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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Spherical aberration is the only form of monochromatic axial wavefront aberration produced by rotationally symmetrical surfaces centered and orthogonal in regard to the optical axis. The spherical attribute is due to this aberration being inherent to the basic optical surface - spherical - for object at infinity. Spherical aberration affects the entire image field, which makes its correction first priority. As shown in Figure 3633, the light or electrons passing through the periphery of the lens are focused more strongly than those passing through the centre and the circle of the least confusion (the best focus point) is located within the range of longitudinal spherical aberration. Note that both the marginal ray and paraxial ray foci can be at the same point if spherical aberrations does not exist.

Figure 3633. Schematic diagram of spherical aberration (SA) and paraxial ray focus.