Before image data are screened for the subsequent exposure of a printing form, files needed for the screening step are changed in such a way that the values of individual elements in the data fields are changed. The number of elements whose values are changed is determined by evaluating contiguous regions within the data fields. These can be, for example, threshold value matrices in which individual matrix elements are set to a value corresponding to the color white, or which, for the half-tones, are calibrated to a reduced value of the full-tone density. In the latter case, the full-tone image points are screened with a frequency-modulated screen.