order from the object side has a concave surface on the image side and a stop is positioned between the first lens and the second lens the second lens has a convex surface on the object side the third lens has a concave surface on the object side and the fourth lens has a convex aspherical surface on the image side such that the refractive power of the fourth lens becomes weaker towards the edge of the lens specified conditions are preferably satisfied in order to provide favorable correction of aberrations application_number US 35823703 A description the invention will first be discussed in general terms the single focus lens of the present invention is formed of in order from the object side a first lens element of negative refractive power having a concave surface on the image side a stop a second lens element of positive refractive power having a convex surface on the object side a third lens element of negative refractive power having a concave surface on the object side and a fourth lens element of positive refractive power having a convex lens surface on the image side that is aspherical such that the refractive power of this surface becomes weaker towards the periphery of the lens and wherein the following conditions 1 and 2 are satisfied r1 is the radius of curvature near the optical axis of the lens surface on the object side of the first lens element r2 is.