This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.
For a perfectly coherent illumination in EMs, the resulting phase contrast transfer function (pCTF) continues to oscillate between +1 and -1 even at very large spatial frequencies, which is not the case with a real microscope.
For real TEMs, the phase contrast transfer function is limited by a number of factors: i) Variation in electrical currents in the lenses; ii) Variation in accelerating voltage and gun emission; iii) Attenuation in the CCD camera; iv) Slightly nonparallel illumination; v) Specimen drift and vibration, vi) Defocus, and vii) Two-fold astigmatism. All these limits can be described by envelope functions, E, which will gradually dampen the phase contrast transfer function and eventually reduce it below the noise at some spatial frequency. The first three factors depend on the inherent properties of the microscope. For all modern microscopes this significant decrease happens at higher spatial frequencies than the point to point resolution and defines the information limit of the microscope beyond which no information is transferred at all. Moreover, early studies of the aberration effects on pCTF can be found in [1,2].
 H.P. Erickson, A. Klug, Measurement and compensation of defocusing and
aberrations by Fourier processing of electron micrographs, Philosophical
Transactions of the Royal Society of London, Series B 261 (105) (1971)
 F. Thon, Phase contrast electron microscopy, Electron Microscopy in Materials
Science (1971) 570–625.