An optical system for imaging pickup includes, sequentially arranged from an object side to an image side: the first lens element with positive refractive power having a convex image-side surface, the second lens element with refractive power having a concave image-side surface, the third lens element with refractive power, the fourth lens element with refractive power having both object-side and image-side surfaces being aspheric, and the fifth lens element with negative refractive power having a concave image-side surface, and both object-side and image-side surfaces being aspheric. By such arrangements, the optical system for imaging pickup satisfies conditions related to shorten the total length and to reduce the sensitivity for use in compact cameras and mobile phones with camera functionalities.