T-Stop ↔ F-Stop (Transmission) Helper

How to Use the T-Stop ↔ F-Stop Helper

1. Enter your lens’s f-stop (aperture value). Type in the f-number (e.g., f/1.4, f/2.8, f/4).
2. Enter the lens transmission efficiency. This is usually given as a percentage (e.g., 90%, 80%), or you can use a known T-stop value if your lens provides it.
3. Click Calculate. The tool will show you:
   • The equivalent T-stop (transmission stop).
   • The difference between the entered f-stop and the calculated T-stop.
   • The exposure gap in stops of light (how much brighter or darker the lens transmits compared to its theoretical value).
4. (Optional) Enter a known T-stop to calculate the reverse: the theoretical f-stop it corresponds to.
5. Use the result to compare lenses, balance exposure across different cameras, or match cine lenses and photo lenses in the same setup.

Tip: F-stops are a mathematical ratio of aperture size, while T-stops measure actual light transmission. Cinema lenses are rated in T-stops for consistency between lenses on set.

Understanding T-Stops vs F-Stops in Lenses

When working with lenses, one of the most important distinctions for photographers and cinematographers to understand is the difference between F-stops and T-stops. Both terms describe aperture values, but they serve slightly different purposes:

• F-stop (f-number): A theoretical value based on the ratio of the lens’s focal length to the diameter of the entrance pupil. It’s a geometric measurement that determines how much light should enter the lens. For example, a 50mm lens set to f/2.0 has an entrance pupil of 25mm.
• T-stop (transmission stop): A practical measurement that accounts for light loss inside the lens due to glass elements and coatings. It indicates the actual amount of light reaching the sensor or film.

For most still photography, f-stops are sufficient because photographers can adjust exposure easily in-camera or in post. However, in cinematography, consistency across multiple lenses is critical. Two different lenses both marked at f/2.8 might not transmit exactly the same amount of light, but if they are rated at T2.9, the cinematographer knows they will match exposure perfectly.

The T-Stop ↔ F-Stop Helper makes it simple to calculate the difference between the two. By entering your f-stop and lens transmission efficiency, the calculator returns the equivalent T-stop, along with the exposure gap in stops. For example, if a lens has an f/2.8 aperture but only transmits 90% of the light, the effective T-stop is closer to T3.0. That slight difference can matter when cutting between lenses in the same scene.

This tool is especially useful for:

• Cinematographers and DPs working with multiple cine lenses across a production.
• Camera assistants who need to ensure exposure consistency when swapping lenses.
• Photographers curious about how much light their glass actually transmits.
• Students learning about the technical and creative aspects of exposure.

By understanding the relationship between F-stops and T-stops, filmmakers and photographers can make more informed lens choices, balance exposure more precisely, and ensure consistency across shots. Whether you’re shooting a feature film, a commercial, or simply testing your lenses, the T-Stop ↔ F-Stop Helper gives you the clarity you need to measure true light transmission.