Abstract: An auto-focus system for focusing reflecting objects which are under investigation in a measuring device. The invention comprises a light source for producing an illumination beam for illuminating an object plane of an objective lens to be focused, an imaging beam and a focusing beam, both originating from the illuminated object, means for separating the focusing beam and the imaging beam, the latter producing an image of the object in an image plane which is evaluated, and an amplitude object not visible in the imaging beam. Means are provided to render visible the amplitude object in two detector planes, where resulting counter moving interference patterns are evaluated and used for focusing the objective lens.