Abstract:
A fixed-focal-length lens system includes first and second lens groups arranged in order from front to rear. The first lens group consists of a plurality of lenses and has a negative refracting power and the second lens group consists of a plurality of lenses and has a positive refracting power. The fixed-focal-length lens system satisfies the following conditions 
     
       0.58&lt;|f.sub.1 /f|&lt;0.95                   (1) 
     
     
       0.50&lt;|f.sub.1 /f.sub.2 |&lt;1.0             (2) 
     
     
       1.6&lt;Bf/f&lt;2.9                                               (3) 
     
     
       1.1&lt;f.sub.2 /f&lt;1.45                                        (4) 
     
     wherein f represents the focal length of the fixed-focal-length lens system, f 1  represents the focal length of the first lens group, f 2  represents the focal length of the second lens group and Bf represents the back focus of the lens system.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a fixed-focal-length lens system which is suitable for use as a projector lens in an enlarger or the like. 
     2. Description of the Prior Art 
     In a photographic enlarger for printing images borne by a negative film on a photographic paper, there has been used an orthometer type fixed-focal-length lens system due to easiness in correction of aberrations such as distortion. 
     However the known orthometer type fixed-focal-length lens system is disadvantageous in that the back focus (the distance between the negative film and the lens surface nearest to the negative film) is short and a mirror which is positioned between the lens system and the negative film to lead light transmitted through the negative film to a light measuring circuit for color correction makes it difficult for the operator to view the negative film when printing images borne by the negative film on a photographic paper. 
     Though, in a retro-focus type fixed-focal-length lens system, the back focus can be relatively long as compared with its focal length as disclosed, for instance, in Japanese Unexamined Patent Publication Nos. 64(1989)-61714 and 61(1986)-188512, the retro-focus type fixed-focal-length lens system is short in its focal length itself (10 mm or so) and accordingly its back focus cannot be sufficiently long. Further the retro-focus type fixed-focal-length lens system is disadvantageous in that it is difficult to correct aberrations when used at high magnifications due to an asymmetric arrangement with respect to the stop. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing observations and description, the primary object of the present invention is to provide a fixed-focal-length lens system which has a high magnification and a long back focus as well as excellent aberration properties. 
     The fixed-focal-length lens system in accordance with the present invention comprises first and second lens groups arranged in this order from front to rear, the first lens group consisting of a plurality of lenses and having a negative refracting power and the second lens group consisting of a plurality of lenses and having a positive refracting power, and satisfies the following conditions 
     
         0.58&lt;|f.sub.1 /f|&lt;0.95                   (1) 
    
     
         0.50&lt;|f.sub.1 /f.sub.2 |&lt;1.0             (2) 
    
     
         1.6&lt;Bf/f&lt;2.9                                               (3) 
    
     
         1.1&lt;f.sub.2 /f&lt;1.45                                        (4) 
    
     wherein f represents the focal length of the fixed-focal-length lens system, f 1  represents the focal length of the first lens group, f 2  represents the focal length of the second lens group and Bf represents the back focus (the distance between the negative film and the lens surface nearest to the negative film) of the lens system. 
     The meaning of the above conditions will hereinbelow be described in detail. 
     Conditions (1) and (2) are for minimizing aberrations and elongating the back focus. That is, if |f 1  /f| falls below 0.58 or |f 1  /f 2  | falls below 0.50, aberrations become so large that the number of the lens elements must be increased to correct the aberrations, which is undesirable in view of simplifying the structure of the lens system. If |f 1  /f| exceeds 0.95 or |f 1  /f 2  | exceeds 1.0, the back focus of the fixed-focal-length lens system cannot be sufficiently long. 
     Condition (3) is for limiting the negative refracting power of the front lens group (the first lens group) as well as ensuring a long back focus. That is, if Bf/f falls below the lower limit, the back focus of the fixed-focal-length lens system cannot be sufficiently long. If it exceeds the upper limit, the negative refracting power of the front lens group becomes so strong that correction of aberrations becomes difficult. 
     Condition (4) is for limiting the positive refracting power of the rear lens group (the second lens group) as well as ensuring a long back focus. That is, if f 2  /f falls below the lower limit, the positive refracting power of the rear lens group becomes so strong that correction of aberrations becomes difficult. If it exceeds the upper limit, the back focus of the fixed-focal-length lens system cannot be sufficiently long. 
     Thus, in the fixed-focal-length lens system in accordance with the present invention which satisfies the above conditions (1) to (4), the back focus can be sufficiently long without deteriorating the image forming performance. 
     Further the fixed-focal-length lens system in accordance with the present invention consists of only two lens groups and is relatively simple in structure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view showing a fixed-focal-length lens system in accordance with a first embodiment of the present invention, 
     FIG. 2 is a schematic view showing an enlarger employing the lens system, 
     FIGS. 3A to 3C respectively show spherical aberration, astigmatism and distortion of the fixed-focal-length lens system in accordance with the first embodiment, 
     FIG. 4 is a cross-sectional view showing a fixed-focal-length lens system in accordance with a second embodiment of the present invention, 
     FIGS. 5A to 5C respectively show spherical aberration, astigmatism and distortion of the fixed-focal-length lens system in accordance with the second embodiment, 
     FIG. 6 is a cross-sectional view showing a fixed-focal-length lens system in accordance with a third embodiment of the present invention, and 
     FIGS. 7A to 7C respectively show spherical aberration, astigmatism and distortion of the fixed-focal-length lens system in accordance with the third embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 2, an enlarger has a negative mask 11 having a rectangular opening 12. A negative film F is fed passing across the opening 12. An image on the negative film F is projected on a photographic paper C through a print lens system 1 and a mirror 2. A mirror 3 is movable between a retracted position shown by the solid line in FIG. 2 and an operative position shown by the broken line. When the image on the negative film F is to be projected onto the photographic paper C, the movable mirror 3 is held in the retracted position, and when the movable mirror 3 is positioned in the operative position, light transmitted through the negative film F is led to a light measuring system 5 and is focused on a photodetector 8 through a lens 6. A part of the light transmitted through the lens 6 is deflected by a split prism 7 and focused on a light receiving surface of an image pickup element 9 for a monitor 10. The image signal output from the image pickup element 9 is input into the monitor 10 and is reproduced on the monitor 10 for positioning of the negative film F and the like. 
     The fixed-focal-length lens system in accordance with the present invention is especially suitable as the print lens system 1, though it can be applied to various other optical instruments. 
     Now fixed-focal-length lens systems in accordance with first to third embodiments of the present invention will be described, hereinbelow. 
     In the description hereinbelow and in the drawings, the radii of curvature of the refracting surfaces, the axial surface separations (the axial air separations or thicknesses of lenses), the refractive indices for the sodium d-line and the Abbe&#39;s numbers of the lenses are respectively designated at R, d, N and ν which are numbered respectively by subscripts in order from front to rear. In this specification, the end of the lens system facing the image forming plane (the photographic paper C) will be referred to as &#34;front&#34;, and the end of the lens system facing the negative film F will be referred to as &#34;rear&#34;. 
     As shown in FIG. 1, the fixed-focal-length lens system in accordance with the first embodiment of the present invention comprises a first lens group I and a second lens group II arranged in this order from front to rear and satisfies the aforesaid conditions (1) to (4). The first lens group I consists of three lenses L1 to L3 arranged in this order from front to rear and has a negative refracting power. The second lens group II consists of four lenses L4 to L7 and has a positive refracting power. 
     The lens L1 is a negative meniscus lens convex to front, the lens L2 is a double-concave lens having the face of greater curvature faced to front, the lens L3 is a double-convex lens having the face of greater curvature faced to rear, the lens L4 is a double-convex lens having the face of greater curvature faced to front, the lens L5 is a double-concave lens having the face of greater curvature faced to rear, the lens L6 is a double-convex lens whose faces have curvatures substantially equal to each other and the lens L7 is a double-convex lens having the face of greater curvature faced to rear. 
     The magnification M, the focal length f(mm), the effective back focus Bf(mm), F-number F NO  and the angle of view 2ω of the fixed-focal-length lens system are as shown in table 1. 
     
                       TABLE 1______________________________________magnification M       -1/7.78focal length f        58.82back focus Bf         126.7F-number F.sub.NO     6.5angle of view 2ω                 14.5°______________________________________ 
    
     The radii of curvature R(mm) of the respective lens surfaces, the axial surface separations d (mm) (the central thicknesses of the lenses or the air separations), the refractive indexes N for the sodium d-line of the lenses and the Abbe&#39;s numbers νd of the lenses of the fixed-focal-length lens system of the first embodiment are as shown in table 2. In table 2, the numbers in the leftmost column represent the subscripts given to the symbols R, d and L in FIG. 1. 
     
                       TABLE 2______________________________________radius of   axial surface                       refracting                                Abbe&#39;sNo.  curvature R separation d                       index N  number νd______________________________________1    71.820      2.35       1.73692  51.22    26.320      6.503    -32.581     2.00       1.77620  49.64    63.302      14.00      1.55156  45.45    -35.147     20.356    58.561      15.00      1.51978  52.27    -147.97     40.508    -60.595     1.50       1.78927  43.79    48.318      6.00       1.49845  81.610   -48.318     1.0011   75.199      5.00       1.48915  70.212   -65.348______________________________________ focal length f1 of the first lens group = -43.77 mm focal length f2 of the second lens group = 74.48 mm 
    
     FIGS. 3A to 3C respectively show spherical aberration, astigmatism and distortion of the fixed-focal-length lens system in accordance with the first embodiment. 
     As can be understood from FIGS. 3A to 3C, the fixed-focal-length lens system in accordance with the first embodiment has excellent optical performances. Further as can be understood from table 1, in the fixed-focal-length lens system in accordance with the first embodiment, the back focus which is the distance between the negative film and the lens face nearest to the negative film can be as long as not shorter than 120 mm, which is substantially long as compared with the conventional fixed-focal-length lens system. 
     The fixed-focal-length lens system in accordance with the second embodiment of the present invention is shown in FIG. 4. As shown in FIG. 4, the fixed-focal-length lens system in accordance with the second embodiment of the present invention comprises a first lens group I and a second lens group II arranged in this order from front to rear and satisfies the aforesaid conditions (1) to (4). The first lens group I consists of three lenses L1 to L3 arranged in this order from front to rear and has a negative refracting power. The second lens group II consists of four lenses L4 to L7 and has a positive refracting power. 
     The lens L1 is a negative meniscus lens convex to front, the lens L2 is a double-concave lens having the face of greater curvature faced to front, the lens L3 is a double-convex lens having the face of greater curvature faced to rear, the lens L4 is a double-convex lens having the face of greater curvature faced to front, the lens L5 is a double-concave lens having the face of greater curvature faced to rear, the lens L6 is a double-convex lens having the face of greater curvature faced to rear and the lens L7 is a double-convex lens having the face of greater curvature faced to front. 
     The magnification M, the focal length f(mm), the effective back focus Bf(mm), F-number F NO  and the angle of view 2ω of the fixed-focal-length lens system are as shown in table 3. 
     
                       TABLE 3______________________________________magnification M       -1/7.78focal length f        60.87back focus Bf         109.66F-number F.sub.NO     6.5angle of view 2ω                 15.7°______________________________________ 
    
     The radii of curvature R (mm) of the respective lens surfaces, the axial surface separations d (mm) (the central thicknesses of the lenses or the air separations), the refractive indexes N for the sodium d-line of the lenses and the Abbe&#39;s numbers νd of the lenses of the fixed-focal-length lens system of the second embodiment are as shown in table 4. In table 4, the numbers in the leftmost column represent the subscripts given to the symbols R, d and L in FIG. 4. 
     
                       TABLE 4______________________________________radius of   axial surface                       refracting                                Abbe&#39;sNo.  curvature R separation d                       index N  number νd______________________________________1    55.121      6.736      1.73341  54.42    26.064      6.203    -34.287     3.11       1.77250  49.54    52.999      13.56      1.54895  44.85    -37.440     21.056    77.306      15.50      1.51931  68.77    -86.491     40.508    -180.01     1.20       1.78426  30.39    97.870      1.4010   555.69      3.50       1.49700  81.611   -54.202     0.5012   75.911      3.79       1.48749  70.213   -235.95______________________________________ focal length f1 of the first lens group = -50.16 mm focal length f2 of the second lens group = 69.84 mm 
    
     FIGS. 5A to 5C respectively show spherical aberration, astigmatism and distortion of the fixed-focal-length lens system in accordance with the second embodiment. 
     As can be understood from FIGS. 5A to 5C, the fixed-focal-length lens system in accordance with the second embodiment has excellent optical performances. Further as can be understood from table 3, in the fixed-focal-length lens system in accordance with the second embodiment, the back focus which is the distance between the negative film and the lens face nearest to the negative film can be as long as not shorter than 100 mm, which is substantially long as compared with the conventional fixed-focal-length lens system. 
     The fixed-focal-length lens system in accordance with the third embodiment of the present invention is shown in FIG. 6. As shown in FIG. 6, the fixed-focal-length lens system in accordance with the third embodiment of the present invention comprises a first lens group I and a second lens group II arranged in this order from front to rear and satisfies the aforesaid conditions (1) to (4). The first lens group I consists of three lenses L1 to L3 arranged in this order from front to rear and has a negative refracting power. The second lens group II consists of four lenses L4 to L7 and has a positive refracting power. 
     The lens L1 is a negative meniscus lens convex to front, the lens L2 is a double-concave lens having the face of greater curvature faced to rear, the lens L3 is a double-convex lens having the face of greater curvature faced to front, the lens L4 is a double-convex lens having the face of greater curvature faced to rear, the lens L5 is a double-concave lens having the face of greater curvature faced to front, the lens L6 is a double-convex lens having the face of greater curvature faced to rear and the lens L7 is a double-convex lens having the face of greater curvature faced to rear. 
     The magnification M, the focal length f(mm), the effective back focus Bf(mm), F-number F NO  and the angle of view 2ω of the fixed-focal-length lens system are as shown in table 5. 
     
                       TABLE 5______________________________________magnification M       -1/7.78focal length f        58.59back focus Bf         134.46F-number F.sub.NO     6.5angle of view 2ω                 13.3°______________________________________ 
    
     The radii of curvature R(mm) of the respective lens surfaces, the axial surface separations d (mm) (the central thicknesses of the lenses or the air separations), the refractive indexes N for the sodium d-line of the lenses and the Abbe&#39;s numbers νd of the lenses of the fixed-focal-length lens system of the third embodiment are as shown in table 6. In table 6, the numbers in the leftmost column represent the subscripts given to the symbols R, d and L in FIG. 6. 
     
                       TABLE 6______________________________________radius of   axial surface                       refracting                                Abbe&#39;sNo.  curvature R separation d                       index N  number νd______________________________________1    80.654      1.85       1.73447  54.42    25.776      6.653    -55.910     3.78       1.77331  51.84    48.812      1.005    52.056      11.75      1.54820  42.86    -56.919     21.727    131.23      10.59      1.61374  51.78    -84.526     42.029    -66.228     1.40       1.78566  40.410   70.870      3.50       1.49700  81.611   -46.321     0.5012   133.13      3.00       1.48749  70.213   -60.082______________________________________ focal length f1 of the first lens group = -39.26 mm focal length f2 of the second lens group = 69.80 mm 
    
     FIGS. 7A to 7C respectively show spherical aberration, astigmatism and distortion of the fixed-focal-length lens system in accordance with the third embodiment. 
     As can be understood from FIGS. 7A to 7C, the fixed-focal-length lens system in accordance with the third embodiment has excellent optical performances. Further as can be understood from table 5, in the fixed-focal-length lens system in accordance with the third embodiment, the back focus which is the distance between the negative film and the lens face nearest to the negative film can be as long as not shorter than 130 mm, which is substantially long as compared with the conventional fixed-focal-length lens system. 
     The arrangement of the fixed-focal-length lens system in accordance with the present invention need not be limited to those described above in conjunction with the first to third embodiments, but the number of the lens elements of each lens group, the radius of curvature of each lens element and the like may be variously modified so long as the aforesaid conditions (1) to (4) are satisfied. 
     Further, though the fixed-focal-length lens system of the present invention is especially useful for an enlarger, it can be applied to various other optical instruments.