Patent ID: 8411367

Claim:
An image forming optical system comprising, in order from the object side to the image side, a first lens group having a positive refracting power, a second lens group having a negative refracting power, and an image side lens group having a positive refracting power, wherein the distance between the first lens group and the second lens group changes during zooming, wherein a cemented optical element C is provided in the first lens group, the cemented optical element C comprises a refractive optical element A having a positive refracting power and an optical element B, the refractive optical element A is located closest to the object side in the first lens group, and the image forming optical system satisfies the following conditional expressions (4-1), (4-2), and (4-3): ν d A <30 (4-1), 0.54 <θgF A <0.92 (4-2), and | f B /f A |>0.08 (4-3), where nd A , nC A , nF A , and ng A are the refractive indices of the refractive optical element A for the d-line, the C-line, the F-line, and the g-line respectively, νd A is the Abbe constant (nd A −1)/(nF A −nC A ) of the refractive optical element A, θgF A is the relative partial dispersion (ng A −nF A )/(nF A −nC A ) of the refractive optical element A, f A is the focal length of the refractive optical element A, and f B is the focal length of the optical element B.