US Patent 7274518 Optical system for taking image

An optical system comprises from the object side: a positive first lens element, a negative second lens element, a positive third lens element, a fourth lens element, and an aperture stop located between the object side of the optical system and the second lens element for controlling the brightness of the optical system. A radius of curvature R3 of a front surface of the second lens element satisfies the relation: −0.02 [1/mm]<1/R3<0.22 [1/mm], an Abbe number V2 of the second lens element is less than 40. A focal length of the fourth lens element is f4, a focal length of the optical system is f, they satisfy the relation: f/f4<0.1, at least one inflection point is formed on the rear surface of the fourth lens element.