Patent ID: 7440196

Claim:
An image forming optical system comprising, in order from an object side: a first lens unit having a positive refractive power; an aperture stop; and a second lens unit having a positive refractive power, the first lens unit being a lens unit which is closest to the object side in the image forming optical system; the second lens unit being a lens unit which is closest to an image side in the image forming optical system, the second lens unit comprising, in order from the object side: a front lens unit in which a surface closest to the image side is a concave surface which faces the image side; and a rear lens unit which has a positive refractive power and in which a surface closest to the object side is a concave surface which faces the object side, the rear lens unit consisting of one positive meniscus lens whose convex surface faces the image side, the image forming optical system satisfying the following conditions (1) and (2): −1.0< R 2/ R 3<−0.01 (1); and 0.3< ΣD 2 G/FL< 1.2 (2), wherein R 2 denotes a paraxial radius of curvature of the surface of the front lens unit closest to the image side, R 3 denotes a paraxial radius of curvature of the surface of the rear lens unit closest to the object side, ΣD 2 G denotes an axial length from the surface closest to the object side to the surface closest to the image side in the second lens unit, and FL denotes a focal length of the image forming optical system.