Is there a "best" adapter?

[WaterGoblin]

I was planning on attaching a Nikon D600 DSLR camera to my trinocular Nikon Optiphot, but I see there are lot of different adapters and ways to attach a camera. Is there a "best one"? is there any difference between the available options apart from cost and appearence?
And what would you recommend I use?

[Scarodactyl]

Most straightforward for full frame is probably a pli 2.5x or pl 2.5xa photo eyepiece. I'd acoid the normal pl 2.5x, it is not amazing. There are some more involved ways to get better field coverage but they'd involve a fair bit of DIY.

[apochronaut]

There are two parts to adapting any specific camera to a microscope. The optic and the adapter. Sometimes there is no optic and just an adapter.

Once the camera is at hand one chooses the optic and pathway next, then purchases whatever adapters are necessary to realize the pathway that the optic needs last. If there is no optic, a suitable optical path still must be designed.

" Best " is whatever fulfills the requirements.

[blekenbleu]

Optiphot objectives do not depend on photo relay lens for corrections.
Nikon Optiphot era photo relay lenses are not great.
Nikon F-mount camera bodies have 46.5mm flange-to sensor distance.
My Canon EOS-M camera has 18mm flange-to-sensor distance
and has room to spare for Optiphot trinocular "F" direct projection with 38mm "DIN" camera port, which adds 20mm.
Your D600 might barely be parfocal for direct projection
if directly on your trinocular "F" photo port with the photo tube unscrewed.
That could be confirmed by focusing a calibration slide in eyepieces,
then in darkened room measure distance from photo port to focused image projected on e.g. waxed paper.

A 1.4x teleconverter would enable Optiphot objective fields to substantially fill a D600 sensor.

The trinocular "F" photo port thread is 42mm,
but recessed into head so that male-male 42mm filter coupler ring cannot engage it.
I have not tried, but it may be possible to improvise an F-mount adapter with 42mm thread extension, using:

* a 42-39mm step-down ring with no flange, e.g. https://www.ebay.com/itm/253981920429
* a 39mm filter with glass removed as extension e.g. https://www.ebay.com/itm/373952962485
* a 39mm male coupler, https://www.ebay.com/itm/165995000407
* a thin 39mm to Nikon F-mount flange adapter, https://www.amazon.ca/dp/B0100QII5C

[Scarodactyl]

Certainly a good teleconverter should give good results if you can make it fit. Another option is to get an ultrawide head and replace the photo lens in the top with a slightly longer lens, like 140ish mm. If you want to go down the rabbit hole these are superior to a 2.5x relay eyepiece since you woll get much better field coverage (assuming you get higher end lenses that can fill it.) Many people are content with the 2.5x though, it depends on how far down tbe rabbithole you want to go.