Wide angle optical system and image pickup apparatus using the same

An optical system comprises, in order from the object side, an aperture stop, a first lens with positive refracting power, a second lens with negative refracting power, and a third lens, both surfaces of the third lens are an aspherical surface in which refracting power varies in accordance with distance from the optical axis in such a way that the both surfaces have a convex shape facing toward the object side in the vicinity of the optical axis and have a concave shape facing toward the object side in the vicinity of the outer circumference, and the following conditions (1) and (2) are satisfied: 2.2< d 2 /d 3 <7  (1) −0.04< f/f 3 <0.04  (2) where d 2 is the thickness of the first lens on the optical axis, d 3 is a space distance between the first and second lenses on the optical axis, f is a focal length of the whole of the wide angle optical system, and f 3 is a focal length of the third lens.