a method for controlling focus on a label side of a light - scribe disc divides the label side into several zones , finds the maximum sbad and the corresponding focus volt and spoke during focusing on each zone , approaches a primary focus volt level curve , compensates a predetermined volt to focus on the label side , checks the maximum sbad and the corresponding focus volt and spoke between the primary focus volt level curve and the compensation volt on each zone , and approaches a calibrating focus volt level curve to reduce errors .