T-stop
Lens & optics
A more accurate measure of a lens's light transmission than the f-number, accounting for light absorption by the glass elements.
F-stops are calculated from the geometric ratio of focal length to aperture diameter. T-stops are measured with a light meter through the actual lens. A lens rated f/2.0 may be T/2.1 or T/2.3 because glass absorbs some light. T-stops are used in cinema and video, where exposure consistency between lenses is critical.
Tools that help with this: