Patent ID: 7016124

Claim:
An image-formation optical system comprising: a reflective optical unit including a plurality of reflective surfaces, each of the reflective surfaces having a curvature and a rotationally asymmetric shape; wherein the following conditions are satisfied: L/{Er ( S− 1)}<2.2 3< L/{Eo ( S− 1)} where L represents an equivalent optical path length in air on a reference axis between a first reflective surface of the reflective optical unit which is closest to an object and a final reflective surface of the reflective optical unit which is closest to an image surface, the reference axis representing a path traveled by a central principal ray passing through a center of a pupil and reaching a center of an image surface, Er represents a first diameter of an entrance pupil of the reflective optical unit in a first direction perpendicular to a first sectional plane including the reference axis, Eo represents a second diameter of the entrance pupil in a second direction parallel to the first sectional plane, the second diameter Eo being smaller than the first diameter Er, and S represents the number of surfaces from the first reflective surface to the final reflective surface.