Measuring a feed spindle length change in a printing exposer, having at least one exposure head on a carrier moved axially along the drum in a feed direction by a stepping motor and the spindle, carries out the measurement by determining the number of stepping motor cycles needed by the carrier to travel through a reference section parallel to the feed direction. During calibration, an optimum number of cycles per revolution of the exposure drum is set, and the number of cycles needed to travel through the reference section is determined. During an operating phase, the number of cycles needed to travel through the reference section is determined and a corrected number of cycles per revolution of the exposure drum is determined in accordance with the relationship K.sub.k=K.sub.0.times.M.sub.x/M.sub.0. The temperature of the components involved in the measurement is kept constant with a temperature control system.