Patent ID: 8472124
Filing Date: 2013-06-25
Classification: G02B,G03B

Abstract:
1. An image forming optical system comprising in order from an object side: a first lens group G a second lens group G a third lens group G a fourth lens group G the image forming optical system comprises maximum of five lens groups, and at the time of zooming from a wide angle end to a telephoto end, when a focal length of the overall image forming optical system is in a range of 1.2 fw to 1.8 fw, the first lens group G in a rectangular coordinate system in which, a horizontal axis is let to be νd (LA) and a vertical axis is let to be nd(LA), when a straight line expressed by is set, nd and νd of the positive lens LA are included in an area which is determined by a straight line when it is a lower limit value of a range of the following conditional expression (1) and a straight line when it is an upper limit value of the range of the following conditional expression (1), and an area determined by the following conditional expression (2) where, fw denotes a focal length of the image forming optical system at wide angle end, νd(LA) denotes Abbe's number (nd(LA)−1)/(nF(LA)−nC(LA)) for the positive lens LA, and nd(LA), nC(LA), nF(LA) denote refractive indices of the positive lens LA for a d-line, a C-line, and an F-line respectively.