A model is developed for the overall DQE of an x-ray screen-film system, and this is related to the component DQE's of the screen and film. The model characterizes the screen in terms of its x-ray absorption efficiency, MTF, and the statistics associated with the conversion of absorbed x-rays to emitted light photons. This model establishes that the film need be characterized solely by its DQE with respect to the emitted light photons. We show that the only explicit exposure dependence of the screen-film DQE results from the film DQE, while the spatial-frequency dependence is dominated by the screen MTF. However, depending on the screen gain, a significant exposure/spatial-frequency domain may exist where the overall DQE is independent of both of these factors and depends mainly on x-ray absorption efficiency.