The f-stop value divided by the square root of the lens transmittance.

A T-stop (for transmission stops, by convention written with capital letter T) is an f-number adjusted to account for light transmission efficiency (transmittance). A lens with a T-stop of N projects an image of the same brightness as an ideal lens with 100% transmittance and an f-number of N. A particular lens' T-stop, T, is given by dividing the f-number by the square root of the transmittance of that lens:

T = f transmittance . {\displaystyle T={\frac {f}{\sqrt {\text{transmittance}}}}.} T={\frac {f}{\sqrt {\text{transmittance}}}}.

For example, an f/2.0 lens with transmittance of 75% has a T-stop of 2.3:

T = 2.0 0.75 = 2.309... {\displaystyle T={\frac {2.0}{\sqrt {0.75}}}=2.309...} T={\frac {2.0}{\sqrt {0.75}}}=2.309...

Since real lenses have transmittances of less than 100%, a lens's T-stop number is always greater than its f-number.

With 8% loss per air-glass surface on lenses without coating, multicoating of lenses is the key in lens design to decrease transmittance losses of lenses. Some reviews of lenses do measure the t-stop or transmission rate in their benchmarks. T-stops are sometimes used instead of f-numbers to more accurately determine exposure, particularly when using external light meters. Lens transmittances of 60%–95% are typical. T-stops are often used in cinematography, where many images are seen in rapid succession and even small changes in exposure will be noticeable. Cinema camera lenses are typically calibrated in T-stops instead of f-numbers. In still photography, without the need for rigorous consistency of all lenses and cameras used, slight differences in exposure are less important; however, T-stops are still used in some kinds of special-purpose lenses such as Smooth Trans Focus lenses by Minolta and Sony.

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