Patent ID: 7869136
Filing Date: 2011-01-11
Classification: G02B

Abstract:
1. 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, characterized in that: a cemented optical element D is provided in the first lens group, the cemented optical element D is made up of an optical element B disposed on the object side, an optical element C disposed on the image side, and a refractive optical element A having a positive refracting power disposed between the optical element B and the optical element C, there is at least one optical element having a positive refracting power located closer to the object side than the cemented optical element D, and the image forming optical system satisfies the following conditional expressions (4-1), (4-2), and (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, nd B , nC B , nF B , and ng B are the refractive indices of the optical element B for the d-line, the C-line, the F-line, and the g-line respectively, nd C , nC C , nF C , and ng C are the refractive indices of the optical element C for the d-line, the C-line, the F-line, and the g-line respectively, νd A is the Abbe number (nd A −1)/(nF A −nC A ) of the refractive optical element A, νd B is the Abbe number (nd B −1)/(nF B −nC B ) of the optical element B, νd C is the Abbe number (nd C −1)/(nF C −nC C ) of the optical element C, νd BA is the Abbe number of the refractive optical element A and the optical element B regarded as a single optical element, OgF A is the relative partial dispersion (ng A −nF A )/(nF A −nC A ) of the refractive optical element A, θgF B is the relative partial dispersion (ng B −nF B )/(nF B −nC B ) of the refractive optical element B, θgF C is the relative partial dispersion (ng C −nF C )/(nF C −nC C ) of the refractive optical element C, θgF BA is the effective relative partial dispersion of the refractive optical element A and the optical element B regarded as a single optical element, f A is the focal length of the refractive optical element A, f B is the focal length of the optical element B, f C is the focal length of the optical element C, f BA is the composite focal length of the optical element B and the refractive optical element A, f t is the composite focal length of the refractive optical element A, the optical element B, and the optical element C, and θgFn is the effective relative partial dispersion f t ×νefn×(θgF BA ×φ BA /ν BA +θgF C ×φ C /ν c ) of the cemented optical element D, wherein