Measure of the actual light transmission of a lens accounting for all optical losses. More precise than F-stop for film production.
T-Stop (Light Transmission)
The T-stop is the actual light transmission ratio of a lens under real-world conditions. Unlike the F-stop, which is a geometric calculation, the T-stop measures the exact amount of light that the lens actually transmits to the sensor or film.
Technical Fundamentals
Measuring T-stops
T-stops are measured photometrically using calibrated light sensors under standardized conditions:
- Standard Illumination: 5600K daylight
- Spectral Range: Full visible spectrum
- Measurement Method: Integral light transmission measurement
- Calibration Standard: SMPTE / ISO 518 for cinematography
T-Stop vs. F-Stop
| Aspect | F-Stop | T-Stop |
|---|---|---|
| Calculation | Geometric (Focal length/Diameter) | Photometrically measured |
| Accuracy | Theoretical | Practical |
| Glass Losses | Not considered | Fully considered |
| Usage | Photography | Film production |
| Lens Marking | Standard photo lenses | Cinema lenses |
Practical Relevance
A lens with f/2.0 might actually be T/2.3 if glass elements and coatings absorb 15% of the light. When changing lenses within a scene, T-stops guarantee identical exposure without additional testing.
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