Optical scanner having symmetry about an oblique divider

A device for optically scanning an information plane (2) having tracks (5). Radiation supplied by a radiation source (7) is focused on the information plane by an objective system (8). The beam reflected by the information plane is incident on a dividing element, for example, a grating (12), which divides the beam into two halves along a dividing line (17). Two sub-gratings (15, 16) at both sides of the dividing line, respectively, each form a sub-beam from one half of the beam, which sub-beams are detected by a detection system (10), which produces detection signals therefrom. A focus error signal is generated from the detector signals. According to the invention, modulation of the focus error signal by the track structure will be minimal if the dividing line (17) is located at an angle between 15 DEG and 80 DEG to the direction of the tracks.