On Fri, 22 Nov 2002 14:06:12 -0330, "Chris Fewer"
>"Neuman - Ruether"
>news:3de04a2f.4097883@newsstand.cit.cornell.edu...
>> On Thu, 21 Nov 2002 23:59:46 -0330, "Chris Fewer"
>>
>> >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...;-)
>Ok, I'm still very slightly confused...
>I knew what f stops were. I meant the f-rating on the lens, it's maximum
>aperature size.
>My questions (Though not stated) was related to temporarily replacing a
>standard lens with a surplus Angineaux or Switar, which apparently, can be
>bought relatively cheap to what they should be worth, just because film
>isn't used the same way it once was.
???????????
An f-stop is an f-stop, barring unusual lens
characteristics that make the T-stops very
different from the f-stop on one of two compared.
In other words, in terms of lens speed, you can
generally compare lenses by the rated maximum
stop. Also of importance for your question is
coverage, which for a film frame is the measured
diagonal of the image area covered. This needs
to be sufficient to cover at least the TV-safe
image-area of the CCD size of the camera you want
to use the lens on. Since this last may be hard to
find, trial may provide the best information about
applicability...