Patent ID: 7382552

Claim:
A zoom optical system, comprising in order from a reduction side to a magnification side: a first lens unit having a positive optical power; a second lens unit having a negative optical power; a third lens unit having a positive optical power; and a fourth lens unit having a positive optical power, wherein respective intervals between the first, second, third and fourth lens units vary during zooming, wherein a magnification-side conjugate position with respect to a reduction-side conjugate position and a position of a pupil of the zoom optical system with respect to the reduction-side conjugate position are substantially immobile respectively across an entire zooming range, wherein said pupil is located outside of the zoom optical system and is located on said magnification conjugate side of said zoom optical system, and wherein the fourth lens unit moves toward the magnification side and an interval between the first lens unit and the fourth lens unit increases during zooming from a wide-angle end to a telephoto end.