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Tube lens

Microscopes orignally used finite optics;
relatively 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
for finite focal distances on both sides (*Case iv*),
often with a so-called tube length (*B'*) of 160mm.

However, any number of useful accessories
may get inserted between objectives and oculars, increasing tube length.
Rather than changing objectives, such accessories insert *another* lens
to refocus objective rays at the increased distance.
Reflected light (episcopic) finite microscopes,
which insert a beamsplitter for illuminating opaque objects,
are typically designed for longer tube length (e.g. 210mm for Nikon).
Alternatively, rays from infinity objectives, being colimated,
are relatively unaffected by distance to tube lens,
which focuses colimated rays to an intermediate image plane for oculars
that in turn set focus distance for observers' eyes.
 

A camera lens of appropriate focal length can serve as a tube lens,
provided that infinity objectives are fully corrected.

Nikon UW head has infinity space around its prism

Afocal uses another lens to obtain virtual images from parallel rays,
and that term also applies to afocal photography,
where rays from oculars to observers' eyes are nearly parallel,

then replacing observer by a camera with lens focused near infinity.
This camera can alternatively be inserted in place of an infinity scope's tube lens.

A camera lens' focal length determines overall image size / magnification.
A 40mm lens focused on a 10X microscope ocular typically fills an APS-C sensor,
depending on objectives' field diameters.
Replacing an infinity microscope objective's tube lens
typically wants a telephoto camera lens (e.g. 100-200mm).

70-200mm zoom:  easy/lazy tube lens

Older Canon f/2.8 70-200mm zooms sell for around US$200;  f/4 even less.

These have both internal focus and zoom:  filter ring and length do not change.
maintained by blekenbleu