Patent ID: 7944625

Claim:
An optical system comprising, in order from an object side to an image side: a first lens unit; an aperture stop; and a second lens unit having a positive refractive power, wherein the first lens unit includes at least one positive lens made of a material whose Abbe number (νdGP) and relative partial dispersion (θgFGP) satisfy the following conditions: θ gFGP −(0.6438−0.001682 ×νdGP )>0.005 60<νdGP, and wherein a back focal length when focusing on an infinitely-distant object point (BF), a focal length of the entire optical system (f), an average refractive index of materials of positive lenses included in the second lens unit (Np), and a distance from a surface of the positive lens of the first lens unit on the image side to the aperture stop on an optical axis (Lp) satisfy following conditions: 1.0 <BF/f< 3.0 1.6<Np 0.01 <Lp/f< 0.80.