I'm fairly certain the 6.5mm opteka lens would produce a full frame fish effect, search around to see if it's any good. I don't know anything about hdslr filming and using slr lenses with threaded fisheyes, so I'll stop here. But i'd go with an actual lens over a screw on fish.
Sorry bud, you won't manage to get an angry response from me :)
Does the OP seem like someone who wants to sacrifice AF and stuff for some slightly noticeable difference in the pictures? Nope, I'm sure some simplicity would be nice and he could still end up taking great pictures.
i don't know if the same holds true for nikon-canon adapters, and i know there are probably adapters with no glass, but many adapters for canon-minolta have a piece of glass that both magnifies the focal length and reduces sharpness.