What is a T-Stop
A related concept from the motion picture industry is the concept of the t-stop. This is related to a f-stop, in that they both have the same physical size and depth of field on a given lens. What differs is that while an f-stop is the theoretical value for exposure, the t-stop takes into account the actual transmission of light through the lens. In any lens, the t-stop will be slightly smaller than the f-stop as all optics cause some amount of light loss. The reason this is rarely mentioned in photography circles and heavily used in video is that in video you switch footage between cameras. If the lenses used on the two different cameras are set to the same f-stop, there can be a visible difference in light levels when switching between the two lenses. Thus, high end video lenses are marked in t-stop to aid in matching exposure between lenses.
What surprised me, was just how much this loss could be! When I compared a few lenses at DXO, there was a big difference in marked aperture versus measured transmission. For example, the new Sigma 18-35 f/1.8 lens has almost zero difference (0.1 EV), while the Canon 17-55mm f/2.8 and the Olympus 12-40mm f/2.8 have half a stop difference (0.5 EV).
There are a few implications here:
1. When looking at images where people are pixel peeping across lenses and brands, keep in mind that one lens might be robbing the camera of half a stop of light versus another. This might make it look like the manufacturer is underrating ISO, or that one has more noise than another, when really it is that you've underexposed.
2. External light meters don't take this into account. I find this particularly interesting as people often use light meters for their 1/10 of a stop accuracy, but you could be off by a full half stop just due to the lens light loss!
3. Markings on the barrel don't necessarily mean what they say. If you go to a fast f/2.8 zoom to gain low light ability, you might really be going from f/3.5 on the wide end of your kit zoom to f/3.2 on the fast zoom! Barely a difference... Keep in mind that light loss does NOT affect DOF comparisons.
As an example - here is a new Rokinon lens intended for cine use, and it is marketed as a 16mm T2.2, or 16mm f/2.0 lens. Zeiss and other have a full line of cine lenses marked in T stops: Master Prime Lenses
Big picture, it doesn't affect still photographers much, but useful to be aware of if you are chasing gear that you think will give you a 1/3 of a stop advantage :)