T-Stop (Transmission Stop / Cinema Standard)
Direct Answer
The T-Stop (Transmission Stop, standardized under ISO 517 for cinematography) is a photometric measurement of a lens's true light transmission ($T = f / \sqrt{\text{Transmittance}}$), correcting the theoretical geometric f-stop for physical light lost to glass reflection and optical element absorption. Standardized across all cine primes (Cooke, ARRI, Zeiss, Angénieux).
Source: ISO 517:2008 / ANSI PH3.48 / ASC (American Society of Cinematographers) Manual
Specs
| Mathematical Formula | T-Number = F-Number / √(Transmittance Percentage as a decimal) |
| Physical Meaning | A lens set to T2.8 transmits the exact same photometric illumination to the sensor as an ideal 100% transparent lens at f/2.8 |
| Complex Zoom Glass Loss | A 18-element zoom marked f/2.8 with 80% transmission has an actual transmission rating of T3.1 (0.3 stops darker than f/2.8) |
| Cinema Iris Ring Markings | Marked strictly in T-stops with linear mechanical spacing and clickless 0.8 MOD gears for follow focus rigs |
| Shot-to-Shot Matching | Switching from a 24mm wide lens to a 85mm telephoto on a movie set at T2.8 produces zero exposure flicker or luminance shift |
Why Cinema Demands T-Stops
In still photography, a $0.2\text{-stop}$ difference between two lenses is masked by auto-exposure. In filmmaking, where multiple cameras shoot the same actor from different angles with different focal lengths, cutting between shots on lenses marked only in theoretical f-stops would create noticeable brightness jumps. Calibrating lenses on a photometer bench in true T-stops guarantees identical exposure across all camera angles.