Provided is an optical system including, in order from an object side to an image side, a first lens unit and a second lens unit with a positive refractive power. One of lenses of the first lens unit is the closest to the object side and has a negative refractive power. The second lens unit includes an aperture diaphragm. One of lenses that is included in the second lens unit, located on the object side or the image side of the aperture diaphragm and the closest to the aperture diaphragm has a concave lens surface that faces the aperture diaphragm. Conditions are satisfied whose values are set by using refractive indexes of a negative lens and a positive lens of the first lens unit that respectively have the smallest Abbe number.