Patent ID: 8654242

Claim:
A single-focus optical system comprising, in order from an object side to an image side: a first lens group fixed with respect to an image plane, the first lens group including: at least one positive lens; and at least one negative lens; a second lens group configured to be moved to focus, the second lens group including: at least one positive lens; a third lens group fixed with respect to the image plane, the third lens group including: at least one lens having at least one aspherical surface having a positive optical power on a periphery thereof, wherein the most object-side lens is a positive lens having a convex object-side surface; and wherein the following relationships are satisfied: 5 <Δv 1<70 0.01 <bf/TL< 0.24 0.04 <T 34 /TL< 0.4 where: Δv1 is a maximum value of a difference between an Abbe number of the positive lens of the first lens group and an Abbe number of the negative lens of the first lens group; bf is an axial distance from a surface apex of the most image-side lens surface to the image plane, wherein when a parallel plate is placed therebetween, an air equivalent thickness of the parallel plate is counted in; TL is an axial distance from a surface apex of the most object-side lens surface to the image plane, wherein when a parallel plate is placed therebetween, an air equivalent thickness of the parallel plate is counted in; and T 34 is an axial distance between the third lens and the fourth lens, wherein when T 34 is variable for focusing, T 34 for focusing at infinity is adopted.