Optical system and image pickup apparatus having the same

An image pickup apparatus comprising:

• an optical system;
• and
• an image sensor configured to receive an image formed by the optical system,
• wherein the optical system includes, in order from an object side to an image side, a first lens unit having a positive refractive power, a diaphragm, a second lens unit having a negative refractive power, and a third lens unit having a positive refractive power,
• wherein each distance between adjacent lens units is changed during focusing,
• wherein the second lens unit is moved to the image side during focusing from an object at infinity to a short-distance object,
• wherein the first lens unit includes a positive lens closest to the object and two or more negative lenses,
• wherein the second lens unit consists of a single negative lens, and
• wherein the following conditional expressions are satisfied:
• 0.2<LD1/LD<0.4,
• BF/f<0.25, and
• 15<vdG1<26
• where LD1 is a distance on an optical axis from a lens surface closest to the object of the first lens unit to a lens surface closest to an image plane of the first lens unit, LD is a distance on the optical axis from the lens surface closest to the object of the first lens unit to the image plane, f is a focal length of the optical system, BF is a back focus of the optical system during focusing on the object at infinity, and vdG1 is an Abbe number of the positive lens.