Double telecentric objective lens

摘要

A double telecentric objective lens having a double telecentric optical system, wherein said double telecentric optical system comprises: a front group (GF) having a positive refractive power as a whole; and a rear group (GR) having a positive refractive power as a whole, wherein the rear focus of said front group (GF) and the front focus of said rear group (GR) coincide with each other, and a diaphragm is placed at the position where the focuses coincide, wherein said front group (GF) includes a first unit (G1) formed of a cemented lens composed of a convex lens (L1) and a concave lens (L2), and having a positive refractive power as a whole; and a second unit (G2) including a cemented lens composed of a convex lens (L3) and a concave lens (L4), and a further concave lens (L5), arranged in that order from an object side, and having a positive refractive power as a whole, wherein said rear group (GR) includes a third unit (G3) including a concave lens (L6) and a cemented lens composed of a concave lens (L7) and a convex lens (L8), and having a positive refractive power as a whole; and a fourth unit (G4) formed of a cemented lens composed of a concave lens (L9) and a convex lens (L10), and having a positive refractive power as a whole, and wherein the following conditions are satisfied: n1n-n1p> 0.1 nu 1p- nu 1n > 25 0.3fF < r2n < 0.5fF 1.4 < (r2p/r2n) < 2.7 n3n < n3p where n1p and nu 1p respectively represent the refractive index and the Abbe's number of said convex lens (L1) of said first unit (G1), n1n and nu 1n respectively represent the refractive index and the Abbe's number of said concave lens (L2) of said first unit (G1), r2p represents the radius of curvature of that surface of said cemented lens composed of said convex lens (L3) and said concave lens (L4) in said second unit (G2) that is furthermost from the object, r2n represents the radius of curvature of the object-side surface of said further concave lens (L5) in said second unit (G2), fF represents the focal length of said entire front group (GF), n3n represents the average refractive index of said concave lenses (L6, L7) in said third unit (G3), and n3p represents the refractive index of said convex lens (L8) in said third unit (G3), the refractive indices and the focal length being values for the d-line having a wavelength of 587.56 nm.