Patent ID: 8018659
Filing Date: 2011-09-13
Classification: G02B

Abstract:
1. An image forming optical system comprising, in order from the object side: a first lens group G a second lens group G a third lens group G a fourth lens group G a rearmost lens group GR, wherein during zooming from the wide angle end to the telephoto end, the third lens group G where βRw is the imaging magnification of the rearmost lens group GR in a state in which the image forming optical system is focused on a certain object point for which the imaging magnification of the entire image forming optical system is equal to or lower than 0.01 at the wide angle end, the rearmost lens group GR consists only of a lens component that is at a substantially constant distance from the image plane during zooming, and the image forming optical system satisfies the following condition: where NRp and NRn are the refractive indices for the d-line of the materials of which a positive lens and a negative lens in the rearmost lens group GR are made respectively.