What happens when you attach a fisheye lens in reverse with a reversing ring?

Question

I'm not familiar with reversing rings and I'm not sure what to expect.

Answer 1

Generally you can't. Most fisheyes have convex/bulgy front elements and no filter threads. E.g., the Samyang/Rokinon 8mm f/3.5:

It can't take any screw-on filters. Which means there's no way to screw on a macro reversal ring.

Answer 2

Optics can be viewed from either end. I.e. the subject can become the sensor, and the sensor can become the subject; the functional light paths/ray traces remain the same.

What a camera lens does is take a larger area from the scene and compress it down to the sensor area. Which is why reversing that function, by reversing the lens, causes the same lens to expand the scene to a larger area (increased magnification).

A telephoto lens takes a smaller area of the scene which has to be compressed less to fit on the sensor. That is why reversing longer lenses is less effective for increasing magnification.

So you can guess what reversing a fisheye would do... it would take a smaller area and expand it even more. And the distortion fisheye lenses cause would be reversed... i.e. barrel distortion would become pincushion distortion when reversed.