Abstract:
The present invention relates to a variable magnification finder comprising an objective lens group having a positive refractive power for enabling a wide angle view of an object. The objective lens group includes a first lens sub-group having either a negative or a positive refractive power; a second lens sub-group having a negative refractive power; a third lens sub-group having a positive refractive power and having a cemented lens with at least two pieces; and a fourth lens sub-group having a positive refractive power. The third lens sub-group is movable for magnifying the image, and the second lens sub-group is movable for compensating the focus of the magnified image. The finder further comprises an eyepiece lens group having a positive refractive power for viewing the image.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a variable magnification finder. More particularly, the present invention relates to a compact variable magnification finder that can obtain a wide angle view for a real image finder optical system of a camera or a video camera. 
     2. Description of the Prior Art 
     Conventionally, most compact cameras employ a virtual image finder such as an Albada finder or an inverse Galilean finder. Such finders provide a relatively wide angle view and have a compact construction because they do not require a prism for erecting the image. However, in such finders, the lens closest to an object must be large and the view field of the finder blurs in the marginal zone. 
     Recently, because of the above-mentioned drawbacks, real image finders are employed instead of virtual image finders. Therefore, real image optical system is more relevant to developing a picture-taking lens of a compact camera with a wide angle view. 
     Real image finders are disclosed in U.S. Pat. No. 4,842,395 entitled &#34;FINDER OF VARIABLE MAGNIFICATION,&#34; Japanese Laid-open Patent No. SH061-156018 entitled &#34;Variable Magnification Finder,&#34; Laid-open Patent No. PYEONG 6-51197 entitled &#34;Real Image Finder,&#34; Laid-open Patent No. PYEONG 6-18780 entitled &#34;Variable Magnification Finder Optical System,&#34; and patent application No. PYEONG 6-102454 discloses an objective lens group comprising four lens groups which perform variable magnification and compensation by moving a second lens group and a third lens group. 
     In the above-mentioned real image finder optical systems, an objective lens group and a condenser lens group form an image of an object, and the enlarged image of an object is observed through an eyepiece group. 
     When a real image finder optical system is compact, a wide angle view may be achieved if an image formed through the objective lens is small by shortening the focal distance of the objective lens at a wide angle position. 
     However, the refractive power of the objective lens increases as the focal distance of the objective lens decreases, so that it is difficult to compensate for aberrations. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a compact variable magnification finder which enables a wide angle view and has a compensated chromatic aberration with a relatively simple construction. 
     In order to achieve the above object, the present invention provides a variable magnification finder with an objective lens group having a positive refractive power for enabling a wide angle view of an object where the objective lens group includes four sub-groups. The first lens sub-group has either a negative or a positive refractive power and the second lens sub-group has a negative refractive power. The third lens sub-group has a positive refractive power and has a junction lens with at least two pieces, and the fourth lens sub-group has a positive refractive power. The third lens sub-group is movable for magnifying the image, and the second lens sub-group is movable for compensating the focus of the magnified image of the variable magnification finder. The variable magnification finder further includes an eyepiece lens group having a positive refractive power for viewing the image. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a schematic sectional view of the lens groups of a variable magnification finder optical system in accordance with a first preferred embodiment of the present invention; FIG. 1B is a schematic sectional view of the lens groups of a variable magnification finder telescope system in accordance with a first preferred embodiment of the present invention; FIGS. 2A-2C are diagrams showing a spherical aberration, astigmatism, and distortion aberration, respectively, of a variable magnification finder optical system in accordance with a first preferred embodiment of the present invention; 
     FIGS. 2D-2F are diagrams showing a spherical aberration, astigmatism, and distortion aberration, respectively, of a variable magnification finder telescope system in accordance with a first preferred embodiment of the present invention; 
     FIG. 3A is a schematic sectional view of the lens groups of a variable magnification finder optical system in accordance with a second preferred embodiment of the present invention; 
     FIG. 3B is a schematic sectional view of lens of a variable magnification finder telescope system in accordance with a second preferred embodiment of the present invention; 
     FIGS. 4A-4C are diagrams showing a spherical aberration, astigmatism, and distortion aberration, respectively, of a variable magnification finder optical system in accordance with a second preferred embodiment of the present invention; 
     FIGS. 4D-4F are diagrams showing a spherical aberration, astigmatism, and distortion aberration, respectively, of a variable magnification finder telescope system in accordance with a second preferred embodiment of the present invention; 
     FIG. 5A is a schematic sectional view of the lens groups of a variable magnification finder optical system in accordance with a third preferred embodiment of the present invention; 
     FIG. 5B is a schematic sectional view of the lens groups of a variable magnification finder telescope system in accordance with a third preferred embodiment of the present invention; 
     FIGS. 6A-6C are diagrams showing a spherical aberration, astigmatism, and distortion aberration, respectively, of a variable magnification finder optical system in accordance with a third preferred embodiment of the present invention; 
     FIGS. 6D-6F are diagrams showing a spherical aberration, astigmatism, and distortion aberration, respectively, of a variable magnification finder telescope system in accordance with a third preferred embodiment of the present invention; 
     FIG. 7A is a schematic sectional view of the lens groups of a variable magnification finder optical system in accordance with a fourth preferred embodiment of the present invention; 
     FIG. 7B is a schematic sectional view of the lens groups of a variable magnification finder telescope system in accordance with a fourth preferred embodiment of the present invention; 
     FIGS. 8A-8C are diagrams showing a spherical aberration, astigmatism, and distortion aberration, respectively, of a variable magnification finder optical system in accordance with a fourth preferred embodiment of the present invention; and 
     FIGS. 8D-8F are diagrams showing a spherical aberration, astigmatism, and distortion aberration, respectively, of a variable magnification finder telescope system in accordance with a fourth preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will become apparent from the following detailed description, when viewed in light of the accompanying drawings. 
     As shown in FIGS. 1A, 1B, 3A, 3B, SA, 5B, 7A, and 7B, a variable magnification finder in accordance with each preferred embodiment of the present invention comprises an objective lens group 10 having a positive refractive power, an eyepiece lens group 30 having a positive refractive power, and a prism 20, mounted between the objective lens group 10 and eyepiece lens group 30, for erecting the image. 
     Objective lens group 10 comprises a fixed first lens subgroup I having a positive or negative refractive power, a movable second lens sub-group II having a negative refractive power, a movable third lens sub-group III having a positive refractive power and a fixed fourth lens sub-group IV having a positive refractive power. 
     Objective lens group 10 changes magnification by moving lens sub-group III as a variable magnification finder section and compensates aberration change resulting from the above-mentioned magnification change by moving lens group II as a compensation section. 
     Lens sub-group III includes a cemented lens having at least two pieces (preferred embodiments 1-4). 
     More specifically, lens sub-group III includes a cemented two piece plastic lens (preferred embodiment 1); a cemented plastic lens and a glass lens (preferred embodiment 2); a cemented two piece glass lens and one piece of plastic lens (preferred embodiment 3); and a cemented two piece and another plastic lens (preferred embodiment 4). 
     The operation of a variable magnification finder whose construction is described above is explained as follows. 
     To achieve an acceptable aberration compensation for chromatic aberration throughout an overall range of variable magnification and to shorten a change of aberration resulting from a variable magnification with a relatively simple lens construction, refractive power must be distributed by dividing an objective lens group into several groups, i.e., variable magnification section, a compensation section, and an image formation section. 
     Particularly, to compensate for chromatic aberration, a variable magnification section which is very sensitive to chromatic aberration must comprise cemented lenses, each having different refractive powers in addition to distribution of refractive power into each group. 
     Generally, a cemented lens group of a variable magnification section compensates chromatic aberration by joining at least one lens having a positive refractive power and at least one lens having a negative refractive power. 
     If a variable magnification section has a positive refractive power, a negative refractive power is joined with the positive refractive power, so that the variable magnification section compensates chromatic aberration by making the positive refractive power larger than the negative refractive power. 
     An amount of a chromatic aberration is generally expressed in the following equation. ##EQU1## CA: a chromatic aberration amount Φ1: a power of a first lens of a variable magnification section 
     ν1: Abbe number of a first lens of a variable magnification section 
     Φ2: a power of a second lens of a variable magnification section 
     ν2: Abbe number of a second lens of a variable magnification section 
     The chromatic aberration amount CA, as stated in the above equation, increases when the value of positive refractive power Φ1 is larger than the value of a negative refractive power Φ2, so that refractive power of a variable magnification section Φ1+Φ2 has a positive value. 
     Therefore, in order to decrease the chromatic aberration amount CA, an Abbe number of a first lens ν1 having a positive refractive power must become large and an Abbe number of a second lens ν2 having a negative refractive power must be small. 
     In other words, a first lens must be made of crown material and a second lens must be made of flint material. 
     Accordingly, chromatic aberration becomes small as chromatic aberration amount is close to zero. 
     If a variable magnification section has a negative refractive power, a lens is constructed by selecting materials with characteristics opposite the above mentioned description. 
     In other words, chromatic aberration must be compensated by selecting materials which make an Abbe number of a first lens small and an Abbe number of a second lens large in order to reduce the chromatic aberration amount CA. 
     A coefficient of an aspherical lens of each lens in accordance with a preferred embodiment which satisfies the above conditions is expressed by the following equation. ##EQU2## X: optical axial thickness from the lens vertex y: vertical thickness of the optical axis 
     C: reciprocal of the radius of curvature 
     K: conic number 
     A 4 , A 6 , A 8 , A 10  : aspherical coefficients 
     The construction of a lens which satisfies the above-mentioned conditions and values is described below. 
     As shown in FIGS. 1A and 1B, a variable magnification finder in accordance with a first preferred embodiment of the present invention comprises objective lens group 10, prism 20, and eyepiece lens group 30. Also, objective lens group 10 includes four lens sub-groups, I, II, III, and IV, and lens sub-group III comprises a cemented lens of two plastic pieces. 
     Values for the first preferred embodiment of the present invention are shown in table 1. 
     A radius of curvature of a refractive surface equals r 1 , a thickness of lens or a distance between lenses equals d 1 ,(I=1-19), a d-line refractive index of a lens equals N, an Abbe number of a lens equals ν, a magnification of an overall lens system equals m and a half viewing angle equals W. 
     In accordance with a first preferred embodiment of the present invention, a diagram value of a viewing angle of a variable magnification finder 2W is 65.24°˜23.72° and magnification is ˜0.33˜0.90. 
     
                       TABLE 1______________________________________  radius ofsurface  a curvature             thickness refractive                               Abbenumber (r.sub.i)  (d.sub.i) index (N)                               number (ν)______________________________________1      135.499    1.50      1.49200 57.42      12.026     variable3      11.632     1.84      1.49200 57.44      6.530      variable5      15.400     0.9       1.58700 31.06      9.000      5.1       1.49200 57.47      -8.420     variable8      20.480     4.90      1.49200 57.49      ∞    4.8010     ∞    25.10     1.49200 57.411     ∞    1.4012     26.432     2.13      1.49200 57.413     -20.524    17.4814     ∞______________________________________ 
    
     In the first preferred embodiment of the present invention, the above-mentioned thickness of lens or distance between lenses vary according to a view angle as follows. 
     
         ______________________________________ 2w = 65.2°               2w = 31.8°                         2w = 23.7°______________________________________d2    3.300         6.864     2.761d4    15.109        4.318     2.224d7    15.255        22.482    28.679______________________________________    aspherical coefficient                  aspherical coefficient    of a second surface                  of a third surface______________________________________K        0.619777      0.4992116EA4       0.1906919E-3  -0.2318559E-2A6       -0.5301008E-4 -0.8585752E-4A8       0.8578080E-6  0.291292BE-5A10      -0.3970000E-8 -0.1581000E-6______________________________________aspherical     aspherical  asphericalcoefficient    coefficient coefficient ofof the fourth  of the fifth                           the twelfthsurface        surface     surface______________________________________K    -0.784158E     -0.3280350E+2                           -0.7000000EA4   -0.4786413E-13 0.3037277E-3                           0A6   -0.3573633E-6  -0.1407511E-4                           0A8   -0.6426686E-6  0.1409382F-6                           0A10  0.5594403E-7   0.1230796E-8                           0______________________________________ 
    
     As shown in FIGS. 3A and 3B, a variable magnification finder in accordance with a second preferred embodiment of the present invention comprises objective lens group 10, prism 20 and eyepiece lens group 30. Also, objective lens group 10 includes four lens sub-groups, I, II, III, and IV. 
     As mentioned above, lens sub-group I includes a two piece cemented lens and lens sub-group III comprises a cemented lens of one glass piece and one plastic piece. 
     Values for the second preferred embodiment of the present invention are shown in table 2. 
     In accordance with a second preferred embodiment of the present invention, a diagram value of a viewing angle of a variable magnification finder 2W is 54.2°˜23.7° and magnification is ˜0.404˜0.898. 
     
                       TABLE 2______________________________________surface radius of thickness  refractive                               an Abbenumber  a curvature             (d)        Index(N)                               number(ν)______________________________________1       24.666    1.74       1.78472                               25.72       96.194    0.80       1.48749                               70.43       9.218     variable4       -40.89    2.15       1.49200                               57.45       36.369    variable6       14.526    4.64       1.49200                               57.47       -7.692    0.80       1.84666                               23.88       -10.728   variable9       13.312    4.50       1.49200                               57.410      31.676    5.1211      ∞   26.40      1.51680                               64.212      ∞   2.2013      30.292    4.47       1.67003                               47.214      -8.184    1.00       1.74077                               27.815      -24.237   19.0016      ∞______________________________________ 
    
     In the second preferred embodiment of the present invention, the above-mentioned thickness of lens or distance between lenses vary according to a view angle as follows. 
     
         ______________________________________ 2w = 54.2°               2w = 27.8°                         2w = 23.7°______________________________________d3    2.962         9.000     7.967d5    19.505        8.915     2.500d8    15.497        20.048    27.497______________________________________aspherical    aspherical   asphericalcoefficient   coefficient  coefficient ofof the fifth surface              of the sixth surface                           the ninth surface______________________________________K    0.61041081E0  -0.15643027E+2                           -0.21607242E0A4   -0.29145657E-3              0.37409431E-3                           -0.12394355E-3A6   -0.11035981E-4              -0.92771152E-5                           0.18165015E-4A8   0.14217520E-5 0.14501134E-6                           -0.10712869E-5A10  -0.67993762E-7              -0.98641230E-9                           0.17124611E-7______________________________________ 
    
     As shown in FIGS. 5A and 5B, a variable magnification finder in accordance with a third preferred embodiment of the present invention comprises objective lens group 10, prism 20 and eyepiece lens group 30. Also, objective lens group 10 includes four lens sub-groups I, II, III and IV. 
     As mentioned above, the lens sub-group I comprises a two piece cemented lens and lens sub-group III comprises a cemented lens of two glass pieces and one plastic piece. 
     Also, eyepiece lens group 30 includes a cemented lens of two pieces. 
     Values for the third preferred embodiment of the present invention are shown in the table 3. 
     In accordance with a third preferred embodiment of the variable magnification finder 2W is 54.2°˜23.7° and magnification is ˜0.409˜0.898. 
     
                       TABLE 3______________________________________surface radius of thickness  refractive                               an Abbenumber  a curvature             (d)        index (N)                               number(ν)______________________________________1       27.632    1.75       1.78742                               25.72       -282.368  0.80       1.48749                               70.43       9.880     variable4       -22.748   2.24       1.49200                               57.45       51.113    variable6       14.780    0.04       1.49200                               57.47       17.129    5.00       1.51680                               64.28       -7.707    1.27       1.84666                               23.89       11.169    variable10      13.171    4.50       1.49200                               57.411      30.402    5.1312      ∞   26.40      1.51680                               64.213      ∞   2.2014      30.292    4.47       1.67003                               47.215      -8.184    1.00       1.74077                               27.816      -24.237   19.0017      ∞______________________________________ 
    
     In the third preferred embodiment of the present invention, the above-mentioned thickness of lens or distance between lens vary according to a view angle as follows: 
     
         ______________________________________ 2w = 54.2°               2w = 27.8°                         2w = 23.7°______________________________________d3    3.400         8.103     6.906d5    17.962        8.946     2.665d8    15.495        19.809    27.285______________________________________aspherical     aspherical  asphericalcoefficient    coefficient coefficient ofof the fifth   of the sixth                           the ninthsurface        surface     surface______________________________________K    -0.10626751E+3 -0.77322103E                           -0.21656963EA4   -0.26040794E-13               0.67306393E-4                           -0.18566387E-3A6   0.10436942E-4  0.24471809E-5                           0.15810196E-4A8   -0.71756008E-6 -0.17205113E-6                           -0.86883071E-6A10  0.94420121E-8  0.3000e187E-8                           0.12740654E-7______________________________________ 
    
     As shown in FIGS. 7A and 7B, a variable magnification finder in accordance with a fourth preferred embodiment or the present invention comprises objective lens group 10, prism 20, and eyepiece group 30. 
     As mentioned above, objective lens group 10 includes four lens sub-groups, I, II, III, and IV, and lens sub-group I includes a two piece cemented lens. 
     Also, lens sub-group III includes a cemented lens of two pieces and another plastic lens. 
     Eyepiece lens group 30 includes a cemented lens of two pieces. 
     Values for the fourth preferred embodiment of the present invention are shown in table 4. 
     In accordance with a fourth preferred embodiment of the present invention, and a value of a viewing angle of a variable magnification finder 2W is 54.2°˜23.7° and magnification is ˜0.401˜0.900. 
     
                       TABLE 4______________________________________surface radius of thickness  refractive                               an Abbenumber  a curvature             (d)        index (N)                               number(ν)______________________________________1       22.176    2.16       1.84666                               23.82       ∞   0.80       1.53172                               48.83       8.641     variable4       -20.513   1.00       1.49200                               57.45       106.938   variable6       -106.938  1.00       1.49200                               57.47       -38.542   0.108       21.393    4.42       1.48749                               70.49       -7.692    0.80       1.84666                               23.810      -10.200   variable11      13.602    4.50       1.49200                               57.412      34.704    5.1313      ∞   26.40      1.51680                               64.214      ∞   2.2015      30.292    4.47       1.67003                               47.216      -8.184    1.00       1.74077                               27.817      -24.237   19.0018      ∞______________________________________ 
    
     In the fourth preferred embodiment of the present invention, the above-mentioned thickness of lens or distance between lenses vary according to a view angle as follows. 
     
         ______________________________________ 2w = 54.2°               2w = 27.8°                         2w = 23.7°______________________________________d3    3.349         9.395     8.494d5    18.972        8.377     2.000d8    15.286        19.836    27.111______________________________________ aspherical   aspherical   aspherical coefficient  coefficient  coefficient of of the fifth of the sixth the ninth surface      surface      surface______________________________________K     0.41931376E+3              -0.70554042E+2                           -0.18304920A4    -0.1723162BE-3              -0.22960926E-3                           -0.21826019E-3A6    0.37534386E-5              0.43988689E-S                           0.11291829E-4A8    -0.20613980E-6              -0.16777814E-6                           -0.56362619E-6A10   0.28579942E-8              0.14003472E-8                           0.79546903E-8______________________________________ 
    
     Consequently, a real image finder optical system in accordance with the preferred embodiments of the present invention, with a compact and relatively simple structure, obtains a wide angle view field and compensates for chromatic aberration using a cemented lens. 
     Accordingly, the present invention provides a variable magnification finder with an acceptable aberration performance throughout the entire range, from an optical angle range to a telescope range, even though it has high magnification.