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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Because of the dependence of beam size on diffraction and aberrations as shown in Figure 2752, in order to achieve the optimum performance the diffraction and aberrations should be balanced as indicated by the optimum convergence semi-angles. For big electron sources, at low convergence semi-angles (α), the diffraction mainly determines the beam size, while at high α the spherical aberration is the dominant factor.
The diffraction term is given by,
        d_d=0.61λ/α ----------------- [2752a]
The spherical aberration term is given by,
        d_s=1/2C_sα³ -------------------------- [2752b]
where,
        λ --  The wavelength of the incident electron beam,
        C_s -- The spherical aberration coefficient,
        α -- The convergence semi-angle.