In this quick video, Carl from ProAV TV in the U.K. talks to Director of Photography and cinematographer Alister Chapman regarding a question that we got many times in the past – what is the difference between F and T stops in lenses.
First, it is important to clarify that for the most part F stop numbers are typically used in stills lenses while T stops are used for Cinema (or video) lenses. both relate to the amount of light the lens allows to get into the camera and onto the sensor.
The difference is that while T stops take into consideration the actual light Transmittance (hence T) of a lens – meaning how much light goes through a lens, F-stop is a more theoretical number that does not take into account some of the light loss a specific lens design has (for example when light bounce inside the internal components of the lens or is absorbed by the coating etc.). If you are a numbers person – Wikipedia has some equations for you.
Put simply, F-stops give you an approximate measurement of how much light a lens can get in while a T-stop gives you an exact and accurate number.
In our experience, the differences between F and T stops might be pretty small (a lens might have an f/1.4 but is actually T1.5) but there can be cases where much bigger differences exist where the design of a lens will prevent more of the light from passing through.
Is there a need to be so exact with the amount of light transferred by a lens? in stills shooting – not that much. In a professional video and cinematic work, this is actually extremely important. The reason is simple – if you are shooting with say a 45mm T2 lens and you want to change to a 90mm T2 lens you want to make sure that at T2 (and for that matter any other T stop) the lens will give you the same exposure – otherwise you will have different exposures in your film in the same scene which can cause a lot of problems in post-production.
So why does not all lens manufacturers just use T-stops? that is a good question and the only answer that we heard (and that Carl and Alister also mention) is cost. Doing exact light transmission measurements for lenses is expensive. Since this isn’t something that most non-pro video shooters care about, most manufacturers just stick to F-stops.
You can find a lot more info and tips about using video equipment on our HDSLR subsection here on LensVid. You can also find more technology-related photo and video news on our photo-technology subsection.