Most metals have small unit cells, and in small unit-cell materials in low-index zones the strength of the diffracted beams is often greater for lighter metals than it is for heavier metals. The reason for this is that the strong double-diffraction in heavy metals gives rise to weak Bragg diffraction. In this paper it is shown that for small unit-cell materials, light atom precipitates can be easily detected in heavy atom matrices; and that in large unit-cell materials, the breakdown in the weak-phase-object approximation in thick crystals can be used to identify atom type at defects.