Patent ID: 7761005

Claim:
A relay finder optical system, of a single-lens reflex camera, comprises a condenser lens group, a prism, and a relay lens group, in this order from a primary imaging plane which is optically equivalent to an imaging plane of the single-lens reflex camera, wherein an image, which is formed on said primary imaging plane, and which is upside down and inverted from left to right, is re-formed on a secondary imaging plane for viewing by a photographer; wherein said relay lens group comprises a positive front lens group, a negative intermediate lens group, and a positive rear lens group, in this order from an object; and wherein the most photographer's-eye side lens element of said positive rear lens group comprises a positive lens element having a convex surface facing toward the eyes of a photographer, and satisfies the following conditions: 1<SF −6< f/fM <−3 wherein SF designates the shaping factor of the most photographer's-eye side positive lens element of said positive rear lens group (SF=(r 1 +r 2 )/(r 1 −r 2 )); r 1 designates the radius of curvature of the object-side surface of the most photographer's-eye side positive lens element of said positive rear lens group; r 2 designates the radius of curvature of the photographer's eye-side surface of the most photographer's-eye side positive lens element of said positive rear lens group; f designates the focal length of said relay finder optical system; and fM designates the focal length of said negative intermediate lens group.