Patent ID: 7715117

Claim:
An optical system comprising, in order from an object side to an image side: a first lens unit having a negative refractive power; and a second lens unit having a positive refractive power, wherein the second lens unit includes, in order from the object side to the image side: a front lens unit; a stop; and a rear lens unit, wherein the front lens unit includes a biconvex positive lens located closest to the image side, and wherein a back focus during focusing on an infinitely-distant object point (BF), a focal length of the entire optical system (f), a focal length of the positive lens of the front lens unit located closest to the image side (fp), a focal length of the front lens unit (f 2 a ), a refractive power of a surface of the rear lens unit located closest to the object side (φf), and a refractive power of the entire optical system (φ) satisfy the following conditions: 1.0< BF/f< 3.0 0.1 <fp/f 2 a< 0.5 <0.7 <|φf/φ|< 1.5.