Yeah you barely need 2.8 on a fisheye. Only place you would find it usable is if you are filming in indoor parks alot. Rokinon 8 and opteka is made with the same optics just diffrent name.
I dont know if this been said yet but if it hans't
Everyone who is shooting hd or some other kind of 16:9 video you have to crop it to 4:3 and export to 480p or else it will look like shiet.
If you dont know how, ill help ya.