Currently, there is significant interest in quantifying the performance of digital radiography systems. Digital radiography systems can be thought of as continuous linear shift-invariant systems followed by sampling. This view, along with the large number of pixels used for flat-panel systems, has motivated much of the work which attempts to extend figures of merit developed for analog systems, in particular, the detective quantum efficiency (DQE) and the noise equivalent quanta (NEQ). A more general approach looks at the system as a continuous-to-discrete mapping and evaluates the signal-to-noise ratio (SNR) completely from the discrete data. In this paper, we study the effect of presampling blur on these figures of merit. We find that even in this idealized model the DQE/NEQ formulations do not accurately track the behavior of the fully digital SNR. Therefore, DQE/NEQ cannot be viewed as indicators of the signal-known-exactly/background-known-exactly (SKE/BKE) detectability. In order to make design decisions by optimizing detectability one must work with the fully digital definition of detectability.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.