1. An image processing apparatus comprising: an imaging device which captures an image; an optical system which causes said imaging device to form an image of an object; and a distance determining unit configured to determine distance of the object between said optical system and the object based on a size of a blur developed on the image, wherein said optical system has a characteristic which simultaneously satisfies both of conditions that (i) variation in magnification is equal to or smaller than the predetermined number of pixels in the case where a focused point is set farthest from and closest to said optical system in a range of the distance of the object determined by said distance determining unit and (ii) variation in a Point Spread Function due to an image height of said optical system is equal to or smaller than a predetermined degree so as not to affect the determination of the distance of the object by said distance determining unit.