METHOD OF DETECTING FOCAL POSITION SHIFT AND OPTICAL PICKUP APPARATUS

摘要

PROBLEM TO BE SOLVED: To provide a method of detecting a focal position shift which is capable of exactly matching the focus of a beam-condensing optical system to an information recording layer of an optical recording medium by detecting the focal shift of the beam-condensing optical system free of offsets. SOLUTION: The focal position shift of a two-elements objective lens 9 is detected in accordance with the light beam passing a region 2b as a region of 60 to 85% of the effective diameter of the beam regulated by the opening of the objective lens around an optical axis OZ of the light beam past the two-elements objective lens 9 described above.