Macroscopic fields in EMs are produced by macroscopic elements, including the solenoids, magnetic multipoles or by the voltages applied to conducting devices, for example cylinders or apertures.
In conventional EM systems, the existence of spherical aberration requires the utility of very small apertures to maximize the spatial resolution, while the resolution will also be limited by diffraction (see Airy disc) if the apertures are too small.
Fresnel fringes are produced along the edges of TEM objects when the objects are out of focus under a coherent illumination. For instance, the edge between the carbon film and vacuum, or the aperture edges in the TEM present such fringes.
In STEM operation, to observe the Ronchigram, the apertures after the specimen are removed and a large probe convergence angle (>100 mrad) is selected, for instance by inserting the largest STEM objective aperture (condenser aperture in CTEM).
The columns of high voltage TEMs are normally higher than 3 m so that the aperture drives and specimen airlock are difficult to reach during microscope operation. Fortunately, computer or remote control devices are provided for modern microscopes; otherwise, a ladder is needed.