An optical system has at least two optical elements whose spatial relation with respect to each other can be changed. At least one of the optical elements comprises a plurality of optical components. The optical system comprises first measuring means for individually measuring an image defect of each optical component, and first computing means for computing first target positions for the plurality of optical components such that an overall image defect of the at least one of the optical elements is below a predetermined threshold value. Second measuring means are provided for measuring an overall image defect of the optical system, and second computing means represent the measured overall image defect as a linear combination of base functions of an orthogonal function set. The second computing means calculate second target position for the at least two optical elements so as to reduce the overall image defect.