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Here we see the positions of the condenser lenses in the illumination system of the microscope.
Gun Crossover Second condenser lens Aperture Specimen
The convergence angle of the beam, β, at the sample describes the angular range over which electrons are incident at each point on the sample.

In order to minimise the convergence angle of the beam at the specimen, the strength of the second condenser lens is changed.
β /line>
With the objective lens focussed on the sample, an image of the source appears in the sample. The convergence angle, β, is simply related to the aperture diameter and its distance from the sample..
d 2a
β = tan-1 a/d
If the objective lens is underfocussed, the beam is focussed at a point beyond the sample.

This decreases the convergence slightly.
If, instead, the lens is overfocussed the image of the crossover appears before the sample. In this case electrons appear to come only from the crossover, and the convergence is decreased greatly.

For this reason the microscope is usually operated with the second condenser lens overfocussed.
β Image crossover