Patent ID: 8587877

Claim:
An imaging optical system comprising: in order from an object side to an image side, a front unit in which a negative lens element is arranged closest to the object side, an aperture diaphragm, and a rear unit having positive optical power, wherein in focusing from an infinite distance object to a short distance object, the negative lens element arranged closest to the object side and the aperture diaphragm are fixed with respect to an image surface, and a part of lens elements included in the front unit or the rear unit moves along an optical axis, the following condition (1) is satisfied: 0.9 <R/f< 2.0 (1) where R is an average between an image side curvature radius of the negative lens element arranged closest to the object side, and an image side curvature radius of a second negative lens element from the object side among all the negative lens elements, and f is a focal length of the entire system, and a lens element which moves along the optical axis in focusing satisfies the following condition (6) or (7): 0.2 <βp <0.8 (6) 1.0 <βn <1.8 (7) where βp is a paraxial imaging magnification in an infinity in-focus condition of a positive lens element which moves in focusing, and βn is a paraxial imaging magnification in an infinity in-focus condition of a negative lens element which moves in focusing.