US Patent 7894313 Optical disc recording and reproducing apparatus

An optical pickup has a photo detector which is divided into first, second, third and fourth quadrants, receives a laser beam reflected from an optical disc via an objective lens, and outputs first, second, third and fourth detection signals from the respective quadrants. A system controller controls a memory to store levels (A, B, C and D) of the first, second, third and fourth detection signals after a focus servo section performs a focus operation by shifting the objective lens in a direction perpendicular to the optical disc surface, and then, calculates a ratio of ((A+D)/(B+C)) in a radial direction of the optical disc. The system controller controls the tracking servo section to perform a tracking operation to shift the objective lens in the radial direction based on a correction table which represents a relationship between correction amount of the lens shift and the calculated ratio ((A+D)/(B+C)).