an imaging lens having a two - group , two - lens - element construction is disclosed that is formed of , in order from the object side without any intervening lens elements : an aperture stop ; a first lens element of positive refractive power with its surface on the object side being aspherical and convex near the optical axis and its surface on the image side being aspherical and either convex or concave near the optical axis ; and a second lens element of positive refractive power with its surface on the object side being aspherical and convex near the optical axis and its surface on the image side being aspherical and concave near the optical axis but convex toward the periphery . specified conditions are satisfied in order to reduce various aberrations and provide a sufficient back focus for the insertion of other optical elements .