skrev:

On Wed, 28 Sep 2005 22:29:43 -0700, "Brion K. Lienhart"
wrote:

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.

So what was your point in bringing up a special case ulikely
to be implemented?

To illustrate the difference in principle between f-stop and t-stop in
an obviuos way, perhaps?

Jan B=F6hme