Patent ID: 7948691

Claim:
An optical system comprising: in order from an object side to an image side, a first lens unit having a positive refractive power; an aperture stop; and a second lens unit, wherein the first lens unit includes: a first-a lens unit having a positive refractive power; and a first-b lens unit having a negative refractive power which moves along an optical axis in focusing, wherein the first-b lens unit includes at least one negative lens and at least one positive lens 1 bp , and wherein the following conditional expression is satisfied: 0.020 <θgF 1−0.6438+0.001682 ×vd 1<0.100, where θgF1 denotes a partial dispersion ratio of a material of the positive lens 1 bp and vd1 denotes Abbe number of the positive lens 1 bp.