Abstract:
An optical system for use in conjunction with a camera viewfinder having a penta-mirror comprises a positive first lens element and a negative second lens element satisfying predetermined conditions.

Description:
REFERENCE TO RELATED APPLICATION 
     This a continuation in-part of U.S. patent application Ser. No. 07,461,512 filed Jan. 5, 1990, now abandoned. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to an optical system for use with a viewfinder in single-lens reflex cameras that employ a penta-mirror in place of a pentaprism. 
     Prior art optical systems for use with a viewfinder having a penta-mirror in place of a pentaprism are described in Japanese Utility Model Publication Nos. 48-32325 and 48-10424. These prior art optical systems are advantageous over those which employ a pentaprism in that they suffer a smaller degree of reduction in magnification. On the other hand, they use so many lens elements that the overall length of the lenses in the eyepiece increases, thus making it impossible to realize a compact camera. Further, the use of expensive glass results in a higher production cost. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to solve the above and other problems of the prior art and to provide an improved optical system for use with a viewfinder that employs a penta-mirror. The system adopts a simple two-unit-two-element composition and yet insures sharp viewing and good performance. Not only is this optical system compact, but it also achieves high magnification and can be manufactured at low cost. 
     The above and other objects of the present invention are attained by an optical system for use with a viewfinder in a single-lens reflex camera, which optical system comprises, in order from the finder screen side, a first lens unit composed of a positive lens element and a second lens unit composed of a negative lens element. This optical system satisfies the following conditions: 
     
         -2&lt;SF1&lt;0                                                   (1) 
    
     
         0&lt;SF2&lt;3                                                    (2) 
    
     where SF1 is the shape factor of the first lens unit and SF2 is the shape factor of the second lens unit. 
     In a preferred embodiment, at least one of the four surfaces delineating said first and second lens units is aspheric. 
     In another preferred embodiment, the optical system further satisfies the following conditions: 
     
         0.05&lt;(d.sub.1 +d.sub.2 +d.sub.3)/f&lt;0.3                     (3) 
    
     where d 1  is the thickness of said first lens in the direction of optical axis, d 2  is the distance between said first and second lenses on the optical axis, d 3  is the thickness of said second lens in the direction of optical axis, and f is the composite focal length of the optical system. 
     Furthermore, preferably, the optical system meets the following condition: 
     
         1.0&lt;f.sub.F /f&lt;1.3                                         (4) 
    
     where f F  is the distance from the finder screen to the first surface of the first lens. 
     The first lens and the second lens may be cemented together. 
     The first lens and second lenses are preferably formed of acrylic resin and polycarbonate resin, respectively, or materials having comparable Abbe numbers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be more clearly understood from the following description in conjunction with the accompanying drawings, wherein: 
     FIGS. 1-13 are simplified cross-sectional views of optical system constructed according to Examples 1-13, respectively; and 
     FIGS. 14-26 are graphs plotting the aberration curves obtained with the optical systems of Examples 1-13, respectively, when the eye ring has a diameter of 4. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     According to the present invention, the optical system comprises, in order from the finder screen side, a first lens unit composed of a positive lens element and a second lens unit composed of a negative lens element. This optical system satisfies the following conditions: 
     
         -2&lt;SF1&lt;0                                                   (1) 
    
     
         0&lt;SF2&lt;3                                                    (2) 
    
     where SF1 is the shape factor of the first lens unit and SF2 is the shape factor of the second lens unit, with the shape factor being given by SF=(R s  +R E )/(R s  -R E ), where R s  is the radius of curvature of the surface of each lens on the finder screen side and R E  is the radius of curvature of the surface of each lens on the eyepoint side. 
     Condition (1) sets forth a requirement that should be satisfied by the shape factor of the first lens unit. If SF1 is less than the lower limit of this condition, coma can be easily compensated but spherical aberration is difficult to compensate. If SF1 exceeds the upper limit of this condition, spherical aberration can be easily compensated but coma is difficult to compensate. Further, the distance between the first and second lens units will increase at their marginal areas to cause an undue increase in the effective aperture of the first lens. 
     Condition (2) sets forth a requirement that should be satisfied by the shape factor SF2 of the second lens unit. If SF2 is less than the lower limit of this condition, coma is difficult to compensate. If SF2 exceeds the upper limit of this condition, the radius of curvature of the surface on the eyepoint side will decrease to produce a virtually short eyepoint. 
     In order to provide better performance, at least one of the four surfaces delineating the first and second lens units is preferably aspheric. If an aspherica surface is not used, marked coma or changes in eyesight with respect to the center will occur in the marginal area of the visual field. The occurrence of coma is particularly noticeable if the distance between the first and second lens units is increased in order to attain a higher magnification. 
     In a preferred embodiment, the aspheric surface is preferably of a shape expressed by: ##EQU1## where X is the distance measured from the apex along the optical axis in the direction in which rays travel, Y is the height from the optical axis, R is the radius of curvature of a reference spherical plane, K is the shape factor of a rotating quadratic curved plane, and A 2m  is the asphericity coefficient of a higher degree. 
     In another preferred embodiment, the optical system further satisfies the following condition: 
     
         0.05&lt;(d.sub.1 +d.sub.2 +d.sub.3)/f&lt;0.3                     (3) 
    
     where d 1  is the thickness of said first lens in the direction of optical axis, d 2  is the distance between said first and second lenses on the optical axis, d 3  is the thickness of said second lens in the direction of optical axis, and f is the composite focal length of the optical system. 
     Condition (3) relates to the ratio of the overall length of the optical system to its composite focal length. If the lower limit of this condition is not reached, a desired high magnification is not attainable. If the upper limit of this condition is exceeded, high magnification is attainable, but the effective aperture of the first lens will become large. 
     Condition (4) defines a distance from the screen of the finder to the first surface of the first lens. If the lower limit would be exceeded, the finder magnification would be decreased and the finder coverage would be unduly decreased. In the worst cases, it would be exceeded, it would be necessary to use a lens having a short radius of curvature, which results in difficulty in manufacturing the lens. 
     In order to achieve effective compensation for chromatic aberration, it is particularly preferred to make the first lens of an acrylic resin or a Crown glass having an Abbe number comparable to that of any acrylic resin, and to make the second lens of a polycarbonate resin or a flint glass having an Abbe number comparable to that of a polycarbonate resin. The use of resin-made lenses offers the added advantage of lighter weight and lower cost. 
     In addition, it is possible to cement the first and second lenses together. Also, these two lenses may be formed of acrylic resin. In this case, the machining of the lenses is improved. 
     Examples 1-13 of the present invention are described below with reference to data sheets, in which SN denotes the surface number, r i  denotes the radius of curvature (in millimeters) of the ith surface, d i  denotes the distance (in millimeters) between the ith and (i+1)th surfaces, n j  denotes the refractive index of the jth lens (optical material) at the d-line, v j  denotes the Abbe number of the jth lens (optical material), K i  denotes the shape factor of a rotating quadratic curved plane for the ith surface, A 4i  denotes the biquadratic asphericity coefficient of the ith surface, and A 6i  denotes the triquadratic asphericity coefficient of the ith surface. In each of Examples 1-12, the distance from the fourth surface to the eyepoint is 15 mm and the effective finder coverage is 95%. In Example 13, the distance from the fourth surface to the eyepoint is also 15 but the effective finder coverage is 92%. The distance from the screen to the first surface is 81.004 mm for Examples 1-10, 74.6 mm for Examples 11 and 12, and 75.013 for Example 13. 
     EXAMPLE 1 
     
         __________________________________________________________________________SN  r.sub.i     d.sub.i         n.sub.j             ν .sub.j                 K.sub.i                      A.sub.6i__________________________________________________________________________1   15.534     6.476         1.49186             57.4                 -0.23684                      -7.74061 × 10.sup.-82   -28.357     2.154       -7.370183   -53.238     1   1.58547             29.94   18__________________________________________________________________________ 
    
     Distance from the screen to the first surface: 81.004 
     Distance from the fourth surface to the eyepoint: 15 
     Effective finder voverage: 95% 
     f=70.229 
     Magnification: 0.740X (52/70.229) 
     SF1=-0.292 
     SF2 =0.495 
     (d 1  +d 2  +d 3 )/f=0.137 
     f F  /f=1.153 
     EXAMPLE 2 
     
         __________________________________________________________________________SN  r.sub.i     d.sub.i         n.sub.j             ν .sub.j                 K.sub.i                      A.sub.6i__________________________________________________________________________1   14.489     6.224         1.49186             57.4                 -0.34447                      -7.28882 × 10.sup.-82   -34.501     2.005       -11.66943   -146.305     1   1.58547             29.94   15__________________________________________________________________________ 
    
     Distance from the screen to the first surface: 81.004 
     Distance from the fourth surface to the eyepoint: 15 
     Effective finder coverage: 95% 
     f=69.961 
     Magnification: 0.743X (52/69.961) 
     SF1=-0.408 
     SF2=0.814 
     (d 1  +d 2  +d 3 )/f=0.132 
     f F  /f=1.158 
     EXAMPLE 3 
     
         __________________________________________________________________________SN  r.sub.i     d.sub.i         n.sub.j             ν .sub.j                 K.sub.i                      A.sub.6i__________________________________________________________________________1   13.231     8.203         1.49186             57.4                 -0.275                      -1.30316 × 10.sup.-72   -24.387     1.231       -9.80573                       2.4702 × 10.sup.-83   -110.535     1   1.71736             29.34   15__________________________________________________________________________ 
    
     Distance from the screen to the first surface: 81.004 
     Distance from the fourth surface to the eyepoint: 15 
     Effective finder coverage: 95% 
     f=67.03 
     Magnification: 0.776X (52/67.03) 
     SF1=-0.297 
     SF2=0.761 
     (d 1  +d 2  +d 3 )/f=0.156 
     f F  /f=1.208 
     EXAMPLE 4 
     
         __________________________________________________________________________SN  r.sub.i     d.sub.i         n.sub.j             ν .sub.j                 K.sub.i                      A.sub.6i__________________________________________________________________________1   14.678     7.6 1.49186             57.4                 -0.38849                      -9.08487 × 10.sup.-82   -28.267     2.153       -9.041083   -333.815     1   1.80518             25.44   18__________________________________________________________________________ 
    
     Distance from the screen to the first surface: 81.004 
     Distance from the fourth surface to the eyepoint: 15 
     Effective finder coverage: 95% 
     f=67.314 
     Magnification: 0.772X (52/67.314) 
     SF1=-0.316 
     SF2=0.898 
     (d 1  +d 2  +d 3 )/f=0.160 
     f F  /f=1.203 
     EXAMPLE 5 
     
         __________________________________________________________________________SN  r.sub.i     d.sub.i         n.sub.j             ν .sub.j                 K.sub.i                      A.sub.6i__________________________________________________________________________1   13.944     7.548         1.49186             57.4                 -0.43628                      -9.28391 × 10.sup.-82   -31.306     1.94        -12.31743   136.934     1   1.80518             25.44   15__________________________________________________________________________ 
    
     Distance from the screen to the first surface: 81.004 
     Distance from the fourth surface to the eyepoint: 15 
     Effective finder coverage: 95% 
     f=66.927 
     Magnification: 0.777X (52/66.927) 
     SF1=-0.384 
     SF2=1.246 
     (d 1  +d 2  +d 3 )/f=0.157 
     f F  /f=1.210 
     EXAMPLE 6 
     
         __________________________________________________________________________SN  r.sub.i     d.sub.i         n.sub.j             ν .sub.j                 K.sub.i                       A.sub.6i__________________________________________________________________________1   12.472     7.237         1.49186             57.4                  -0.49027                       -8.40087 × 10.sup.-82   -69.886     1.944       -53.61693   30.528     1   1.80518             25.44   11.288__________________________________________________________________________ 
    
     Distance from the screen to the first surface: 81.004 
     Distance from the fourth surface to the eyepoint: 15 
     Effective finder coverage: 95% 
     f=65.58 
     Magnification: 0.793X (52/65.58) 
     SF1=-0.697 
     SF2=2.173 
     (d 1  +d 2  +d 3 )/f=0.155 
     f F  /f=1.235 
     EXAMPLE 7 
     
         __________________________________________________________________________SN  r.sub.i     d.sub.i         n.sub.j             ν .sub.j                 K.sub.i                      A.sub.6i__________________________________________________________________________1   16.589     5.989         1.49186             57.4                 -0.88321                      -1.00077 × 10.sup.-72   -38.051     3.1443   -142.669     1   1.58547             29.94   18__________________________________________________________________________ 
    
     Distance from the screen to the first surface: 81.004 
     Distance from the fourth surface to the eyepoint: 15 
     Effective finder coverage: 95% 
     f=70.066 
     Magnification: 0.742X (52/70.066) 
     SF1=-0.393 
     SF2=0.776 
     (d 1  +d 2  +d 3 )/f=0.145 
     f F  /f=1.156 
     EXAMPLE 8 
     
         __________________________________________________________________________SN  r.sub.i     d.sub.i         n.sub.j             ν .sub.j                 K.sub.i                      A.sub.6i__________________________________________________________________________1   15.091     6.814         1.49186             57.42   -24.665     1.820       -6.00198                      -8.98476 × 10.sup.-63   -31.262     1   1.58547             29.94   20.510__________________________________________________________________________ 
    
     Distance from the screen to the first surface: 81.004 
     Distance from the fourth surface to the eyepoint: 15 
     Effective finder coverage: 95% 
     f=70.273 
     Magnification: 0.740X (52/70.273) 
     SF1=-0.241 
     SF2=0.208 
     (d 1  +d 2  +d 3 )/f=0.137 
     f F  /f=1.152 
     EXAMPLE 9 
     
         __________________________________________________________________________SN  r.sub.i     d.sub.i         n.sub.j             ν .sub.j                 K.sub.i                      A.sub.6i__________________________________________________________________________1   13.991     6.168         1.49186             57.42   -51.422     2.2503   -86.065     1   1.58547             29.9                 58.9175                      -5.27521 × 10.sup.-54   18__________________________________________________________________________ 
    
     Distance from the screen to the first surface: 81.004 
     Distance from the fourth surface to the eyepoint: 15 
     Effective finder coverage: 95% 
     f=69.757 
     Magnification: 0.745X (52/69.757) 
     SF1=-0.572 
     SF2=0.654 
     (d 1  +d 2  +d 3 )/f=0.135 
     f F  /f=1.161 
     EXAMPLE 10 
     
         __________________________________________________________________________SN  r.sub.i     d.sub.i         n.sub.j             ν .sub.j                 K.sub.i                      A.sub.6i__________________________________________________________________________1   12.463     6.502         1.49186             57.42   -73.265     1.8863   -60.334     1   1.58547             29.94   18.165            4.40044                      -4.80934 × 10.sup.-7__________________________________________________________________________ 
    
     Distance from the screen to the first surface: 81.004 
     Distance from the fourth surface to the eyepoint: 15 
     Effective finder coverage: 95% 
     f=68.342 
     Magnification: 0.761X (52/68.342) 
     SF1=-0.709 
     SF2=0.537 
     (d 1  +d 2  +d 3 )/f=0.137 
     f F  /f=1.185 
     EXAMPLE 11 
     
         __________________________________________________________________________SN r.sub.i  d.sub.i     n.sub.j         ν .sub.j            K.sub.i                 A.sub.6i A.sub.6i__________________________________________________________________________1  12.818  8.5     1.49186         57.4            0.22455                 -5.72694 × 10.sup.-6                          -8.00138 × 10.sup.-82  63.022  03  63.022  1.5     1.58547         29.94  18            4.08541       -2.04666 × 10.sup.-7__________________________________________________________________________ 
    
     Distance from the screen to the first surface: 74.6 
     Distance from the fourth surface to the eyepoint: 15 
     Effective finder coverage: 95% 
     f=65.211 
     Magnification: 0.797X (52/65.211) 
     SF1=-1.511 
     SF2=1.800 
     (d 1  +d 2  +d 3 )/f=0.153 
     f F  /f=1.144 
     EXAMPLE 12 
     
         __________________________________________________________________________SN r.sub.i  d.sub.i     n.sub.j         ν .sub.j            K.sub.i                 A.sub.6i A.sub.6i__________________________________________________________________________1   15.175  8.5     1.49186         57.4            0.51174                 -6.7107 × 10.sup.-6                          -4.61234 × 10.sup.-82  737.592  03  737.592  1.5     1.58547         29.94   25.562       7.51188       -2.60332 × 10.sup.-8__________________________________________________________________________ 
    
     Distance from the screen to the first surface: 74.6 
     Distance from the fourth surface to the eyepoint: 15 
     Effective finder coverage: 95% 
     f=68.746 
     Magnification: 0.756X (52/68.746) 
     SF1=-1.042 
     SF2=1.072 
     (d 1  +d 2  +d 3 )/f=0.145 
     f F  /f=1.085 
     EXAMPLE 13 
     
         __________________________________________________________________________SN  r.sub.i     d.sub.i         n.sub.j             ν .sub.j                 K.sub.i                      A.sub.6i__________________________________________________________________________1   19.175     5.70         1.49186             57.4                 -0.781                      -1.523 × 10.sup.-82   -82.5 2.103   82.5  1.5 1.49186             57.44   18__________________________________________________________________________ 
    
     Distance from the screen to the first surface: 75.013 
     Distance from the fourth surface to the eyepoint: 15 
     Effective finder coverage: 92% 
     f=70.719 
     Magnification: 0.735X (52/70.719) 
     SF1=-0.623 
     SF2=1.558 
     (d 1  +d 2  +d 3 )/f=0.132 
     f F  /f=1.061 
     As described above, the optical system of the present invention adopts a simple two-unit-two-element composition and yet it is capable of achieving higher magnification without using a pentaprism. In spite of its small size, this optical system insures good performance as is clear from the graphs in FIGS. 14-26 which plot the aberration curves obtained with this system. If both the first and second lenses are made of resins, the additional advantage of very low cost is attained.