F-Stop and T-Stop
So what is the difference between the f-stop and T-stop? It is pretty simple really. The f-stop is the ratio between the focal length of the lens divided by the diameter of the aperture opening. So if the focal length of the lens is 50mm and the diameter of the opening is 25mm then the f-stop is f/2.0.
This is pretty straight forward, but what is really important is how much light is getting to the sensor. In a perfect world the f-stop would be a good gauge for all lenses, but in reality each piece of glass had different properties that change the amount of light that goes through the lens. The best glass loses the least amount of light and is closer to the f-stop. T-stop is just the measure of the actual light that gets transmitted through the lens, no matter what the glass. So, while it is possible to get different amounts of light through different lenses with the same f-stop, with same T-stops, you should get the same exposure no matter what lens you use.
In short: f-stop is the mathematical formula and T-stop is the actual measured transmission.