a lens module include : a first lens having positive refractive power ; a second lens having positive refractive power ; a third lens having refractive power ; a fourth lens having positive refractive power ; a fifth lens having refractive power ; and a sixth lens having negative refractive power and having an inflection point formed on an image - side surface thereof . the lens module may satisfy the following conditional expressions : 0 . 36 & lt ; sd / f & lt ; 0 . 48 , where sd is a size of a stop opening and f is an overall focal length of the lens module .