Patent ID: 7672065

Claim:
A lens system comprising, sequentially in order from a magnifying side, a first lens group which makes up a substantially afocal optical system as a whole and a second lens group having a positive refractive power as a whole, wherein the first lens group comprises, sequentially in order from a magnifying side, a first sub-lens group ( 1 a ) having a negative refractive power as a whole and a second sub-lens group ( 1 b ) having a positive refractive power as a whole, wherein the second lens group comprises, sequentially in order from a magnifying side, a first sub-lens group ( 2 a ) having a negative refractive power as a whole and a second sub-lens group ( 2 b ) having a positive refractive power as a whole, and wherein the following conditional expression (1) is satisfied with respect to a power set for the first lens group, the following conditional expression (2) is satisfied with respect to an afocal magnification set for the first lens group, the following conditional expression (3) is satisfied with respect to a power set for the first sub-lens group ( 1 a ) of the first lens group, the following conditional expression (4) is satisfied with respect to a power set for the first sub-lens group ( 2 a ) of the second lens group, the following conditional expression (5) is satisfied with respect to a power set for the second sub-lens group ( 2 b ) of the second lens group, and the following conditional expression (6) is satisfied with respect to a thickness dimension at an optical axis of the first lens group: −0.3 ≦f/f I ≦0.3 (1) 0.35 ≦h IE /h IX ≦0.45 (2) −1.4 ≦f/f Ia ≦0.8 (3) −0.5 ≦f/f IIa ≦0.1 (4) 0.35 ≦f/f IIb ≦0.65 (5) 1.9 ≦T I /f≦ 2.5 (6) where: f: Composite focal length of the whole lens system; f I : Composite focal length of the first lens group; h IE : Height of paraxial ray incident onto a magnifying-side surface of a lens element which is disposed outermost on the magnifying side of the first lens group; h IX : Height of a paraxial ray emerging from a contracting-side surface of a lens element which is disposed outermost on a contracting side of the first lens group; f Ia : Composite focal length of the first sub-lens group (1 a ) of the first lens group; f IIa : Composite focal length of the first sub-lens group ( 2 a ) of the second lens group; f IIb : Composite focal length of the second sub-lens group ( 2 b ) of the second lens group; and T I : Distance on the optical axis between the magnifying-side surface of the lens element which is disposed outermost on the magnifying side of the first lens group to the contracting-side surface of the lens element which is disposed outermost on the contracting side of the first lens group.