Defocus determination in the STEM by phase contrast methods

The imaging parameters of the scanning transmission electron microscope (STEM), in particular the defocus, can be obtained from a focal series of phase contrast images of an amorphous carbon film. By using a tilted specimen the phase contrast transfer characteristic (PCTC) and the mean-square contrast have been determined for a specific instrument. The PCTC contains all the information about the defocus dependence of the phase contrast. The results agree well with those obtained from a focal series of a non-tilted object and with the theoretical predictions.