tube lens 10 this intermediate image is then observed or electronically detected by apparatus not shown in the drawing as a result of the longitudinal chromatic aberration of the optical elements taking part in the imaging the optimal position of the structure 4 to be imaged differs for different illumination wavelengths and or emission wavelengths and these differences must be compensated by displacing the structure 4 along the optical axis 12 as is indicated by the arrows 13 the displacement is generally carried out by motor operated means which are not shown in the drawing and which are controlled by a computer 13 which carries out the necessary calculations the correction status and therefore the longitudinal chromatic aberration of objectives and other optical systems can be parameterized with knowledge of the optical design in this way reference positions for a structure to be projected can be determined from a set of parameters describing the correction state for every combination of objective excitation wavelength emission wavelength or corresponding spectral regions so that it is no longer necessary to calibrate all of these combinations the method according to the invention is illustrated in the following using the example of the longitudinal chromatic aberration chl and the imaging of a structure for example a transmission grating in the object plane in order to determine the above mentioned parameter set the chl is calculated.