<p>An optical information recording and reproduction system uses an objective lens (5) to focus a light beam onto the information-carrying medium (6). To ensure the beam is focused correctly part of the reflected beam is transmitted to an optical component (10) which has a reflecting surface (11) with reflectivity or transmissivity which varies continuously with respect to the angle of incidence. A photo-detector (9) senses the distribution of intensity of the beam reflected by the surface (11).</p>
