Abstract:
A rear conversion lens which is to be mounted between a main lens and a predetermined image plane for attaining a larger resultant focal distance than a focal distance of the main lens basically comprises, in the order taken from an object, a first lens unit having a positive refractive power and a second lens unit having a negative refractive power, and has a negative refractive power as a whole. 
     The structure meets the following conditional formulas. 
     
       0.5&lt;f.sub.1 /|f.sub.RCL |&lt;0.6, f.sub.RCL &lt;0 (1) 
     
     
       1.2&lt;r.sub.2 /r.sub.3 &lt;1.4                                  (2) 
     
     where f RCL  is a focal distance of the rear conversion lens, f 1  is a focal distance of the first lens unit, r 2  is a radius of curvature of the first lens unit facing an image, and r 3  is a radius of curvature of the second lens unit facing an object.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a rear conversion lens to be mounted in rear of a primary lens, and more particularly to a compact rear conversion lens which can be mounted in a compact camera or video camera. 
     2. Related Background Art 
     A conversion lens to be mounted in front of or in rear of a main lens in order to expand a focal distance of the main lens has been known. 
     A so-called front conversion lens which is mounted in front of the main lens can readily expand the focal distance of the main lens but it leads to an increased diameter of the front conversion lens. Accordingly, it is difficult to mount it in a compact camera. 
     On the other hand, a so-called rear conversion lens (RCL) which is mounted in rear of the main lens is basically of a structure which is advantageous to compactness. 
     As a result, many proposals have been made, for example by Japanese Laid-Open Patent Applications No. 60-214327, No. 60-179712 and No. 58-195817 and Japanese Patent Publication No. 61-45207. 
     Although many efforts have been done in those known techniques, they are still not enough in the following aspects. 
     In the Laid-Open Patent Application No. 60-214327, a magnification is as low as 1.2 and effect as the RCL is small. 
     In the Laid-Open Patent Application No. 60-179712, the magnification of the RCL is 1.5-2.0 but the compactness of the structure is not satisfactory because it uses three lenses, positive, negative and negative lenses. 
     On the other hand, in the Laid-Open Patent Application No. 58-195817, the RCL comprises two lenses, positive and negative lenses but a magnification is as low as 1.15. Thus, an effect as the RCL is small. 
     In the Patent Publication No. 61-45207, a magnification of 1.5 is attained with two lenses, positive and negative lens but a correction for an outer coma aberration which is caused by the RCL is insufficient and a satisfactory performance is hard to attain. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a high performance rear conversion lens (RCL) which solves all of the above problems, has a structure which permits a simple and compact shape, provide a high magnification and can be mounted in a compact camera. 
     In accordance with the present invention, the rear conversion lens which is to be mounted between a main lens and a predetermined image plane for attaining a larger resultant focal distance than a focal distance of the main lens basically comprise, in the order taken from an object, a first lens unit having a positive refractive power and a second lens unit having a negative refractive power, and has a negative refractive power as a whole. 
     The structure meets the following conditional formulas. 
     
         0.5&lt;f.sub.1 /|f.sub.RCL |&lt;0.6, f.sub.RCL &lt;0 (1) 
    
     
         1.2&lt;r.sub.2 /r.sub.3 &lt;1.4                                  (2) 
    
     where f RCL  is a focal distance of the rear conversion lens, f 1  is a focal distance of the first lens unit, r 2  is a radius of curvature of the first lens unit facing an image, and r 3  is a radius of curvature of the second lens unit facing an object. 
     The present invention provides a rear conversion lens which is light, simple and compact in structure, has a sufficient magnification, has an excellent imaging characteristic and can be removably mounted in a compact camera. 
     Other objects, features and advantages of the present invention will be apparent from the following description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a principle of an RCL in which a conversion lens is mounted in rear of a main lens, 
     FIG. 2 shows a structure of an optical system in which an RCL in first, second or third embodiment of the present invention is mounted on a triplet type main lens shown in Table 1, 
     FIG. 3 shows a structure of an optical system in which an RCL of a fourth embodiment of the present invention is mounted on the triplet type main lens shown in Table 1, and 
     FIG. 4 shows a structure of an optical system in which an RCL of fifth or sixth embodiment of the present invention is mounted on a tessor type main lens shown in Table 6. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The rear conversion lens (RCL) usually has a negative refractive power and it is mounted between a main lens ML which is operable by itself and has a focal distance f and a rear focal point O 1  of the main lens ML so that a light flux which passes through the main lens ML and is directed to the rear focal point O 1  is focused by the RCL at a resultant focal point O 2  on an image plane I which is in rear of the focal point O 1 , as shown in FIG. 1. Thus, where the RCL is mounted in rear of the main lens ML, the resultant focal distance of the main lens ML and the RCL is expanded from f to F. 
     In an embodiment of the present invention, the RCL comprises, in the order taken from the object, a first lens unit G 1  having a positive refractive power and a second lens unit G 2  having a negative refractive power, as shown in FIGS. 2 to 4. 
     The RCL of the present invention meets the following conditional formulas 
     
         0.5&lt;f.sub.1 /|f.sub.RCL |&lt;0.6, f.sub.RCL &lt;0 (1) 
    
     
         1.2&lt;r.sub.2 /r.sub.3 &lt;1.4                                  (2) 
    
     where f RCL  is a focal distance of the RCL, f 1  is a focal distance of the first lens unit G 1 , r 2  is a radius of curvature of the first lens unit G 1  facing an image, and r 3  is a radius of curvature of the second lens unit G 2  facing an object. 
     In order to attain the expanded focal distance by mounting the RCL in rear of the main lens ML, the RCL must have a negative refractive power as a whole. In order to maintain a good imaging characteristic when the RCL is mounted on the main lens ML, the RCL must be properly corrected for aberration. 
     In general, however, when the RCL is mounted on the main lens ML which is properly corrected for aberration, a spherical aberration is overcorrected, a petzval sum becomes negative, a positive image curvature and a positive distortion aberration occur, and a coma aberration occurs at a periphery of the lens having the negative refractive power. In order to cancel those aberrations, the RCL must include at least one positive lens. In order to allow removable mounting of the RCL in a still camera or video camera, a compact structure is essential. 
     Thus, in order to construct the RCL as small number of lenses as possible and correct the aberration in the RCL, the RCL must comprise a lens unit having a positive refractive power and a lens unit having a negative refractive power. Thus, the RCL may be a combination of concave/convex lenses or convex/concave lenses. 
     In the former combination of the concave/convex lenses, an air lens formed between the negative lens unit and the positive lens unit affects to an off-axis light so that the correction for an off axis aberration over a wide angle is difficult to attain. Further, a diameter of the positive lens located in rear of the negative lens unit is increased by a dissipation action of the negative lens. 
     Accordingly, the RCL in the present invention basically comprises convex/concave lenses. 
     In order to adequately correct the aberration of the RCL with this structure, it is necessary to meet the above conditional formulas. 
     The conditional formulas of the present invention are now explained in detail. 
     The conditional formula (I) defines the proper focal distance of the first lens unit G 1  of the RCL. If the upper limit of the conditional formula (1) is exceeded, the petzval sum of the entire system including the main lens ML is a large negative value and a significant degradation is observed in a sadital image plane. On the other hand, when the lower limit is exceeded, the correction of an outer coma aberration generated in the RCL is difficult to attain. 
     The conditional formula (2) defines a proper ratio of the radius of curvature r 2  of the first lens unit G 1  facing an image and the radius of curvature r 3  of the second lens unit G 2  facing an object required to keep an optimum balance between the spherical aberration and a meridional image plane. If the upper limit of the conditional formula (2) is exceeded, the spherical aberration is over corrected and the balance to the negative meridional image plane is hard to keep. On the other hand, if the lower limit of the conditional formula (2) is exceeded, the spherical aberration is under-corrected and the balance to the negative meridional image plane is hard to keep. 
     In order to attain sufficient aberration correction and compactness, the following conditional formulas are to be preferably met. 
     
         0.70&lt;d.sub.0 /Bf.sub.ML &lt;0.97                              (3) 
    
     
         0.05&lt;D/|f.sub.RCL |&lt;0.15                 (4) 
    
     
         1.73&lt;n.sub.2                                               (5) 
    
     
         0.018&lt;d.sub.2 /|f.sub.RCL |&lt;0.028        (6) 
    
     
         0.025&lt;d.sub.1 /|f.sub.RCL |&lt;0.065        (7) 
    
     
         12&lt;ν.sub.2 -ν.sub.1 &lt;20                              (8) 
    
     where 
     Bf ML  : back focus of the main lens ML 
     d o  : distance from a plane of the rear conversion lens (RCL) facing the object to the rear focal point O 1  of the main lens ML 
     f RCL  : focal distance of the rear conversion lens (RCL) 
     D: On-axis thickness of the rear conversion lens (RCL) 
     n 2  : refractive index of the second lens unit G 2   
     d 2  : On-axis distance between the plane of the first lens unit G 1  of the rear conversion lens (RCL) facing the image and the plane of the second lens unit G 2  facing the object 
     d 1  : On-axis thickness of the first lens unit G 1   
     ν 1  : Abbe number of the first lens unit G 1   
     ν 2  : Abbe number of the second lens unit G 2   
     The conditional formula (3) defines the mount position of the RCL relative to the main lens ML. If the upper limit of the conditional formula (3) is exceeded, it is advantageous for compactness of the RCL but a sufficient air gap D o  between the main lens ML and the RCL is not maintained and the RCL cannot be removed. On the other hand, if the lower limit of the conditional formula (3) is exceeded, the diameter of the RCL increases and it is difficult to keep a sufficiently high magnification. 
     The conditional formula (4) defines the on-axis thickness of the RCL required for the compactness of the RCL. If the upper limit of the conditional formula (4) is exceeded, the size of the RCL increases. On the other hand, if the lower limit of the conditional formula (4) is exceeded, the correction of the off-axis aberration is hard to attain. In order to better correct, 
     
         1.75&lt;n.sub.2                                               (5&#39;) 
    
     is preferably to be met. 
     The conditional formula (5) defines the refractive index of the second lens unit G 2  required to maintain a proper petzval sum of the entire system including the main lens ML. If the limit of the conditional formula (5) is exceeded, the correction of the petzval sum is difficult to attain. 
     The conditional formula (6) defines the on-axis distance d 2  between the plane of the first lens unit G 1  of the RCL facing the image and the plane of the second lens unit G 2  facing the object. If the upper limit of the conditional formula (6) is exceeded, the correction of the negative image curvature is difficult to attain. On the other hand, if the lower limit of the conditional formula (6) is exceeded, the correction of the positive image curvature is difficult to attain. In order to attain better correction, 
     
         0.027&lt;d.sub.1 /|f.sub.RCL |&lt;0.050        (6&#39;) 
    
     is preferably to be met. 
     The conditional formula (7) defines the on-axis thickness d 1  of the first lens unit G 1  of the RCL required to properly correct a chrominance aberration. If the limit of the conditional formula (7) is exceeded, it is difficult to simultaneously correct the on-line chrominance aberration and magnification chrominance aberration. In order to attain better correction, 
     
         12&lt;ν.sub.2 -ν.sub.1 &lt;18                              (7&#39;) 
    
     is preferably to be met. 
     The conditional formula (8) defines a difference between the abbe number ν 1  of the first lens unit G 1  and the abbe number ν 2  of the second lens unit G 2  required to correct the on-axis chrominance aberration. If the upper limit of the conditional formula (8) is exceeded, the chrominance aberration is under corrected. On the other hand, if the lower limit is exceeded, the chrominance aberration is over-corrected. 
     Embodiments of the present invention are now explained. First to fourth embodiments of the RCL of the present invention shown in Tables 2 to 5 are designed for a triplet main lens ML shown in the Japanese Laid-Open Patent Application No. 60-177313 filed by the same assignee of the present invention, as shown in Table 1. 
     In those tables, 2ω is an image angle, f is a focal distance of the main lens ML, f RCL  is a focal distance of the RCL, Bf ML  is a back focus of the main lens ML, F is a resultant focal distance of the main lens ML and the RCL, Bf is a back focus to the image plane, D o  is an air gap between the last plane of the main lens ML and the first plane of the RCL, d o  is a distance from the plane of the RCL facing the object to the rear focal point O 1  of the main lens ML, and β is a magnification of the RCL. 
     
                       TABLE 1______________________________________(main lens)f = 36.00, F number 3.5, 2ω = 63.2°Radius of  Center Thickness                        Refractive                                 AbbeCurvature  Distance     Index    NumberNo.  r          d            n        ν______________________________________1    13.070     5.100        1.79668  45.422    29.010     1.0103    -39.670    1.400        1.72825  28.344    13.740     1.8005    35.000     2.900        1.80218  44.746    -24.200    (Bf.sub.ML)Bf.sub.ML = 28.716______________________________________ 
    
     A stop S (behind stop) is arranged at a position of 1.000 from the last plane of the main lens ML. 
     
                       TABLE 2______________________________________(First embodiment)F = 51.33, F number 5.0, 2ω = 44.8°Radius of  Center Thickness                        Refractive                                 AbbeCurvature  Distance     Index    NumberNo.  r          d            n        ν______________________________________1    -127.320   2.000        1.61750  30.752     -20.600   1.5003     -16.380   1.000        1.79668  45.424    -103.850   (Bf)D.sub.o = 3.00, Bf = 31.25, β = 1.426d.sub.o = 25.716, f.sub.RCL = -69.93______________________________________ 
    
     
                       TABLE 3______________________________________(Second embodiment)F = 51.46, F number 5.0, 2ω = 44.8°Radius of  Center Thickness                        Refractive                                 AbbeCurvature  Distance     Index    NumberNo.  r          d            n        ν______________________________________1    -120.000   2.000        1.62588  35.642     -21.000   1.5003     -16.500   1.000        1.77279  49.444    -120.000   (Bf)D.sub.o = 3.00, Bf = 31.35, β = 1.429d.sub.o = 25.716, f.sub.RCL = -69.93______________________________________ 
    
     
                       TABLE 4______________________________________(Third embodiment)F = 51.26, F number 5.0, 2ω = 44.8°Radius of  Center Thickness                        Refractive                                 AbbeCurvature  Distance     Index    NumberNo.  r          d            n        ν______________________________________1    -214.920   2.200        1.64831  33.772     -22.450   1.5003     -17.030   1.000        1.77279  49.444    -229.340   (Bf)D.sub.o = 3.00, Bf = 30.87, β = 1.424d.sub.o = 25.716, f.sub.RCL = -68.83______________________________________ 
    
     
                       TABLE 5______________________________________(Fourth embodiment)F = 55.00, F number 5.3, 2ω = 41.8°Radius of  Center Thickness                        Refractive                                 AbbeCurvature  Distance     Index    NumberNo.  r          d            n        ν______________________________________1     250.000   3.700        1.61750  30.752    -23.000    1.5003    -17.500    1.000        1.84042  43.304     855.000   (Bf)D.sub.o = 2.00, Bf = 32.59, β = 1.528d.sub.o = 26.716, f.sub.RCL = -57.38______________________________________ 
    
     Fifth and sixth embodiments of the RCL of the present invention shown in Tables 7 and 8 are designed for a tessor type main lens ML shown in the Japanese Laid-Open Patent Application No. 60-176011 filed by the assignee of the present invention. 
     In those tables, 2ω is an image angle, f is a focal distance of the main lens ML, f RCL  is a focal distance of the RCL, Bf ML  is a back focus of the main lens ML, F is a resultant focal distance of the main lens ML and the RCL, Bf is a back focus to the image plane, D o  is an air gap from the last plane of the main lens ML to the first plane of the RCL, d o  is a distance from the plane of the RCL facing the object to the rear focal point O 1  of the main lens ML, and β is a magnification of the RCL. 
     
                       TABLE 6______________________________________(main lens)f = 36.00, F number 3.0, 2ω = 61.0°Radius of  Center Thickness                        Refractive                                 AbbeCurvature  Distance     Index    NumberNo.  r          d            n        ν______________________________________1    11.690     3.800        1.77279  49.442    25.640     1.2803    -57.950    1.440        1.71736  29.484    11.060     0.9205    21.940     3.800        1.79668  45.426    -13.690    0.920        1.51680  64.12-125.200   (Bf.sub.ML)Bf.sub.ML =  27.15______________________________________ 
    
     A stop S (behind stop) is arranged at a position of 0.720 from the last plane of the main lens ML. 
     
                       TABLE 7______________________________________(Fifth embodiment)F = 48.60, F number 4.0, 2ω = 46.4°Radius of  Center Thickness                        Refractive                                 AbbeCurvature  Distance     Index    NumberNo.  r          d            n        ν______________________________________1    -110.940   2.260        1.64831  33.772     -23.740   1.5403     -18.890   1.030        1.77279  49.444    -128.570   (Bf)D.sub.o = 1.720, Bf = 29.741, β = 1.350d.sub.o = 25.895, f.sub.RCL = -81.52______________________________________ 
    
     
                       TABLE 8______________________________________(Sixth embodiment)F = 48.60, F number 4.0, 2ω = 46.6°Radius of  Center Thickness                        Refractive                                 AbbeCurvature  Distance     Index    NumberNo.  r          d            n        ν______________________________________1    -101.460   3.100        1.61750  30.752    -22.890    1.6503    -18.040    1.000        1.84042  43.304    -69.820    (Bf)D.sub.o = 1.720, Bf = 29.089, β = 1.350d.sub.o = 25.895, f.sub.RCL = -80.78______________________________________ 
    
     The values corresponding to the conditional formulas (1)-(8) in the first to sixth embodiments are given below. 
     
                                           TABLE 9__________________________________________________________________________(Values corresponding to conditions)(1)  (2)        (3)   (4)  (5) (6)  (7)  (8) Embodi- ment ##STR1##      ##STR2##         ##STR3##               ##STR4##                    n.sub.2                        ##STR5##                             ##STR6##                                  ν.sub.2 - ν.sub.1__________________________________________________________________________1    0.5634     1.258        0.8955              0.06416                   1.79668                       0.02139                            0.02852                                 14.672    0.5771     1.273        0.8955              0.06435                   1.77279                       0.02145                            0.02860                                 13.803    0.5593     1.318        0.8955              0.06829                   1.77279                       0.02179                            0.03196                                 15.674    0.5976     1.314        0.9304              0.10806                   1.84042                       0.02614                            0.06449                                 12.555    0.5657     1.257        0.9377              0.05925                   1.77279                       0.01889                            0.02772                                 15.676    0.5838     1.269        0.9377              0.07118                   1.84042                       1.02043                            0203838                                 12.55__________________________________________________________________________ 
    
     The RCL in each embodiment has a compact structure and it can be mounted in a still camera or video camera. 
     Where the RCL of the present invention is mounted, the focusing may be done by driving the main lens ML toward the object, driving the main lens ML and the RCL in union, or driving the RCL alone along the optical axis. 
     The RCL in each embodiment of the present invention comprises two lenses, positive and negative lenses, although at least one lens unit of the RCL of the present invention may be a bonded lens. Further, at least one lens unit of the RCL of the present invention may be a plastic lens. 
     The present invention is not limited to the above embodiments but the lenses may be bonded lenses.