Abstract: A sheet feeder of a printing machine has sheet separating and conveying facilities and a stack table which raises a stack of sheets by means of a controlled lift drive so that a respective uppermost sheet of the stack is in an operating position for the sheet separating and conveying facilities, the sheet separating facility including separating suckers for raising, in timed sequence, a respective trailing sheet end, as viewed in sheet conveying direction through the feeder, a suction line connecting the separating suckers to a suction source, a pressure measuring device connected in the suction line for determining an actual instant of time at which a respective sheet is sucked up by the separating suckers, a comparator device for comparing the actual suction instant of time with a nominal suction instant of time generated in synchronism with rotation of the printing machine to determine a difference value, a comparator device for comparing the difference value with a value corresponding to an optimum height