Patent ID: 7800832
Filing Date: 2010-09-21
Classification: G02B

Abstract:
1. An image forming optical system comprising: a positive lens group; a negative lens group; and an aperture stop, wherein the negative lens group is disposed at an object side of the aperture stop, and the negative lens group includes a cemented lens which is formed by cementing a plurality of lenses, and in a rectangular coordinate system in which, a horizontal axis is let to be Nd and a vertical axis is let to be νd, when a straight line indicated by Nd=α×νd+β where α=−0.017 is set, Nd and νd of at least one lens forming the cemented lens are included in both of areas namely, an area which is determined by a line when a lower limit value is in a range of a following conditional expression (1a) and a line when an upper limit value is in a range of the following conditional expression (1a), and an area determined by following conditional expressions (2a) and (3a): where Nd denotes a refractive index, and νd denotes an Abbe's number.