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Parfocal DSLR trinocular port

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Since CF objective introduction,
many Nikon trinocular heads support video camera attachment without relay lens.
This so-called direct projection wants e.g. CS or C-mount with roughly 16 to 24mm diagonal sensor,
which happens to be nearly APS-C, with advantage to Canon's slightly smaller sensors.

Flange to sensor distance

C-mount has 17.526 flange-to-sensor distance.
Most mirrorless camera bodies have compatible specifications,
.. but SLRs require space for mirrors:

A DSLR attached to a C-mount phototube will not be parfocal with eyepieces.
Refocusing objectives for the DSLR is not only inconvenient;
captured images are liable to more aberrations
from objectives beyond their designed working distances.
Problematic aberrations typically increase with N.A. (numeric aperture).

        While not parfocal, this Canon 6D DSLR attached to a Ergo trinocular head
yields acceptable images with a CF Plan 20x objective:

That phototube appears to be integral to its trinocular head casting;
for higher magnifications, only mirrorless camera bodies should apply...


Meanwhile, this Nikon approx 50mm phototube:

 .. attaches to its trinocular photo port by 3 screws:
A C-to-M42 adapter was sacrificed for this exercise.

 Its inner diameter closely matches Nikon's photoport;
an 11/16 socket held them concentric
while drilling a first hole:

the photo port is slightly recessed:
 an M42 extension tube connects the focus helicoid:


Parts:
... which with thin M42 to EF flange adapter, yields DSLR parfocality:
           
maintained by blekenbleu