Convert between T-stop and F-stop using lens transmission percentage or exposure loss, or calculate transmission from known T-stop and F-stop values.

Required: T-stop and lens transmission.

Enter the percentage of incident light reaching the image sensor or film.

T-Stop to F-Stop Formula

The following formula is used to convert a T-Stop value to an F-Stop value.

F - Stop = T - Stop ร— sqrt(Transmission / 100)

Variables:

  • T-Stop is the T-Stop value of the lens.
  • Transmission is the percentage of light that is transmitted through the lens.
  • F-Stop is the equivalent F-Stop value.

To convert T-Stop to F-Stop, multiply the T-Stop value by the square root of the lens transmission percentage divided by 100.

T-Stop to F-Stop Conversion Table (Assumes Transmission Factor t = 0.90)
T-Stop (T) F-Stop (f)
1.00.949
1.11.044
1.21.138
1.41.328
1.61.518
1.81.708
2.01.897
2.22.087
2.52.372
2.82.656
3.23.036
3.53.320
4.03.795
4.54.269
5.04.743
5.65.313
6.35.977
7.16.736
8.07.589
9.08.538
* Rounded to 3 decimals. Assumes constant transmission factor t = 0.90. Formula: f = T ร— โˆšt โ‡’ at t = 0.90, โˆšt โ‰ˆ 0.948683, so f โ‰ˆ 0.948683 ร— T.

What is T-Stop?

T-Stop, or Transmission Stop, is a measurement that indicates the actual amount of light that passes through a lens and reaches the camera sensor or film. Unlike F-Stop, which is a theoretical value based on the lens aperture size, T-Stop takes into account the light loss due to the lens glass and coatings. T-Stop provides a more accurate exposure measurement, which is especially useful in cinematography where consistent exposure is crucial across different lenses.

How to Calculate F-Stop from T-Stop?

The following steps outline how to calculate the F-Stop from a T-Stop value.

  1. First, determine the T-Stop value of the lens.
  2. Next, determine the lens transmission percentage.
  3. Use the formula F-Stop = T-Stop / sqrt(Transmission / 100).
  4. Finally, calculate the equivalent F-Stop value.
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem:

Use the following variables as an example problem to test your knowledge.

T-Stop value of the lens = 2.8

Lens transmission percentage = 90%

Using the formula, the equivalent F-Stop value is calculated as follows:

F-Stop = 2.8 / sqrt(90 / 100) = 2.8 / sqrt(0.9) โ‰ˆ 2.8 / 0.949 โ‰ˆ 2.95

Therefore, the equivalent F-Stop value is approximately 2.95.