Patent ID: 7054254

Claim:
A single-element objective lens for an optical pick-up, said objective lens directing an incident light beam to a data recording surface of an optical disc through a cover layer to form a beam spot, a numerical aperture of said objective lens being 0.7 or more, said objective lens being configured such that a characteristic of aberration due to decentering of surfaces of said objective lens is substantially comparable to a characteristic of aberration due to an inclination of an optical axis of said objective lens and a central axis of the light beam incident on the data recording surface of the optical disc with respect to the optical disc, and such that there exists a coefficient α with which condition: - 0.01 < X 1 + X 2 × ( - 1 + n 0 2 + ( n 0 2 - 1 ) ⁢ X 2 ) - h ⁡ ( n 0 - 1 ) ⁢ ⁢ c + α ⁡ ( tNA ⁡ ( - 1 n d + 1 - NA 2 n d 2 - NA 2 ) ) < 0.01 is satisfied, wherein, X represents a differentiation, with respect to h, of a function g(h), which is a SAG amount of the first surface of the objective lens at a height h from the optical axis thereof, c represents the differentiation of X with respect to h, n 0 represents a refractive index of the objective lens, n d represents a refractive index of the cover layer, t represents a thickness along the optical axis of the cover layer, NA represents a function NA(h) which is defined as NA(h)=h/f, f being a focal length of the objective lens, h being any value within a range 0<h≦h max , wherein h max is a maximum effective radius of a light beam on the first surface.