Quote Originally Posted by MattKing View Post
T-stops are used primarily for motion picture lenses - they provide a more exact measurement of the light transmission of a lens then the slightly more theoretical f-stops.

The difference is usually small, and generally only relevant in the exacting circumstances of motion picture work.
Exactly. It's the standard measurement used for cinema lenses, it's the F-stop corrected for absorbance and reflectance. It is usually very close to the F-stop, I was just curious as to whether some manufacturers' lenses might be more affected by absorbance and reflectance than others. But I guess guess if the T-stop was never given by the manufacturer, it'd be pretty hard to figure out.