--- Jaros�aw Brzezi�ski <[EMAIL PROTECTED]> wrote:
> Bob Talbot <[EMAIL PROTECTED]> napisa� /
> wrote:
>
> >
> > The two lenses could have the same f-number but
> different t-numbers
> > ... due to transmission
> >
> > The strict definition of f-number may relate to
> axial "beams" of
> light
> > only.
f-number has absolutely nothing to do with beams of
light, axial or otherwise. Nor does it have anything
to do with light transmission, vignetting or filters.
An f-stop is simply the ratio between the focal length
of the lens and the diameter of the aperture. Nothing
more, nothing less.
It's not at all unusual for a zoom lens to transmit
less light than an equivilent fixed lens at the same
aperture. All those extra glass surfaces increase
internal reflection and reduce transmission. With
modern lens coatings the effect is much smaller than
it used to be, but it's still there.
=====
Bob Meyer
I wish I knew what I know now, when I was younger...
http://www.meyerweb.net/epson
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