Optical system and image pickup apparatus including the same

An optical system comprising, in order from an object side to an image side:

• a first lens unit having positive refractive power;
• and
• a second lens unit having negative refractive power,
• wherein the first lens unit moves and the second lens unit does not move during focusing,
• wherein the first lens unit includes a positive lens arranged closest on the object side of the first lens unit,
• wherein the second lens unit includes three or more lenses including at least two negative lenses,
• wherein when the first lens unit is divided at a largest interval and a second largest interval of intervals between adjacent lenses in the first lens unit, the first lens unit consists of a first subunit having positive refractive power, a second subunit having negative refractive power, and a third subunit having positive refractive power arranged in order from the object side to the image side, and
• wherein the following conditional expressions are satisfied:
• 0.14<sk/f<0.30
• 0.59<f13/f<1.21
• where sk represents a distance on an optical axis from a lens surface closest to the image side of the second lens unit to an image plane, f represents a focal length of the optical system in an in-focus state on an infinite-distance object, and f13 represents a focal length of the third subunit, and
• wherein the following conditional expression is satisfied:
• 0.33<|f12|/f<1.44
• where f12 represents a focal length of the second subunit.