Based on observation of the test surface over a specified field of view, focussing the highest peaks and lowest valleys by movement of the objective. Determination of the distance (peak-to-valley height) between the highest peak and lowest valley by movement of the objective in order to focus on the lowest valley within the same field of view. Repetition of the procedure to obtain values for a further 19 different fields of view, calculation of the arithmetic mean. The focussing microscope is also specified. The procedure is also applicable to the determination of the surface profile of essentially planar blast-cleaned steel.