Patent ID: 7605983

Claim:
A viewing optical system, comprising: a screen on which an object image taken via a taking lens is formed, an image erection optical system for erecting the object image formed on said screen, and an eyepiece lens optical system of positive refracting power for guiding a light beam leaving said image erection optical system to a viewer's eyeball, wherein: said image erection optical system comprises a prism having an internal reflective surface, and said eyepiece optical system comprises, in order from a screen side to an exit pupil side, a first lens component of negative refracting power wherein an exit pupil side surface has a paraxial-radius-of-curvature absolute value smaller than a paraxial-radius-of-curvature absolute value of a screen side surface, a second lens component having a double-convex shape, and a third lens component having a meniscus shape concave on an exit pupil side, with satisfaction of the following conditions (1), (2) and (3): 1.55<ndp (1) 0.35 <fe/Dip< 0.60 (2) 1.0 <D 3/( D 1 +D 2)<1.8 (3) where ndp is a d-line refractive index of the prism; fe is a focal length of the eyepiece lens system provided that when diopter adjustment is implemented by movement of the lens components, it is going to be the focal length of the eyepiece lens system at −1 diopter; Dip is an optical path length along an optical axis from an image formation position on the screen to an exit surface of the prism; D 1 is a thickness on the optical axis of the first lens component; D 2 is a thickness on the optical axis of the second lens component; D 3 is a thickness on the optical axis of the third lens component; and the lens components are lenses that are identifiable by an air contact surface, wherein each lens is a single lens or a cemented lens.