The influence on image contrast, tube load and patient mean absorbed dose of different ways of shaping diagnostic X-ray spectra by placing filters in the beam is derived fro two radiographic models (abdominal screen-film radiography and intra-oral, dental radiography) using a computational model. The filters are compared at either equal tube load (keeping tube potential constant) or equal contrast (adjusting the tube potential with the different filters), but always at equal energy imparted per unit area to the image receptor. Compared at equal tube load and relative to standard aluminum filtration, reductions in the mean absorbed dose in the patient of 15-25% can be achieved using filters of Cu, Ti, W and Au (increasing the tube load by 30-40% compared with standard aluminium filtration). However, contrast is also reduced by 7%. Compared at equal contrast, the dose reductions are smaller, about 10%. Filters of copper are generally recommended, as are filters of aluminium. The use of bandpass filters (K-edge filters) should be restricted to examinations where the need for substantial variation in tube potential from patient to patient is small. The benefit of using thicker filters than those commonly used today (increasing tube load by factors of 1.4-2.0 compared with no added filter) is small as the dose reduction is most rapid for small initial values of added filters, and the increase in tube load increases steadily with increasing filter thickness.