Defocusing phase contrast effects in electron microscopy

Abstract

SUMMARY The theory of defocusing phase contrast is presented in terms of Scherzer's (1949) ‘integrated‐ray’ and Heidenreich's (1964) ‘single‐ray’ approach to spherical aberration; contrast transfer theory in terms of the Scherzer expression is included. Fine structure in transverse sections, negative stain and carbon film is shown to be the result of defocusing phase contrast. Examples of interpretative confusion are given for various macromolecular systems and quantitative support is found for a simplified ‘rule‐of‐thumb’ approach to phase‐contrast enhancement. High‐resolution studies of carbon‐fibre lattices demonstrate contrast reversal, hence it is possible to show that a practical expression for the phase change due to spherical aberration falls between the Scherzer and the Heidenreich terms; the defocus term is also found to require some adjustment.