Patent ID: 8107001
Filing Date: 2012-01-31
Classification: G02B

Abstract:
1. An image forming optical system comprising in order from an object side to an image surface; a first lens group having a positive refracting power; a second lens group having a negative refracting power; and a third lens group having a positive refracting power, wherein at the time of zooming from a wide angle end to a telephoto end, a distance between the first lens group and the second lens group increases, and a distance between the second lens group and the third lens group decreases, and the first lens group has a cemented lens which includes a first lens element E in the cemented lens, the first lens element E the first lens element E the image forming optical system satisfies the following conditional expressions (1), (2), (3), and (4) where, ν fE f double denotes a combined focal length of the cemented lens when a boundary surface on an image surface side of the second lens element E θgF θgF θgF nC nC nC ν deff and θgFeff denote equivalent Abbe's number and equivalent partial dispersion ratio respectively when the cemented lens includes only the first lens element E where, fG ft denotes a focal length at the telephoto end of the image forming optical system, and enz denotes a distance from a first lens surface of the image forming optical system up to an entrance-pupil position.