An imagery optical system, sequentially from an object side to an image side on an optical axis comprising: the first lens element with positive refractive power, the second lens element with positive refractive power, the third lens element, the fourth lens element, and the fifth lens element having at least one inflection point. Each of the five lens elements may be made of plastic and comes with bi-aspheric surfaces. The imagery optical system satisfies conditions related to shorten the total length and to reduce the sensitivity for use in compact cameras and mobile phones with a camera function.