Randall Ainsworth wrote:
In article , Brion K. Lienhart
wrote:

Nope. F:2.8 is solely based on the size of the lens. Obsidian is glass,
you can grind it into a lens shape, but if you use it as an element in a
lens, you're going to get some reallllllly slow shutter speeds. Of
course that's an extreme case, the difference between plain old glass
glass, and exotic flouro-silicates is only a few fractions of a percent.

An f/2.8 lens is going to allow a specific amount of light to go
through regardless of what kind of glass it's made of.

Actually no, but in most cases the difference is unimportant.

One case where the difference generally is important is with
a mirror lens. A mirror lens is generally only about 60%
efficient compared with around 90%+ efficiency for a glass
lens with ten multicoated air-glass surfaces.

A 500mm f/6.3 mirror lens may let in about the same light
as a 500mm f/8 coated glass lens. This can be a significant
factor when deciding what lens to buy. A mirror lens does
not gather quite as much light as you would expect from
the aperture.

Peter.
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