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← Creative Hut LibraryPhotography basics · Aperture

Part 10 of 13 in the aperture guide

What is the difference between an f-stop and a T-stop?

Written by Abin Alex, Founder and Director, and Anu Joseph, Faculty, Creative Hut Academy. Reviewed 29 October 2026.

Short answer. An f-stop is calculated from the size of the opening. A T-stop is the f-number corrected for the light the lens glass absorbs, so it shows the light that actually reaches the sensor. A T-stop is always larger than the f-number.

  • Real lenses transmit 60% to 95% of the light, because some is lost inside the glass.[2]
  • T = f-number ÷ √transmittance.
  • Two lenses at the same f-stop can have different T-stops. The Zeiss Otus 55 mm and 85 mm are both f/1.4, but measure about T1.5 and T1.7.[2]
  • Light-meter readings, depth-of-field tables and EV charts all use f-stops.[1]
T = f ÷ √transmittance
An f/2 lens that passes 75% of the light: T = 2 ÷ √0.75 = 2 ÷ 0.87, about T2.3.[1][2]
Light enteringLight reaching sensorf/2 here is T2.3 (75% passes)
Figure 7. A lens marked f/2 that loses a quarter of the light behaves like T2.3.
Lensf-stopTransmittanceT-stop
Any lens (example)f/275%T2.3
50 mm prime (example)f/2.875%T3.2
70 to 200 mm zoom (example)f/2.870%T3.3

The percentages are teaching examples. A zoom has more glass than a prime, so it usually loses more light.

In photography
The difference is rarely more than about a third of a stop, and the camera’s built-in meter reads the real light, so it corrects for it automatically.[2]
In cinematography
Cine lenses are marked in T-stops, because a small exposure difference between lenses would show when shots are cut together and costs money to correct in post. See the aperture page for the DOP.[2][2]
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[Lecture example slot: an f-stop and T-stop scale on a lens barrel, if the college has a cine lens to photograph]
Check yourself

A lens marked f/2.8 transmits 70% of the light. What is its T-stop?

Show the answer

about T3.3

Questions people ask

Is a T-stop always larger than the f-number?

Yes. Some light is lost inside the glass, so the T-stop number is always a little larger than the f-number.

Why do cinema lenses use T-stops?

So that every lens in a set exposes the same. A small difference between lenses would show when shots are cut together.

References

  1. Creative Hut Academy (2025) Aperture. Lecture slides by Abin Alex and Anu Joseph.
  2. PetaPixel. F-Stops vs T-Stops: The Difference Explained in Plain English. petapixel.com/2016/12/30/f-stops-vs-t-stops-difference-explained-plain-english

All 13 questions in this guide

  1. What is aperture, and what does it do?
  2. What is a diaphragm, and how does it work?
  3. What is the difference between absolute and relative aperture?
  4. How is the f-number written, and why does a bigger number mean a smaller opening?
  5. Why does each f-stop multiply by 1.4 and not by 2?
  6. What are full, half and third stops of aperture?
  7. What is effective aperture?
  8. What is lens speed, and what does maximum aperture mean?
  9. How do I set the aperture on my camera?
  10. What is the difference between an f-stop and a T-stop?
  11. Does a smaller aperture always make a sharper picture?
  12. How does aperture affect depth of field?
  13. How do cinematographers use aperture?
Previous: part 9Next: part 11
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Guide by Abin Alex and Anu Joseph, Creative Hut Academy.