In X-ray-to-light conversion digital radiography, we compared the image quality of a system in which photodetection is done from the X-ray incident surface (hereafter referred to as a front exposure system) and a system in which photodetection is done from the back side opposite the X-ray incident surface (hereafter referred to as a back exposure system). Modulation transfer function (MTF) and detective quantum efficiency (DQE) measurements were performed using the method IEC prescribes. Both MTF and DQE were higher with the front exposure system than with the back exposure system, with the former delivering better image quality. This difference can be accounted for by differences in the distribution of absorbed X-ray doses in the phosphor layer, the readout efficiency, which varies as a function of depth in the phosphor layer, and depth-dependent blurs of light. Furthermore, we determined changes in image quality incurred by varying the quality of X-rays, the thickness of the phosphor layer and the crystal structure of phosphors. The advantage of the front exposure system becomes more pronounced with decreasing X-ray tube voltage, increasing phosphor layer thickness, and the use of phosphors in powder form.