Abstract: An optical detector (110) is disclosed, comprising: at least one optical sensor (122) adapted to detect a light beam (120) and to generate at least one sensor signal, wherein the optical sensor (122) has at least one sensor region (126), wherein the sensor signal of the optical sensor (122) is dependent on an illumination of the sensor region (126) by the light beam (120), wherein the sensor signal, given the same total power of the illumination, is dependent on a width of the light beam (120) in the sensor region (126); at least one focus-tunable lens (128) located in at least one beam path (130) of the light beam (120), the focus-tunable lens (128) being adapted to modify a focal position of the light beam (120) in a controlled fashion; at least one focus-modulation device (136) adapted to provide at least one focus-modulating signal (138) to the focus-tunable lens (128), thereby modulating the focal position; at least one imaging device (140) being adapted to record an image; and at least one evaluation