--- Bob Talbot <[EMAIL PROTECTED]> wrote:
>
> <<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.>>
> Strictly, it should not. But when you actually
> start testing leght
> transmission for yourself you can begin to wonder.
> The two have
> become so synonomous that, with progressive dumbing
> down of
> photography, the term has all but been redefined.
AGAIN, let me emphasize: F-STOP has NOTHING to do
with measured light transmission. PERIOD. It is a
mathematical relationship. It may be true that most
people don't realize that, but that doesn't change the
facts. Not one single camera maker measure the actual
light transmission of their lenses and adjusts the
actual diameter of the diaphragm to try to match light
transmission from one lens to the next.
>
> And actually, the axial / off-axial effect is
> important. The *quoted*
> f-number ismjust one number ... it means for axial
> beams (for which it
> is measured?) two lenses may indeed have the same
> light transmission
> for the same f number BUT lens design factors mean
> that off-axis beams
> are restricted more in one than another.
Again, this is wrong. You refer to the "axial beam
for which [the f-stop is measured]." F-stop isn't
measured with respect to beams of light, either axial
or otherwise. It's the relationship between focal
length and the diameter of the aperture. Yes, it gets
a bit more complicated with zooms and internal lenses,
in the the apparent diameter of the aperture can vary
from the physical. Nonetheless, f-stop has nothing to
do with actual, measured light transmission.
> <<An f-stop is simply the ratio between the focal
> length
> of the lens and the diameter of the aperture.
> Nothing
> more, nothing less.>>
>
> Since this reads like you are contradicting ...
I'm not contradicting anything I said. This is in
complete agreement, and simply reinforces what I said,
in somewhat different words.
>
> WRONG!
>
> It is the ratio between the *effective* aperture and
> the focal length.
> The actual (physical) diameter of the aperture need
> not be the same
> ... it depends on exactly where the blades sit in
> the light path.
Yes, I simplified a bit for the sake of the
discussion. but nothing here changes the fact the
f-stop is related to the SIZE of two things, not to
actual light transmission, which is the point I was
making.
> <<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.>>
>
> Ah, then why not acknowlege that is precisely what
> was meant by
> t-number?
Way back in the dim, dark history of photography, some
vendors did mark T numbers on their lenses. But the
trend never caught on in still photography, for
reasons unknown. In the movie industry, by
comparison, lenses are always marked with T-Numbers,
because it's critical that the exposure on film not
change as the director selects different focal length
lenses within the middle of a shoot.
Let me say it as simply as I can:
F-Stop is NOT related to the actual or measured
transmission characteristics of a lens. It's
calculated by mathematical formula. Two lenses with
the same focal length and f-stop can have markedly
different measured transmission characteristics, due
to things like vignetting, internal reflections,
different coatings, etc.
=====
Bob Meyer
I wish I knew what I know now, when I was younger...
http://www.meyerweb.net/epson
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