Patent ID: 6775071
Filing Date: 2004-08-10
Classification: G02B

Abstract:
An image forming optical system comprising, in order from an object side toward an image side:a first lens unit with positive refracting power; a second lens unit with positive refracting power; a third lens unit with negative refracting power; a fourth lens unit with positive refracting power; and an aperture stop interposed between the third lens unit and the fourth lens unit, a variable magnification optical system being provided in which spacings between the first lens unit and the second lens unit, between the second lens unit and the third lens unit, and between the third lens unit and the fourth lens unit are changed to thereby vary an imaging magnification, wherein the image forming optical system changes the imaging magnification while constantly keeping an object-to-image distance thereof, and in at least one variable magnification state where the imaging magnification is changed, satisfies the following conditions: |En|/L>0.4 |Ex|/|L/&bgr;|>0.4 where En is a distance from a first lens surface on the object side of the variable magnification optical system to an entrance pupil of the image forming optical system, L is the object-to-image distance of the image forming optical system, Ex is a distance from a last lens surface on the image side of the variable magnification optical system to an exit pupil of the image forming optical system, and &bgr; is a magnification of a whole of the image forming optical system.