Abstract:
A retrofocus type standard lens includes a front lens group having a positive power, a diaphragm, and a rear lens group having a negative power, in this order from the object side. The focusing is carried out by moving the front lens group without moving the diaphragm and the rear lens group. The front lens group has a negative subgroup and a positive subgroup. The retrofocus type standard lens satisfies the relationships: 
     
       0.5&lt;f/f.sub.F &lt;1.0; 
     
     
       -0.7&lt;f/f.sub.FN &lt;-0.3, 
     
     wherein &#34;f&#34; designates a focal length of an entire lens system; &#34;f&#34; designates a focal length of the front lens group; and &#34;f FN  &#34; designates a focal length of the negative subgroup.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to retrofocus type standard lens and wide angle lens. 
     2. Description of Related Art 
     In a conventional focusing system of a lens, there are entire advance type in which the lens groups are entirely moved upon focusing, and floating types in which the front and rear lens groups are independently moved to vary the distance between the lens groups upon focusing. However, in either type, since the diaphragm unit is also moved during the focusing, it is difficult to simplify the structure of the lens barrel. 
     There is also known a front lens advance type in which only the front lens group is moved without moving the diaphragm. To simplify the lens barrel, it is desirable to use a front lens advance type to thereby restrict the aberration. To this end, it is necessary to reduce the absolute value of the aberration factors of the front lens group and the rear lens group. However, in the conventional retrofocus type lenses, it is difficult to restrict the aberration fluctuation during the focusing. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a retrofocus type standard lens in which there is no movement of the diaphragm during the focusing operation from the infinity to the close object distance to thereby restrict the aberration fluctuation caused by the focusing operation. 
     Another object of the present invention is to provide a retrofocus type wide angle lens in which there is no movement of the diaphragm during the focusing operation from the infinity to the close object distance to thereby restrict the aberration fluctuation caused by the focusing operation. 
     To achieve the object mentioned above, according to an aspect of the present invention, there is provided a retrofocus type standard lens including a front lens group having a positive power, where the front lens group includes a negative subgroup and a positive subgroup, a diaphragm, and a rear lens group having a positive power, in this order from the object side. The focusing is carried out by moving the front lens group without moving the diaphragm and the rear lens group, and wherein the retrofocus type standard lens satisfies the following formulae (1) and (2S): 
     
         0.5&lt;f/f.sub.F &lt;1.0;                                        (1) 
    
     
         -0.7&lt;f/f.sub.F N &lt;-0.2,                                    (2S) 
    
     wherein &#34;f&#34; designates the focal length of the entire lens system; &#34;f F  &#34; designates the focal length of the front lens group; and, &#34;f F  N &#34; designates the focal length of the negative subgroup belonging to the front lens group. 
     Preferably, the positive subgroup of the front lens group has two positive lenses. 
     Preferably, a first negative lens belonging to the negative subgroup of the front lens group satisfies the following formula (3S): 
     
         1.75&lt;n,                                                    (3S) 
    
     wherein &#34;n&#34; designates the refractive index of the first negative lens at the d-line. 
     According to another aspect of the present invention, there is provided a retrofocus type wide angle lens including a front lens group having a positive power, where the front lens group being comprised of a negative subgroup and a positive subgroup, a diaphragm, and a rear lens group having a positive power, in this order from the object side. The focusing is carried out by moving the front lens group without moving the diaphragm and the rear lens group, and the retrofocus type wide angle lens satisfies the following formulae (1) and (2W): 
     
         0.5&lt;f/f.sub.F &lt;1.0;                                        (1) 
    
     
         -1.2&lt;f/f.sub.F N &lt;-0.7,                                    (2W) 
    
     wherein &#34;f&#34; designates the focal length of the entire lens system; &#34;f F  &#34; designates the focal length of the front lens group; &#34;f F  N &#34; designates the focal length of the negative subgroup belonging to the front lens group. 
     Preferably, the negative lens group of the front lens group has two negative lenses and satisfies the following formula (4): 
     
         -0.6&lt;f/f.sub.1 &lt;-0.35,                                     (4) 
    
     wherein &#34;f 1  &#34; designates the focal length of a first negative lens of the two negative lenses, the first negative lens being located closer to an object. 
     Preferably, the positive lens group of the front lens group has two positive lenses. 
     Moreover, the first negative lens of the negative subgroup preferably satisfies the following formula (3W): 
     
         1.8&lt;n,                                                     (3W) 
    
     wherein &#34;n&#34; designates the refractive index of the first negative lens at the d-line. 
     The present disclosure relates to subject matter contained in Japanese patent application Nos. 5-307978 and 5-307979 (both filed on Dec. 8, 1993) which are expressly incorporated herein by reference in their entirety. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described below in detail with reference to the accompanying drawings, in which; 
     FIG. 1 is a schematic view of a lens arrangement of a retrofocus type standard lens, according to a first embodiment of the present invention; 
     FIG. 2 shows various aberration diagrams of a retrofocus type standard lens shown in FIG. 1 at the infinite object distance; 
     FIG. 3 shows various aberration diagrams of a retrofocus type standard lens shown in FIG. 1, at the closest object distance; 
     FIG. 4 shows a comparative example of various aberration diagrams of a retrofocus type standard lens in which two lenses (having a negative power as a whole) of the front lens group that is located on the object side in FIG. 1 constitute a focusing lens group; 
     FIG. 5 is a schematic view of a lens arrangement of a retrofocus type standard lens, according to a second embodiment of the present invention; 
     FIG. 6 shows various aberration diagrams of a retrofocus type standard lens shown in FIG. 5 at the infinite object distance; 
     FIG. 7 shows various aberration diagrams of a retrofocus type standard lens shown in FIG. 5, at the closest object distance; 
     FIG. 8 shows a comparative example of various aberration diagrams of a retrofocus type standard lens in which two lenses (having a negative power as a whole) of the front lens group that is located on the object side in FIG. 5 constitute a focusing lens group; 
     FIG. 9 is a schematic view of a lens arrangement of a retrofocus type wide angle lens, according to a third embodiment of the present invention; 
     FIG. 10 shows various aberration diagrams of a retrofocus type wide angle lens shown in FIG. 9 at the infinite object distance; 
     FIG. 11 shows various aberration diagrams of a retrofocus type wide angle lens shown in FIG. 9, at the closest object distance; 
     FIG. 12 shows a comparative example of various aberration diagrams of a retrofocus type wide angle lens in which three lenses (having a negative power as a whole) of the front lens group that is located on the object side in FIG. 9 constitute a focusing lens group; 
     FIG. 13 is a schematic view of a lens arrangement of a retrofocus type wide angle lens, according to a fourth embodiment of the present invention; 
     FIG. 14 shows various aberration diagrams of a retrofocus type wide angle lens shown in FIG. 13, at the infinite object distance; 
     FIG. 15 shows various aberration diagrams of a retrofocus type wide angle lens shown in FIG. 13, at the closest object distance; and, 
     FIG. 16 shows a comparative example of various aberration diagrams of a retrofocus type wide angle lens in which three lenses (having a negative power as a whole) of the front lens group that is located on the object side in FIG. 13 constitute a focusing lens group. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A retrofocus type standard lens according to the present invention is comprised of a front lens group having a positive power, a diaphragm, and a rear lens group having a positive power, arranged in this order from the object side. One of the most significant features of the present invention resides in that the front lens group has a positive power, and that the front lens group is comprised of a negative subgroup and a positive subgroup. The retrofocus type standard lens further meets the requirements specified in the formulae (1) and (2S). 
     With this feature, the absolute value of the aberration factor of the front lens group can be reduced to thereby restrict the aberration fluctuation during the focusing. If the front lens group has a negative power contrary to the requirement mentioned above, the aberration would considerably vary during the focusing. 
     If the ratio defined in the formula (1) exceeds the upper limit, it becomes difficult to correct coma. If the ratio is below the lower limit in the formula (1), the displacement of the lens group for the focusing becomes so large that the aberration considerably varies. 
     If the ratio defined in the formula (2S) is above the upper limit, the distortion (positive) is too large and it is difficult to obtain a sufficient back focal distance. Conversely, if the ratio is smaller than the lower limit, astigmatism can not be sufficiently corrected. 
     It is necessary to increase the power of the negative lens within the divergent optical system, i.e., it is necessary to decrease the radius of curvature of the negative lens to obtain angle of view and back focal distance larger than predetermined values. However, if the radius of curvature of the negative lens is reduced, an increased distortion is caused. To correct the distortion, it is necessary to provide a lens having a positive refractive power in the divergent optical system. To obtain a back focal distance more than a predetermined value, the radius of curvature of the negative lens must be further reduced, thus resulting in an adverse influence on the spherical aberration and coma, etc. 
     According to an aspect of the present invention, the positive subgroup belonging to the positive front lens group is made of two positive lenses. Hence, the convex surface is located away from the diaphragm, so that the distortion can be corrected. Moreover, spherical aberration and coma in a large aperture lens whose F number is approximately F1.4 can be effectively corrected. 
     It is preferred that the negative subgroup of the first lens group has at least one negative lens which is located at an object side. 
     When the first negative lens has a refractive index which satisfies the formula (3S), the radius of curvature of the first negative lens is prevented from being too small. If the ratio defined in the formula (3S) exceeds the upper limit, it becomes very difficult to correct astigmatism. Conversely, if the ratio is smaller than the lower limit, the radius of curvature of the first negative lens is too small to easily produce the same. 
     Another aspect of the present invention is addressed to a retrofocus type wide angle lens, which will be discussed below. The following discussion partly overlaps the foregoing. 
     A retrofocus type wide angle lens according to the present invention is comprised of a front lens group having a positive power, a diaphragm, and a rear lens group having a positive power, arranged in this order from the object side. One of the most significant features of the present invention resides in that front lens group has a positive power and in that the front lens group is comprised of a negative subgroup and a positive subgroup. The retrofocus type wide angle lens further meets the requirements specified in the formulae (1) and (2W). 
     With this feature, the absolute value of the aberration factor of the front lens group can be reduced to thereby restrict the aberration fluctuation during the focusing. If the front lens group has a negative power contrary to the requirement mentioned above, the aberration would vary considerably during the focusing. 
     If the ratio defined in the formula (1) exceeds the upper limit, it becomes difficult to correct coma. If the ratio is below the lower limit in the formula (1), the displacement of the lens group for the focusing is so large that the aberration considerably varies. 
     If the ratio defined in the formula (2W) is above the upper limit, the distortion (negative) can be reduced but no sufficient back focal distance can be obtained. Conversely, if the ratio is smaller than the lower limit, the negative power is too large to effectively correct the distortion (negative). 
     It is necessary to increase the power of the negative lens within the divergent optical system, i.e., it is necessary to decrease the radius of curvature of the negative lens to obtain angle of view and back focal distance larger than predetermined values. However, if the radius of curvature of the negative lens is reduced, an increased distortion is caused. To correct the distortion, it is necessary to provide a lens having a positive refractive power in the divergent optical system. To obtain a back focal distance more than a predetermined value, the radius of curvature of the negative lens must be further reduced, thus resulting in an adverse influence on the spherical aberration and coma, etc. 
     According to an aspect of the present invention, the negative lens group belonging to the front lens group that constitutes a divergent optical system is made of two negative lenses. Hence, the negative power is distributed so as to effectively correct the distortion. 
     If the ratio defined in the formula (4) exceeds the upper limit, it becomes difficult to correct astigmatism. Conversely, if the ratio is smaller than the lower limit, the radius of curvature of the first negative lens is too small to easily produce the same. 
     The positive subgroup of the front lens group can be made of two positive lenses, so that the convex surface is located away from the diaphragm to correct the distortion. Moreover, spherical aberration and coma in a large aperture lens whose F number is approximately F1.4 can be effectively corrected. 
     When the first negative lens has a refractive index which satisfies the formula (3W), the radius of curvature of the first negative lens is prevented from being too small. 
     Four embodiments of the present invention will be discussed below. The first and second embodiments are directed to a retrofocus type standard lens. 
     FIG. 1 shows a lens arrangement of a retrofocus type standard lens according to a first embodiment in which the diaphragm S is provided between the positive front lens group 10 and the positive rear lens group 20. The front lens group 10 is comprised of a first negative lens 11 which constitutes a negative subgroup, and first and second positive lenses 12 and 13 that constitute a positive subgroup, in this order from the object side. The rear lens group 20 is comprised of a negative lens 21, a positive lens 22, and a positive lens 23, in this order from the object side. There is a plane-parallel plate 24 behind the positive lens 23. The plane-parallel plate 24 can be dispensed with. The focusing is carried out by moving only the front lens group 10 forwardly from the infinite object distance to the close object distance. No movement of the diaphragm S and the rear lens group 20 occurs during the focusing. 
     Numerical data of the lens system shown in FIG. 1 is shown in Table 1 below. Various aberrations thereof at the infinite object distance and the closest object distance (the object distance from the first surface=100) are shown in FIGS. 2 and 3, respectively. 
     In FIGS. 2 and 3, &#34;SA&#34; designates the spherical aberration, &#34;SC&#34; the sine condition, &#34;d-line&#34;, &#34;g-line&#34; and &#34;C-line&#34; the chromatic aberration represented by the spherical aberration and the transverse chromatic aberration, at the respective wavelengths, &#34;S&#34; the sagittal ray, and &#34;M&#34; the meridional ray, respectively. 
     In the tables and the drawings, &#34;F No  &#34; designates the f-number, &#34;f&#34; the focal length, &#34;ω&#34; the half angle of view, &#34;Y&#34; the image height, &#34;f B  &#34; the back focal distance, &#34;f BP  &#34; distance between a surface of an image side of plane-parallel plate 24 and an image plane, &#34;r&#34; the radius of curvature of each lens surface, &#34;d&#34; the distance between the lenses, &#34;N d  &#34; the refractive index of the d-line, and &#34;υ d  &#34; the Abbe number of the d-line, respectively. 
     
                       TABLE 1______________________________________F.sub.NO = 1:1.4f = 5.27ω = 29.20° (Y = 3.0)f.sub.B = d.sub.12 + d.sub.13 = 4.51f.sub.BP = 0surface No.      r        d         N.sub.d                               ν.sub.d______________________________________1          9.100    1.00      1.80400                               46.62          3.894    2.28      --    --3          -83.620  2.94      1.62004                               36.34          -14.300  0.58      --    --5          8.450    2.35      1.77250                               49.66          -10.788  0.71      --    --STOP       ∞  1.82      --    --7          -5.310   0.70      1.84666                               23.88          6.564    0.28      --    --9          24.750   2.19      1.77250                               49.610         -5.310   0.10      --    --11         10.770   2.41      1.77250                               49.612         -17.646  3.22      --    --13         ∞  1.29      1.51633                               64.114         ∞  --        --    --______________________________________ 
    
     In the first embodiment, when the focusing is carried out by the front lens group 10, d6 is equal to 0.71 (d6=0.71) at the infinite object distance, and d6=1.05 at the closest object distance (object distance from the first surface=100), respectively. Namely, Δd6=0.34 which is extremely small. If only the first negative lens 11 and the first positive lens 12, having a negative power as a whole and belonging to the front lens group 10, are moved as a focusing lens group, Δd4=2.83 (d4=0.58→3.41), which is considerably larger than Δd6=0.34. The aberrations thereof are shown in FIG. 4 as a comparative example. 
     As can be seen in FIG. 4, if the negative lens group is used as a focusing lens group, there is a large variation of the aberration during the focusing from the infinity to the closest object distance. 
     FIG. 5 shows a lens arrangement of a retrofocus type standard lens according to a second embodiment of the present invention. The basic lens arrangement in the second embodiment is substantially identical to that of the first embodiment. 
     Numerical data of the lens system shown in FIG. 5 is shown in Table 2 below. Diagrams of various aberrations thereof at the infinity and the closest object distance (object distance from the first surface=100) are shown in in FIGS. 6 and 7, respectively. 
     
                       TABLE 2______________________________________F.sub.NO = 1:1.4f = 5.31ω = 28.8° (Y = 3.0)f.sub.B = d.sub.12 + d.sub.13 = 3.85f.sub.BP = 0surface No.      r        d         N.sub.d                               ν.sub.d______________________________________1          9.954    1.00      1.80400                               46.62          3.890    1.32      --    --3          52.299   2.47      1.51742                               52.44          -18.963  0.88      --    --5          7.505    2.42      1.77250                               49.66          -9.700   0.70      --    --STOP       ∞  1.66      --    --7          -4.732   0.70      1.84666                               23.88          6.704    0.17      --    --9          18.734   2.27      1.77250                               49.610         -4.847   0.10      --    --11         9.428    3.04      1.77250                               49.612         -12.851  2.56      --    --13         ∞  1.29      1.51633                               64.114         ∞  --        --    --______________________________________ 
    
     In the second embodiment, when the focusing is carried out by the front lens group 10, d6=0.70 at the infinite object distance, and d6=1.06 at the closest object distance, respectively. Namely, Δd6=0.36 which is extremely small. If only the first negative lens 11 and the first positive lens 12, having a negative power as a whole and belonging to the front lens group 10, are moved as a focusing lens group, Δd4=1.78 (d4=0.88→2.66), which is considerably large. The aberrations thereof are shown in FIG. 8 as a comparative example. 
     As can be seen in FIG. 8, if the negative lens group is used as a focusing lens group, there is a large variation of the aberration during the focusing from the infinity to the closest object distance. 
     The values of the formulae (1), (2S), and (3S) in the two embodiments are shown in Table 3 below. 
     
                       TABLE 3______________________________________      Embodiment 1                Embodiment 2______________________________________formula (1)  0.91        0.89formula (2S) -0.54       -0.62formula (3S) 1.80        1.80______________________________________ 
    
     As can be seen from Table 3 above, the two embodiments satisfy the requirements defined by the formulae (1), (2S) and (3S). Moreover, according to the present invention, the various aberrations at the infinity and the closest object distance can be correctly compensated in a retrofocus type standard lens. 
     As may be understood from the foregoing, according to the retrofocus type standard lens of the present invention, no movement of the diaphragm takes place during the focusing, so that the aberration fluctuation caused by the focusing from the infinity to the closest object distance can be minimized. 
     The third and fourth embodiments are directed to a retrofocus type wide angle lens. 
     FIG. 9 shows a lens arrangement of a retrofocus type wide angle lens according to a third embodiment of the present invention. In this embodiment, the diaphragm S is provided between the positive front lens group 10 and the positive rear lens group 20. The front lens group 10 is comprised of a first negative lens 11 and a second negative lens 12 which constitute a negative subgroup, and first and second positive lenses 13 and 14 that constitute a positive subgroup, in this order from the object side. The rear lens group 20 is comprised of a negative lens 21, a positive lens 22, and a positive lens 23, in this order from the object side. There is a plane-parallel plate 24 behind the positive lens 23. The plane-parallel plate 24 can be dispensed with. The focusing is carried out by moving only the front lens group 10 forwardly from the infinite object distance to the close object distance. No movement of the diaphragm S and the rear lens group 20 occurs during the focusing. 
     Numerical data of the lens system shown in FIG. 9 is shown in Table 4 below. Diagrams of various aberrations thereof at the infinity and the closest object distance (object distance from the first surface=100) are shown in FIGS. 10 and 11, respectively. 
     
                       TABLE 4______________________________________F.sub.NO = 1:1.4f = 3.54ω = 42.8° (Y = 3.0)f.sub.B = d.sub.12 + d.sub.13 = 4.51f.sub.BP = 0Surface No.      r        d         N.sub.d                               ν.sub.d______________________________________1          12.342   0.90      1.88300                               40.82          4.161    2.83      --    --3          45.985   0.70      1.65844                               50.94          5.830    1.44      --    --5          76.110   1.99      1.83481                               42.76          -9.600   1.10      --    --7          9.066    2.31      1.77250                               49.68          -10.447  0.70      --    --STOP       ∞  1.82      --    --9          -5.310   0.70      1.84666                               23.810         6.564    0.28      --    --11         24.750   2.19      1.77250                               49.612         -5.310   0.10      --    --13         10.770   2.41      1.77250                               49.614         -17.646  3.22      --    --15         ∞  1.29      1.51633                               64.116         ∞  --        --    --______________________________________ 
    
     In the third embodiment, when the focusing is carried out by the front lens group 10, d8=0.70 at the infinite object distance, and d8=0.85 at the closest object distance (object distance from the first surface=100), respectively. Namely, Δd8=0.15 which is extremely small. If the first negative lens 11, the second negative lens 12 and the first positive lens 13, having a negative power as a whole and belonging to the front lens group 10, are moved as a focusing lens group, Δd6=1.72 (d6=1.10→2.82), which is considerably large. The aberrations thereof are shown in FIG. 12 as a comparative example. 
     As can be seen in FIG. 12, if the negative lens group is used as a focusing lens group, there is a large variation of the aberration during the focusing from the infinity to the closest object distance. 
     FIG. 13 shows a lens arrangement of a retrofocus type wide angle lens according to a fourth embodiment of the present invention. The basic lens arrangement in the fourth embodiment is substantially identical to that of the first embodiment shown in FIG. 1. 
     Numerical data of the lens system shown in FIG. 9 is shown in Table 5 below. Diagrams of various aberrations thereof at the infinity and the closest object distance (object distance from the first surface=100) are shown in FIGS. 14 and 15, respectively. 
     
                       TABLE 5______________________________________F.sub.NO = 1:1.4f = 3.51ω = 43.4° (Y = 3.0)f.sub.B = d.sub.12 + d.sub.13 = 3.85f.sub.BP = 0surface No.      r        d         N.sub.d                               ν.sub.d______________________________________1          12.715   0.90      1.88300                               40.82          4.094    3.00      --    --3          53.677   0.70      1.60311                               60.74          5.267    1.02      --    --5          34.434   1.88      1.83481                               42.76          -11.890  0.42      --    --7          7.506    2.42      1.77250                               49.68          -9.699   0.70      --    --STOP       ∞  1.66      --    --9          -4.732   0.70      1.84666                               23.810         6.704    0.17      --    --11         18.734   2.27      1.77250                               49.612         -4.847   0.10      --    --13         9.428    3.04      1.77250                               49.614         -24.851  2.56      --    --15         ∞  1.29      1.51633                               64.116         ∞  --        --    --______________________________________ 
    
     In the fourth embodiment, when the focusing is carried out by the front lens group 10, d8=0.70 at the infinite object distance, and d8=0.85 at the closest object distance (object distance from the first surface=100), respectively. Namely, Δd8=0.15 which is extremely small. If the first negative lens 11, the second negative lens 12 and the first positive lens 13, having a negative power as a whole and belonging to the front lens group 10 are moved as a focusing lens group, Δd6=0.81 (d6=0.42→1.23), which is considerably large. The aberrations thereof are shown in FIG. 16 as a comparative example. 
     As can be seen in FIG. 16, if the negative lens group is used as a focusing lens group, there is a large variation of the aberration during the focusing from the infinity to the closest object distance. 
     The values of the formulae (1), (2W), (3W) and (4) in the third and fourth embodiments are shown in Table 6 below. 
     
                       TABLE 6______________________________________      Embodiment 3                Embodiment 4______________________________________formula (1)  0.91        0.89formula (2W) -0.96       -1.01formula (4)  -0.47       -0.49formula (3W) 1.88        1.88______________________________________ 
    
     As can be seen from Table 6 above, the two embodiments satisfy the requirements defined by the formulae (1), (2W), (3W), and (4). Moreover, according to the present invention, the various aberrations at the infinity and the closest object distance can be correctly compensated in a retrofocus type wide angle lens. 
     As may be understood from the foregoing, according to retrofocus type wide angle lens of the present invention, no movement of the diaphragm occurs during the focusing, so that the aberration fluctuation caused by the focusing from the infinity to the closest object distance can be minimized.