Patent ID: 7898743

Claim:
An image forming optical system comprising: a positive lens group; a negative lens group; and an aperture stop, wherein the positive lens group is disposed at an object side of the aperture stop, and the positive 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 is set, where α=−0.017 and β is a predetermined constant, 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) 1.45<β<2.15 (1a) 1.59885≦Nd <2.20 (2a′″) 3<νd <12 (3a) where, Nd denotes a refractive index, and νd denotes an Abbe's number.