A relay finder optical system, of a single-lens reflex camera, includes a condenser lens group, a prism, and a relay lens group, in this order from a primary imaging plane.
An image, which is formed on the primary imaging plane, and is upside down and inverted from left to right, is re-formed on a secondary imaging plane for viewing by a photographer.
The relay lens group includes a positive front lens group, a negative intermediate lens group, and a positive rear lens group, in this order from the object.
The most photographer's-eye side lens element of the positive rear lens group comprises a positive lens element having a convex surface facing toward the image, and satisfies the following conditions:
1<SF (1)
−6<f/fM<−3 (2)
SF: the shaping factor of the most photographer's-eye side positive lens element (SF=(r1+r2)/(r1−r2)); r1: the radius of curvature of the object-side surface of the most photographer's-eye side positive lens element; r2: the radius of curvature of the photographer's eye-side surface of the most photographer's-eye side positive lens element; f: the focal length of the relay finder optical system; and fM: the focal length of the negative intermediate lens group.