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In TEM imaging, the conventional modes of highresolution imaging are mainly determined by the objective lens defocus. There are four generally used defocus settings:
i) “Standard” defocus.
ii) Scherzer defocus maximizing the phase contrast of a weakphase object,
iii) Lichte defocus of least confusion, minimizing contrast delocalization,
iv) Minimum phasecontrast defocus.
Except the first defocus is infocus, 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.
The defocus value (Δf) can be determined from the position of the first crossover of contrast transfer function (refer to Equation 4974b):
 [3874]
Here, n = 0, ±1. That means different integers n give different defoci. For instance, a TEM system has a spherical aberration constant C_{s} 1.38 mm, is operating at 200 kV (the wavelength is 0.0025 nm), and has the first crossover of Fourier transform at u = 2.75 nm^{1}. Therefore, three possible defocus values are 15 nm (for n = 1), 70 nm (for n = 0), and 53 nm (for n = 1).
The values of Δf (defocus value), C_{s} (spherical aberration coefficient), and D (standard deviation of the Gaussian distribution of defocus due to the chromatic aberration) can be obtained by TEM image deconvolution.
Table 3874. Selections of focusing/defocusing conditions in TEM imaging.
Best/optimum defocus condition for HRTEM imaging 
page4232 
Optimal defocus for minimizing the delocalization in (HR)TEM imaging 
page1715 
Minimum (phase)contrast in (HR)TEM imaging with Gaussian defocus 
page4233 
Maximum (phase)contrast in (HR)TEM imaging (with Scherzer defocus) 
page2779 
