Method for automatic focusing an imaging optical system on the surface of a sample

A sample P is illuminated by a measurement light beam with an incidence angle differing from 0°; light reflected therefrom is detected; intensity values are recorded and evaluated; and the opening of a field stop is imaged onto a receiving surface, thereby generating an image. In such a method, a value of "light" or "dark" is allocated to each intensity value as a function of a brightness threshold; the smallest rectangle that encloses all "light" positions on the receiving surface is determined; the geometric center point of that rectangle is determined; the position of that point is compared with the position on the receiving surface that corresponds to the geometric center point of the image generated by the field stop in the focused state; and a change in the distance between the sample and the imaging optical system corresponding to the distance between the two points is effected.