system, there is a tendency for the image angle more or less to increase at very close photographic distances. Hence, when the power of the aforementioned air lens is great, the angle at which the ray with the largest image angle emerges from the first lens at a position very close to the end of the wide angle position is abruptly increased. Thus, the diameter of the front lens must be quite large. This is a disadvantage in a compact lens. Moreover, in such a lens, the foremost lens is negative so that the position of the rear principal point of the front group is quite large from the rearmost face of the image end of the front group toward the object end. Therefore, it is necessary to provide a large principal point distance between the front group and the rear group at the extreme wide angle position. This results in a large total length that is inconvenient in a compact system. As a further point, the incident height of the principal ray away from the optical axis upon the lens face is small. Hence, it is difficult to correct the distortion at the end of the wide angle position, at which the largest image angle is as large as 84°. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to obtain a wide image angle at the short focal length. 
     Another object of the present invention is to make the whole system compact. 
     Further another object of the present invention is to correct the aberrations, particularly the distortion and the astigmatism aberration at the wide angle side. 
     In many cases of the zoom lens whose frontmost lens is positive, the frontmost lens and the rearmost lens in the front group are positive lenses, between which a plural number of negative lenses are arranged, while in accordance with the present invention a positive lens for correcting various aberrations is arranged between the negative lenses. 
     In the invention, a front group having a negative refracting power and a rear group having a positive refracting power are arranged in sequence from the object end. The front group consists of a first positive lens having a strong positive refracting power at the object side, a second negative meniscus lens having a strong negative refracting power at the image end, a third lens having a strong positive refracting power at the image end, a fourth bi-concave lens having a strong negative refracting power at the image end and a fifth positive lens having a strong positive refracting power at the object end in sequence from the object end, while the rear group consists of two positive lenses, a bi-concave lens and more than one positive lens in sequence from the object end. 
     Because the first lens and the third lens are in the front group, the positive lenses are used in the correction of distortion in the wide angle position. It is now possible to adequately correct the astigmatism, something which contributes much to the realization of the compact and highly efficient system. Specifically, by arranging two positive lenses it becomes possible to correct the aberrations without making the power of the air lens formed between the first and the second lens strong. Hence, the diameter of the front lens can be decreased while the distortion and the astigmatism aberration from the middle image angle to the largest image angle can effectively be corrected. 
     In the above, the relations: 
     
         f1≧1.2 f3                                           (1) 
    
     
         N3&lt;N4                                                      (2) ##EQU1## are established. Here; 
    
     ft is the focal length of the whole system at the end of the telephoto side. 
     f1 is the focal length of the first lens. 
     f3 is the focal length of the third lens. 
     N3 is the refracting power of the third lens (d-line). 
     N4 is the refracting power of the fourth lens (d-line). 
     r6 is the radius of curvature at the image side of the third lens in the front group. 
     r7 is the radius of curvature at the object side of the fourth bi-concave lens in the front group. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a section of the lens in accordance with a first embodiment. 
     FIG. 2 shows a section of the lens in accordance with a second embodiment. 
     FIG. 3 shows a section of the lens in accordance with a third embodiment. 
     FIG. 4 shows a section of the lens in accordance with a fourth embodiment. 
     FIG. 5 shows a section of the lens in accordance with a fifth embodiment. 
     FIG. 6 shows a section of the lens in accordance with a sixth embodiment. 
     FIGS. 7A-7C respectively show the aberrations of the first embodiment at the short focal length. 
     FIGS. 8A-8C respectively show the aberrations of the first embodiment at the middle focal length. 
     FIGS. 9A-9C respectively show the aberrations of the first embodiment at the long focal length. 
     FIGS. 10A-10C respectively show the aberrations of the second embodiment at the short focal length. 
     FIGS. 11A--11C respectively show the aberrations of the second embodiment at the middle focal length. 
     FIGS. 12A-12C respectively show the aberrations of the second embodiment at the long focal length. 
     FIGS. 13A-13C respectively show the aberrations of the third embodiment at the short focal length. 
     FIGS. 14A-14C respectively show the aberrations of the third embodiment at the middle focal length. 
     FIGS. 15A-15C respectively show the aberrations of the third embodiment at the long focal length. 
     FIGS. 16A-16C respectively show the aberrations of the fourth embodiment at the short focal length. 
     FIGS. 17A-17C respectively show the aberrations of the fourth embodiment at the middle focal length. 
     FIGS. 18A-18C respectively show the aberrations of the fourth embodiment at the long focal length. 
     FIGS. 19A-19C respectively show the aberrations of the fifth embodiment at the short focal length. 
     FIGS. 20A-20C respectively show the aberrations of the fifth embodiment at the middle focal length. 
     FIGS. 21A-21C respectively show the aberrations of the fifth embodiment at the long focal length. 
     FIGS. 22A-22C respectively show the aberrations of the sixth embodiment at the short focal length. 
     FIGS. 23A-23C respectively show the aberrations of the sixth embodiment at the middle focal length. 
     FIGS. 24A-24C respectively show the aberrations of the sixth embodiment at the long focal length. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In case of the lenses shown in FIGS. 1 to 6, the front group corresponds to the lens surfaces r1 to r10, while the rear group corresponds to the lens surfaces r11 to r20 respectively r11 to r22. 
     At the time of the zooming operation the front group and the rear group are moved at the same time, and the distance between the front and the rear group is varied in order to change the focal length of the whole system. The position of the image surface is mechanically compensated. The focussing is adjusted by moving the front group. 
     Below, the meaning of the conditions (1) to (3) will be explained. 
     With reference to condition (1), the distortion correction effect for the first and third lens in the front group is larger in the wide angle condition as explained above, so that the incident height of the ray, distant from the optical axis, is larger upon the first lens than upon the third lens. 
     Consequently, even if the power of the first lens is selected weaker than that of the third lens, the equivalent correction effect can be obtained. Hence, unless the condition should be fulfilled, the power of the first lens would be so strong that when the distortion at the largest image angle at the end of the wide angle range is properly corrected, much barrel distortion remains at the middle image angle. On the other hand, when the distortion at the middle image angle is properly corrected, the astigmatic aberration at the largest image angle is quite large so that it is difficult to correct the distortion and the astigmatic aberration simultaneously. 
     Conditions (2) and (3) relate to the air lens formed with the third and the fourth lenses. The air lens has the effect of correcting the distortion by means of the difference between the incident height of the ray distant from the optical axis upon the surface r6 and that upon the surface r7. In order to obtain a difference between the incident height of the ray distant from the optical axis upon the r6 surface and that upon the r7 surface, it is possible to think of a method for making the radius of curvature of the r6 surface almost equal to that of the r7 surface and increasing the air gap between the r6 surface and the r7 surface. However, this is quite disadvantageous for realizing the compact system because of an increase of the lens diameter and the whole length. Also, the astigmatic aberration at the middle of the picture is quite large, beyond correction. This is also disadvantageous. Consequently, in accordance with the present invention, the distance between the lens surface along the optical axis is not made large but the radius of curvature of the r6 surface is made different from that of the r7 surface in order to obtain a difference between the incident height of the rays distant from the optical axis. In order to meet the above purpose, the condition r7&gt;r6&gt;0 is used at r6 and r7. Thus, if the condition (2) is not fulfilled due to the strongly convex surface of the r6 surface the ray most distant from the optical axis is apt to be totally reflected. Further, the condition (2) is also essential in order to keep the field curvature with the front group by keeping the negative Petzval&#39;s sum as small as possible. Beyond the upper limit of the condition (3) the positive power of the air lens is so strong that the astigmatic aberration at the end of the wide angle side is under corrected. Beyond the lower limit the barrel distortion at the middle image angle or the astigmatic aberration at the largest image angle is so great that the distortion or the aberration is under corrected to the point that it becomes difficult to adequately correct the astigmatic aberration and the distortion at the end of the wide angle range. 
     Now, let us suppose that the front group consists of a positive, a negative, a positive, a negative and a positive lens in sequence and the focal lengths of the first to the fifth lens are f1, f2, f3, f4 and f5. When the relation f1&gt;f3&gt;f5 is applied to the positive lenses, the relation f2&gt;f4 is applied to the negative lenses, the power of the lenses upon which the ray distant from the optical axis is incident at large height is selected strong and that of the lens upon which the ray distant from the optical axis is incident at small height is selected weak, the ray distant from the optical axis is not refracted very strongly by any lens so that the ray distant from the optical axis is refracted smoothly by the first to the last lens in such a manner that the aberration distant from the optical axis can be made small. 
     Further, the rear group is a variation of the triplet lens, so that positive lenses with strong power are arranged before negative lenses so as to position the front principal point as close to the front group as possible in order to realize a compact system on a practical basis. 
     The above construction fulfills basic conditions. By fulfilling further conditions a much higher efficiency can be obtained. 
     The rear group is formed of a sixth positive lens having a strong positive refractive force at the object side, a seventh meniscus lens group having one or two positive meniscus lenses convex toward the object, an eighth negative lens having a strong negative force at the image side and a ninth positive lens group having one or two positive lenses. Now, let the refractive indices (d-line) of the first, the second, the fifth, the sixth and the eighth lens be N1, N2, N5, N6 and N8, have mean values of the refractive indices of the seventh, and the ninth lens group be N7, N9, the Abbe&#39;s number of the sixth and the eighth lens be ν5, ν6 and ν8 and the mean value of the Abbe&#39;s number of the seventh lens be ν7, so the following conditions are fulfilled: ##EQU2## 
     The relation (4) is essential in order to keep the Petzval&#39;s sum small. If this relation is not fulfilled, the distortion on the image surface becomes too large. Further, although the front group has as a whole a negative power, the number of the positive lenses is larger than that of the negative lenses, so that unless the refractive index of the negative lenses should be increased so as to increase the radius of curvature of the surfaces of the negative lenses aberrations of a higher degree would take place, which makes the smooth correction difficult. 
     The relations (5) and (6) are essential in order to obtain a small Petzval&#39;s sum as a whole by correcting the positive Petzval&#39;s sum of the rear group with the negative Petzval&#39;s sum of the front group. Beyond the upper limit of the relation (5) and/or (6) the distortion on the image surface becomes too small. As far as the ordinary optical glass is concerned, a proper combination of glasses is difficult under the conditions (8) and (9). Further, beyond the lower limit the positive Petzval&#39;s sum of the rear group becomes so small that the Petzval&#39;s sum of the whole system becomes negative. 
     The relation (7) is essential in order to adequately correct the chromatic aberration along the optical axis of the front group by making the fifth lens with glass material of high dispersion. Although it is effective to make the positive lenses with glass material of high dispersion in order to correct the over-corrected chromatic aberration taking place in the front group, if the first and the third lens are made of glass material with very high dispersion chromatic aberration of magnification is apt to take place. This is disadvantageous. Further, when the relation is not fulfilled the variation of the chromatic aberration due to zooming and focussing becomes very large, beyond the permitted value. 
     The relations (8) and (9) concern to the correction of the chromatic aberration in the rear group. It is desirable to place the positive lens group closer to the object than the eighth negative lens with glass material with low dispersion because the power of the lens group is strong and the eighth negative lens is with a glass material with high dispersion. If the relations are not fulfilled, the under-corrected chromatic aberration (short wave length, for example g-line) takes place in the near group. Even if this under-correction is cancelled with the over-corrected chromatic aberration taking place in the front group, the under-correction is cancelled only at one position during the zooming so that the variation of the chromatic aberration due to the zooming becomes large. 
     As explained above, in accordance with the present invention compact, high efficiency wide angle zoom lenses with zoom ratio of about 2 at the image angle about 84° at the end of the wide angle side, which has so far been considered to be difficult to realize, can be realized as in case of the embodiments. 
     The lens compositions are shown in FIGS. 1 to 6. The spherical aberration, the Sine condition, the astigmatic aberration and the distortion at the wide angle side, at the middle and at the telephoto side for an object at infinitive distance are respectively shown in FIGS. 7 to 9, FIGS. 10 to 12, FIGS. 13 to 15, FIGS. 16 to 18, FIGS. 19 to 21 and FIGS. 22 to 24. 
     EXAMPLE 1 
     
         ______________________________________Focal Length f = 100-198                F.No. = 3.5Image Angle 2ω = 84°-47°Radius of Curvature           Surface   Refractive DispersionNo.  r              Distance d                         Index n  ν______________________________________ 1   1761.568       10.51     1.6228   572    -4909.441      0.43    413.75         9.68      1.72342  384    102.351        40.775    -617.581       14.44     1.61293  376    -204.96        1.367    -265.984       7.26      1.7725   49.68    128.702        14.849    146.573        20.24     1.6668   3310   -837.348       d1011   198.573        12.95     1.51633  64.112   -588.064       8.0613   104.194        13.63     1.51633  64.114   390.064        0.415   96.625         22.14     1.51633  64.116   220.726        11.1317   -503.407       15.18     1.80518  25.418   72.823         7.1419   292.117        11.05     1.62606  39.120   -117.764Focal Length f   100       140      198Variable Distance d10            134.13    57.39    4.09______________________________________ 
    
     EXAMPLE 2 
     
         ______________________________________Focal Length f = 100-198                F.No. = 3.5Image Angel 2ω = 84°-47°Radius of Curvature           Surface   Refractive DispersionNo.  r              Distance d                         Index n  ν______________________________________ 1   473.75         22.06     1.6668   332    -3737.097      0.43    4028.226       9.92      1.72342  384    109.355        47.545    -1378.871      17.30     1.60323  42.36    -234.516       1.217    -410.992       7.78      1.7725   49.68    134.052        15.189    144.407        22.67     1.6727   32.110   3193.096       d1011   213.786        8.19      1.51633  64.112   1546.169       0.4313   113.71         27.18     1.51633  64.114   1069.274       0.415   89.435         15.08     1.51633  64.116   766.573        13.3917   -368.048       18.23     1.71736  29.518   67.323         13.3119   -688.911       8.63      1.62041  60.320   -135.04        0.421   175.766        8.63      1.62374  47.122   618.77Focal Length f   100       140      198Variable Distance d10            144.45    61.64    4.11______________________________________ 
    
     EXAMPLE 3 
     
         ______________________________________Focal Length f = 100-198                F.No. = 3.5Image Angle 2ω = 84°-47°Radius of Curvature           Surface   Refractive DispersionNo.  r              Distance d                         Index n  ν______________________________________ 1   1060.363       19.03     1.6668   332    -2827.419      0.373    322.581        8.57      1.72342  384    85.14          39.235    2120.887       14.92     1.60323  42.36    -285.242       1.127    -425.887       6.69      1.7725   49.68    129.194        13.359    128.427        19.56     1.78472  25.710   423.456        d1011   202.379        11.96     1.63854  55.412   -1137.621      7.6913   99.129         15.43     1.51633  64.114   323.891        0.3915   85.242         20.16     1.51633  64.116   260.564        8.4717   -795.847       6.45      1.80518  25.418   66.641         11.8619   237.198        21.37     1.60323  42.320   -124.515Focal Length f   100       140      198Variable Distance d10            118.66    51.84    5.43______________________________________ 
    
     EXAMPLE 4 
     
         ______________________________________Focal Length f = 100-198                F.No. = 3.5Image Angle 2ω = 84°-47°Radius of Curvature           Surface   Refractive DispersionNo.  r              Distance d                         Index n  ν______________________________________ 1   611.5508       19.03     1.66680  332    -3225.81       0.373    685.484        8.57      1.72342  384    83.8133        51.135    16791.25       14.9      1.60323  42.36    -233.2258      1.137    -255.605       6.69      1.7725   49.68    148.456        4.039    127.806        19.56     1.6727   32.110   -4849.38       d1011   195.685        11.98     1.63854  55.412   -728.024       7.7113   98.956         15.43     1.51633  64.114   348.297        0.3915   94.968         20.16     1.51633  64.116   189.552        7.0217   -866.323       15.1      1.80518  25.418   69.419         9.0719   193.996        21.37     1.60323  42.320   -139.032Focal Length f   100       140      198Variable Distance d10            117.3     50.48    4.07______________________________________ 
    
     EXAMPLE 5 
     
         ______________________________________Focal Length f = 100-198                F.No. = 3.5Image Angle 2ω = 84°-47°Radius of Curvature           Surface   Refractive DispersionNo.  r              Distance d                         Index n  ν______________________________________1    711.279        22.07     1.6668   332    -3737.096      0.443    794.133        9.93      1.72342  384    114.294        42.895    1230.661       17.26     1.60323  42.36    -366.423       1.297    -1178.951      7.78      1.7725   49.68    131.815        32.269    154.446        22.66     1.6727   32.110   448.812        d1011   212.245        8.19      1.51633  64.112   -5424.395      0.413   113.742        27.18     1.51633  64.114   879.355        0.4315   95.464         15.08     1.51633  64.116   798.145        13.3717   -426.774       21.90     1.71736  29.518   64.919         13.7919   -820.318       8.63      1.62041  60.320   -143.238       0.4321   163.177        8.63      1.62374  47.122   437.844Focal Length f   100       140      198Variable Distance d10            143.84    61.03    3.51______________________________________ 
    
     EXAMPLE 6 
     
         ______________________________________Focal Length f = 100-195                F.No. = 3.5Image Angle 2ω = 82.2°-48.2°Radius of Curvature           Surface   Refractive DispersionNo.  r              Distance d                         Index n  ν______________________________________ 1   598.981        16.421    1.60311  60.72    4138.586       0.4033    271.278        9.274     1.6968   55.54    83.54          34.35    1499.682       14.538    1.55963  61.26    -349.565       3.8917    -425.787       7.258     1.7725   49.68    128.512        16.7819    129.962        13.784    1.7552   27.510   371.853        d1011   191.133        12.517    1.6968   55.512   -494.492       8.06413   90.5           31.428    1.65844  50.914   1206.049       7.19715   -224.771       12.097    1.80518  25.416   82.093         13.12417   -6882.406      10.081    1.53172  48.918   -162.768       0.60519   2151.097       12.097    1.53172  48.920   -118.965Focal Length f   100       141      195Variable Distance d10            105.58    44.64    3.951______________________________________