T-stop
A measured aperture value that accounts for the light a lens actually transmits, rather than the geometric ratio an f-number implies.
Formula / rule
T = N / √(transmittance)
Measured in
T-stops
In depth
Every air-to-glass surface reflects and absorbs a little light, so a lens marked f/2 might only transmit as much as a theoretically perfect T/2.2. Cine lenses are marked in T-stops because a colourist cannot tolerate a brightness jump when cutting between two lenses set to the same nominal aperture. Stills lenses are marked in f-stops because depth of field depends on the geometric ratio, not the transmission.
Examples in practice
A complex 24-70 mm zoom with eighteen elements may measure T/3.1 at its marked f/2.8, while a simple six-element prime marked f/2.8 measures close to T/2.9.