This paper explores how noise equivalent quanta (NEQ) can be estimated conveniently in digital imaging systems, and provides two very different examples of its application. A principal result is that, if an imaging system has a flat noise power spectrum (NPS) prior to down-sampling, and if pre-filtration is used to control aliasing while maintaining reasonable sharpness, the NPS will again become roughly flat after down-sampling, with the magnitude reduced by approximately the square of the down-sampling factor. This result allows the NEQ of a digital system to be conveniently estimated as the square of the product of the capture modulation transfer function (MTF), the linear pixel signal to noise ratio (SNR), the resampling factor, and the pre-filter MTF. Two examples of applications of this approximation are described: (1) understanding likely performance of pedestrian detection algorithms as a function of automotive image sensor properties; and (2) developing a new usability metric for mobile imaging, digital zoom factor, that combines the information contained in the commonly used parameters of megapixels and SNR₁₀.