On Thu, 21 Nov 2002 20:39:50 -0800, "Paul Tauger" wrote:
>"FDR" <_remove_spam_block_rzitka@hotmail.com> wrote in message
>news:aVhD9.158356$c51.47209868@twister.nyroc.rr.com...

>> It's the size of largest aperture opening. The larger the opening, the
>> higher the shutter speed can go and the shorter the depth of field is.

>Ummm . . . not quite.

Wellllll......, not to be a nudge, but the above really
is correct in terms of the lens rating, as in, an f1.4
lens passes more light at maximum opening than an f1.8,
and all else being equal, this results in the use of a
higher shutter speed to compensate, and in slightly less
DOF...;-)

>f measures the light transmission ability of the
>lens -- the lower the number, the more light it transmits.

F-stop is the simple relationship of the aperture
diameter of the lens divided into the focal-length,
as in, a 50mm lens at f4 has an opening diameter of
12.5mm (or any variation of this equation). The "true
speed" of the lens can be different due to materials
and design, but the difference is generally small
enough to ignore (especially with TTL auto and manual
exposure systems, which compensate for lens transmission
automatically...).

>When the iris
>closes down, it limits the amount of light transmitted by the lens, so the f
>gets higher.

Yes. If you stop down two stops (f8 in the above
example), the lens opening diameter halves (to 6.25mm),
and the lens passes 1/4 of the original amount of
light...

>Lenses which differ based on the number of elements, groupings or physical
>characteristics of the glass can have different f values for the same
>diameter iris opening.

This is true, but meaningless unless you add "and for
the same focal length"...;-) The "true speed" (marked on
pro film lenses, since exposure with these is not TTL,
but based on the numbers alone) is generally very close
to the marked speed, but with the greatest error being
near wide-open where the design of the lens structure has
the greatest effect on the accuracy of the marked aperture.
At small stops, the error is mostly in glass transmission
if the aperture itself is accurate, but at small stops,
aperture size errors usually predominate...