Patent ID: 7848027

Claim:
An objective lens comprising, in order from an object side: a first lens group having positive refractive power; a second lens group having positive refractive power; a diffractive optical element having a diffractive optical surface; and a third lens group having negative refractive power; the most object side surface of the first lens group forming a concave surface facing the object side; a principal ray crossing an optical axis between the second lens group and the third lens group, and the diffractive optical element being disposed such that the diffractive optical surface is disposed in the vicinity of the position where the principal ray crosses the optical axis; and the following conditional expressions being satisfied: h<α×L× 0.3 H<[H 3+{φ/(2 ×fa )+ H 3 /|f 3 |}×L]× 0.85 where α=φ/2×{1/fa−(L/f 12 − 1 )/|f 3 |}, H 3 =fa×NA, h denotes the maximum height of a principal ray on the diffractive optical surface, φ denotes a real field of view, L denotes a distance along the optical axis between the second lens group and the third lens group, fa denotes a focal length of the objective lens, H denotes the maximum effective radius on the diffractive optical surface, NA denotes a numerical aperture, f 12 denotes a combined focal length of the first lens group and the second lens group, and f 3 denotes a focal length of the third lens group.