An optical system which forms an optical image on an image pickup element, comprising in order from an object side, a first lens unit having a positive refractive power, which includes a plurality of lenses, a stop, and a second lens unit which includes a plurality of lenses, wherein the first lens unit includes a first object-side lens which is disposed nearest to an object, and the second lens unit includes a second image-side lens which is disposed nearest to an image, and the first lens unit includes a negative lens, and a positive lens which is disposed on the object side of the negative lens, and the following conditional expressions are satisfied: β≦−1.1  (15) 0.08