The degree and amount of the information revealed by electron microscopes (EMs) mainly depends on six factors:
i) The resolving power of the microscope (e.g. usually < 0.3 nm for TEM);
ii) The energy spread of the electron beam;
iii) The quality of the EM sample (e.g. the cleanness and thickness of the specimen for TEM);
iv) The knowledge of electron-matter interaction and electron optics;
v) The operation skills of EMs;
vi) The composition and stability of the specimen.
i) and ii) depend mainly on how thick your pocket is, meaning the more money you spend, the better the microscope parameters; iii) - v) depends on your experimental skill and background; while vi) depends on your luck and/or choice of experimental system.
Since some EM operation tips from experiences and theoretical understandings are very tricky, and thus can hardly be described in words, for instance, in literature or through emails, it is suggested to visit experienced laboratories to ask for assistance.
In aberration-corrected EMs (electron microscopes) a combination of hexapole or octupole lenses are used in a aberration corrector which lacks rotational (circular) symmetry and thus doesn’t have to have a positive spherical aberration like conventional, round magnetic lenses.
Some "smart" TEM experiments which are not normally performed are:
i) Energy-selecting slit
before the ADF detector. 
ii) Angular selecting aperture before the spectrometer. 
iii) Off-axis positioned filter entrance aperture. 
 Haider M 1989 Ultramicroscopy 28, 240.
 Bleloch AL, Castell MR, Howie A and Walsh CA 1994 Ultramicroscopy 54, 107.
 Walther T and Humphreys CJ 1997 Inst. Phys. Conf. Ser. 153, 303.