Patent ID: 8064319

Claim:
An objective lens for converging used light on a desired position of an optical recording medium which information is recorded in and reproduced from, the objective lens consisting of: a single lens element having at least one aspheric surface, wherein the following conditional expressions (1) to (3) are satisfied 0.70<NA<0.98 (1) 1.10 ≦d/f≦ 1.28 (2′) 0.48<X<0.55 (3) where NA denotes a numerical aperture of the objective lens on an optical recording medium side, d denotes a thickness of the objective lens on an optical axis in mm, f denotes a focal length of the objective lens in mm, X is equal to (X1−X2)·(n−1)/(NA·f) n denotes a refractive index of the objective lens at a wavelength of the used light, X1 denotes a distance, in mm and in an optical axis direction, between (a) a first tangential plane that is perpendicular to the optical axis and is tangent to a vertex of a light source side surface of the objective lens and (b) a most outside position within an effective diameter of the light source side surface, which is a position on the light source side surface on which a marginal ray having the NA is incident, and X1 is defined so that a direction toward the optical recording medium from the first tangential plane serving as a reference point is a positive direction and that a direction toward the light source from the first tangential plane is a negative direction, and X2 is a distance, in mm and in the optical axis direction, between (a) a second tangential plane that is perpendicular to the optical axis and is tangent to a vertex of an optical recording medium side surface and (b) a most outside position within an effective diameter of the optical recording medium side surface, which is a position on the optical recording medium side surface on the marginal ray having the NA is incident, and X2 is defined so that a direction toward the optical recording medium from the second tangential plane serving as a reference point is a positive direction and a direction toward the light source from the second tangential plane is a negative direction.