Patent ID: 8432617

Claim:
An image forming optical system comprising in order from an object side to an image side: a first lens group having a positive refractive power; a second lens group having a negative refractive power; and an image-side lens group having a positive refractive power, wherein a distance between the first lens group and the second lens group changes at the time of zooming, and a refractive optical element A having a positive refractive power is positioned in the first lens group, the first lens group comprises a cemented optical element D which is positioned on a most object-side, the cemented optical element D is arranged such that the refractive optical element A is positioned between an optical element B which is positioned on the object side and an optical element C which is positioned on the image side, and the image forming optical system satisfies the following conditional expression (1-1), conditional expression (1-2), and conditional expression (2) νd A <30 (1-1) 0.54<θgF A <0.9 (1-2) | fG 1 /fG 2|>6.4 (2) where, nd A , nC A , nF A , and ng A denote refractive indices of the refractive optical element A for a d-line, a C-line, an F-line, and a g-line respectively, νd A denotes Abbe's number (nd A −1)/(nF A −nC A ) for the refractive optical element A, θgF A denotes a partial dispersion ratio (ng A −nF A )/(nF A −nC A ) of the refractive optical element A, fG1 denotes a focal length of the first lens group, and fG2 denotes a focal length of the second lens group.