On Sun, 01 Apr 2001 16:29:22 GMT, "Alexander Ibrahim" wrote:

>That and most motion picture cameras use T-stops, I am told there
>is a subtle difference, but I do not know what it is supposed to
>be.

Actual light transmission of lenses varies slightly from
the marked f-stops; the T-stops give standard transmission
values compensating for the lens variables, so exposure can
be based on calibrated meters, known shutter speeds, and the
T-stops - without having to figure in the lens light
transmission errors... (an "f1.4" lens may be marked "T-1.6"
wide open, with the other stops on the lens similarly
remarked from normal, but with "T-4", etc., being moved
from the f-stop positions to have the same transmission as the
theoretical f4, etc.). With TTL metering, this is not
necessary - and lenses can even be allowed to change "speed"
with zooming...