This question already has an answer here:
As I understand it, the F-number of a lens is equal to the aperture diameter divided by the focal length. So f/1 would mean that the aperture is as wide as the lens is long.
Question: Is it physically possible for the aperture to somehow be wider than that? For example, something like an f/0.5 lens?
I gather that F-number is already the number one cause of expensive lenses, so even if such a lens is physically possible, it's likely to be far to expensive to actually buy. (To say nothing about whether it's even useful in the first place.) I'm just curious about the physics of the matter.