From: "Nicholas Wright"
Subject: 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.
The maximum aperture is derived by dividing the diameter of the
aperture wide open into the focal length. In theory, it will
tell you something about how much light hits the film.
However, a lens with a lot of glass (a zoom lens with a dozen or
more lens elements) is bound to transmit less light than a lens
with less glass (a prime lens may have as little as 4 elements,
depending on design and focal length) because of light loss
caused by glass surface reflections.
This was the thing that lens coatings, and especially multi
coating is supposed to cure.
If the zoom lens is multicoated throughout, the light loss due
to internal reflections may be so insignificant as to be
unmeasurable. I have seen cheapo junk lenses that transmitted a
full stop less light than what they should have, probably due to
a combination of the lens being slower than marked, and internal
reflections.
William Robb
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