Tube lens

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Microscopes orignally used finite optics;
simple lenses which focus parallel (infinite) rays to a point
may be repurposed for converging rays on either side, becoming finite.
Over 100 years ago, compound microscope objective lenses were optimized
to work best with finite focal lengths on both sides,
often with a so-called tube length of 160mm.

However, there are any number of useful accessories which could be inserted
between objectives and whatever might be beyond that tube length.
Rather than change objectives, these accessories would modify rays.
An alternative is to use infinity objectives,
which are largely unaffected by changes in distance between them and
a so-called tube lens, which converts those parallel rays back to finite,
which oculars in turn change back to nearly parallel rays for observers’ eyes.

Using another lens to obtain virtual images from parallel rays is called afocal,
and that term is also used for Afocal photography,
where rays from oculars to observers’ eyes are nominally parallel,
so the observer can be replaced by a camera with lens focused near infinity.
This camera can alternatively be inserted in place of an infinity scope’s tube lens.
Changing the camera lens’ focal length changes overall magnification.
A 40mm lens typically fills an APS-C sensor when focused over a micrscope ocular.
When used in place of an infinity microscope’s tube lens,
longer camera focal lengths are typically wanted.