Two-prism viewfinder for photographic or video camera - produces real image in neighbourhood of frame inserted between adjacent faces of polygonal and triangular prisms

摘要

Negative and positive lenses (2a,2b) of the viewing lens (1) are both movable along their common axis (1a) ahead of a prism unit (3). An incident beam of light from the viewing lens strikes a first prism (6) on its entry face (6a) which is curved to form, with the viewing lens, a telecentric optical system. After internal reflections at successive faces (6b,6c) it enters (7a) a second prism (7), undergoing three reflections (7b-7d) before emerging (7e) into an eyepiece (5) through which an upright image is seen. ADVANTAGE - Non-inverted real image produced in plane of frame (8) by optics with smaller dimensions in both vertical and horizontal directions.