In article ,
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"Anton Shepelev" wrote in
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Hello all,

As I know, and I hope am correct about it, a lens's
aperture is the ratio of the front pupils' diameter
to the focus length. And it is this number that is
indicated as the maximum aperture and is also marked
on the aperture ring, while the actual illuminance
produced by a lens on its focal plane depends on
many factors like (multi)coating, the number of
optical elements e.t.c.

Taking this into accout, the reading of an ideal
external light-meter may differ from that of an
ideal internal (say TTL) one. My question is whether
this correction coefficient (or difference, if
expressed in logarithmic terms) has a name for it in
the photographic slang.

Thanks in advance,
Anton



Cinematographers use "T-Stops" (T for Transmission)not so much for
lenses which depart slightly from geometric F-stop values, but largely
to compensate for the fact they often use several filters at once. They
do not have TTL metering, generally, but their lenses are calibrated in
compensated T-stop values, and their meters read T-Stops. They have
tables and calculators to compute their filter packs and shutter angles.