Method for calibrating a measuring device which is configured for measuring optically smooth faces, with the steps: - moving with finite precision and hence positioning errors up to various measurement positions which lie in a test volume of the measuring device and can be distinguished by location and/or angular coordinates, - production of measurement signals in the respective measurement positions, and - determination of parameters of a calculation model of the measuring device from the measurement signals and from the location and/or angular coordinates of the measurement positions, wherein the calculation model allows calculation of the optical travel length lying between several light sources and a detector for each possible light beam which passes through the test volume and is reflected, diffracted or refracted on a calibration object , characterised in that - from the measurement positions approached with positioning errors, a coordinate system for the test volume is determined which is established by six secondary conditions (N1 - N6), wherein precisely six coordinate values of three or more measurement positions approached with positioning errors are assigned to the coordinate system, - for each light source , the calibration object is positioned in additional measurement positions in the test volume once in the zero test and then positioned defocused, - wherein the defocused additional measurement positions are selected such that on measurement, the measurement signal contains information from several light sources , - one or more calibration objects are used which have curved surfaces with different radii of curvature or diffractive optics which form different wave fronts. - moving with finite precision and hence positioning errors up to various measurement positions which lie in a test volume of the measuring device and can be distinguished by location and/or angular coordinates, - production of measurement signals in the respective measurement positions, and - determination of parameters of a calculation model of the measuring device from the measurement signals and from the location and/or angular coordinates of the measurement positions, wherein the calculation model allows calculation of the optical travel length lying between several light sources and a detector for each possible light beam which passes through the test volume and is reflected, diffracted or refracted on a calibration object , - from the measurement positions approached with positioning errors, a coordinate system for the test volume is determined which is established by six secondary conditions (N1 - N6), wherein precisely six coordinate values of three or more measurement positions approached with positioning errors are assigned to the coordinate system, - for each light source , the calibration object is positioned in additional measurement positions in the test volume once in the zero test and then positioned defocused, - wherein the defocused additional measurement positions are selected such that on measurement, the measurement signal contains information from several light sources , - one or more calibration objects are used which have curved surfaces with different radii of curvature or diffractive optics which form different wave fronts. <EOS>