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 conventional SEM systems, the most common SEM objective lenses are asymmetric pinhole lens (or called asymmetric conical lens) as shown in Figure 3597a. In order to keep the magnetic field within the lens and to provide a field-free region above the SEM specimen for SE (secondary electron) collection, the final polepiece has a very small bore (called pinhole). The specimen has to be kept as close as possible to the polepiece because its aberrations increase with working distance.

Figure 3597a. Pinhole lens as the objective lens in SEMs 

Figure 3597b shows the schematic illustration of spatial resolutions of SEMs with various lenses, including conventional W (tungsten) filament SEMs, pinhole lens, snorkel lens, and in-lens. The best resolution can be obtained by in-lens SEMs.

Figure 3597b. Schematic illustration of spatial resolutions of SEMs with various lenses.