beam 15 in intermediate image plane 16 0044 fig2 a shows the shape of a focus spot 4 a on specimen 4 the direction of travel of focus spot 4 a on specimen 4 is depicted by an arrow p focus spot 4 a is constituted by zero order focus spot 9 0 a negative first order focus spot 9 u2212 1 and a positive first order focus spot 9 1 no regions having differing reflectances are present on specimen 4 only one high reflectance region 100 is provided in general specimen 4 to be examined is arranged on a specimen stage the optimum focal position is established by generating a relative motion between the specimen stage and objective 14 of the microscope based system this relative motion is performed over a small region around the optimum focal position as a result of the relative motion focus spot 4 a moves over the surface of specimen 4 in the manner indicated by arrow p in fig2 b the intensity recorded in intermediate image plane 16 is plotted as a function of focal position the pixel number of the individual photosensitive elements of the ccd element is plotted on abscissa 17 in fig2 b the intensity of the light reflected from the surface of the sample is plotted in arbitrary units on ordinate 18 the curve that yields the optimum focal position is shown as a.