A Need for Bandwidth Limitations in Electron Microscopes

In light-optical microscopes, numerical apertures (apertures in the back focal plane of the objective lens) are an integral part. Often, these apertures can be adjusted continuously, e.g., in single lens reflex (SLR) cameras. In electron microscopy however, and despite much larger magnification ranges, there is only a limited selection of fixed apertures. This is due a) to the pm wave length of the electrons resulting in very small scattering angles from objects with lattice constants in of about 100-400pm and b) to manufacturing limitations and contamination issues as apertures with decreasing hole sizes tend to contaminate easily. While many transmission electron microscopes have a selection of only 4-6 apertures, they offer tremendous magnification ranges. Yet, for most magnifications, suitable numerical apertures are not available. This actually will either become or already is a problem.