----- Original Message -----
From: "Joao Ribeiro" <jribe...@greco.com.br>
> I read somewhere that the f stop is actually the "apparent diameter" of the
> opening of the lens when viewed by the front side
Joao,
You are correct, that apparent diameter is called entrance pupil and
is the virtual image of the diaphragm opening formed by the glass in front on
it. Light doesn't "know/care" how big the actual opening of the aperture is.
Light is only able to "see" the virtual opening and enters only through the
area enclosed by that opening. That is why this is the diameter we are
interested in using to find the f/stop.
> divided by its focal distance at infinity.
The other way around, actually. Focal distance at infinity divided by the
apparent diameter. I know that is what you meant, but thought to mention it
for clarity sake.
Guillermo
