The illumination system of the conventional transmission electron microscope is designed to provide an adequate level of coherent illumination over the field of view being examined. The additional requirement of providing the maximum possible current into very small (sub-micron) focussed spots, as is required by X-ray microanalysis for example, necessitates a probe forming lens with a very short working distance and has previously been met by using either a mini-lens or the prefield of the objective lens when used in the Single-Field Condenser-Objective mode.

The SFCO gives the smallest spot size and aberration parameters but suffers from the disadvantage that the illumination and imaging fields are not independently controllable.