For photographers, F-stop is a very familiar term, as it indicates the size of the aperture opening at a given focal length. The smaller the number, the wider the aperture is open, meaning more light reaches the sensor. However, it doesn't tell you how much light actually reaches the sensor. The fact that this varies between lenses isn't a problem for photographers, as they compensate for it with the exposure time, making the difference imperceptible in the final photograph. However, filming is quite different, as you film with fixed ISO and aperture values ​​to achieve the ideal cinematic look (more on this here: Are cinematic videos possible on a budget?), and therefore significant differences can occur. That's why cinematic lenses (lenses for filming) are labeled with a T-stop, which indicates how much light reaches the sensor. Since different lenses, due to varying numbers of elements and different designs, should deliver the same amount of light to the sensor, T-stops were developed to specify how much light actually reaches the sensor. Basically, T and F are comparable in that the more light falls on the sensor, the wider the aperture is open, and the better the background blur. However, two lenses with the same F-stop setting can have different brightness levels.