A method of measuring a deviation of an optical test surface from a target shape is provided. The method includes directing an incoming beam of electromagnetic radiation onto the test surface to generate a measuring beam that has interacted with the test surface, causing the ray that has interacted with the test surface to pass through an interferometer on a deviated path, performing an interferometric measurement by superimposing a reference beam with the measuring beam to determine a wave front deviation of the measuring beam from the reference beam, determining a retrace error in the wave front deviation, and correcting the measured wave front deviation by eliminating the retrace error therefrom. The differences in aberrations accumulated by the ray having traveled on the deviated path from fictitious aberrations that would have been accumulated by a ray that had traveled on an undeviated path cause the retrace error.