On Fri, 22 Nov 2002 21:19:00 -0500, Stacey wrote:
>On Fri, 22 Nov 2002 10:23:42 -0500, "RGBaker"
>wrote:
David Ruether wrote:

[...]
>>> 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...
>>> DR

>>If memory serves, 'actual' light transimission apertures measured in
>>T-stops; 'derived' transmission figures as described in this thread are
>>called F-stops.

>Right. The old uncoated lenses that did have variations at the same
>physical f stop size due to light loss through multiple glass air
>surfaces were rated in T stops so a photographer could use the same
>settings on a 3 element and a 5+ element lens and get the same
>exposure. This light loss is why multi element zoom lenses weren't
>possible until multicoated glass was developed and why 3 and later 4
>element with 2 cemented together were so popular before coatings were
>developed. The 5 element lenses were sharper but too much light was
>lost and they had problems with flare. This multicoating (and really
>plain single coatings) is also why this T stop light transmission
>rating system was dropped as the difference in light loss between
>different design coated lenses is almost immeasurable.

Ummmm.... I don't think I agree with most of
the above, except the first word...;-) The losses
at air/glass surfaces, even with no coating, is
relatively minimal, though flare can become an issue
with very complex lenses, mostly satisfied with
single-coating. Lens structure at wide stops and
diaphragm area errors at small stops tend to be more
important than glass transmission and reflections
for affecting true speed at particular stops (which
is why a single-number "T-stop" is not very accurate
in describing lens characteristics). As with careful
view camera users who map the true shutter speeds,
for accurate exposure setting on lenses with non-TTL
metering, the diaphragm accuracy should also be mapped,
along with the speed errors due to lens design...