<<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.
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.
<<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 ...
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.
<<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?
But of course, if you were just agreeing ... well, we came to the same
conclusion in different words
Bob
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