An optical system includes, a front lens unit having a positive refractive power, an aperture stop, and a rear lens unit having a positive refractive power, wherein the front lens unit includes an FB lens system, and an FA lens system, the FB and FA lens systems being configured to move appropriately, when shifting a focus from an object at a long distance to an object at a short distance. A focal length fFB of the FB lens system and a focal length f of the entire optical system are appropriately set.