Optics / Exposure
T-Stop ↔ F-Stop (Transmission) Helper
Convert between f-number and T-stop for any lens transmission efficiency, see the exposure gap in stops, and check a full T-stop ladder for matching cine and photo lenses live as you type.
Convert
Reference
f-stop is a geometric ratio of focal length to aperture diameter — the same on paper regardless of lens design. T-stop measures actual light transmitted through the glass, accounting for coating and element losses. Cine lenses are rated in T-stops so multiple lenses can be intercut without an exposure jump.
Results
T-Stop Ladder at This Transmission
How this works: T-stop = N ÷ √transmission, where transmission is entered as a fraction of light that reaches the sensor (a 90% efficient lens transmits 0.90 of the light a perfect lens would). The exposure gap in stops is −log₂(transmission) — how much extra light you’re losing to glass and coatings, expressed in the same units as aperture stops.
T-Stops vs F-Stops: Why Cinematographers Use Both
Aperture is usually described with an f-number, a purely geometric ratio of focal length to entrance pupil diameter. Two lenses set to the same f-number theoretically pass the same amount of light — but in practice, glass elements and coatings absorb a small amount of it, and different lens designs lose different amounts. A T-stop measures the light that actually reaches the sensor, correcting for that loss.
The T-Stop ↔ F-Stop Helper converts between the two in either direction for any lens transmission efficiency, shows the exposure gap in stops, and lays out a full T-stop ladder so you can match exposure across lenses of different designs.
The Formula
T-stop = N ÷ √transmission, where N is the f-number and transmission is the fraction of light the lens actually passes (a lens rated 90% transmission passes 0.90 of the light a theoretically perfect lens would). A lens marked f/2.8 with 90% transmission has a true T-stop of about T2.95 — close enough that most photographers never notice, but far enough to matter when intercutting between two different cine lenses on set.
Why Cine Lenses Are Rated in T-Stops
Photography lenses are almost always marked in f-stops because still photographers can chimp a shot and adjust. Cinematographers cutting between multiple lenses in the same scene don’t have that luxury — an unnoticed half-stop shift in brightness between two shots reads as a mistake in the edit. Cine lens manufacturers measure actual transmission on a bench and print the T-stop directly on the barrel so every lens in a set matches exactly.
This tool is especially useful for:
- Cinematographers and DPs working with multiple cine lenses across a production.
- Camera assistants confirming exposure consistency when swapping lenses mid-shoot.
- Anyone mixing cine and photo lenses on the same body who needs to reconcile the two rating systems.