Every photographer knows about F-stops. But what is a T-Stop and why is it something we should know about?
T-stops (or Transmission Stops) are featured on cinematographer lenses and are more accurate at determining exposure than F-stops. In order to explain what a T-stop is and how it works, we have to do a quick recap on F-stops.
An F-stop is a measure of the size of the iris of your lens. It’s a dimensionless value that you can use to work out the aperture of your lens. It’s simple math. For example, if you have a 100mm f/2.0, you would divide 100 by 2 and your aperture would be 50mm. A 50mm f/2.0 has a 25mm aperture, a 35mm f/2.0 is 17.5mm.
Ideally, by classing the lenses as f/2.0, they would all create equal exposures. However, because each brand builds camera lenses differently, the way they absorb light is different for each lens.
This is where T-stops are introduced, it is a measurement not an equation. The T-stop is an F-stop corrected for the amount of light reflected or absorbed the lens. It is the true speed of the lens. A 100mm lens at f/4.0 would behave differently than a 50mm lens at f/4.0, but a 100mm at t/4.0 and a 50mm at t/4.0 would have the same exposure.
The T-stop values across lenses are the same even if the F-stop value of the lenses is different.
As light passes through a lens, there is always loss (never gain) so a T-stop is always slower than an F-stop. For example, an f/2.8 lens could be t/3.2 and another f/2.8 lens can have t/3.4.
Picture Correct claims the Tamron 70-200mm f/2.8 has a T-stop of 3.2, the Nikon Nikkor 20-700mm f/2.8 has a T-stop of 3.3 and the Canon 70-200mm f/2.8 has a T-stop of 3.4. That is a lot of light lost.
With lens coatings having come a long way in the last 80 years, the differences aren’t as noticeable as they once were. If you are filming on a DSLR, interchanging lenses or just looking at buying a new lens, T-stops are worth knowing about, because when you use a f/2.8, you really want it to be f/2.8, not an f/3.4.
- Further reading
- A Crash Course In How Camera Lenses Work