Patent ID: 8482867

Claim:
An imaging optical system consisting of, in order from an object side thereof, a first lens group of positive refracting power, a second lens group of positive or negative refracting power, and a third lens group of positive refracting power, wherein: an aperture stop is located between an object-side surface in the first lens group and an object-side surface in the second lens group; a lens on the most object side of the first lens group, and a lens on the most image side of the third lens group remains constantly fixed; upon focusing from a far distance to a near distance, only the second lens group moves on an optical axis; the third lens group comprises a front sub-lens component of positive refracting power and a rear sub-lens component of negative refracting power; and an axial air separation between the front sub-lens component of positive refracting power and the rear sub-lens component of negative refracting power is larger than an axial air separation between other lenses in the third lens group; wherein the first lens group and the third lens group remain constantly fixed.