Abstract:
The disclosed finder system includes, from front to rear, a first objective lens of negative power, an optical member having a half-mirror, an eyepiece of positive power and a viewfinder frame arranged near the eyepiece to be observed by the eyepiece with the aid of a reflection from the half-mirror. The finder system satisfies the condition 0.2&lt;d/D&lt;0.6 where D is the air separation between the first objective lens and the eyepiece lens and d is the air separation between the optical member and the eyepiece lens. Thus the first objective lens is exchangeable for a second objective lens of different power from that of the first objective lens to change the magnification of the finder system.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to finder systems, and more particularly to finder systems having interchangeable components of different refracting power to change the magnification power of the entire system. 
     2. Description of the Prior Art 
     Known prior art cameras have a number of inter-changeable objective lenses, for example, standard and telephoto lenses, to permit shooting with two or more different focal lengths. In such a prior art camera, it is desirable to make the magnification of an object image and the angular field coverage of the finder coincide with those on the film plane as the focal length of the objective lens changes. 
     For example, discretely changeable magnification power finder systems of the reverse Galilean type are known from Japanese Laid-Open Patent Application No. 52-137331, Japanese Utility Model Application No. 54-66541 and U.S. Pat. No. 4,277,158. The operating mechanism for these finder systems is, however, quite complicated, because changing of the magnification power requires axially displacing the two components constituting the objective lens while retracting the other component from the optical path. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an ojbect of the present invention to provide a reverse-Galilean finder system of very simple form while still maintaining the possibility of changing the magnification power of the finder to be achieved with the limitation of the size to a minimum. 
     To accomplish the object of the invention, a principal feature of the finder system is that a first objective lens of negative power is followed by an eyepiece of positive power after a space in which is arranged an optical member having a half-mirror. A viewfinder frame is arranged near the eyepiece to be observed by the eyepiece lens through a reflection of the half-mirror of the optical member. Letting D denote the air separation between the first objective lens and the eyepiece lens, and d the air separation between the optical member and the eyepiece lens, the following condition is satisfied: 
     
         0.2&lt;d/D&lt;0.6 
    
     When the first objective lens is exchanged by a second objective lens having a different refracting power from that of the first objective lens, the magnification power of the entire system changes. 
     In a preferred embodiment, the second objective lens is constructed with a positive and a negative component. 
     In the present invention, in order to achieve a minimization of the variation of the aberrations of the finder system with the change of the focal length, the ratio &#34;m&#34; of the maximum to the minimum possible magnification power of the finder lies in the following range: 
     
         1&lt;m≦2.5 
    
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 to 3 are schematic diagrams illustrating a change of the power distribution in the prior art finder system. 
     FIG. 4 is a schematic diagram of a power distribution according to the present invention. 
     FIGS. 5 and 7 are longitudinal section views of a first specific embodiment of the finder system according to the present invention in the wide angle and telephoto positions respectively. 
     FIGS. 6 and 8 are graphic representations of the aberrations of the finder system of FIGS. 5 and 7 in the wide angle and telephoto positions respectively. 
     FIGS. 9 and 11 are longitudinal section views of a second specific embodiment of the finder system according to the present invention in the wide angle and telephoto positions respectively. 
     FIGS. 10 and 12 are graphic representations of the aberrations of the finder system of FIGS. 9 and 11 in the wide angle and telephoto positions respectively. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will next be described in connection with the prior known Albada finder system, but, of course, can be applied to the mark type finder. 
     For the purpose of easy understanding, an example of the reverse-Galilean type is described where the finder system constitutes a perfect afocal system. Referring to FIG. 1, for the finder system of which the magnification power is γ, and in which the interval between the principal points of the objective lens and the eyepiece lens is e, the refractive power ρ1 of the objective lens and the refractive power ρ2 of the eyepiece lens are expressed by the following equations: ##EQU1## 
     Let us now consider a finder system operating with selection of two different magnification powers for wide angle and telephoto settings. Such is derived by replacing the objective lens constituting part of the aforesaid reverse-Galilean finder system with another objective lens of different refractive power. 
     For the finder system having a minimum magnification γW and a maximum magnification γT with the principal point intervals eW and eT between the objective lens and the eyepiece when in the wide angle and telephoto positions repsectively, while the refractive power ρ2 of the eyepiece is constant with the change of the magnification, from equations (1) and (2) we have the following equations for the refractive powers ρ1W and ρ1T of the objective lens when in the wide angle and telephoto positions respectively and for the refractive power ρ2 of the eyepiece lens: ##EQU2## Under the conditions that γT&lt;1 and γT&gt;γW, from the equations (3) and (4), ##EQU3## and from equation (5) ##EQU4## 
     It is to be understood from the equations (6) and (7) that in the case of the reverse-Galilean finder system, to increase the magnification power of the entire system by replacing the objective lens by another or second objective lens of different refractive power, the second objective lens for the telephoto setting lens must be given a weaker refractive power than that of the first objective lens for the wide angle setting, and must be positioned nearer to the eyepiece lens. 
     It should be also pointed out that in this case, as illustrated in FIGS. 2 and 3, the value of the refractive power ρ1T of the objective lens for the telephoto setting and the value of the prinicipal point interval eT between the objective lens and the eyepiece lens are determined unequivocally for the given value of the magnification γT by the equations (4) and (5), and, therefore, have no degree of freedom. 
     In order to make the finder system compact, when the eW is taken at a smaller value, the required value of the eT becomes still smaller as will be understood from the equation (7). Particularly in application to the Albada finder system, it becomes difficult to secure a space large enough to accommodate the frame reflection mirror between the objective lens and the eyepiece lens. 
     Also, even when the requirement for the availability of the space the frame reflection mirror occupies is fulfilled, the separation between the frame reflection mirror and the eyepiece lens has to be sufficiently long. Otherwise, the observation of the frame will be objectionably deteriorated. 
     For the above-described reason, in this case the eW must be taken at a somewhat large value in excess of the desired one, and that a compact finder system is difficult to achieve. 
     In an embodiment of the present invention illustrated in FIG. 4, the objective lens for the telephoto setting is constructed with two components. The first component has a positive refractive power ρA and the second has a negative refractive power ρB. Hence we have the following equations: ##EQU5## As is evident from the equation (9), the axial separation e2 between the objective lens and the eyepiece lens has some degree of freedom. That is, by choosing appropriate values of the e1 and ρA, it is possible to widen the axial separation e2. Even for the reverse-Galilean finder, as in this embodiment, and further for the Albada one, an air space large enough to accommodate the frame reflection mirror can be secured between the objective lens and the eyepiece lens. If the above-stated inequalities of condition for all the optical elements are satisfied, it is possible to achieve a magnification power-changeable compact finder system. 
     The ratio &#34;m&#34; of the maximum to the minimum magnification of the finder system of the invention is, for good stability of aberration correction with the change of the magnification, preferably limited to the following range: 
     
         1&lt;m ≦2.5 
    
     Two different examples of specific finders systems of the invention can be constructed in accordance with the numerical data given in the following tables for the radii of curvature R, the axial thicknesses and air separations D, the refractive indices N, Abbe numbers ν, of the glasses of the lens elements. The subscripts are numbered consecutively from front to rear. 
     In these specific numerical examples, the finder system is provided with an aspherical surface to achieve good correction of aberrations. Particularly when the first and second objective lenses have their one lens surface made aspheric, good optical performance is advantageously obtained. 
     Taking an X-axis as the optical axis and a Y-axis in a direction perpendicular to the optical axis as the direction in which the light advances as positive with an original point at the vertex of the lens surface, an equation for the aspheric surface may be expressed by: ##EQU6## where R* is the radius of curvature of the paraxial region of the lens surface, and A, B, C, D, E, A&#39;, B&#39;, C&#39; and D&#39; are aspherical coefficients. 
     EXAMPLE 1 
     
         ______________________________________(Magnification ratio: 1.64)Wide Angle Setting with Angular Magnification: 0.45______________________________________R1 = ∞      D1 = 2.00   N1 = 1.49171                              ν1 = 57.4R2 = ∞      D2 = 1.00R3 = ∞      D3 = 2.00   N2 = 1.49171                              ν2 = 57.4R*4 = 11.578      D4 = 15.00R5 = 30.129      D5 = 1.50   N3 = 1.52300                              ν3 = 58.6R6 = 34.000      D6 = 12.00R7 = ∞      D7 = 2.50   N4 = 1.52300                              ν4 = 58.6R8 = -30.631      D8 = 16.00R9 = Eye Point______________________________________ Note: R3 and R*4 define a first objective lens, R5 and R6 a frame reflection mirror lens, and R7 and R8 an eyepiece lens. R*4 is the aspheric surface. 
    
     
         ______________________________________Aspherical Coefficients______________________________________A          B         C            D   E______________________________________0.0        0.13218E-03                0.67528E-05  0.0 0.0______________________________________A&#39;         B&#39;        C&#39;           D&#39;______________________________________0.18664E-03      0.50757E-04                0.36556E-06  0.0______________________________________ 
    
     
         ______________________________________Telephoto setting with Angular Magnification: 0.738______________________________________R1 = ∞      D1 = 2.00   N1 = 1.49171                              ν1 = 57.4R2 = ∞      D2 = 9.00R3 = 46.723      D3 = 3.00   N2 = 1.49171                              ν2 = 57.4R*4 = -74.282      D4 = 2.00R5 = -46.262      D5 = 1.50   N3 = 1.49171                              ν3 = 57.4R6 = 14.078      D6 = 2.50R7 = 30.129      D7 = 1.50   N4 = 1.52300                              ν4 = 58.6R8 = 34.000      D8 = 12.00R9 = ∞      D9 = 2.50   N5 = 1.52300                              ν5 = 58.6R10 = -30.631      D10 = 16.00R11 = Eye Point______________________________________ Note: R3 to R6 define a second objective lens and R4* is the aspheric surface. 
    
     
         ______________________________________Aspherical Coefficients______________________________________A          B         C            D   E______________________________________0.0        0.10242E-03                0.58042E-05  0.0 0.0______________________________________A&#39;         B&#39;        C&#39;           D&#39;______________________________________0.14617E-03      0.38004E-04                0.35183E-06  0.0______________________________________ 
    
     EXAMPLE 2 
     
         ______________________________________(Magnification Ratio: 1.64)Wide Angle Setting with Angular Magnification: 0.45______________________________________R1 = ∞ D1 = 2.00  N1 = 1.49171                              ν1 = 57.4R2 = ∞ D2 = 1.00R3 = ∞ D3 = 2.00  N2 = 1.49171                              ν2 = 57.4R*4 = 14.566 D4 = 17.86R5 = 46.748  D5 = 1.50  N3 = 1.52300                              ν3 = 58.6R6 = 41.000  D6 = 14.00R7 = ∞ D7 = 2.50  N4 = 1.52300                              ν4 = 58.6R8 = -33.282 D8 = 16.00R9 = ∞ (Eye Point)______________________________________ Note R3 and R*4 define an objective lens, R5 and R6 a frame reflection mirror lens, and R7 and R8 an eyepiece lens. R*4 is the aspheric surface. 
    
     
         ______________________________________Aspherical Coefficients______________________________________A       B         C          D       E______________________________________0.0     0.35239E-03             0.64612E-05                        0.13608E-06                                0.17168E-08______________________________________A&#39;      B&#39;        C&#39;         D&#39;______________________________________0.21139E-03   0.10711E-03             0.15885E-05                        0.17304E-07______________________________________ 
    
     
         ______________________________________Telephoto Setting with Angular Magnification: 0.738______________________________________R1 = ∞       D1 = 2.00  N1 = 1.49171                              ν1 = 57.4R2 = ∞       D2 = 12.37R3 = 42.773 D3 = 3.00  N2 = 1.49171                              ν2 = 57.4R4 = -185.190       D4 = 1.99R5 = -70.804       D5 = 1.50  N3 = 1.49171                              ν3 = 57.4R6 = 16.545 D6 = 2.00R7 = 46.748 D7 = 1.50  N4 = 1.52300                              ν4 = 58.6R8 = 41.000 D8 = 14.00R9 = ∞       D9 = 2.50  N5 = 1.52300                              ν5 = 58.6R10 = -33.282       D10 = 16.00R11 = Eye Point______________________________________ Note: R3 to R6 define an objective lens