The wikipedia for "Fisheye Lens"
(
https://en.m.wikipedia.org/wiki/Fisheye_lens ) has an interesting section on the "mapping function" of different models of fisheye lenses. They categorize lens examples as either
Stereographic, Equidistant, Equisolid angle, or
Orthographic.
Does the perfect fisheye exist? What is its mapping function? If not, what would it be?
How fishy is too fishy?
Probably from familiarity, I have a bias toward the Century Mk 1 on a VX1000, in the right hands. I'm not saying it's perfect, but when I see fisheye footage shot with camera & lens setups that are significantly different from it, I am at least a little bit turned off. This includes the HD Panasonics with the Xtreme.
Some factors:
Aspect ratio--4:3 is probably my ideal, but 16:9 and 3:2 can also be cool if they can be shot close while somehow not also cutting off heads
Sensor format--Whatever works, bigger the better. Alexa 65 could be interesting.
Vignetting--I think I want the choice to have none. In reality a bit in the corners might be nice. Optional, ideally.
Peripheral compression of subject/spot--I think high compression on the edges of the frame is good for stability of shots, and I like how it makes spots look. Some lenses, like the stereographic Samyang 12mm and 8mm can stretch things out a bit on the sides, which I think can accentuate the filmer's wobbles.
Fishiness--There's a 4mm Laowa, and some of the iPhone attachment lenses, somehow they look a little too fishy to me. Though that guy in Chicago uses his 4mm pretty well. The Samyang/Rokinons don't seem fishy enough for me. They're a little flat. I suppose this could be warped a bit in post either way, though that seems a little sacreligious.
Ability to get very close to the skater and not cut their head off--Essential
Anyway, I'm rambling.