Apertu geometric vs. real

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

P.S.: Asking this because I recently got a tele-
scopic lens with two mirrors in it, and the
declared apperture of f/8 seems to be about
one step too light...