Abstract:
A compact zoom lens of high magnification comprises four lens groups. In succession from the object side, the first lens group has a positive refractive power and a focusing function, the second lens group has a negative refractive power and a magnification changing function and consists, in succession from the object side, of a negative lens having its concave surface of sharper curvature facing the image side, a negative doublet lens consisting of a biconcave lens and a positive lens, and a negative lens having its concave surface of sharper curvature facing the object side, the third lens group has a positive refractive power and is reciprocable to correct the movement of the image point resulting from zooming, and the fourth lens group has a negative refractive power. Correction of aberrations is well accomplished by suitably selecting the focal length of the doublet lens in the second lens group and the curvature radii of the third, fourth and sixth lens surfaces in the second lens group as viewed from the object side.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a compact photographic zoom lens of high magnification for a 35 mm film still camera. 
     2. Description of the Prior Art 
     A zoom lens can freely change the magnification of an object without interchanging the lens. That is, it can photograph an object while changing the object to a predetermined dimension on the spot in accordance with the photographer&#39;s intention. Therefore, zoom lenses have become more and more widely used. Usually, zoom lenses having a magnification change ratio of 5 to 10 times are used with 8 mm cinecameras, and zoom lenses having a magnification change ratio of 10 to 30 times are used with television cameras. In the case of still cameras having great picture plane dimensions and chiefly photographing stationary objects, as compared with 8 mm cameras or television cameras having small picture dimensions and chiefly photographing moving objects, an optically higher performance is required and it is therefore difficult to obtain zoom lenses of a great magnification change ratio. This is because it is difficult to accomplish well correction of aberrations in zoom lenses of great magnification change ratio for still cameras. Particularly, it is often the case that the quality of photographs by a zoom lens in which only the magnification change ratio is made great, is remarkably lower than the quality of photographs by a lens of a single focal length, and an effort to make the zoom lens compact in order to improve the operability thereof often results in a disadvantage that aberration fluctuation becomes great. Compact photographic zoom lenses of high magnification improved to overcome the above-noted disadvantages have been proposed in U.S. Pat. No. 4,189,213 and Japanese Patent Publication No. 2941/1971. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a high performance zoom lens in which the magnification change ratio from the standard angle of view to the long telephoto is great when use is made of film of a size of the order of 35 mm and whose total length is shorter than the focal length of the telephoto side end. In its specific embodiments which will hereinafter be described, the present invention achieves a zoom lens whose magnification changes from a focal length of 50 mm to a focal length of 300 mm as a lens for a 35 mm single lens reflex camera. 
     In recent years, there has been a strong desire for compactness of photographic lenses. To make a zoom lens compact, there are two methods, namely, a method of shortening the magnification changing portion thereof and a method of shortening the relay portion thereof. The present invention has succeeded in making the lens compact while minimizing the fluctuations of aberrations occurring during magnification change by using an appropriate lens form as the magnification changing portion in the former method. 
     The invention will become more fully apparent from the following detailed description thereof taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of the lens according to an embodiment of the present invention. 
     FIGS. 2A, 2B, 2C and 2D to FIGS. 4A, 4B, 4C and 4D show the various aberrations in Embodiment 1 of the present invention. 
     FIGS. 5A, 5B, 5C and 5D to FIGS. 7A, 7B, 7C and 7D show the various aberrations in Embodiment 2 of the present invention. 
     FIGS. 8A, 8B, 8C and 8D to FIGS. 10A, 10B, 10C show the various aberrations in Embodiment 3 of the present invention. 
     FIGS. 11A, 11B, 11C and 11D to FIGS. 13A, 13B, 13C and 13D show the various aberrations in Embodiment 4  of the present invention. 
     FIG. 14 shows the chromatic aberrations in Embodiments 1 and 4. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention comprises, in succession from the object side, a positive first lens group, a negative second lens group, a positive third lens group and a negative fourth lens group. The first lens group has a focusing function, the second lens group has the function of effecting magnification change by moving on the optical axis, and the third lens group is reciprocable to correct the movement of the image plane resulting from the movement of the second lens group. The second lens group has, in succession from the object side, a negative lens having its concave surface of sharper curvature facing the image side, a negative doublet lens consisting of a biconcave lens and a positive lens, and a negative lens having its concave surface of sharper curvature facing the object side. The third lens group has, in succession from the object side, a biconvex lens, a doublet lens consisting of a negative meniscus lens and a biconvex lens, and a positive lens having its convex surface of sharper curvature facing the object side. The lens system satisfies one or more of the following conditions: 
     (1) 0&lt;Fw/|F II .sbsb.2 |&lt;0.1, but F II .sbsb.2 &lt;0 
     (2) 0.85&lt;R II .sbsb.4 /Fw&lt;1.2 
     (3) 0.2&lt;R II .sbsb.6 /R II .sbsb.3 &lt;0.6 
     (4) 0.08&lt;Fw/F III .sbsb.2 &lt;0.35 
     (5) 1.0&lt;|R III .sbsb.2 |/R III .sbsb.1 &lt;1.6, but R III .sbsb.2 &lt;0 ##EQU1## (7) 38&lt;ν III .sbsb.3 -ν III .sbsb.2 &lt;60 where F II .sbsb.2 represents the focal length of the doublet lens in the second lens group, R II .sbsb.i represents the curvature radius of the ith lens surface of the second lens group, F III .sbsb.2 represents the focal length of the doublet lens in the third lens group, R III .sbsb.j represents the curvature radius of the jth lens surface of the third lens group, N III .sbsb.j represents the refractive index of the glass of the jth lens in the third lens group for d-line, ν III .sbsb.j represents the Abbe number of the glass of the jth lens in the third lens group, and Fw represents the focal length of the wide angle end (short focal length) of the entire lens system. 
     The meaning of each condition will now be described. 
     To make the magnification changing portion compact, the power of the second lens group is made greater than the distribution usually adopted by a zoom lens of this type. Generally, if the power is made greater, the aberration fluctuation accompanying the movement in the direction of the optical axis for magnification change is liable to become great. 
     Therefore, the problem of aggravated aberrations has been solved by forming the second lens group by three groups and setting up condition (1) in the second group. If the lower limit value of this condition is exceeded, the negative distortion will be remarkably increased on the focal length side of the wide angle and, if the upper limit value of this condition is exceeded, the high order aberrations such as coma and astigmatism will be remarkably created. 
     The cemented surface of the second group in the second lens group contributes to the correction of spherical aberration and on-axis chromatic aberration and provides an optimal aberration correction by satisfying condition (2). If the upper limit value of this condition is exceeded, correction of spherical aberration and on-axis chromatic aberration will become difficult and, if the lower limit value of this condition is exceeded, inwardly directed coma and high order chromatic difference of magnification will be remarkably created in the focal length on the wide angle side. 
     On the other hand, by effecting an appropriate power distribution in the second lens group, the spherical aberration which is liable to be over-corrected in the focal length on the telephoto side can be appropriately corrected and this is realized by condition (3). If the upper limit value of this condition is exceeded, spherical aberration will be remarkably over-corrected in the focal length on the telephoto side and, if the lower limit value of this condition is exceeded, the fluctuation of coma and chromatic difference of magnification accompanying the magnification change will become remarkable. The third lens group reciprocates on the optical axis to correct the movement of the image plane accompanying the magnification change and therefore, like the second lens group, it may cause creation of aberration fluctuation. Moreover, the power of the third lens group has been made great to make the lens system compact and this has led to a form in which aberration fluctuation is more liable to occur. 
     Therefore, the aberration fluctuation is suppressed by forming the third lens group by three groups to divide the power thereof and satisfying conditions (4), (5), (6) and (7). 
     Condition (4) divides the power without causing curvature of image field to be created and, if the upper limit value of this condition is exceeded, the image plane will fall down in the negative direction and, if the lower limit value of this condition is exceeded, the meaning of power division will become lessened and the fluctuation of spherical aberration, etc. accompanying the magnification change will become remarkable. 
     Condition (5) by balancing corrects the fluctuations of spherical aberration and astigmatism accompanying the magnification change and, if the upper limit value of this condition is exceeded, the fluctuation of spherical aberration will be aggravated and, if the lower limit value of this condition is exceeded, the fluctuation of astigmatism will be aggravated. 
     Condition (6) effectively corrects spherical aberration by making the cemented surface into a convex surface facing the object side and by providing that surface with a strong negative power without causing creation of coma and, if the upper limit value of this condition is exceeded, spherical aberration will be over-corrected and, if the lower limit value of this condition is exceeded, spherical aberration will be under-corrected. Condition (7) corrects the on-axis chromatic aberration and, if the lower limit value of this condition is exceeded, chromatic aberration will be remarkably created and, if the upper limit value of this condition is exceeded, chromatic aberration will be over-corrected. What offers a problem particularly in a case where the optical performance of a telephoto lens having a relatively long focal length is discussed is a secondary spectrum. In the zoom lens of the present invention as well, the focal length is considerably great on the telephoto side and therefore, the secondary spectrum must be taken into consideration. Especially, the first, second and third lens groups are moved separately on the optical axis and therefore, the aberrations in these lens groups are independently minimized to minimize the aberration fluctuation accompanying the magnification change and as also regards the secondary spectrum, similar independent aberration correction is necessary. As is well-known, an effective method of correcting the secondary spectrum is to use abnormal dispersion glass as the first lens group of a zoom lens. 
     In addition to this, to cope with the task of minimizing the fluctuation of the secondary spectrum, the present invention has succeeded in correcting the secondary spectrum on the telephoto side as well as in each focal length by using abnormal dispersion glass as the third lens group having a positive power. 
     The condition therefore is 
     -0.0023&lt;(θ III .sbsb.3 -θ III .sbsb.2)/(ν III .sbsb.3 -ν III .sbsb.2)&lt;0 
     where θ III .sbsb.j is the partial dispersion ratio of the jth lens in the third lens group. If the lower limit value of this condition is exceeded, the secondary spectrum on the telephoto side will of course be increased and the fluctuation of chromatic aberration accompanying magnification change will become great. In FIG. 6, the difference in fluctuation of on-axis chromatic aberration is shown, and A and B in the Figure correspond to Embodiments 1 and 4, respectively. A is nearer to the lower limit value of this condition and is greater in fluctuation than B. 
     Embodiments of the present invention will hereinafter be described, and a cross-section of the lens of Embodiment 1 is shown in FIG. 1, and the spherical aberration, astigmatism, distortion and chromatic difference of magnification in Embodiments 1, 2, 3 and 4 are shown in FIGS. 2A, 2B and 2C to FIGS. 5A, 5B and 5C. In FIG. 1, I designates the first lens group, II designates the second lens group, III designates the third lens group and IV designates the fourth lens group. In FIGS. 2 to 5, A shows the longitudinal aberration of the wide angle side with respect to an object at infinity, B shows the longitudinal aberration of the intermediate side with respect to an object at infinity, and B shows the longitudinal aberration of the telephoto side with respect to an object at infinity. 
     In the numerical data shown below, Ri represents the curvature radius of each lens surface, Di represents the thickness or air space of each lens, nd represents the refractive index of the material of each lens for d-line, and νd represents the Abbe number of the material of each lens for d-line. 
     NUMERICAL DATA OF EMBODIMENT 1 
     
         ______________________________________f = 51.3 ˜ 292.8 F-number 1: 4.5   R          D          nd      νd______________________________________1       544.953    4.00       1.83400 37.22       114.136    13.17      1.49700 81.63       -288.850   0.20       1.4       106.829    8.54       1.56873 63.15       -8731.945  D.sub.5    1.6       428.922    2.20       1.77250 49.67       60.279     5.04       1.8       -213.914   2.00       1.71300 53.99       51.823     6.22       1.80518 25.410      -778.386   2.49       1.11      -73.697    2.00       1.69680 55.512      -846.906   D.sub.12   1.13      99.436     3.72       1.56873 63.114      -151.646   0.15       1.15      93.467     1.70       1.80518 25.416      41.434     6.06       1.48749 70.117      -190.628   0.15       1.18      77.335     3.05       1.56873 63.119      -8794.965  D.sub.19   1.20      -50.443    1.50       1.60311 60.721      153.228    0.20       1.22      21.649     2.99       1.75520 27.523      38.504     1.01       1.24      93.360     1.40       1.69680 55.525      22.509     9.03       1.26      -492.983   1.50       1.67270 32.127      38.003     4.98       1.58913 61.128      -34.457               1.______________________________________b.f. = 98.96 Total length 287.85The diaphragm surface lies 4.29 ahead of the 20th surface.f       51.3          122.4   292.8______________________________________D.sub. 5   1.64          44.14   86.64D.sub.12   96.97         43.99   1.73D.sub.19   6.99          17.46   17.22______________________________________ 
    
     NUMERICAL DATA OF EMBODIMENT 2 
     
         ______________________________________f = 51.3 ˜ 292.8 F-number 1: 4.5   R          D          nd      νd______________________________________1       496.402    4.00       1.83400 37.22       114.938    13.23      1.49700 81.63       -302.726   0.20       1.4       108.573    8.76       1.56873 63.15       -7840.922  D.sub.5    1.6       1304.506   2.20       1.77250 49.67       59.272     4.42       1.8       -260.092   2.00       1.71300 53.99       47.500     5.50       1.80518 25.410      800.305    2.91       1.11      -65.811    2.00       1.69680 55.512      -155.709   D.sub.12   1.13      115.000    3.53       1.56873 63.114      -125.000   0.15       1.15      87.070     1.70       1.80518 25.416      40.377     5.34       1.48749 70.117      -839.975   0.15       1.18      75.463     3.33       1.56873 63.119      -417.685   D.sub.1920      -49.421    1.50       1.60311 60.721      153.254    0.20       1.22      21.647     2.80       1.75520 27.523      38.616     1.22       1.24      92.102     1.40       1.69680 55.525      22.530     9.05       1.26      -539.261   1.50       1.67270 32.127      38.242     5.02       1.58913 61.128      -34.376               1.______________________________________b.f. = 100.04 Total length 287.89The diaphragm surface lies 4.2 ahead of the 20th surface.f       51.3          122.4   292.8______________________________________D.sub.5 2.44          44.94   87.44D.sub.12   96.38         43.4    1.14D.sub.19   6.9           17.37   17.13______________________________________ 
    
     NUMERICAL DATA OF EMBODIMENT 3 
     
         ______________________________________f = 51.3 ˜ 292.8 F-number 1: 4.5 R             D         nd      νd______________________________________1     527.100       4.00      1.83400 37.22     114.444       13.21     1.49700 81.63     -294.039      0.20      1.4     106.635       8.52      1.56873 63.15     -0.45E + 05   D.sub.5   1.6     324.033       2.20      1.77250 49.67     59.094        5.57      1.8     -153.555      2.00      1.69680 55.59     57.763        6.05      1.80518 25.410    -397.846      2.21      1.11    -75.115       2.00      1.69680 55.512    -1633.633     D.sub.12  1.13    100.716       3.69      1.56873 63.114    -152.367      0.15      1.15    95.402        1.70      1.80518 25.416    41.641        5.95      1.48749 70.117    -225.502      0.15      1.18    78.911        3.26      1.56873 63.119    -660.382      D.sub.19  1.20    -51.571       1.50      1.60311 60.721    154.008       0.20      1.22    21.617        2.99      1.75520 27.523    38.570        1.10      1.24    90.343        1.40      1.71300 53.925    22.500        9.53      1.26    -625.395      1.50      1.67270 32.127    38.971        4.93      1.58913 61.128    -35.315                 1.______________________________________b.f. = 98.15 Total length 287.86The diaphragm surface lies 4.32 ahead of the 20th surface.f       51.3          122.4    292.8______________________________________D.sub.5 1.91          44.41   86.91D.sub.12   96.77         43.8    1.54D.sub.19   7.02          17.49   17.25______________________________________ 
    
     NUMERICAL DATA OF EMBODIMENT 4 
     
         ______________________________________f = 51.3 ˜ 292.8 F-number 1: 4.5   R          D          nd      νd______________________________________1       554.732    4.00       1.83400 37.22       113.716    13.25      1.49700 81.63       -287.872   0.20       1.4       106.765    8.58       1.56873 63.15       -6235.324  D.sub.5    1.6       527.828    2.20       1.77250 49.67       60.733     4.93       1.8       -231.597   2.00       1.69680 55.59       51.726     6.28       1.80518 25.410      -1012.080  2.62       1.11      -75.605    2.00       1.69680 55.512      -1546.937  D.sub.12   1.13      116.652    3.51       1.56873 63.114      -158.719   0.15       1.15      85.589     1.70       1.80518 25.416      42.120     6.09       1.49700 81.617      -178.919   0.15       1.18      78.817     3.13       1.51633 64.119      -1026.131  D.sub.19   1.20      -54.921    1.30       1.69680 55.521      209.218    0.20       1.22      21.900     3.14       1.75520 27.523      39.727     1.15       1.24      92.936     1.50       1.69680 55.525      22.391     9.55       1.26      -872.268   1.30       1.67270 32.127      38.896     4.92       1.58913 61.128      -35.733               1.______________________________________b.f. = 98.49 Total length 287.87The diaphragm surface lies 4.19 ahead of the 20th surface.f       51.3          122.4   292.8______________________________________D.sub. 5   1.69          44.19   86.69D.sub.12   96.95         43.98   1.72D.sub.19   6.89          17.36   17.12______________________________________ 
    
     TERTIARY ABERRATION COEFFICIENT 
     Values when f=51.3 of Embodiment 1 has been regularized to 1. 
     
         ______________________________________SA          CM       AS       PT      DS______________________________________1     0.0002    0.0019   0.0174 0.0428  0.55332     -0.0306   0.0014   -0.0001                           -0.0552 0.00263     0.0019    -0.0119  0.0749 0.0590  -0.84154     0.0226    0.0202   0.0181 0.1741  0.17205     0.0157    -0.0707  0.3189 0.0021  -1.44816     -0.0054   0.0369   -0.2521                           0.0521  1.36557     -0.3985   0.0295   -0.0022                           -0.3709 0.02768     0.000     0.0010   0.1045 -0.0998 0.47569     0.1827    -0.0818  0.0367 0.0295  -0.029610    -0.0071   -0.0284  -0.1136                           0.0294  -0.337311    0.0006    -0.0017  0.0050 -0.2859 0.841812    -0.2084   -0.1365  -0.0894                           0.0249  -0.042313    6.7102    1.1157   0.1855 0.1870  0.061914    1.7593    -0.6397  0.2326 0.1226  -0.129215    0.0298    0.0131   0.0057 0.2448  0.109716    -7.0869   -0.5480  -0.0424                           -0.1465 -0.014617    3.3373    -1.0040  0.3021 0.0882  -0.117418    -0.1726   -0.1078  -0.0674                           0.2405  0.108219    7.6413    -1.9665  0.5061 0.0021  -0.130820    -46.7429  5.1241   -0.5617                           -0.3826 0.103521    -0.0524   -0.0480  -0.0441                           -0.1260 -0.155922    47.7728   6.2441   0.8161 1.0196  0.239923    0.5654    0.4473   0.3538 -0.5733 -0.173624    -1.1404   0.7176   -0.4516                           0.2256  0.142225    -45.0060  -7.4711  -0.2402                           -0.9359 -0.361226    0.0077    0.0276   0.0991 -0.0418 0.205627    -4.3545   -0.9221  -0.1953                           -0.0424 -0.050328    36.8402   -0.6334  0.0109 0.5519  -0.0097Total -0.3178   0.1085   0.0275 0.0361  0.5679______________________________________ 
    
     SA . . . Spherical aberration coefficient 
     CM . . . Coma coefficient 
     AS . . . Astigmatism coefficient 
     PT . . . Petzval sum 
     DS . . . Distortion coefficient 
     Embodiment 1   f=122.4 
     
         ______________________________________SA          CM       AS       PT      DS______________________________________1     0.0067    0.0084   0.0106 0.0428  0.06732     -0.9932   0.3592   -0.1299                           -0.0552 0.06693     0.0614    -0.0895  0.1305 0.0590  -0.27614     0.7314    -0.1435  0.0282 0.1741  -0.03975     0.5084    -0.5823  0.6670 0.0021  -0.76656     -0.1844   0.2762   -0.4136                           0.0521  0.54127     -2.6534   -0.0406  -0.0006                           -0.3709 -0.00578     0.0030    -0.0117  0.0463 -0.0998 0.21149     1.4531    -0.1795  0.0222 0.0295  -0.006410    -0.0107   -0.0300  -0.0845                           0.0294  -0.155211    -0.1663   0.1404   -0.1185                           -0.2859 0.341312    -1.1905   -0.3818  -0.1225                           0.0249  -0.031313    16.8042   1.2608   0.0946 0.1870  0.021114    1.3940    -0.5174  0.1920 0.1226  -0.116815    0.8978    0.1996   0.0444 0.2448  0.064316    -16.0414  -0.2818  -0.0050                           -0.1465 -0.002717    3.3332    -1.0066  0.3040 0.0882  -0.118418    -0.0860   -0.0262  -0.0080                           0.2405  0.070919    8.8849    -2.2866  0.5885 0.0021  -0.152020    -46.7429  5.1241   -0.5617                           -0.3826 0.103521    -0.0524   -0.0480  -0.0441                           -0.1260 -0.155922    47.7728   6.2441   0.8161 1.0196  0.239923    0.5654    0.4473   0.3538 -0.5733 -0.173624    -1.1404   0.7176   -0.4516                           0.2256  0.142225    -45.0060  -7.4711  -1.2402                           -0.9359 -0.361226    0.0077    0.0276   0.0991 -0.0418 0.205627    -4.3545   -0.9221  -0.1953                           -0.0424 -0.050328    36.8402   -0.6334  0.0109 0.5519  -0.0097Total 0.6422    0.1529   0.0328 0.0361  -0.3458______________________________________ 
    
     Embodiment 1   f=292.8 
     
         ______________________________________SA          CM       AS       PT      DS______________________________________1     0.2195    0.0201   0.0018 0.0428  0.00412     -32.5142  6.2311   -1.1941                           -0.0552 0.23943     2.0103    -0.7703  0.2952 0.0590  -0.13574     23.9434   -3.8962  0.6340 0.1741  -0.13155     16.6418   -5.4692  1.7974 0.0021  -0.59146     -5.4020   2.0681   -0.7918                           0.0521  0.28327     -5.3461   -0.3500  -0.0229                           -0.3709 -0.02588     -0.4748   0.2647   -0.1476                           -0.0998 0.13809     5.2686    -0.0352  0.0014 0.0295  -0.000510    0.0238    -0.0339  0.0483 0.0294  -0.110511    -5.3356   1.4292   -0.3828                           -0.2859 0.179112    -1.5145   -0.4984  -0.1640                           0.0249  -0.045813    16.4923   1.2647   0.0970 0.1870  0.021814    1.4027    -0.5203  0.1930 0.1226  -0.117115    0.8639    0.1951   0.0440 0.2448  0.065216    -15.7727  -0.2950  -0.0055                           -0.1465 -0.002817    3.3344    -1.0069  0.3041 0.0882  -0.118518    -0.0934   -0.0289  -0.0099                           0.2405  0.071719    8.8563    -2.2793  0.5866 0.0021  -0.151520    -46.7429  5.1241   -0.5617                           -0.3826 0.103521    -0.0524   -0.0480  -0.0441                           -0.1260 -0.155922    47.7728   6.2441   0.8161 1.0196  0.239923    0.5654    0.4473   0.3538 -0.5733 -0.173624    -1.1404   0.7176   -0.4516                           0.2256  0.142225    -45.0060  -7.4711  -1.2402                           -0.9359 -0.361226    0.0077    0.0276   0.0991 -0.0418 0.205627    -4.3545   -0.9221  -0.1953                           -0.0424 -0.050328    36.8402   -0.6334  0.0109 0.5519  -0.0097Total 0.4940    -0.2748  0.0720 0.0361  -0.4882______________________________________ 
    
     The correspondence between each embodiment and various conditions will be shown below. 
     
         __________________________________________________________________________       Embodiment              Embodiment                     Embodiment                            Embodiment       1      2      3      4__________________________________________________________________________(1) Fw/|FII.sub.2 |       0.0258 0.093  0.0307 0.0054(2) RII.sub.4 /Fw       1.01   0.93   1.13   1.01(3) RII.sub.6 /RII.sub.3       0.345  0.253  0.489  0.326(4) Fw/FIII.sub.2       0.181  0.104  0.154  0.251(5) |RIII.sub.2 |/RIII.sub.1       1.53   1.09   1.51   1.36 ##STR1##   0.393  0.404  0.391  0.375  (7) νIII.sub.3 - νIII.sub.2       44.7   44.7   44.7   56.2 ##STR2##   -0.0022              -0.0022                     -0.0022                            -0.0016  θIII.sub.2       1.323  1.323  1.323  1.323θIII.sub.3       1.221  1.221  1.221  1.233__________________________________________________________________________