Schematic of the specimen used for our calculations. Within the `feature slab' of thickness tf, we assume that we have a pixel of width Δp with a feature f next to pure background material b. In the planes above and below, we may have a mixed background material b′. In the case of copper features f in a matrix of silicon, we assume that both b and b′ are silicon. In the case of a biological specimen, we assume that the feature f is protein embedded in a background b of ice, while the mixed background material b′ above and below (referred to as `tissue' in this manuscript) is 70% ice and 30% protein in accordance with the typical water fraction of the human brain (Shah et al., 2008).