On Tue, 17 Dec 2002 18:29:41 +0000 (UTC), glhansen@steel.ucs.indiana.edu (Gregory L. Hansen) wrote:

>Zoom lenses usually seem to have a variable widest aperture. And I
>thought that was kind of weird until I remembered that f/stop is lens
>length divided by aperture diameter. When the lens gets longer, unless
>the front lens grows the f/stop will go up.

Not the front, but the clear aperture...

>So I guess the real question is why can I have an 80-200mm zoom that opens
>up to f/4 all the way through its range?

It is actually around f2.8 (+) at the short end, but
a compensating mechanism has been added to the diaphragm
to close it appropriately to keep the relative aperture
the same as the lens is zoomed. This is done to keep the
aperture constant for metering convenience, or to prevent
the lens from being used at a stop that would result in
poor quality images (most of the constant-aperture zooms
are expensive and high-quality - and also manual-focus
[more likely to be used with manual-metering, perhaps...]).