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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 16 to 24mm diagonal sensor,
or Canon's 22.3×14.9mm APS-C sensors.

Flange to sensor distance

C-mount flange-to-sensor distance is 17.526mm;
many interchangeable lens mirrorless camera bodies have similar specifications,
.. but SLR mirror box requires more space:

A DSLR attached to a C-mount phototube CANNOT be parfocal with eyepieces.
Refocusing objectives for the DSLR is not only inconvenient;
captured images are liable to more aberrations
from objectives outside their designed geometries.
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:

Eclipse head bundled with Optiphot 150 EPI illuminator
Parfocality fine tuning by focusing helicoid 		.. attaches to Eclipse trinocular photo port by 3 screws:
A C-to-M42 adapter was sacrificed for this exercise.

 C-mount 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 focusing helicoid:


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