Patent ID: 8885270

Claim:
A single-focal-length imaging lens system comprising, in order from an object side: a first lens group, the first lens group being composed of, in order from the object side, a first lens element having a positive refractive power and convex to the object side and a second lens element having a negative refractive power and concave to the image side; a second lens group which includes at least one positive lens element, the second lens group being composed of, in order from the object side, a third lens element convex to the object side and a fourth lens element having a positive refractive power and convex to the image side; and a third lens group which includes at least one aspherically shaped lens element having an inflection point at a position other than an intersection with an optical axis, the third lens group being composed of a fifth lens element having a negative refractive power and concave to the image side, wherein the imaging lens system performs focusing by, while keeping the first and third lens groups stationary relative to an image plane, moving the second lens group in an optical axis direction, an air lens formed by an interval between the first and second lens groups has a negative refractive power, an air lens formed by an interval between the second and third lens groups has a positive refractive power, and conditional formula (1) below is fulfilled; 0 ≦|Fi 1 /Fi 2 |−|Fm 1 /Fm 2|≦10 (1) where, when the air lens formed by the interval between the first and second lens groups is referred to as a first air lens, and the air lens formed by the interval between the second and third lens groups is referred to as a second air lens, then Fi1 represents a focal length of the first air lens when focusing on an infinite object distance; Fi2 represents a focal length of the second air lens when focusing on the infinite object distance; Fm1 represents the focal length of the first air lens when focusing on a closest object distance; and Fm2 represents the focal length of the second air lens when focusing on the closest object distance.