An optical system comprises, in order from an object side to an image side, a first lens group includes a first lens element with positive refractive power having a convex object-side surface; and a second lens group in order from the object side to the image side includes a second lens element with negative refractive power having a concave image-side surface, a third lens element with positive refractive power having a convex image-side surface, and a fourth lens element with negative refractive power having a concave object-side surface and a concave image-side surface, wherein the image-side surface of the fourth lens element changes from concave at a paraxial region to convex at a peripheral region, and both of the object-side surface and the image-side surface of the fourth lens element are aspheric.