Projection optical system with enlargement at a varying magnification

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
• θ1 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.