Methods and apparatus are provided for generating focus, tracking, and magnification error signals in an multi-track optical disk reader. An optical pickup assembly for the disk reader includes an optical sensor having multiple sensor elements. Some of the sensor elements contain multiple divisions whose outputs are summed to produce tracking, focus, and magnification error signals. Electronic and optical mechanical systems are provided for correcting or compensating for magnification errors caused by an optical pickup assembly of a multi-track optical disk reader.