US Patent 7692877 Optical lens system for taking image

An optical lens system for taking image comprises four lens elements from the object side to the image side: a first lens element with positive refractive power having a convex object-side surface; a second lens element with negative refractive power having a concave object-side surface and a concave image-side surface, at least one of the object-side surface and the image-side surface of the second lens element being aspheric, inflection points being formed on the image-side surface of the second lens element; a third lens element with positive refractive power having a convex object-side surface and a convex image-side surface; a fourth lens element with negative refractive power; and an aperture stop being located between the object side and the second lens element.