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

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The Scherzer phase plate is equivalent to the Zernike phase plate in light optics [1 - 3] and thus alters the phase of the diffracted electron wave. The weak potential of thin TEM specimens imposes the phase change of the scattered electron wave by π/2 with respect to the transmitted wave.

[1] Zernike, F. (1942). Phase contrast, a new method for the microscopic observation of transparent objects, Part I. Physica 9, 686–698.
[2] Zernike, F. (1942). Phase contrast, a new method for the microscopic observation of transparent objects, Part II. Physica 9, 974–986.
[3] Zernike, F. (1955). How I discovered phase contrast. Science 121, 345–349.