Patent ID: 7239598

Claim:
An objective lens for an optical pick-up which is used to record data to and/or to reproduce data from at least three types of optical discs by selectively using one of at least three light beams having different wavelengths, the at least three types of optical discs having at least two different thicknesses of cover layers, when a thickness of a first optical disc of the at least three types of optical discs to be used for recording data and/or reproducing data using a first light beam of the at least three light beams having a wavelength shortest of all the wavelengths of the at least three light beams is represented by t 1 , a thickness of a second optical disc of the at least three types of optical discs to be used for recording data and/or reproducing data using a second light beam of the at least three light beams having a wavelength longer than that of the first light beam is represented by t 2 , and a thickness of a third optical disc of the at least three types of optical discs to be used for recording data and/or reproducing data using a third light beam of the at least three light beams having a wavelength longest of all of the wavelengths of the at least three light beams is represented by t 3 , t 1 being substantially equal to 0.6 mm, t 2 being substantially equal to 0.6 mm, and t 3 being substantially equal to 1.2 mm, when a numerical aperture required for recording data to and/or reproducing data from the first optical disc is represented by NA 1 , a numerical aperture required for recording data to and/or reproducing data from the second optical disc is represented by NA 2 , and a numerical aperture required for recording data to and/or reproducing data from the third optical disc is represented by NA 3 , a relationship NA 1 ≧NA 2 >NA 3 being satisfied, when the first optical disc is used, a substantially collimated light beam of the first light beam being incident on said objective lens, when the second and third optical discs are used, diverging beams of the second and third light beams being incident on said objective lens, respectively, given that magnification and a focal length of said objective lens are respectively represented by M 1 and f 1 when the first optical disc is used, the magnification and the focal length of said objective lens are respectively represented by M 2 and f 2 when the second optical disc is used, and the magnification and the focal length of said objective lens are respectively represented by M 3 and f 3 when the third optical disc is used, following conditions (1), (2) and (3) being satisfied: −0.02 <f 1 ×M 1<0.02 (1) −0.10 <f 2 ×M 2<−0.04 (2) −0.29 <f 3 ×M 3<−0.19 (3).