On Fri, 22 Nov 2002 21:07:05 -0500, Stacey
>On Thu, 21 Nov 2002 20:39:50 -0800, "Paul Tauger"
>
>>"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. f measures the light transmission ability of the
>>lens -- the lower the number, the more light it transmits. When the iris
>>closes down, it limits the amount of light transmitted by the lens, so the f
>>gets higher.
>>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.
>????
>
>Where did you find this info? Iris size vs focal length is how f stops
>are determined, period. It has nothing to do with measurement of light
>transmission to determine what a lenses f ratings are. The number of
>elements, groupings or other physical characteristics (other than
>physical vignetting) have nothing to do with f stop ratings. Light
>transmission used to vary -slightly- from lens to lens at the same f
>stop depending on the design but this doesn't occur with modern
>multicoated optics as the loss of light with multicoating is so small
>you'd never be able to see it. Even with those old (pre 1940's)
>uncoated optics that did vary, they still had the same f stop ratings
>Stacey
Yes - but it is easy to see that most lenses are
very noticeably "slower" at their widest stop
than the f-stop rating would indicate (and this
amount varies among lenses...), and they are also
often noticeably incorrect at small stops due to
diaphragm irregularities if you base the exposure
on non-TTL metering and run a sequence of exposures
(with an accurate shutter) from widest to smallest
aperture. In other words, MOST lenses are noticeably
slower than their rated maximum f-stop wide open,
may approximate their true speed through the middle
stops, and may become erratic near the smallest stop...