An apparatus and a method are presented for detecting the focal position of an optical system (10) with a radiation source (12), a focusing imaging system (16), an at least partially reflective surface (18) on the focus (18a), a digital camera (24) for recording an image reflected by said surface (18), a computer (C) for evaluating the image recorded by the camera (24), and with an optical element (34 36) in the beam path of the optical system (10) upstream of the focusing imaging system (16), which element influences said image depending on the focal position.
