Patent ID: 8493663

Claim:
An optical system comprising: in order from an enlargement conjugate side to a reduction conjugate side via a largest air gap, a front unit having a negative power, and a rear unit having positive power, wherein the optical system satisfies the following conditions: 1.75 <Nd n1 <2.05; 0.02 <ΞgF n1 −(0.6438−0.001682 ×vd n1 )<0.08; 2.5 <|f n1 /F|< 5.0; and BF/F> 3.5, where f nl represents a focal length of a negative lens in the front unit, Nd nl represents a refractive index of a material of the negative lens, vd n1 represents an Abbe number of the material of the negative lens, θgF n1 represents a partial dispersion ratio of the material of the negative lens, F represents a focal length of the optical system, and BF represents an air-equivalent back focus in the optical system.