The influence of two methods of an automatic exposure control (AEC) setup using a simple measure of detectability is examined as a function of x-ray beam quality for a computed radiography (CR) system and for an indirect conversion digital radiography (DR) system. The regimes assessed were constant air kerma at the detector and the constant contrast noise ratio (CNR). A low scatter geometry was employed with x-ray spectra varying from 60 kV and 1 mm Cu to 125 kV and 2 mm Cu. The CNR was measured using a 2 mm thick Al square of dimension 1 cm by 1 cm. Detectability was quantified via a nominal contrast for a fixed beam quality of 70 kV and 1 mm added Cu filtration, taken from c-d curves measured using the Leeds TO20 test object, for the four x-ray spectra. In addition, objective image quality parameters including modulation transfer function (MTF), noise power spectrum (NPS) and detective quantum efficiency (DQE) were also measured for both systems at the four different x-ray spectra. It was found that the constant air kerma strategy did not maintain threshold nominal contrast (simple detectability) constant as the x-ray beam mean energy increased, for either the CR system or the DR system. For the CR detector, the threshold nominal contrast for a 1 mm disc increased by a factor of 4.4 from 3.50% to 15.4% as the tube potential was raised from 60 kV to 125 kV, while for the DR detector, the threshold nominal contrast increased by a factor of 3.4, from 2.27% to 7.67% as the tube potential increased from 60 kV to 120 kV. The constant CNR method was more successful at maintaining constant detectability for the c-d discs. The threshold nominal contrast for the 1 mm disc changed by a factor of 1.2, from 4.80% to 5.70% for the CR system, as the spectrum changed from 60 kV to 125 kV. For the indirect conversion detector, the threshold nominal contrast increased from 2.27% to 5.66% (a factor of 1.4 increase). The constant CNR strategy required an increase in air kerma by factors of 14.5 and 4.5 for the CR and DR detectors, respectively. The Rose SNR was used to show that the air kerma at the detector should be modulated by the inverse of the measured contrast loss squared and by the inverse of the reduction in the low frequency DQE of the detector, in order to maintain the Rose SNR constant. Calculations of air kerma required for a constant Rose SNR, made using the measured contrast and the extrapolated DQE(0), were within 25% of the air kerma for the measured CNR method for both detectors. These results suggest that the constant CNR strategy keeps simple object detectability constant by increasing air kerma to overcome (a) the contrast loss and (b) the photon loss, due to a reduction in DQE as the mean energy is increased. AEC systems set up to hold air kerma constant or nearly constant at the x-ray detector will not maintain a constant level of contrast-detail detectability as x-ray energy changes. The constant CNR method is considerably more successful if fixed contrast-detail detectability is required for different x-ray beam qualities.