An apparatus and a method are presented for detecting thefocal position of an optical system (10) with a radiationsource (12), a focusing imaging system (16), an at leastpartially reflective surface (18) on the focus (18a), adigital camera (24) for recording an image reflected bysaid surface (18), a computer (C) for evaluating the imagerecorded by the camera (24), and with an optical element(34； 36) in the beam path of the optical system (10) upstreamof the focusing imaging system (16), which elementinfluences said image depending on the focal position.
