An optical system for taking image comprises three lens elements with refractive power, from the object side to the image side: a first lens element with negative refractive power, a second lens element with positive refractive power, and a third lens element with positive refractive power, wherein an aperture stop is located behind the first lens element, each of at least two lens elements is provided with at least one aspheric surface. By such arrangements, the optical system can obtain a good image quality, maintain enough back focal length and increase the field of view of the optical system.