On Thu, 21 Nov 2002 23:59:46 -0330, "Chris Fewer" wrote:

>Could somone refresh my mind as to what the f rating on a lens means again?
>And the difference between f2.5 and f1.5 ... it's related to the iris,
>correct?

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). 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. 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...). The "true speed" (marked on pro film
lenses, since exposure with these is not TTL, but is
based on the numbers alone), while generally very close
to the marked speed, generally has 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. In small video cameras, though,
much of the above is not a real concern...;-)