Mind your f/s and T-s

So there are a seemingly infinite amount of acronyms, backronyms, contractions, letters and numbers used on set and it can be a mind bender trying to untangle them all. Eventually you tend to get them all but there are two letters in particular that confuse trainees and experienced ACs alike; the f/stop and the T-stop. When I’m on set and the Cinematographer says, “give me 4,” or “show me 5.6 and a half,” I will always repeat back, “T-4!” or “T-5.6 and one-half”. Sometimes people come up to me and ask, “why do you say ‘T-something’ instead of f/something?”

The answer to this question is both incredibly simple and moderately complicated (WARNING! MATH AHEAD). To understand the difference we need to know exactly what each of these things are.

From the moment people start learning about cameras they learn the term f/stop. For most people using stills cameras it’s the dial that makes things brighter and darker, for more experienced folk it’s called an aperture or iris dial and they know that f/2.8 makes the image twice as bright as f/4, which is twice as bright as f/5.6 and so on but they probably don’t know the math behind it.

Stills lenses are nearly always measured in f/stops

The f/stop is calculated very simply. It’s the focal length divided by the aperture diameter. In more mathematical terms it’s expressed as N=f/D where f = focal length of the lens (in mm) and D = diameter of the aperture. Therefore 100mm lens at f/4 would have an aperture diameter of 25mm. The reason that f/stop is written with a slash in it is because it’s the relationship between ‘f’ and ‘D’ and this has been used on stills camera lenses since before most of us were born.

This number is great for determining the amount of light entering the aperture but it has some drawbacks. It’s a simple mathematical equation because it does not take into account anything else within an optical system except the focal length and the exposure at the aperture itself. A lens is a much more subtle and complicated system. This is where the T-stop comes in.

When light enters any lens element there is a certain level of absorption. Light entering any transparent object will lose some light to reflection and some more light to absorption. Even though modern lens elements have anti reflex coatings which take their refractive properties up to about 99.9% (John Fauer's Film & Digital Times - 'Cooke Book' Jan 2013, pg 14)  there is still always some light loss before the light gets to the film plane. On lenses with a lot of elements, such as big zooms, this light loss can actually be significant, a ½ stop or more. (http://www.dxomark.com/Lenses/Nikon/Nikon-AF-S-Nikkor-70-200mm-F28-G-ED-VR-II)

Arri's Master Primes are built, measured and tested using T-stops

A T-stop stands for a ‘transmitted’ stop. It takes into account the light hitting the film plane after it has passed through the ALL the lens elements both in front of and behind the aperture. The aperture diameter would be wider than if you simply calculated N=f/D as we are now taking into account light being lost through the front elements and the light that has yet to be lost through the rear elements. A lens with a T-stop of 2.8 will have exactly that light hitting the film plane.


Through modern testing regimes and precision manufacturing it is now possible to check each individual lens that comes out of the workshop. Companies can put them on their test stands and mark the aperture ring with the correct T-stop during testing (John Fauer's Film & Digital Times - 'Cooke Book' Jan 2013, pg 18).

So that’s it in a nutshell. When the Cinematographer asks the lens to be set to T2.8 they have absolute confidence that their exposure is absolutely correct. Any questions or comments are absolutely welcome and I certainly hope you've enjoyed reading this as much as I've enjoyed writing it.


James L.