"If you know of any other extremely fast lenses, please share it with us.
Thank you for this interesting link. The champion is the Zeiss 40mm F0.33, but this one is not useable for photograhpy.
The late Lex Werkheim told us the fastest lens possible would theoretically be a F0.5, but he did not tell us how this was calculated.
Anyway, I think 0.85 is already too fast to be useable in real life photography, except possibly aerial photography (over a flat coutryside) or x-ray photograhy. The only advantage of such speed that I see is that it makes focusing easier in the dark.
I do aerial photography, and I can't imagine ever needing such a fast aperture. Of course what I do is in daytime. It would be useful for night or low light aerial photography (or perhaps during very dark, severe storms...assuming you're crazy enough to fly in them!).
Interesting tidbits about the Zeiss and the fast aperture theory. Zeiss also made a 90mm f/.9 or something, didn't they?
I for one, would like a 30-120 zoom with constant f/0.9, autofocus, and image stabilization...with bokeh as smooth as Canon's 135 f/2! Would that be possible? The front element would be 130mm or so, and it would be heavy...but surely it's not physically impossible...