A measuring device (1) for measuring the shape, contour and/or roughness of a workpiece is based on a contact-less optical probe having a large numeric aperture. The probe has at least two different focal points with which at least two photo receptors are associated. The latter generate a differential signal for controlling a positioning device (13) for tracking the optical probe in such a manner that the workpiece surface is maintained within the measuring range of the probe. The differential signal is proven to result in a rapid and accurate tracking of the position of the sensor arrangement (3).