What is a T-stop, or Transmission stop?

A transmission stop, or T-stop, is a measurement term that refers to the actual amount of light that passes through the lens and reaches the sensor on a camera. T-stop is a tested and calculated value used by filmmakers. It is different from an F-stop, which is a theoretical value used in photography. For example, two lenses with the same F-stop rating can have two different T-stop values.

In most photography situations, the T-stop is not commonly used because minor exposure differences between two lenses are compensated for by light metering technologies onboard a camera. However, for filmmakers, T-stop information is valuable due to the need for real-world lens comparison values when shooting multiple scenes with multiple angles on multiple days and using multiple lenses.

A particular lens's T-stop, T, is given by dividing the F-stop by the square root of the transmittance of that lens: for example, an f/2.0 lens with a transmittance of 75% has a T-stop of 2.3. Since real lenses have transmittances of less than 100%, a lens's T-stop number is always greater than its F-stop number.