1. A lens system having an F number equal to or smaller than 2.4, wherein a positive leas element is disposed closest to an object side; a diaphragm disposed in a widest air space in the lens system; in a rear unit including all lens elements disposed on the image side relative to the diaphragm, a lens element having a lens surface of a smallest curvature radius is a negative lens element, and a lens element having a lens surface of a second smallest curvature radius is a positive lens element; the negative lens element and the positive lens element in the rear unit are adjacent to each other; and the lens system satisfies the following conditions: 
 0.05< L — 1/ L 13 TH< 0.21 
 1.5< L — TH/Y< 8 where, L_ 1 is an interval from a lens surface located closest to the object side to a lens surface located on the object side relative to the diaphragm, L_TH is an interval from the lens surface located closest to the object side to a lens surface located closest to an image side, and Y is a maximum image height.