= Solution
\b[Atomic-resolution contrast is primarily <electron phase contrast>.] <elastic scattering> by the specimen potential changes the phase of the transmitted <electron> wave. <interference> between transmitted and diffracted components, modified by the objective lens, converts that phase change into intensity variations. Appropriate <defocus>, sufficiently small <optical aberrations>, good <spatial optical coherence> and transfer of the <graphene> <spatial frequencies> are needed. At exact focus an ideal weak phase object need not give a strong visible lattice.
Mass–thickness or amplitude contrast can also distinguish thicker overlap regions, folds, contamination and support film, especially when scattered <electrons> are excluded by an aperture. It is comparatively weak for a clean monolayer of low-$Z$ <carbon>. <diffraction contrast> becomes important in crystalline, multilayer or differently oriented regions when Bragg-scattered beams are selected or excluded. Relative rotation of layers can additionally produce a <moiré pattern>.
A dark region alone is not a reliable layer count: <electron phase contrast> can change sign with <defocus>, and contamination also changes intensity. Use fold-edge fringes, <diffraction> information and calibrated image simulations or thickness-sensitive measurements together. Defect identification should likewise compare a structural model with the actual imaging conditions, rather than reading every dark lattice site as a missing atom.
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