The “f” in f-stop

The “f” in f-stop stands for the focal length of the lens. While focal length itself refers to the field of view of a lens, f-stop is about how much light you allow to hit the sensor via the aperture opening. The aperture is the hole in the middle of the lens, made up of rotating blades that open to let in light when the press the shutter release. The diameter of the aperture determines how much light gets through and thus how bright your exposure will be.