Patent ID: 7855837

Claim:
A viewing optical system comprising: a screen on which an object image by a taking lens is formed, an image erection optical system for erecting the object image formed on said screen, and an eyepiece lens system of positive refracting power for guiding a light beam leaving said image erection optical system to the eyeball of a viewer, wherein: said image erection optical system comprises a plurality of reflecting surfaces, said eyepiece optical system comprises, in order from a screen side to an exit pupil side, a first lens element of negative refracting power, a second lens element convex on both the screen side and the exit pupil side, and a third lens element that is of negative refracting power and has a meniscus shape concave on the exit pupil side, and said third lens element satisfies the following condition (1): 0.13 <DL 3 /fe< 0.30 (1) where DL3 is an axial thickness of the third lens element, and fe is a focal length of the eyepiece lens system provided that when the eyepiece lens system has a diopter adjustable by movement of the lenses, fe is going to be a focal length at −1 diopter.