In order to reduce the influence of magnetic interference fields on the position of an x-ray focus of an x-ray tube, the deviation of a current focus position from a target focus position or of a measurement value proportional to the deviation is determined, the focus position by reducing the deviation of the current focus position from the target focus position, or the measurement value proportional to the deviation, on the basis of the determined deviation or the determined measurement value proportional to the deviation. The deviation of the current focus position from the target focus position or the measurement value proportional to the deviation is regulated to a value of essentially zero via repeated determination and correction.