Patent ID: 8780462

Claim:
A fixed focal length image forming lens comprising: an aperture stop; a first lens group arranged on an object side relative to the aperture stop; and a second lens group having a positive power arranged on an image side relative to the aperture stop, the first lens group comprising a first F lens group having a negative power and a first R lens group having a positive power arranged in this sequence from the object side with a widest air space within the first lens group between the first F lens group and the first R lens group, the first F lens group comprising at least two negative lenses, the first R lens group comprising at least one positive lens, the second lens group comprising a second F lens group having a positive power and a second R lens group arranged in this sequence from the object side, the second F lens group comprising a first positive lens, a first negative lens, a second negative lens, and a second positive lens continuously arranged in this sequence from the object side, wherein the second R lens group comprises one positive lens or one negative lens, and at least one negative lens in the second F lens group satisfies the following relationships (6) and (7): 30 <νdn< 41 (6) Δθ gFn< 0 (7) where νdn represents an Abbe number of a material forming at least one of the negative lenses in the second F lens group and ΔθgFn represents an abnormal dispersion property of the material, where the abnormal dispersion property ΔθgF is defined as a deviation from a reference line defined as a straight line connecting a coordinate point of a reference glass species K7, where νd=60.49 and θgF=0.5436, and a coordinate point of a reference glass species F2, where νd=36.26 and θgF=0.5828, on a two-dimensional coordinate having two orthogonal axes of a Y axis representing a partial dispersion ratio θgF and an X axis representing an Abbe number νd, and the partial dispersion ratio θgF is defined by the following relationship: θ gF =( ng−nF )/( nF−nC ), where ng represents a refraction index of Fraunhofer line to g line, nF represents a refraction index of Fraunhofer line to F line, and nC represents a refraction index of Fraunhofer line to C line.