Patent ID: 8018659

Claim:
An image forming optical system comprising, in order from the object side: a first lens group G 1 that is fixed during zooming and includes a reflecting optical element for bending an optical path; a second lens group G 2 having a negative refracting power and movable during zooming; a third lens group G 3 having a positive refracting power; a fourth lens group G 4 having a positive refracting power, and a rearmost lens group GR, wherein during zooming from the wide angle end to the telephoto end, the third lens group G 3 moves on the optical axis toward the object side, characterized in that the rearmost lens group GR satisfies the following condition: 0.95<βRw<2.5 (1-1), 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: 0.3 <NRn−NRp <0.9 (1-2), 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.