A method of preparing an apparatus for material processing by generating optical breakthroughs in an object. The apparatus includes a variable focus adjustment device. A contact element is mounted to the apparatus, the contact element has a curved contact surface having a previously known shape. The position of the contact surface is determined prior to processing the object, by focusing measurement laser radiation near or on the surface by the variable focus adjustment device, and the focus position is adjusted in a measurement surface intersecting the expected position of the contact surface. Radiation from the focus of the measurement laser radiation is confocally detected. The position of points of intersection between the measurement surface and the contact surface is determined from the confocally detected radiation to determine the position of the contact surface from the position of the points of intersection and the previously known shape of the contact surface.