Patent ID: 8746897

Claim:
An optical projection system for projecting an enlarged image on a projection surface, the optical projection system, comprising: an image forming element configured to form an image to be enlarged by the optical projection system; and a coaxial optical system and a concave mirror arranged in this order on an optical path from the image forming element to the projection surface, the coaxial optical system including a plurality of lens groups and an aperture stop that share an optical axis, wherein the lens groups include a first lens group and other lens groups, the first lens group having a negative refractive power and configured to independently move in an optical axis direction for adjusting a focus of the optical projection system, wherein the aperture stop is arranged at a position closer to the image forming element than the first lens group, the first lens group being arranged closest to the concave mirror among the lens groups having a negative refractive power, wherein if the other lens groups have negative refractive power, the focus is adjusted by moving the first lens group in a predetermined direction of the optical axis direction while moving the other lens groups in a direction opposite to the predetermined direction of the optical axis direction, wherein if the other lens groups have positive refractive power, the focus is adjusted by moving the first lens group in the predetermined direction of the optical axis direction while moving the other lens groups in a direction the same as the predetermined direction of the optical axis direction, and wherein if the other lens groups include a second lens group having positive refractive power and a third lens group having negative refractive power, the focus is adjusted by moving the first lens group in the predetermined direction of the optical axis direction while moving the second lens group in a direction the same as the predetermined direction of the optical axis direction and the third lens group in a direction opposite to the predetermined direction of the optical axis direction.