When it comes to lenses, you will see markings indicating the focal length, and another indicating the F-Stop or in the case of Cinema lenses, the T-Stop.
The F-Stop on a still photography lens is a measurement of how much light is allowed to enter the lens and fall onto the camera sensor based on the focal length compared to the size of the opening in the lens. To calculate the F-Stop of a lens, you take the focal length of the lens divided by the diameter of the aperture.
For example, a 28mm lens with an aperture diameter of 20mm will have an f-stop of f/1.4.
A T-Stop on the other hand is a measurement of the amount of light that actually goes through the lens and onto the sensor of the camera.