A BRIEF LOOK AT IMAGING AND CONTRAST TRANSFER

The development of the basic notions of electron microscope imaging and of contrast transfer theory are described. The effects of finite source size and chromatic instabilities (defocus fluctuations) can be represented by envelope functions which attenuate the oscillating contrast transfer function and which have significant effects on resolution. Beam-tilt effects are described. It is shown that there is a close relationship between holography and the contrast transfer representation of the imaging of weak-phase objects. This is a personal account which attempts to give an extremely condensed review of the development of the subject with particular emphasis on the last twenty-five years or so, and on matters of interest in biological macromolecular structure investigations.