Patent ID: 7206143
Filing Date: 2007-04-17
Classification: G02B

Abstract:
1. An image-formation optical system comprising, in order from an object side thereof, a first positive lens, a second negative lens of double-concave shape, a third positive lens and a fourth positive meniscus lens convex on an object side thereof, including at least one aspheric surface, said aspheric surface located at the first positive lens and satisfying the following condition: where r sp is an axial radius of curvature of the aspheric surface, and r asp is a value that is obtained when a difference between a radius of curvature with the aspheric surface taken into account and an axial radius of curvature reaches a maximum in an optically effective range, wherein a total number of lens elements is four and a composite focal length of the first positive lens and the second negative lens is positive.