Patent ID: 7804653

Claim:
An imaging optical system comprising, in a sequence from an object side to an image side: a stop; a first lens which is convex at the object side and has positive refractive power and both surfaces of which are aspherical; a second lens which is convex at the image side and has negative refractive power; and a third lens having a concave surface at its central portion facing the image side and a convex surface at its peripheral portions facing the image side, wherein the refractive power of the third lens does not change within an effective aperture that denotes a distance from the optical axis to a highest location on a surface of the third lens, wherein the imaging optical system satisfies the conditions of the following Inequalities: 0.8 < L 2 ⁢ Y < 1 0.5 < fl f < 0.86 where L denotes the distance from the stop to the image focal point, 2Y denotes the diagonal length of the effective image surface, f denotes the overall focal distance of the imaging optical system, and f 1 denotes the focal distance of the first lens.