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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Practically, the large, and positive third-order spherical aberration can be combined with a optimized defocus setting (namely, sample height Z = C_1,0) to form the Scherzer λ/4 phase plate,

           ---------------------------- [2769a]

So that the point resolution is,

           ---------------------------- [2769b]

In this case, the object information up to a spatial frequency 1/d_Sch is transferred with the same sign of the aberration function, yielding dark-atom contrast for a TEM specimen that can satisfy weak phase object approximation (WPOA) condition.

Based on C_s correction, extending Scherzer’s point resolution to information limit of the microscope can optimize the positive phase contrast from a weak-phase object. Assuming g_Sch = g_max and a defocus of Z_Sch = -8/(3λg²_max) , one can obtain, [1]

          ---------------------- [2769c]

The resulting contrast delocalization is given by,

          ---------------------- [2769d]

In summary, due to the positive spherical aberration in traditional TEMs, the only way to produce phase contrast from a thin object is to use an underfocus setting. The resulting aberration function from underfocus is negative and the phase change is positive. The positive phase contrast from a weak phase object is thus dark with respect to the background (mean intensity).

The phase-contrast term -isin2πχ(g) in the contrast transfer function becomes zero if C_S=0 and Δf=0, while the amplitude-contrast term cos2πχ(g) is maximum, 1. Therefore, for aberration-corrected microscope, the TEM images present atomic structures by amplitude-contrast rather than by phase-contrast, and thus in HRTEM images the projected atom column are imaged in bright contrast on a dark background.

Table 2769. More readings.

[1] Lentzen, M., Jahnen, B., Jia, C.L., Thust, A., Tillmann, K. & Urban, K. (2002). High-resolution imaging with an aberrationcorrected
transmission electron microscope. Ultramicroscopy 92, 233–242.