This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.
In TEM imaging, the conventional modes of high-resolution imaging are mainly determined by the objective lens defocus. There are four generally used defocus settings:
Except the first defocus is in-focus, underfocuses are applied to all the other defocus settings because of the existing, positive spherical aberration of the objective lens, which has to be counterbalanced in a certain way by a negative defocus aberration related to underfocus.
i) “Standard” defocus.
ii) Scherzer defocus maximizing the phase contrast of a weak-phase object,
iii) Lichte defocus of least confusion, minimizing contrast delocalization,
iv) Minimum phase-contrast defocus.
Nowadays, high resolutions (e.g. ~ 1.2 Å) are routinely reached with the hexapole correctors and the best contrast for crystalline materials has been obtained by tuning the third-order coefficient (C3,0) of the spherical aberration to negative values .
Figure 3643 shows the calculated phase contrast transfer functions (pCTF) at Scherzer defoci in 200-keV TEMs with LaB6 and FEG sources and with different spherical aberrations (Cs).
Figure 3643. Typical pCTFs for 200-keV TEMs: (a) At Scherzer defocus in LaB6 TEM with large positive Cs (1.23 mm), (b) At Scherzer defocus in FEG TEM with large positive Cs (1.23 mm), (c) At Scherzer defocus in FEG TEM with small positive Cs (0.083 mm), and (d) At Scherzer defocus = Lichte defocus in FEG TEM with small negative Cs. Adapted from 
 C. L. Jia, M. Lentzen, K. Urban, Science 299 (2003) 870.
 Markus Lentzen, Progress in Aberration-Corrected High-Resolution Transmission Electron Microscopy Using Hardware Aberration Correction, Microsc. Microanal. 12, 191–205, 2006.