----- Original Message -----
From:
Subject: Re: lens brightness
> > I have heard remarks that a 200/2.8 prime will transmit more
> > light than an 80-200/2.8 zoom. Is this true? I thought a
> > 200/2.8 was a 200/2.8 whether it be a prime or part of a
zoom.
> > Thanks for any help.
>
> I'm with you 2.8 is 2.8. Two different lenses, if calibrated
properly,
> should deliver the same amount of light at the film plane when
set to 2.8.
> No ifs, ands, or buts! Otherwise photography doesn't work
with
> interchangeable lenses.
It would be nice, but unfortunately, it just doesn't work this
way. A 200mm f/4 lens will have a maximum aperture of 50mm
diameter, as will an 80-200mm f/4 lens, if both are calibrated
correctly. This does not mean they will both deliver the same
amount of light to the film, only that the maximum apertures are
the same. The T-stop may be quite different, depending on the
efficiency of the lens.
Constant aperture zooms are very nice for fill flash, even with
auto everything cameras, as the background will tend to stay at
the same density, no matter what focal length you are shooting
at.
William Robb
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