This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.

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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:
        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.

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.