Nikon F to female T2 (M42x0.75) thread

Eyepieces, camera lenses, telescopes all do a same thing, but they are optimized for different tasks. You can find a plenty of adapters to use camera lens as a telescope tube, but why not try using lens as an eyepiece?

I made this adapter just for fun. Turns out that 50mm/1.4 lens is an extremely long eye relief superwide eyepiece (albeit 50mm is too much, but nothing a Barlow can't fix). It works. It may be even useful for you. But don't get your hopes too high.

Printing 0.75mm pitch threads is not an easy feat. Ensure you have small layer height where threads are. I used variable layer height.

Whether your lens will be a usable eyepiece depends on some of its innate characteristics. A fast lens won't nescessary be good. You can use entry pupil size and its apparent distance as a rough reference. Entry pupil is a picture that you see through the lens when you look at its business end. F number is relation between its diameter and focus length. For it to work it should not be larger than your eyepiece entrance or too small to see anything either. It shouldn't be too “deep” in the lens either, or it will be uncomfortable.

50/1.4 with 2x barlow works great. 50/1.8 should work great. In theory something like 35/1.8 or faster should work far better, but i don't have one. I do have 135/2.0 and it does not make sense since ~70mm entrance pupil is way bigger than nescessary. And you get very low maginification. 

Nikon F geometry by @michelegz.