Patent ID: 8149522

Claim:
An image forming optical system comprising: a lens component which comprises a lens L 1 which is made of a transparent material having Abbe's number νd 1 , an intermediate layer L 2 which is made of a transparent material having Abbe's number νd 2 , and a lens L 3 which is made of a transparent material having Abbe's number νd 3 , wherein the intermediate layer L 2 is in a close contact with an optical surface B of the lens L 1 , and a shape of an optical surface C of the intermediate layer L 2 is an aspheric shape, and the lens L 3 is in a close contact with the optical surface C, and the image forming optical system satisfies the following conditions 0.012<1 /νd 3−1 /νd 1<0.090 (1) 0.010<1 /νd 3−1 /νd 2 (2) 1.65<nd1 (4) where, νd 1 denotes Abbe's number (nd 1 −1)/(nF 1 −nC 1 ) of the lens L 1 , νd 2 denotes Abbe's number (nd 2 −1)/(nF 2 −nC 2 ) of the intermediate layer L 2 , and νd 3 denotes Abbe's number (nd 3 −1)/(nF 3 −nC 3 ) of the lens L 3 , where, nd 1 , nC 1 , and nF 1 denote refractive indices of the lens L 1 for a d-line, a C-line, and an F-line respectively, nd 2 , nC 2 , and nF 2 denote refractive indices of the intermediate layer L 2 for the d-line, the C-line, and the F-line respectively, and nd 3 , nC 3 , and nF 3 denote refractive indices of the lens L 3 for the d-line, the C-line, and the F-line respectively; wherein in a rectangular coordinate system with a horizontal axis let to be νd 3 and a vertical axis let to be θgF 3 , when a straight line expressed by θgF 3 =α 3 ×νd 3 +βgF 3 (provided that α 3 =−0.00566) is set, an area which is determined by a straight line when it is a lower limit value of a range of the following conditional expression (7) and a straight line when it is an upper limit value of the range of the following conditional expression (7), and an area determined by the following conditional expression (9) include θgF 3 and νd 3 of the lens L 3 , in a rectangular coordinate system with a horizontal axis let to be νd 3 and a vertical axis let to be nd 3 , when a straight line expressed by nd 3 =a 3 ×νd 3 +b 3 (provided that a 3 =—0.0267) is set, an area which is determined by a straight line when it is a lower limit value of a range of the following conditional expression (8) and a straight line when it is an upper limit value of the range of the following conditional expression (8), and an area determined by the following conditional expression (9) include nd 3 and νd 3 of the lens L 3 0.6520<βgF3<0.8150 (7) 2.0<b3<2.4(provided that nd3>1.3) (8) 10<νd3<35 (9) where, θgF 3 denotes a partial dispersion ratio (ng 3 −nF 3 )/(nF 3 −nC 3 ) of the lens L 3 , νd 3 denotes Abbe's number (nd 3 −1)/(nF 3 −nC 3 ) of the lens L 3 , and nd 3 , nC 3 , nF 3 , and ng 3 denote refractive indices of the lens L 3 for the d-line, the C-line, the F-line, and the g-line respectively.