Abstract:
A variable magnification finder has an objective lens group having an overall positive power. A first lens group and a second lens group each have a negative refractive power; a third lens group and a fourth lens group each have a positive refractive power; and an eyepiece lens group has a positive power. The second and third lens groups move to zooming from the wide angle position to the telephoto position, and compensate the change of an aberration resulting from such movement of the second and third lens groups.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a variable magnification finder for use in cameras. More particularly, the invention relates to a finder wherein an objective lens is constructed simply and compactly, by which means a wide angle field of view can be obtained. 
     2. Description of the Related Art 
     Conventionally, most compact cameras employ a virtual image finder such as an Albada finder or a reverse Galilean finder. Such finders provide a relatively wide angle of view and have a compact construction because a prism for erecting the image is not required. However, in such finders, the closest lens to an object must be large in size, and the view field of the finder blurs in the marginal zone. 
     Because of the drawbacks in virtual image finders and continuous requirements to improve compact cameras, cameras may employ a real image finder instead of a virtual image finder. A panorama function is also available in a real image finder. 
     U.S. Pat. No. 4,842,395 entitled &#34;Finder of Variable Magnification&#34;; Japanese Patent Laid-Open No. SHO 61-156018 entitled &#34;Variable Magnification Finder&#34;; Laid-Open No. PYEONG 6-151197 entitled &#34;Real Image Finder&#34;; and Laid-Open No. 6-18780 entitled &#34;Variable Magnification Finder Optical System&#34; disclose real image finders. 
     In the disclosed finders, an objective lens group and a condenser lens group form an objective image, and the image is observed through an enlarged eyepiece. However, the image formed by the objective lens group is positioned on the marginal zone of the view field, so that the view field is subject to restriction, i.e., the image is divided by the marginal zone of the view field. When constructing a finder without the division of the image, however, the resultant finder becomes large and complicated. 
     When a camera is made compact, a moderately wide field of view may be achieved by shortening the focal distance of the objective lens at the wide angle position. However, the image formed through the objective lens is small. Also, the refractive power of the objective lens increases as the focal length shortens, and it is difficult to compensate aberration. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the problems and disadvantages of the prior art by providing a variable magnification finder which is compact and enables a wide angle of view. Manufacturing costs are lowered because the lens is made from inexpensive material, while still permitting high magnification and good aberration performance throughout the range at the wide angle position. 
     In order to achieve the above object, the present invention, as embodied and broadly defined herein, provides a variable magnification finder comprising an objective lens group having an overall positive power, including, sequentially from an object, a first lens group and a second lens group each having a negative refractive power, a third lens group and a fourth lens group each having a positive refractive power, and an eyepiece lens group having a positive refractive power. The second and third lens groups move to zooming from the wide angle position to the telephoto position, and compensate the change of an aberration resulting from such movement of the second and third lens groups. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention. In the drawings, 
     FIG. 1 is a schematic longitudinal sectional view of an embodiment of an optical system according to the present invention at wide, normal, and telephoto fields of view; 
     FIGS. 2(a)-2(f) are diagrams showing the aberrations of the optical system of FIG. 1; 
     FIG. 3 is a schematic longitudinal sectional view of another embodiment of an optical system according to the present invention at wide, normal, and telephoto fields of view; 
     FIGS. 4(a)-4(f) are diagrams showing the aberrations of the optical system of FIG. 3; 
     FIG. 5 is a schematic longitudinal sectional view of still another embodiment of an optional system according to the present invention at wide, normal, and telephoto fields of view; and 
     FIGS. 6(a)-6(f) are diagrams showing the aberrations of the optical system of FIG. 5. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the present invention is described in detail below with reference to the accompanying drawings. 
     The variable magnification finders shown in FIGS. 1, 3, and 5 each include, a fixed first lens group I having a negative refractive power, a movable second lens group II having a negative refractive power, a movable third lens group III having a positive refractive power, and a fixed fourth lens group IV having a positive refractive power. The first, second, third, and fourth lens groups constitute an objective lens 10 having an overall positive refractive power. The first lens group I is closest to the object. 
     The finders further include an eyepiece 30 having a positive refractive power and a reflecting means 20 mounted between the objective lens 10 and the eyepiece 30 for rendering an objective image as an erect image. The reflecting means 20 may be a prism. 
     Each lens group I, II, III, and IV may be made from a single material, preferable a plastic. Particularly, in this embodiment, a plastic material called &#34;PMMA&#34; may be used. While using only one material decreases manufacturing costs, using only one material also increases the difficulty of compensating for chromatic aberration magnification. This difficulty can be overcome by a suitable arrangement of each lens and by using an asperipherical objective lens 10 and an asperipherical eyepiece 30. 
     The features of the invention are described in detail below. In a preferred embodiment, the first lens group I having a positive refractive power is fixed from an object to simplify the configuration of a lens barrel. The first lens group I serves as a finder cover glass which prevents the second lens group II and the third lens group III from being contaminated by dust, etc. The second lens group II serves as a compensation part, while the third lens group III serves as a magnification part. The refractive power of the second lens group II, having a negative refractive power, is divided with the first lens group I so that the resulting aberration from the compensation part having the larger power of the two is reduced. 
     The object side of the second lens group II is convex. Thus, the second lens group II (the compensation part) compensates for COMA aberration even though the first lens group becomes thick. Good image performance can be obtained throughout the whole range of a finder without a drop in image performance resulting from the movement of a pupil within the view. 
     In the embodiment shown in FIG. 1, the objective lens 10 magnifies with a positive refractive power. In particular, the fourth lens group IV has a positive refractive power and serves as a condenser lens. The fourth lens group IV is in the rear of the magnification part to minimize the size of the eyepiece 30 and the prism 20 in the compact finder system. 
     The image is focused on the image side of the fourth lens group IV on the image plane 40. Accordingly, the image side of the fourth lens group IV is plane. This construction allows the four lens groups to be used even if the zone mark or the marginal frame to the viewing field is not formed on its side. Additionally, a simply constructed panorama system can be achieved when a panorama means is installed on its side. 
     Accordingly, the fourth lens group IV has a plane side and a convex side toward an object. 
     Further, in the real image finder, dust on a focusing side, i.e., a first image forming side, may be undesirable seen through the finder. In this embodiment, to solve this problem, a condenser lens is constructed as thick as possible so that dust toward an object is out of the view field. 
     The foregoing features accomplish objects of the invention. To construct the lens of the variable magnification finder relatively simply, while still preserving acceptable aberration performance throughout the variable magnification range, the variable magnification finder may also incorporate the following features. 
     In the objective lens 10, the front surface of the first lens group I is convex toward an object, or the first lens group I is one lens with one plane surface having a negative refractive power. Further, the second lens group II has one lens or a plurality of lenses having a negative refractive power with one surface concave toward an eyepiece. Still further, the third lens group III includes at least one lens having a positive refractive power with one surface convex toward an object and an eyepiece. Still further, the fourth lens group IV has one lens with a positive refractive power, a convex surface toward an object, and a plane surface toward an eyepiece. 
     When the foregoing conditions are satisfied, the objective lens 10 including the first, second, third, and fourth lens groups I to IV has an overall positive refractive power. Also, the objective lens 10 has a larger overall positive refractive power when an image is focused near the fourth lens group IV, i.e., the condenser lens group where the image is formed initially. 
     The objective lens 10 achieves good aberration performance by: dividing the overall refractive power between the first to fourth lens groups I to IV, compensating the spherical aberration generated by the third lens group III and the fourth lens group IV with the second lens group III, and dividing the negative refractive power of the first and second lens groups I and II. 
     The objective lens 10 which meets the foregoing conditions is varied by moving the third lens group III having a positive refractive power. At that time, resulting parallax is compensated by moving the second lens group II having a negative refractive power. 
     Consequently, the finder provides good overall aberration performance. 
     In this invention, the aspherical lens of the variable magnification finder is expressed as following: ##EQU1## 
     In this equation, X denotes the optical axial thickness from the lens vertex; y the vertical thickness of the optical axis; C the reciprocal number of the radius of curvature; K the conic number; and A 4 , A 6 , A 8  and A 10  the aspherical coefficients. 
     FIGS. 2(a)-2(f) show the aberrations of the optical system shown in FIG. 1. FIGS. 3 and 5 show different embodiments of the optical system in accordance with this invention, and FIGS. 4(a)-4(f) and 6(a)-6(f) show the aberrations of the optical systems shown in FIGS. 3 and. 
     Three examples of specific optical systems are shown in Tables 1-24. Tables 1-8 correspond to the embodiment shown in FIG. 1. Tables 9-16 correspond to the embodiment shown in FIG. 23, and tables 17-24 correspond to the embodiment shown in FIG. 5. 
     In tables 1-24, the half viewing angle equals ω, the axial thicknesses of air separations equals D. The refractive indices and the Abbe numbers of the glasses of the lens elements are given by N and v. The subscripts are numbered consecutively from object to image (left to right in FIGS. 1, 3, and 5). In Tables 1-24 all distances are given in millimeters. 
     Tables 2, 10, and 18 show the air separation for different viewing angles 20 at a wide angle of view, 21.0°, a normal angle of view 31.8°, and a telephoto angle of view, 65.9°. 
     Tables 3-8, 11-16, and 19-24 show the conic number K and the aspherical coefficients for the lens elements. 
     
                       TABLE 1______________________________________Magnification = -0.39 to -1.172                  Refractive  AbbeRadius    Distance     Index       Number______________________________________R1 = 47.021     D1 = 0.80    N1 = 1.49200                              v1 = 57.4R2 = 21.718     D2 = VariableR3 = 20.000     D3 = 0.80    N3 = 1.49200                              v3 = 57.4R4 = 15.364     D4 = 1.03R5 = 16.684     D5 = 0.80    N5 = 1.49200                              v5 = 57.4R6 = 7.169     D6 = VariableR7 = 17.049     D7 = 2.07    N7 = 1.49200                              v7 = 57.4R8 = -15.364     D8 = 1.65R9 = -19.619     D9 = 2.45    N9 = 1.49200                              v9 = 57.4R10 = -9.006     D10 = VariableR11 = 25.574     D11 = 5.50   N11 = 1.49200                              v11 = 57.4R12 = ∞     D12 = 4.80R13 = ∞     D13 = 14.80  N13 = 1.49200                              v13 = 57.4R14 = ∞     D14 = 11.11R15 = 32.868     D15 = 2.34   N7 = 1.49200                              v15 = 57.4R16 = -21.495     D16 = 18.95______________________________________ 
    
     
                       TABLE 2______________________________________D       2ω = 21.0°                2ω = 31.8°                          2ω = 65.9°______________________________________D2 =    1.000        6.427     6.236D6 =    1.000        6.657     14.000D10 =   35.904       27.819    17.670______________________________________ 
    
     
                       TABLE 3______________________________________Aspherical Coefficient of R2______________________________________   K = 0.3069989 × 10   A.sub.4 = -0.10019439 × 10.sup.-3   A.sub.6 = -0.86741314 × 10.sup.-6   A.sub.8 = 0   A.sub.10 = 0______________________________________ 
    
     
                       TABLE 4______________________________________Aspherical Coefficient of R5______________________________________   K = -0.19107742 × 10.sup.+2   A.sub.4 = -0.46861886 × 10.sup.-2   A.sub.6 = 0.25666468 × 10.sup.-3   A.sub.8 = -0.10889761 × 10.sup.-4   A.sub.10 = 0.21014519 × 10.sup.-6______________________________________ 
    
     
                       TABLE 5______________________________________Aspherical Coefficient of R6______________________________________   K = -0.13216630 × 10.sup.+2   A.sub.4 = -0.20765375 × 10.sup.-2   A.sub.6 = 0.99140566 × 10.sup.-4   A.sub.8 = -0.35976467 × 10.sup.-5   A.sub.10 = 0.61721120 × 10.sup.-7______________________________________ 
    
     
                       TABLE 6______________________________________Aspherical Coefficient of R7______________________________________   K = -0.59009749 × 10.sup.+2   A.sub.4 = 0.81157548 × 10.sup.-3   A.sub.6 = -0.42650485 × 10.sup.-4   A.sub.8 = 0.12636884 × 10.sup.-5   A.sub.10 = -0.19564138 × 10.sup.-7______________________________________ 
    
     
                       TABLE 7______________________________________Aspherical Coefficient of R9______________________________________   K = 0.16088563 × 10   A.sub.4 = -0.10512548 × 10.sup.-3   A.sub.6 = 0.17724245 × 10.sup.-6   A.sub.8 = -0.15983230 × 10.sup.-6   A.sub.10 = 0.47707047 × 10.sup.-8______________________________________ 
    
     
                       TABLE 8______________________________________Aspherical Coefficient of R15______________________________________   K = 0.17411573 × 10.sup.+2   A.sub.4 = -0.32605942 × 10.sup.-4   A.sub.6 = -0.27274708 × 10.sup.-6   A.sub.8 = 0.12476988 × 10.sup.-7   A.sub.10 = -0.14327874 × 10.sup.-9______________________________________ 
    
     
                       TABLE 9______________________________________Magnification = -0.359 to -1.168                              AbbeRadius    Distance    Refractive Index                              Number______________________________________R1 = 43.176     D1 = 0.80   N1 = 1.49200 v1 = 57.4R2 = 17.975     D2 = VariableR3 = 22.335     D3 = 0.80   N3 = 1.49200 v3 = 57.4R4 = 8.091     D4 = VariableRS = 19.193     D5 = 1.93   N5 = 1.49200 v5 = 57.4R6 = -31.296     D6 = 0.68R7 = -29.469     D7 = 2.35   N7 = 1.49200 v7 = 57.4R8 = -9.926     D8 = VariableR9 = 22.723     D9 = 5.50   N9 = 1.49200 v9 = 57.4R10 = ∞     D10 = 4.80R11 = ∞     D11 = 14.80 N11 = 1.49200                              v11 = 57.4R12 = ∞     D12 = 11.60R13 = 30.822     D13 = 3.90  N13 = 1.49200                              v13 = 57.4R14 = -22.191     D14 = 17.88______________________________________ 
    
     
                       TABLE 10______________________________________D      2ω = 21.0°                2ω = 31.8°                          2ω = 65.9°______________________________________D2     2.746         7.879     6.739D4     1.000         3.566     14.834D8     36.190        28.490    18.370______________________________________ 
    
     
                       TABLE 11______________________________________Aspherical Coefficient of R2______________________________________   K = -0.95305221   A.sub.4 = -0.21215771 × 10.sup.-4   A.sub.6 = -0.13377052 × 10.sup.-5   A.sub.8 = 0   A.sub.10 = 0______________________________________ 
    
     
                       TABLE 12______________________________________Aspherical Coefficient of R3______________________________________   K = -0.30403221 × 10.sup.+2   A.sub.4 = -0.30403221 × 10.sup.-2   A.sub.6 = 0.25928152 × 10.sup.-3   A.sub.8 = -0.12109943 × 10.sup.-4   A.sub.10 = -0.19868759 × 10.sup.-6______________________________________ 
    
     
                       TABLE 13______________________________________Aspherical Coefficient of R4______________________________________   K = -0.17191929 × 10.sup.+2   A.sub.4 = -0.15238223 × 10.sup.-2   A.sub.6 = 0.99704906 × 10.sup.-4   A.sub.8 = -0.46051012 × 10.sup.-5   A.sub.10 = 0.83222117 × 10.sup.-7______________________________________ 
    
     
                       TABLE 14______________________________________Aspherical Coefficient of R5______________________________________   K = -0.88023953 × 10.sup.+2   A.sub.4 = 0.67463557 × 10.sup.-3   A.sub.6 = -0.38507764 × 10.sup.-4   A.sub.8 = 0.11678678 × 10.sup.-5   A.sub.10 = -0.19868759 × 10.sup.-7______________________________________ 
    
     
                       TABLE 15______________________________________Aspherical Coefficient of R7______________________________________   K = -0.13188563 × 10.sup.+2   A.sub.4 = -0.47491803 × 10.sup.-4   A.sub.6 = -0.82788475 × 10.sup.-6   A.sub.8 = -0.11775149 × 10.sup.-7   A.sub.10 = 0.31178180 × 10.sup.-8______________________________________ 
    
     
                       TABLE 16______________________________________Aspherical Coefficient of R13______________________________________   K = 0.24200984 × 10   A.sub.4 = 0.13902687 × 10.sup.-4   A.sub.6 = -0.76988788 × 10.sup.-6   A.sub.8 = 0.12884705 × 10.sup.-7   A.sub.10 = -0.41708505 × 10.sup.-11______________________________________ 
    
     
                       TABLE 17______________________________________                              AbbeRadius    Distance    Refractive Index                              Number______________________________________R1 = ∞     D1 = 0.95   N1 = 1.49200 V1 = 57.4R2 = 16.040     D2 = VariableR3 = 10.277     D3 = 0.80   N3 = 1.49200 V3 = 57.4R4 = 6.316     D4 = VariableR5 = 17.422     D5 = 0.96   N5 = 1.49200 V5 = 57.4R6 = -36.331     D6 = 0.68R7 = -28.717     D7 = 3.49   N7 = 1.49200 V7 = 57.4R8 = -8.786     D8 = VariableR9 = 18.325     D9 = 5.50   N9 = 1.49200 V9 = 57.4R10 = ∞     D10 = 3.71R11 = ∞     D11 = 24.80 N11 = 1.49200                              V11 = 57.4R12 = ∞     D12 = 1.89R13 = 23.470     D13 = 2.63  N13 = 1.49200                              V13 = 57.4R14 = -20.877     D14 = 20.50______________________________________ 
    
     
                       TABLE 18______________________________________D      2ω = 23.7°                2ω = 31.8°                          2ω = 63.0°______________________________________D2     3.595         6.646     2.975D4     1.185         3.000     15.166D8     29.645        24.779    16.290______________________________________ 
    
     
                       TABLE 19______________________________________Aspherical Coefficient of R2______________________________________   K = -0.20582991 × 10.sup.+2   A.sub.4 = 0.51936485 × 10.sup.-3   A.sub.6 = -0.161237500 × 10.sup.-6   A.sub.8 = 0   A.sub.10 = 0______________________________________ 
    
     
                       TABLE 20______________________________________Aspherical Coefficient of R3______________________________________   K = -0.39780656 × 10   A.sub.4 = -0.43872858 × 10.sup.-2   A.sub.6 = 0.23981111 × 10.sup.-3   A.sub.8 = -0.10931149 × 10.sup.-4   A.sub.10 = 0.18411862 × 10.sup.-6______________________________________ 
    
     
                       TABLE 21______________________________________Aspherical Coefficient of R4______________________________________   K = -0.10701318 × 10.sup.+2   A.sub.4 = -0.013126990 × 10.sup.-2   A.sub.6 = 0.50047221 × 10.sup.-4   A.sub.8 = -0.26951821 × 10.sup.-6   A.sub.10 = 0.49415504 × 10.sup.-7______________________________________ 
    
     
                       TABLE 22______________________________________Aspherical Coefficient of R5______________________________________   K = -0.68979563 × 10.sup.+2   A.sub.4 = 0.85397460 × 10.sup.-3   A.sub.6 = -0.46607284 × 10.sup.-4   A.sub.8 = 0.11684395 × 10.sup.-5   A.sub.10 = -0.14591877 × 10.sup.-7______________________________________ 
    
     
                       TABLE 23______________________________________Aspherical Coefficient of R7______________________________________   K = -0.74726710 × 10   A.sub.4 = -0.18939166 × 10.sup.-3   A.sub.6 = 0.56234163 × 10.sup.-5   A.sub.8 = -0.13148234 × 10.sup.-6   A.sub.10 = 0.20636297 × 10.sup.-8______________________________________ 
    
     
                       TABLE 24______________________________________Aspherical Coefficient of R13______________________________________   K = -0.11102512 × 10.sup.+2   A.sub.4 = 0.13702066 × 10.sup.-4   A.sub.6 = 0.30673633 × 10.sup.-5   A.sub.8 = -0.11332189 × 10.sup.-6   A.sub.10 = 0.11713777 × 10.sup.-8______________________________________ 
    
     Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with the true scope and spirit of the invention being indicated by the following claims.