Patent ID: 8164674
Filing Date: 2012-04-24
Classification: G02B

Abstract:
1. An image forming optical system characterized by comprising a positive first lens group, a negative second lens group, and an image side lens group disposed on the image side of the negative second lens group, wherein the image side lens group has a positive composite refracting power, the first lens group is composed of a cemented lens made up of one positive lens and one negative lens arranged in order from the object side, and the image forming optical system satisfies the following conditional expressions (1-1) and (2-1): where T1g is a thickness of the first lens group on the optical axis, Flt is a focal length of the entire image forming optical system at the telephoto end, and θgF is a partial dispersion ratio (ng1−nF1)/(nF1−nC1) of the negative lens, where nC1, nF1, ng1 are refractive indices of the negative lens respectively for the C-line, F-line, and g-line.