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Here we see the positions of the condenser lenses in the
illumination system of the microscope.

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.

In order to minimise the convergence angle of the beam at the specimen, the strength of the second condenser lens is changed.

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..

If the objective lens is underfocussed, the beam is focussed
at a point beyond the sample.

This decreases the convergence slightly.

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.

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