Glaser’s famous bell-shaped model suggested that in order to attain the minimum focal length and the minimum aberrations of a magnetic lens used in a TEM the specimen should be put at the middle of the gap of a pair of symmetrical polepieces , e.g. called objective polepiece.
A strong upper-objective polepiece is needed to create a convergent beam to form a CBED (convergent-beam electron diffraction) pattern.
Figure 4258 shows the schematic illustration of EBSD setup in a SEM system. The objective pole piece of the system is indicated. The working distance for EBSD is roughly between 8 and 45 mm.
Figure 4258. The schematic illustration of EBSD setup in a SEM system.
Note that in some cases, special objective pole pieces need to be designed for particular applications, for instance, in situ STM-TEM applications.
 Cosslett, V. E. 1991. Fifty years of instrumental development of the electron microscope, in Advances in Optical and Electron Microscopy, Barer, R. and Cosslett, V. E. Eds., Academic Press, London, 215–267.