Output
You have probably heard about F-stops, a value that indicates how much light a lens can take in relative to the length of the lens. So a short lens with a large diameter has large maximum aperture (low F-stop number).
A good example is the 50mm f/1.8 G lens from Nikon – it has a maximum aperture value of 1.8 and hence a lens opening of around 50/1.8 = 28mm. Had the lens been shorter, say 35mm, with the same lens opening at the front, the maximum aperture would have been 35/28 = 1.25.
However, then light travels from the front of the glass to the rear, some light is always lost. There are many reasons for this – coatings is one – but lets for now just accept that as a fact. And then the F-stop changes in upwards direction as the lens lets in less light.
Where F-stop describes the theoretical light a lens takes in, the T-stop is at the far end of the lens, measuring the actual lights that gets through the lens. So T-stops has to be measured – it is no longer a mathematical relationship between lens opening and length.
If we take the example from before, the 1.8G 50mm lens from Nikon with an F-stop value of 1.8. When you look at DXOMark and their measurements of that lens, they report the T-stop value to be 2.0. In other words, from front to back light equal to 0.2 F-stops is lost.
Why should you care about this? First of all, if you buy a fast lens with a large maximum aperture, then my guess is that you do so also because of the ability to take in a lot of light to help you out in low light situations. If the benefit of a fast lens is diminished as the light passes through the lens, then my guess is that you want to know about it?
Photographers in general do not pay much attention to T-stop values. Videographers much more so. The reason being that when you shoot film or video, you often change lens during the same scene, and you don’t want to do a lot of post processing to even out the exposure, if the lenses you use have different T-stop values. So videographers naturally look for lenses that have common T-stop values. And in my view, the less light is lost from front to back, the better lens. I guess that is also why DXOMark has this as one of their key performance indicators when they present their findings.
Related reading
What is aperture? And why important?
What is a stop of light in photography?