Abstract: An optical disc drive (I) comprises an optical pickup (3) with a 4-quadrant photo-detector (20); an objective lens (14) mounted displaceably with respect to the photo-detector (20); an optical displacement actuator (30) for displacing the objective lens (14); and a servo controller (70) for controlling the focus actuator (30). The controller (70) receives respective detector signals (S1, S2, S3, S4) from the photo-detector (20). The controller (70) is adapted to process these detector signals to produce a focal error signal (FE) which is zero if these signals all have identical magnitude. The controller (70) is further adapted to subtract/add an offset parameter (?off) from/to said focal error signal (FE), and to provide the difference/sum as a control output signal.