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 the SEMs with symmetric immersion lens as shown in Figure 3594, a specimen smaller than <5 mm is placed inside the lens gap, reducing significantly the focal length to the range of 2 - 5 mm. The configuration with these lens and specimen positions provides the lowest aberrations, the smallest probe size, and thus the highest spatial resolution comparing with the other objective lens designs in SEM systems because the lens aberrations scale with focal length. Collection of secondary electrons (SEs) in this configuration takes advantage of the fact that the SEs can spiral upward in the strong magnetic field to a SE detector above the lens. With this "through-the-lens" detector, the SEs can be separated from backscattered electrons (BSEs) because the highly energetic BSEs are less strongly confined by the magnetic field. Since the BSEs do not reach the detector they don’t affect the spatial resolution of imaging.

Figure 3594. Symmetric immersion lens as objective lens in SEM systems.

Virtual objective apertures (VOAs) are required for the immersion and the snorkel lenses and they stay further from the specimen chamber and thus are clean.