Compound objective lens having two focal points and apparatus using the lens

摘要

A focus control method to perform focus control on at least two kinds of information media having layers of different thicknesses using an optical lens having a plurality of regions corresponding to the different thicknesses and having a plurality of numerical apertures corresponding to the different thicknesses, the focus control method comprising: a step to change a distance between an information medium and the optical lens: and a step to start focus control when a focus error signal detected from the information medium surpasses a predetermined level so as to perform focus control corresponding to the different thicknesses.