F-stop
The lens's aperture setting can be adjusted. The number can be calculated by taking the focal length of the lens and dividing it by the diameter of the aperture. As a consequence of this, higher f-stop numbers correspond to aperture diameters that are more restricted. By using F-stop numbers, exposure settings for a particular scene can be represented without the need to know the focal length of the lens that was used to take the picture. The name "F-stop" originates from the Waterhouse stop, which consisted of a series of circular holes in strips of metal and "stopped" some of the light that was traveling through the lens. John Waterhouse (1806-1879) is credited with the invention of the system in 1858, but the hole sizes do not coincide with the present f-stop values.
Do you have a better definition? Share it in the comments.
