Abstract:
An optical system for a viewfinder camera is provided. The objective lens system is divided into at least two separate lens groups. The first lens group provides an insufficient correction to aberration while the second lens group is designed specifically to correct the aberration transmitted through the first lens group. The complimentary viewfinder system is also likewise designed to correct the same aberration as the second lens group.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical system for use in a single-lens reflex camera, and more particularly, it pertains to a single-lens reflex camera optical system which includes a beam path splitter inside a photographing lens optical system, as in a motion picture camera and a compact single-lens reflex camera, so that a photographer can obtain through a view finder system a light split by the beam path splitter after having passed through a lens group located before the beam path splitter toward an object. 
     2. Description of the Prior Art 
     Until now for this type of single-lens reflex camera optical system, a photographing lens optical system, i.e., the most important element of a camera, is first designed to achieve the best corrected aberration to ensure high image forming performance on a film surface. A view finder system is then designed in consideration of aberration conditions of the lens group on the object side of a beam path splitter in the photographing lens optical system so that a favorable finder image can be obtained. As is widely known, however, a view finder system consists mainly of a combination of positive lens elements. This causes the view finder system itself to produce longitudinal aberration when the construction is simplified. Therefore, even if the lens group, located before the beam path splitter, is achromatized, longitudinal chromatic aberration occurs in a finder image on the focal plane. To obtain a favorable finder image accordingly, the longitudinal chromatic aberration must be corrected inside the view finder system, which, as a result, requires a specific number of elements for an achromatic lens, thus causing the entire optical system for a single-lens reflex camera to become a considerably complex lens construction. 
     U.S. Pat. No. 3,583,785 and U.S. Pat. No. 4,086,000 are cited of general interest. 
     SUMMARY OF THE INVENTION 
     The primary object of the present invention is to provide an optical system for use in a single-lens reflex camera, which obviates the above drawback and is simple in optical construction, as a whole. 
     According to the present invention, both a photographing lens optical system and a view finder optical system are designed in close relationship to each other, the photographing lens optical system being maintained equal in optical construction to the conventional system, while the view finder optical system is simplified, thereby contributing to the simplification of an entire optical system for a single-lens reflex camera. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 represents a schematic cross sectional view of an optical system for explaining the concept of the present invention. 
     FIG. 2 represents a schematic cross sectional view of a first embodiment of the present invention. 
     FIG. 3 represents a schematic cross sectional view of an exemplary prior art. 
     FIG. 4 represents a schematic cross sectional view of a part of a second embodiment of the present invention. 
     FIG. 5 represents a schematic cross sectional view of a part of a third embodiment of the present invention. 
     FIG. 6 represents a schematic cross sectional view of a part of a fourth embodiment of the present invention. 
     FIGS. 7a to 7d represents graphic plots of various aberrations of the objective lens system in the first embodiment. 
     FIGS. 8a to 8c represent graphic plots of various aberrations of the finder system in the first embodiment. 
     FIGS. 9a to 9d represent graphic plots of various aberrations of the objective lens system in the exemplary prior art. 
     FIGS. 10a to 10c represent graphic plots of various aberrations of the finder system in the exemplary prior art. 
     FIGS. 11a to 11c represent graphic plots of various aberrations of the finder system in the second embodiment. 
     FIGS. 12a to 12c represent graphic plots of various aberrations of the finder system in the third embodiment. 
     FIGS. 13a to 13c represent graphic plots of various aberrations of the finder system in the fourth embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Description will now be given of the present invention according to the embodiments shown in the drawings. FIG. 1 shows a schematic view of one embodiment of the present invention for a motion picture camera with a built-in zoom lens system. With reference to FIG. 1, portions 1 and 3 together constitute a photographing lens optical system for focusing an object on a film surface 4. Provided inside the photographing lens optical system is a beam path splitter 2 consisting of a half mirror disposed between a first lens group 1 and a second lens group 3. First lens group 1 consists of a focal zoom lens system which is of the prior art optical construction. However, first lens group 1 sets aberrations according to the features of the present invention so that the longitudinal chromatic aberration may be corrected in excess inside the lens group. This aberration setting is possible by means of the prior art design technique. Second lens group 3 consists of a master lens system. As is well known, the master lens system has a positive refraction which causes longitudinal chromatic aberration to occur inside the system. According to ordinary design, the longitudinal chromatic aberration is corrected therein by using a required number of lens elements for achromatism in the master lens system. According to the features of the present invention, however, the longitudinal chromatic aberration is overcorrected or corrected in excess inside the first lens group 1. This causes the correction excess in the first lens group 1 to offset the longitudinal chromatic aberration occurring in the second lens group 3 which is the master lens system. Therefore, the correction of longitudinal chromatic aberration in the second lens group 3 is eliminated, whereby the number of lens elements required inside the second lens group 3 can be reduced from the conventional system. 
     According to the present invention, it is natural that longitudinal chromatic aberration is required to be corrected in excess inside the first lens group 1. This causes the first lens group 1 to require a larger number of elements than the conventional system. In other words, the present invention features the entire optical system for a photographing lens wherein the number of elements required for the second lens group 3 of the conventional system has been transferred to the first lens group 1. Therefore, as far as the photographing lens optical system is concerned, the number of elements is equal to the conventional system without any simplification in optical construction. In addition, aberration performance is equal between the photographing lens optical system according to the present invention and the conventional one. 
     Referring again to FIG. 1, view finder systems are labeled 5, 6 and 7. The light passing through the first lens group 1 which is the zoom lens system in the photographing lens optical system is split by beam path splitter 2 and focused as primary image 8 by objective lens systems having positive refraction. Primary image 8 is further focused on focal plane 9 as a secondary image by erect lens system 6 with positive refraction. A virtual image of the secondary image on focal plane 9 is then observed through eyepiece system 7 by eye 10 of a photographer. As is clear from the foregoing, the optical system up to focal plane 9 in the view finder system consists mainly of a positive lens element combination and longitudinal chromatic aberration occurs in an image on focal plane 9. According to ordinary design, the longitudinal chromatic aberration is corrected inside the view finder system, which must use a required number of lens elements for an image, whose longitudinal chromatic aberration has been corrected, to be focused on focal plane 9. In contrast, according to the features of the present invention, the longitudinal chromatic aberration is corrected in the excess in first lens group 1 of the photographing lens optical system, whereby longitudinal chromatic aberration occurring in positive lens elements 5 and 6 of the view finder system is offset by the correction in excess. Therefore, the correction of longitudinal chromatic aberration occurring in the view finder system lens elements 5 and 6 in relation to an image focused on focal plane 9 is eliminated, whereby the number of lens elements required inside the view finder system can be reduced from the conventional system. Thus, the number of lens elements for the entire optical system for a single-lens reflex camera can be decreased. 
     Until now a photographing lens optical system and a view finder system are designed independently of each other, and longitudinal chromatic aberrations occurring in the master lens system and view finder system of the photographing lens optical system which is originally the positive lens system are separately corrected in the respective systems. In contrast, the present invention simplifies construction of the entire optical system for a single-lens reflex camera by concentrating the burden of aberration correction on the lens group located before the beam path splitter toward an object. 
     It should be noted that in FIG. 1, the first lens group 1 is not required to consist of a focal zoom lens system alone and may consist of a combination of the zoom lens system and part of the lens elements in the master lens system next to the zoom lens system. In this case, beam path splitter 2 is arranged inside the master lens system. In addition, if beam path splitter 2 is a movable mirror replaced for a half mirror, the embodiment in FIG. 1 may be used as an optical system for a compact still camera. It is to be understood that although description was made of longitudinal chromatic aberration alone, the technological concept of the present invention is, needless to say, applicable to other aberrations. 
     FIG. 2 shows a first embodiment of the present invention. Objective lens system is constructed by an object side part 1 and an image side part 3, an image forming light path being passed through the object side and image side parts 1 and 3 toward image 4. The image forming light path is branched between the object side part 1 and the image side part 3 by semitransparent mirror 2 of the prism P. The branched light path is received by a branch optical system including elements 5, 6, 9 and 7, which forms a finder system in combination with the object side part 1 of the objective lens system. The front lens group 5 serves as an objective of the branch optical system, and the intermediate lens group 6 serves as an erecting system for converting the inverted primary real image formed by the front lens group 5 into an erecting secondary real image on the reticle at the eye side of the focusing plate 9. Finally, the rear lens group 7 serves as an ocular for watching the image on the reticle. 
     Table 1-L shows a lens construction of the objective lens system along the light path from an object to the image 4 in FIG. 2. Table 1-F shows a lens construction of the finder system along the light path from an object to the eye 10 in FIG. 2. 
     
                       TABLE 1-L______________________________________(Embodiment 1-L)f = 33.00  F.sub.No. = 1.26  2 = 12°  L.B = 5.68Radius of      Axial    Refractive AbbeCurvature      Distance Index      Number______________________________________      r.sub.1            78.512  d.sub.1                        1.5L.sub.1                           N.sub.1                                  1.80518                                        ν.sub.1                                            25.43      r.sub.2            33.152  d.sub.2                        1.5      r.sub.3            38.724  d.sub.3                        6.2L.sub.2                           N.sub.2                                  1.64050                                        ν.sub.2                                            60.08      r.sub.4            ∞ d.sub.4                        0.1      r.sub.5            26.281  d.sub.5                        6.9L.sub.3                           N.sub.3                                  1.64050                                        ν.sub.3                                            60.08      r.sub.6            169.775 d.sub.6                        17.7      r.sub.7            123.964 d.sub.7                        1.0L.sub.4                           N.sub.4                                  1.64050                                        ν.sub.4                                            60.08      r.sub.8            9.026   d.sub.8                        4.4      r.sub.9            -30.992 d.sub.9                        0.9L.sub.5                           N.sub.5                                  1.56873                                        ν.sub.5                                            63.1      r.sub.10            22.881  d.sub.10                        0.6      r.sub.11            17.214  d.sub.11                        2.2L.sub.6                           N.sub.6                                  1.80518                                        ν.sub.6                                            25.43      r.sub.12            39.676  d.sub.12                        0.8      r.sub.13            28.000  d.sub.13                        4.2L.sub.7                           N.sub.7                                  1.62041                                        ν.sub.7                                            60.29      r.sub.14            -13.300 d.sub.14                        1.0L.sub.8                           N.sub.8                                  1.73300                                        ν.sub.8                                            28.24      r.sub.15            -64.057 d.sub.15                        6.3      r.sub.16            ∞ d.sub.16                        7.0P                                 N.sub.9                                  1.62588                                        ν.sub.9                                            35.70      r.sub.17            ∞ d.sub.17                        3.5      r.sub.18            26.419  d.sub.18                        2.0L.sub.9                           N.sub.10                                  1.64050                                        ν.sub.10                                            60.08      r.sub.19            -39.539 d.sub.19                        0.2      r.sub.20            20.518  d.sub.20                        3.5L.sub.10                          N.sub.11                                  1.62135                                        ν.sub.11                                            61.28      r.sub.21            71.826  d.sub.21                        1.5      r.sub.22            -26.604 d.sub.22                        3.0L.sub.11                          N.sub.12                                  1.74000                                        ν.sub.12                                            28.26      r.sub.23            -168.070                    d.sub.23                        1.5      r.sub.24            -974.393                    d.sub.24                        3.6L.sub.12                          N.sub.13                                  1.64050                                        ν.sub.13                                            60.08      r.sub.25            -73.788 d.sub.25                        0.2      r.sub.26            19.202  d.sub.26                        3.5L.sub.13                          N.sub.14                                  1.64050                                        ν.sub.14                                            60.08      r.sub.27            39.469______________________________________ 
    
     
                       TABLE 1-F______________________________________(Embodiment 1-F)(f = 33.00)  Diopter = 0.0  Magnification = 2.33Radius of      Axial     Refractive AbbeCurvature      Distance  Index      Number______________________________________      r.sub.1             78.512 d.sub.1                         1.5L.sub.1                            N.sub.1                                   1.80518                                         ν.sub.1                                             25.43      r.sub.2             33.152 d.sub.2                         1.5      r.sub.3             38.724 d.sub.3                         6.2L.sub.2                            N.sub.2                                   1.64050                                         ν.sub.2                                             60.08      r.sub.4             ∞                    d.sub.4                         0.1      r.sub.5             26.281 d.sub.5                         6.9L.sub.3                            N.sub.3                                   1.64050                                         ν.sub.3                                             60.08      r.sub.6             169.775                    d.sub.6                         17.7      r.sub.7             123.964                    d.sub.7                         1.0L.sub.4                            N.sub.4                                   1.64050                                         ν.sub.4                                             60.08      r.sub.8             9.026  d.sub.8                         4.4      r.sub.9             -30.992                    d.sub.9                         0.9L.sub.5                            N.sub.5                                   1.56873                                         ν.sub.5                                             63.1      r.sub.10             22.881 d.sub.10                         0.6      r.sub.11             17.214 d.sub.11                         2.2L.sub.6                            N.sub.6                                   1.80518                                         ν.sub.6                                             25.43      r.sub.12             39.676 d.sub.12                         0.8      r.sub.13             28.000 d.sub.13                         4.2L.sub.7                            N.sub.7                                   1.62041                                         ν.sub.7                                             60.29      r.sub.14             -13.300                    d.sub.14                         1.0L.sub.8                            N.sub.8                                   1.73300                                         ν.sub.8                                             28.24      r.sub.15             -64.057                    d.sub.15                         6.3      r.sub.16             ∞                    d.sub.16&#39;                         11.5P&#39;                                 N.sub.9                                   1.62588                                         ν.sub.9                                             35.70      r.sub.17&#39;             ∞                    d.sub.27                         18.0      r.sub.28             17.000 d.sub.28                         5.0L.sub.14                           N.sub.15                                   1.62041                                         ν.sub.15                                             60.29      r.sub.29             -11.000                    d.sub.29                         2.0L.sub.15                           N.sub.16                                   1.73300                                         ν.sub.16                                             28.24      r.sub.30             -69.700                    d.sub.30                         43.2      r.sub.31*             20.000 d.sub.31                         4.0L.sub.16                           N.sub.17                                   1.49140                                         ν.sub.17                                             57.9      r.sub.32*             -20.000                    d.sub.32                         42.3      r.sub.33*             20.000 d.sub.33                         4.0L.sub.17                           N.sub.18                                   1.49140                                         ν.sub.18                                             57.9      r.sub.34*             -20.000                    d.sub.34                         16.9      r.sub.35             ∞                    d.sub.35                         4.0G                                  N.sub.19                                   1.49140                                         ν.sub.19                                             57.9      r.sub.36             ∞                    d.sub.36                         21.1      r.sub.37             25.700 d.sub.37                         5.0L.sub.18                           N.sub.20                                   1.62041                                         ν.sub.20                                             60.29      r.sub.38             -10.500                    d.sub.38                         1.0L.sub.19                           N.sub.21                                   1.73300                                         ν.sub.21                                             28.24      r.sub.39             -23.000                    d.sub.39                         18.0Eye Point______________________________________ *ε = -1.35, iΣ Ciφ.sup.2i = 0    Between Tables 1-L and 1-F, the construction from r.sub.1 to r.sub.15 is identical with each other. Further, prism P in Table 1-L is substantially identical with prism P&#39; in Table 1-F, except for the effective thickness, i.e., r.sub.1- to r.sub.17 in case of Table 1-L, and r.sub.16 to r&#39;.sub.17 in case of Table 1-F, which difference is caused by the light path branching from the semitransparent mirror 2. 
    
     According to Table 1-L or 1-F, the object side part 1, i.e., the lens construction from r 1  to r 15 , shows an excessive correction of longitudinal chromatic aberration of F-line to d-line, -0.43 diopter, in terms of finder. On the contrary, the final longitudinal chromatic aberration of F-line to d-line of the whole objective lens system at film 4 is +0.026 mm. This means that the excessive correction of longitudinal chromatic aberration in the object side part 1 is cancelled with the undercorrected or insufficient correction of the same in the image side part 3. 
     On the other hand, the branch optical system 5 to 7 in the finder system has an undercorrected insufficiently corrected longitudinal chromatic aberration of F-line to d-line, +0.55 diopter, by itself. This insufficient correction of longitudinal chromatic aberration is cancelled with the excessive correction of the same in the object side part 1 of the objective lens system, and the longitudinal chromatic aberration of F-line to d-line of the whole finder system, i.e., the object side part 1 plus the branch optical system, is -0.07 diopter. In addition, the longitudinal chromatic aberration of F-line to d-line of the whole finder system at the reticle of focusing plate 9 is +0.030 mm. 
     According to the general requirement for an objective lens system, the absolute value for the longitudinal chromatic aberration of F-line to d-line should be less than about 0.07 mm. The objective lens system of the present invention fulfills this requirement as a whole lens system and also has a characteristic image side part 3 of an insufficiently corrected longitudinal chromatic aberration to be cancelled with the excessive correction of the same in the image side part 1. The degree of the insufficiency of the correction for the longitudinal chromatic aberration in the image side part 3 thus characterizes the present invention, i.e., the absolute value for the insufficient correction of the longitudinal chromatic aberration of F-line to d-line of the image side part 3 is greater than 0.0004 f R  according to the present invention, wherein f R  represents the focal length of the image side part 3. 
     In other words, the present invention is also characterized by the absolute value of the excessive correction of longitudinal chromatic aberration of F-line to d-line in the object side part 1 of the objective lens system, which absolute value is greater than 0.3 diopter in terms of finder. In case of the embodiment in FIG. 2, such an excessive correction for longitudinal chromatic aberration is mainly caused by the doublet composed of lens elements L 7  and L 8 . 
     The ocular, or the rear lens group, 7 of the branch optical system in FIG. 2 adopts a doublet for correcting the chromatic aberration, which would otherwise occur at the ocular itself, to prevent the blue coloration of the inner edge of the finder frame. Another doublet is also adopted as the objective, or the front lens group, 5 of the branch optical system. The pair of doublets 5 and 7 make the branch optical system symmetric to prevent the occurrence of lateral chromatic aberration in the finder image. 
     In the erecting system 6 of the branch optical system, a pair of identically shaped single lenses are adopted, each of the single lenses being a symmetric lens formed by aspherical surfaces to correct the field curvature of the finder image. In Table 1-F, the aspherical surface is indicated by a star (*), e.g., r 31  *, and the necessary information of the aspherical surface are given at the bottom of Table 1-F. An aspherical surface is generally given by the following formula: ##EQU1## Wherein C 0  represents the radius of curvature of the aspherical surface at the optical axis, 
     φ represents the distance from the optical axis, 
     χ represents the displacement of the aspherical surface from the spherical surface of radius of curvature C 0  in the direction of the optical axis, 
     and ε and C i  represent various coefficients for determining the aspherical surface. 
     Table 1-F shows that all aspherical surfaces r 31  * to r 34  * are identical, and ε is -1.35 and C 1 , C 2 , C 3  - - - are all zero in the above formula (1). 
     By the way, the identical aspherical surface can be given by another expression when C 1 , C 2 , C 3  - - - are all zero in the formula (1), the expression being: ##EQU2## Thus, the aspherical surfaces of Table 1-F can also be expressed in a form that Σ i  C i  φ 2i  is not zero. 
     The above explanation for the expression of the aspherical surface can be applied to other embodiments of the present invention, and further explanation will be omitted. 
     As already mentioned, in the branch optical system of FIG. 2, the longitudinal chromatic aberration is left in an insufficiently corrected condition to avoid any addition of further lens elements and to simplify the lens construction of the branch optical system. 
     FIGS. 7a to 7d represent the various aberrations of the lens system of the first embodiment in FIG. 2, and FIGS. 8a to 8c correspond to those for the finder system. These figures prove that the first embodiment in FIG. 2 is a highly useful and practical system. 
     FIG. 3 shows an exemplary prior art, the lens construction of the objective lens system and the finder system being shown in Tables 2-L and 2-F, respectively. 
     
                       TABLE 2-L______________________________________(Exemplary Prior Art - L)f = 33.00  F.sub.No. = 1.26  2 = 12°  L.B. = 12.29Radius of      Axial    Refractive AbbeCurvature      Distance Index      Number______________________________________      r.sub.1            78.512  d.sub.1                        1.5L.sub.1                           N.sub.1                                  1.80518                                        ν.sub.1                                            25.43      r.sub.2            33.152  d.sub.2                        1.5      r.sub.3            38.724  d.sub.3                        6.2L.sub.2                           N.sub.2                                  1.64050                                        ν.sub.2                                            60.08      r.sub.4            ∞ d.sub.4                        0.1      r.sub.5            26.281  d.sub.5                        6.9L.sub.3                           N.sub.3                                  1.64050                                        ν.sub.3                                            60.08      r.sub.6            169.775 d.sub.6                        17.7      r.sub.7            123.964 d.sub.7                        1.0L.sub.4                           N.sub.4                                  1.64050                                        ν.sub.4                                            60.08      r.sub.8            9.026   d.sub.8                        4.4      r.sub.9            -30.992 d.sub.9                        0.9L.sub.5                           N.sub.5                                  1.56873                                        ν.sub.5                                            63.10      r.sub.10            22.881  d.sub.10                        0.6      r.sub.11            17.214  d.sub.11                        2.2L.sub.6                           N.sub.6                                  1.80518                                        ν.sub.6                                            25.43      r.sub.12            39.676  d.sub.12                        0.8      r.sub.13            50.653  d.sub.13                        3.2L.sub.7                           N.sub.7                                  1.70030                                        ν.sub.7                                            47.67      r.sub.14            -62.704 d.sub.14                        8.3      r.sub.15            ∞ d.sub.15                        7.0P                                 N.sub.8                                  1.62588                                        ν.sub.8                                            35.70      r.sub.16            ∞ d.sub.16                        3.5      r.sub.17            15.872  d.sub.17                        2.0L.sub.8                           N.sub.9                                  1.64050                                        ν.sub.9                                            60.08      r.sub.18            36.085  d.sub.18                        0.1      r.sub.19            30.865  d.sub.19                        3.3L.sub.9                           N.sub.10                                  1.62135                                        ν.sub.10                                            61.28      r.sub.20            155.809 d.sub.20                        1.1      r.sub.21            - 22.817                    d.sub.21                        3.0L.sub.10                          N.sub.11                                  1.74000                                        ν.sub.11                                            28.26      r.sub.22            16.622  d.sub.22                        1.6      r.sub.23            36.602  d.sub.23                        0.8L.sub.11                          N.sub.12                                  1.67270                                        ν.sub.12                                            32.22      r.sub.24            24.764  d.sub.24                        2.8L.sub.12                          N.sub.13                                  1.67790                                        ν.sub.13                                            55.38      r.sub.25            -17.259 d.sub.25                        0.1      r.sub.26            16.271  d.sub.26                        3.5L.sub.13                          N.sub.14                                  1.64050                                        ν.sub.14                                            60.08      r.sub.27            685.641______________________________________ 
    
     
                       TABLE 2-F______________________________________(Exemplary Prior Art - F)(f = 33.00)  Diopter = 0.0  Magnification = 2.43Radius of      Axial     Refractive AbbeCurvature      Distance  Indent     Number______________________________________      r.sub.1             78.512 d.sub.1                         1.5L.sub.1                            N.sub.1                                   1.80518                                         ν.sub.1                                             25.43      r.sub.2             33.152 d.sub.2                         1.5      r.sub.3             38.724 d.sub.3                         6.2L.sub.2                            N.sub.2                                   1.64050                                         ν.sub.2                                             60.08      r.sub.4             ∞                    d.sub.4                         0.1      r.sub.5             26.281 d.sub.5                         6.9L.sub.3                            N.sub.3                                   1.64050                                         ν.sub.3                                             60.08      r.sub.6             169.775                    d.sub.6                         17.7      r.sub.7             123.964                    d.sub.7                         1.0L.sub.4                            N.sub.4                                   1.64050                                         ν.sub.4                                             60.08      r.sub.8             9.026  d.sub.8                         4.4      r.sub.9             -30.992                    d.sub.9                         0.9L.sub.5                            N.sub.5                                   1.56873                                         ν.sub.5                                             63.1      r.sub.10             22.881 d.sub.10                         0.6      r.sub.11             17.214 d.sub.11                         2.2L.sub.6                            N.sub.6                                   1.80518                                         ν.sub.6                                             25.43      r.sub.12             39.676 d.sub.12                         0.8      r.sub.13             50.653 d.sub.13                         3.2L.sub.7                            N.sub.7                                   1.70030                                         ν.sub.7                                             47.67      r.sub.14             -62.704                    d.sub.14                         8.3      r.sub.15             ∞                    d.sub.15&#39;                         11.5P&#39;                                 N.sub.8                                   1.62588                                         ν.sub.8                                             35.70      r.sub.16&#39;             ∞                    d.sub.27                         6.8      r.sub.28             45.455 d.sub.28                         5.3L.sub.14                           N.sub.15                                   1.62280                                         ν.sub.15                                             56.98      r.sub.29             -9.823 d.sub.29                         5.0L.sub.15                           N.sub.16                                   1.68300                                         ν.sub.16                                             31.52      r.sub.30             -29.147                    d.sub.30                         38.9      r.sub.31             -687.999                    d.sub.31                         4.5L.sub.16                           N.sub.17                                   1.49140                                         ν.sub.17                                             57.9      r.sub.32             -14.000                    d.sub.32                         39.2      r.sub.33             30.682 d.sub.33                         1.0L.sub.17                           N.sub.18                                   1.73300                                         ν.sub.18                                             28.24      r.sub.34             9.700  d.sub.34                         3.0L.sub.18                           N.sub.19                                   1.62280                                         ν.sub.19                                             56.98      r.sub.35             -26.234                    d.sub.35                         17.0      r.sub.36             16.227 d.sub.36                         3.0L.sub.19                           N.sub.20                                   1.49140                                         ν.sub.20                                             57.9      r.sub.37             211.100                    d.sub.37                         13.4      r.sub.38             ∞                    d.sub.38                         4.0G                                  N.sub.21                                   1.49140                                         ν.sub.21                                             57.9      r.sub.39             ∞                    d.sub.39                         4.7      r.sub.40             -51.853                    d.sub.40                         4.0L.sub.20                           N.sub.22                                   1.49140                                         ν.sub.22                                             57.9      r.sub.41             -13.925                    d.sub.41                         14.4      r.sub.42             21.598 d.sub.42                         1.0L.sub.21                           N.sub.23                                   1.73300                                         ν.sub.23                                             28.24      r.sub.43             8.311  d.sub.43                         4.3L.sub.22                           N.sub.24                                   1.62280                                         ν.sub.24                                             56.98      r.sub.44             -31.397                    d.sub.44                         15.0Eye Point______________________________________ 
    
     In the exemplary prior art, the objective lens system is conventionally designed, i.e., the longitudinal chromatic aberration (F-line to d-line) in the object side part 1 is +0.23 diopter in terms of finder, and the final longitudinal chromatic aberration (F to d) in the whole objective lens system at film 4 is -0.043 mm. This means that the inherent longitudinal chromatic aberration of the image side part 3 is sufficiently corrected within the image side part 3 by itself. In case of the exemplary prior art, such a correction is mainly due to the doublet composed of L 11  and L 12 . 
     As apparent from the comparison between the first embodiment of the present invention and the exemplary prior art, the final longitudinal chromatic aberration at film 4 is substantially of the same degree and the number of lens elements required to construct the objective lens system is identical with each other. Therefore, the objective lens system of the present invention is equivalent to the prior art as far as the final result with which the objective lens system by itself is concerned. 
     However, any simplified branch optical system such as in FIG. 2 cannot be combined with the objective lens system of the exemplary prior art in FIG. 3. The longitudinal chromatic aberration is left in an insufficiently corrected condition in the branch optical system of FIG. 2, and this insufficient correction would not be improved by any means and would remain in the final image in case of a finder system made by combining the simplified branch optical system of FIG. 2 with the conventional object side part 1 of FIG. 3. 
     Therefore, the exemplary prior art in FIG. 3 inevitably adopts a branch optical system in which the number of elements is increased to correct the inherent longitudinal chromatic aberration of the branch optical system by itself. According to Table 2-F, the longitudinal chromatic aberration (F-d) of the branch optical system by itself is -0.39 diopter, and the longitudinal chromatic aberration (F-d) of the whole finder system is -0.023 diopter. In addition, the longitudinal chromatic aberration (F-d) of the whole finder system at the reticle of focusing plate 9 is -0.049 mm. FIGS. 9a to 9d and 10a to 10c show various other aberrations of the exemplary prior art. 
     As apparent from the above comparison, the present invention successfully provides an optical system for use in single lens reflex cameras in which the construction of the whole system is simplified with necessary aberration corrections sufficiently achieved. 
     FIGS. 4 to 6 show finder systems of second to fourth embodiments of the present invention, Tables 3 to 5 corresponding to the second to fourth embodiments, respectively. 
     
                       TABLE 3______________________________________(Embodiment 2)(f - 33.00)  Diopter = 0.0  Magnification = 1.75Radius of      Axial     Refractive AbbeCurvature      Distance  Index      Number______________________________________      r.sub.16             ∞                    d.sub.16&#39;                         11.5P&#39;                                 N.sub.9                                   1.62588                                         ν.sub.9                                             35.70      r.sub.17&#39;             ∞                    d.sub.27                         22.0      r.sub.28*              20.000                    d.sub.28                         4.0L.sub.14                           N.sub.15                                   1.49140                                         ν.sub.15                                             57.9      r.sub.29*             -20.000                    d.sub.29                         17.0      r.sub.30             ∞                    d.sub.30                         4.0G                                  N.sub.16                                   1.49140                                         ν.sub.16                                             57.9      r.sub.31             ∞                    d.sub.31                         58.0      r.sub.32              27.000                    d.sub.32                         3.0L.sub.15                           N.sub.17                                   1.61762                                         ν.sub.17                                             52.70      r.sub.33             -10.700                    d.sub.33                         1.0L.sub.16                           N.sub.18                                   1.73300                                         ν.sub.18                                             28.24      r.sub.34             -30.000                    d.sub.34                         71.3      r.sub.35*              20.000                    d.sub.35                         4.0L.sub.17                           N.sub.19                                   1.49140                                         ν.sub.19                                             57.9      r.sub.36*             -20.000                    d.sub.36                         27.0Eye Point______________________________________ *ε = -1.25, iΣ Ciφ.sup.2i = 0 
    
     
                       TABLE 4______________________________________(Embodiment 3)(f = 33.00)  Diopter = 0.0  Magnification = 1.94Radius of      Axial     Refractive AbbeCurvature      Distance  Index      Number______________________________________      r.sub.16             ∞                    d.sub.16&#39;                         11.5P&#39;                                 N.sub.9                                   1.62588                                         ν.sub.9                                             35.70      r.sub.17&#39;             ∞                    d.sub.27                         18.0      r.sub.28*              20.000                    d.sub.28                         4.0L.sub.14                           N.sub.15                                   1.49140                                         ν.sub.15                                             57.9      r.sub.29*             -20.000                    d.sub.29                         16.8      r.sub.30             ∞                    d.sub.30                         4.0G                                  N.sub.16                                   1.49140                                         ν.sub.16                                             57.9      r.sub.31             ∞                    d.sub.31                         20.1      r.sub.32*              20.000                    d.sub.32                         4.0L.sub.15                           N.sub.17                                   1.49140                                         ν.sub.17                                             57.9      r.sub.33*             -20.000                    d.sub.33                         34.0      r.sub.34*              20.000                    d.sub.34                         4.0L.sub. 16                          N.sub.18                                   1.49140                                         ν.sub.18                                             57.9      r.sub.35*             -20.000                    d.sub.35                         39.0      r.sub.36*              20.000                    d.sub.36                         4.0L.sub.17                           N.sub.19                                   1.49140                                         ν.sub.19                                             57.9      r.sub.37*             -20.000                    d.sub.37                         20.0Eye Point______________________________________ *ε = -0.95, iΣ Ciφ.sup.2i = 0 
    
     
                       TABLE 5______________________________________(Embodiment 4)(f = 33.00)  Diopter = 0.0  Magnification = 1.95Radius of      Axial     Refractive AbbeCurvature      Distance  Index      Number______________________________________      r.sub.16             ∞                    d.sub.16&#39;                         11.5P&#39;                                 N.sub.9                                   1.62588                                         ν.sub.9                                             35.70      r.sub.17&#39;             ∞                    d.sub.27                         18.0      r.sub.28*              16.000                    d.sub.28                         4.0L.sub.14                           N.sub.15                                   1.49140                                         ν.sub.15                                             57.9      r.sub.29             -30.000                    d.sub.29                         40.1      r.sub.30              30.000                    d.sub.30                         4.0L.sub.15                           N.sub.16                                   1.49140                                         ν.sub.16                                             57.9      r.sub.31*             -16.000                    d.sub.31                         41.8      r.sub.32*              16.000                    d.sub.32                         4.0L.sub.16                           N.sub.17                                   1.49140                                         ν.sub.17                                             57.9      r.sub.33             -30.000                    d.sub.33                         40.1      r.sub.34              30.000                    d.sub.34                         4.0L.sub.17                           N.sub.18                                   1.49140                                         ν.sub.18                                             57.9      r.sub.35*             -16.000                    d.sub.35                         20.0Eye Point______________________________________ *ε = -0.85, iΣ Ciφ.sup.2i = 0 
    
     In these embodiments, the object side part 1 (r 1  to r 15 ) of the objective lens system and the air space d 15  in the finder system are identical with those in the first embodiment, and not shown, but the branch optical system is only shown. Further, in FIGS. 4 to 6, the elements of the branch optical system are shown along a straight optical axis equivalent to the practical optical axis including reflection on mirrors. It should be noted that the embodiments in FIGS. 5 and 6 are each formed by identically shaped single lenses. Further, in case of the FIG. 5 embodiment, each single lens is symmetrically shaped. In addition, the focusing plate G having a reticle on its image side surface is located at the position of the primary real image in case of FIGS. 4 and 5 embodiments as indicated by 9&#39;. FIGS. 11a to 11c, 12a to 12c, and 13a to 13c correspond to the second to fourth embodiments, respectively. Necessary data for the longitudinal chromatic aberration of F-line to 3-line of the second to fourth embodiments are summarized in the following table with those of the first embodiment and the exemplary prior art. 
     
         ______________________________________Object        WholeSide Part     Ob-                     Atof            jective Branch   Whole  ReticleObjective     lens    Optical  Finder of FinderLens System   system  System   System System(diopter)     (mm)    (diopter)                          (diopter)                                 (mm)______________________________________1st emb.  -0.43       0.026  0.55   -0.07   0.0302nd emb.  &#34;          &#34;       0.78   0.43   -0.0433rd emb.  &#34;          &#34;       2.17   1.75   -0.0434th emb.  &#34;          &#34;       2.12   1.69   --Prior Art   0.23      -0.043  -0.39  -0.023 -0.049______________________________________ 
    
     The above table discloses that a simplified branch optical system of insufficient correction of longitudinal chromatic aberration can be successfully utilized to form a high class of finder system in accordance with the present invention. 
     By the way, the objective lens system in FIGS. 2 and 3 are zoom lens systems, and the position in the Figures or Tables 1-L, 1-F, 2-l, and 2-F are of telephoto position. In case of wide angle photography, d 6  =0.7, d 12  =17.0, d 15  =7.1 and f=8.73 in Table 1-L and 1-F, while d 6  -0.7, d 12  =17.0, d 14  -9.1 and f-8.74 in Tables 2-L and 2-F. In FIGS. 2 and 3, the focusing lens group, the variator lens group, the compensator lens group, and the relay group are identified with F, V, C and R, respectively.