Provided is an optical system including, in order from an object side to an image side: a first lens unit having a positive refractive power; a second lens unit having a negative refractive power configured to move during focusing; and a third lens unit. An interval between each pair of adjacent lens units is changed during focusing. The optical system includes an aperture stop. The first lens unit includes at least three positive lenses including a positive lens (G1P) arranged closest to the object side. A distance LD on an optical axis from a lens surface closest to the object side of the optical system to an image plane, a focal length “f” of the optical system, a refractive index ndG1P of a material of the positive lens (G1P), and an Abbe number νdG1P of the material of the positive lens (G1P) are each appropriately set.