Patent ID: 6813088
Filing Date: 2004-11-02
Classification: G02B

Abstract:
A projection optical system having an optical axis and comprised of, in order:a first lens group; a second lens group; a diaphragm; a third lens group; and a fourth lens group; whereinsaid second lens group and said third lens group are movable along the optical axis of the projection optical system in order to vary the magnification of the projection optical system; the range of movement of said second lens group and said third lens group along the optical axis includes a position where the first lens group and the second lens group as a first lens unit and the third lens group and the fourth lens group as a second lens unit are positioned symmetrically on opposite sides of a plane that includes said diaphragm so that the first and second lens units are mirror images of one another with regard to said plane so that the optical projection system achieves at least very nearly unity magnification when said second lens group and said third lens group are at said position; and the projection optical system is telecentric or very nearly telecentric on both sides.