JPS6444412A - Albada system inverted galilean finder - Google Patents

Abstract

PURPOSE:To suppress ghost with a simple constitution and to obtain a compact finder device of high efficiency by constituting an eye-piece system of three groups of three lenses having positive refractive force as a whole and setting a specified conditional inequality among a focal distance, the thickness of a 2nd group and the entire distance of image plane. CONSTITUTION:The titled finder is constituted of a 1st group having negative refractive force, the 2nd group of a thick positive lens and a 3rd group having positive refractive force from an object side and the surface 3 of the 2nd group is formed in an Albada reflecting surface and the surface 4 is formed in the frame of a visual field. The light of an object respectively forms the image of the object on an EP with a path shown by a continuous line and the image of the frame of the visual field on the EP after the said light is once reflected on the surface 4 with a path shown by a dotted line. Three inequalities are set among the focal distance (f), the thickness d3 of the 2nd group and an entire intersurface distance L. In the inequality I the assignment of the refractive force of the 2nd and the 3rd groups is set in a certain range so as to suppress the diopter of a frame, in the inequality II it is made to compact and in the inequality III the correction of astigmation is made favorable. The ghost does not occur in the path shown by a dotted line forming the frame of the visual field. Thus, the device can be made compact and the high efficiency can be obtained.