Patent ID: 8767312

Claim:
An image forming optical system essentially consisting of, in order from an object side: a first lens group having a positive refractive power; a second lens group having a negative refractive power; a third lens group having a positive refractive power; and a fourth lens group having a positive refractive power, wherein a refractive index for a d-line of a positive single lens having the largest refractive power among all lenses in the second lens group, is the smallest refractive index among all lenses in the second lens group, the positive single lens having the largest refractive power among all lenses in the second lens group satisfies the following conditional expressions: 1.45 ≦nd p≦ 1.75 (1) 0.40 ≦fp /( fw·ft ) 1/2 ≦0.90 (4) where, nd p denotes the refractive index for the d-line of the positive single lens in the second lens group, fp denotes a focal length of the positive single lens in the second lens group, fw denotes a focal length of the overall optical system at a wide angle end, and ft denotes a focal length of the overall optical system at a telephoto end.