Abstract:
An inverted Galilean type variable magnification zoom finder has a field frame, which can be moved in accordance with a change in magnification of a finder optical system. The field frame may be moved integral with, or independently of, a lens or a lens unit.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a variable magnification zoom finder chiefly used in a compact camera, and in particular, to an inverted Galilean type variable magnification zoom finder. 
     2. Description of Related Art 
     An example of an optical system of a conventional virtual image mode variable magnification zoom finder for compact cameras is shown in FIGS. 1A-1C. The optical system of this finder includes a first fixed lens 1, a second fixed lens 2, and a third fixed lens 3 which are arranged on an optical axis a; a moving lens 4 disposed between the first fixed lens 1 and the second fixed lens 2 to be movable along the optical axis a; and a fixed field frame 5 disposed such that it is stationary on the side of a pupil b of the first fixed lens 1. 
     In the above optical system, a height of a chief ray from the optical axis a at the fixed field frame 5 has a minimum h 1  where the moving lens 4 is located at a wide-angle position (FIG. 1A). As the moving lens 4 is moved from the wide-angle position toward a telephoto position (FIG. 1B), a height h 2  of the chief ray from the optical axis a at the fixed field frame 5 is progressively increased and reaches a maximum h 3  where the moving lens 4 is located at the telephoto position (FIG. 1C). In short, the conventional virtual image mode variable magnification zoom finder is merely designed so that the single fixed field frame 5 is placed to accommodate a change in magnification of the finder. 
     In general, an angle made by the optical axis with the chief ray emerging from an eyepiece system and reaching a pupil is set to be constant in order to make a field factor constant over the range from the wide-angle position to the telephoto position. In the conventional finder optical system shown in FIGS. 1A-1C, however, a field area is determined by the single field frame 5, and thus the height of the chief ray from the optical axis a at a lens (or a lens unit) is varied in accordance with the magnification change from the wide-angle position to the telephoto position (namely, as shown in FIGS. 1A-1C, h 1  &lt;h 2  &lt;h 3 ). For this reason, the angle of the chief ray reaching the pupil does not become constant and unwanted light reaches the pupil, causing the problem of hindering the determination of the composition of photography. 
     In order to avoid this problem, albada type variable magnification zoom finders have been often used in recent years. FIGS. 2A-2C show an example of the albada type variable magnification zoom finder. This albada type finder includes a first fixed lens 6 and a second fixed lens 7 which are arranged on the optical axis a; a half mirror 8 interposed between the first fixed lens 6 and the second fixed lens 7, having a half mirror surface 8a; and a moving lens 9 movable along the optical axis a between the first fixed lens 6 and the half mirror 8. 
     The albada type variable magnification zoom finder has the advantage that when an optical frame c is attached to the eyepiece system, a photographing area becomes clear and the determination of the composition can be accurately made. However, the albada type variable magnification zoom finder, which imperatively requires the half mirror surface 8a to be situated at an arbitrary place, has the problem that the transmittance of the entire finder system is reduced and as a result, the entire visual field becomes dark, detrimental light such as ghost attributable to the half mirror surface 8a is produced, or a rise in manufacturing cost is caused because of the placement of the half mirror. 
     Japanese Utility Model Preliminary Publication No. Sho 60-128329 discloses an inverted Galilean type finder in which two field frames of different sizes are provided and alternately placed on the optical axis, thereby allowing the magnification to be changed. 
     This inverted Galilean type finder, however, requires a complicated mechanism for alternately inserting the two field frames in the optical axis and thus is not necessarily suitable for the latest camera requiring compact design. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide a variable magnification zoom finder, notably an inverted Galilean type variable magnification zoom finder, which can bring about a proper field area over the range from the wide-angle position to the telephoto position without using the arrangement of the albada type variable magnification zoom finder. 
     In order to achieve this object, the inverted Galilean type variable magnification zoom finder according to the present invention is constructed so that a field frame is provided and variable in accordance with the magnification change. Specifically, the field frame is designed to be movable along the optical axis. 
     In the inverted Galilean type variable magnification zoom finder of the present invention, a moving lens (or a lens unit) is moved in accordance with a change in focal length of a photographing lens optical system, and in keeping with this, the position of the field frame is shifted to thereby secure a proper field area with respect to a photographing area, no matter whether the focal length of the photographing lens optical system corresponds to the wide-angle position or the telephoto position. 
     For example, the field frame can be designed so that it can be moved integrally with a lens or lens unit, or independently of it. 
     In a technique of moving the field frame integrally with a lens or lens unit, a fixed field frame is disposed at an arbitrary place, the height of the chief ray from the optical axis at the telephoto position and a movable field frame constructed integrally with a moving lens is determined, the height of the chief ray from the optical axis at the wide-angle position is determined by the arrangement in the finder optical system. By doing so, the optical system which is simple in arrangement and has the same function as in the albada type variable magnification zoom finder can be obtained. However, the positions of the field frames at the wide-angle and telephoto positions may be reversed, depending on the lens type. 
     In a technique of moving the field frame independently of a lens or lens unit, on the other hand, since the chief ray can be intercepted at almost the same height as that of the chief ray determining the field factor over the entire region from the wide-angle position to the telephoto position, this technique can linearly accommodate the entire region, and the field area can be secured with respect to the photographing area. 
     The field frame may be constructed by mechanical means or may also be constructed in such a way that paint is applied to the side or end surface of a lens, or a treatment may be carried out such that the lens has a diffusing surface. 
     This and other objects as well as the features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiment when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A, 1B, and 1C are schematic views showing operating conditions of a conventional inverted Galilean type variable magnification zoom finder; 
     FIGS. 2A, 2B, and 2C are schematic views showing operating conditions of a conventional albada type variable magnification zoom finder; and 
     FIGS. 3A, 3B, and 3C are schematic views showing operating conditions of an inverted Galilean type variable magnification zoom finder of an embodiment in the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of the inverted Galilean type variable magnification zoom finder according to the present invention Is shown in FIGS. 3A-3C. The inverted Galilean type variable magnification zoom finder in this embodiment, as shown in these figures, includes a first fixed lens 10, a second fixed lens 11, and a third fixed lens 12 which are arranged on the optical axis a and a moving lens 13 disposed to be movable along the optical axis a between the first fixed lens 10 and the second fixed lens 11. A fixed field frame 14 is placed on the pupil side of the first fixed lens 10, and a field frame 15 is attached to the surface of the moving lens 13 facing the pupil. That is, the field frame 15 can be moved, together with the moving lens 13, along the optical axis a. 
     When the focal length of the photographing lens optical system corresponds to the wide-angle position, the moving lens 13 in the finder optical system, as shown in FIG. 3A, is located on the object side, namely closest to the first fixed lens 10. As the focal length of the photographing lens optical system is changed to transfer the optical system from the wide-angle position to the telephoto position, the moving lens 13, as shown in FIG. 3B, is moved from the object side toward a pupil 16, namely from the first fixed lens 10 toward the second fixed lens 11. When the focal length of the photographing lens optical system corresponds to the telephoto position, the moving lens 13, as shown in FIG. 3C, is located on the side of the pupil 16, namely closest to the second fixed lens 11. 
     The inverted Galilean type variable magnification zoom finder of the embodiment thus constructed functions as follows: 
     Calling H 1  the height of the chief ray from the optical axis a at the fixed field frame 14 when the moving lens 13 is located at the wide-angle position (FIG. 3A), H 2  the height of the chief ray from the optical axis a at the fixed field frame 14 when the moving lens 13 is located midway between the wide-angle position and the telephoto position (FIG. 3B), and H 3  the height of the chief ray from the optical axis a at the fixed field frame 14 when the moving lens 13 is located at the telephoto position (FIG. 3C), the following condition is established: 
     
         H.sub.1 &lt;H.sub.2 &lt;H.sub.3 
    
     Specifically, the height of the chief ray from the optical axis a at the fixed field frame 14 is minimized when the moving lens 13 is located at the wide-angle position, is progressively increased when the moving lens 13 is moved from the wide-angle position toward the telephoto position, and is maximized when the moving lens 13 is located at the telephoto position. 
     In contrast to this, when T 1  denotes the height of the chief ray from the optical axis a at the moving field frame 15 where the moving lens 13 is located at the wide-angle position (FIG. 3A), T 2  the height of the chief ray from the optical axis a at the moving field frame 15 where the moving lens 13 is located midway between the wide-angle position and the telephoto position (FIG. 3B), and T 3  the height of the chief ray from the optical axis a at the moving field frame 15 where the moving lens 13 is located at the telephoto position (FIG. 3C), the following condition is established: 
     
         T.sub.1 &gt;T.sub.2 &gt;T.sub.3 
    
     Specifically, the height of the chief ray from the optical axis a at the moving field frame 15 is maximized when the moving lens 13 is located at the wide-angle position, is progressively decreased as the moving lens 13 is moved from the wide-angle position toward the telephoto position, and is minimized when the moving lens 13 is located at the telephoto position. 
     In this way, the field area at the wide-angle position, as shown in FIG. 3A, is determined by the behavior of the moving field frame 15 intercepting rays of light outside the field area, while the field area at the telephoto position, as shown in FIG. 3C, is determined by the behavior of the fixed field frame 14 intercepting rays of light outside the field area. 
     Thus, according to the present invention, a proper field area can be obtained with respect to the photographing area, no matter whether the focal length of the photographing lens optical system corresponds to the wide-angle position or the telephoto position. The above behavior may be reversed, depending on the lens type, but in this case, it is only necessary to interchange the field frames 14 and 15.