Patent ID: 7922340

Claim:
A projection optical system receiving light from a display element to project an image displayed by the display element onto a screen with enlargement at a varying magnification achieved by varying a projection distance to the screen, the projection optical system comprising: a refractive optical system composed of one or more refractive lenses and having a positive optical power; and a concave-surfaced mirror disposed to a screen side of the refractive optical system and having a plane-symmetric reflective surface, wherein the projection optical system includes at least one optical element designed to be movable for focusing, wherein the projection optical system forms an intermediate image of the displayed image between the refractive optical system and the concave-surfaced mirror, and wherein conditional formula (1) below is fulfilled: 4°<θ1<20° (1) where θ 1 represents an angle of incidence, as measured on a cross-section of the concave-surfaced mirror along a plane of symmetry thereof, at which a ray, among rays incident on the screen when projection is performed at a shortest distance, that forms a smallest angle with a line normal to a screen surface is incident on the concave-surfaced mirror.