Convert T-stop to f-stop, or f-stop to T-stop, using a typical 90% lens transmission or your own value, or find transmission from known T- and f-stops.

90% is a typical value for coated cine and photo lenses (about 0.15 stop of loss). Use your lens’s data if you know it.

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T-Stop to F-Stop Formula

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

Fโˆ’Stop = Tโˆ’Stop ร— โˆš(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 geometric f-number at the same actual aperture setting.

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

Choose F from T and transmission, T from F and transmission, or transmission from known F and T measured at the same actual aperture setting. With transmission fraction ฯ„, the relationships are F = T ร— โˆšฯ„, T = F รท โˆšฯ„, and ฯ„ = (F รท T)ยฒ. For a passive lens, 0 < ฯ„ โ‰ค 1 and T โ‰ฅ F. A T value alone does not uniquely determine either geometric F or transmission.

Transmission may be supplied as a percentage, 100 ร— ฯ„, or as exposure loss L in stops, where L = -logโ‚‚(ฯ„) and ฯ„ = 2โปแดธ. Zero loss is valid and means 100% transmission. Required numeric inputs start blank; a blank required field does not imply zero loss. Enter positive F/T values, a transmission percentage above 0 through 100%, or nonnegative exposure loss.

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?

The geometric f-number F is focal length divided by entrance-pupil diameter. T is the transmission-adjusted f-number used for photometric exposure, accounting for lens transmission as described by Schneider-Kreuznach. These are two descriptions of the same actual aperture setting, not interchangeable depth-of-field values. Depth-of-field calculations use the geometric f-number together with focal length, focus distance and a chosen blur criterion; matching T-stop alone does not establish matching depth of field.

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 ร— โˆš(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 ร— โˆš(90 / 100) = 2.8 ร— โˆš(0.9) โ‰ˆ 2.656313โ€ฆ โ‰ˆ 2.656

Therefore, the geometric f-number at the same aperture setting is approximately f/2.656.