Refers to the speed or ability of a lens to pass light. It is calculated by dividing the focal length of the lens by its diameter.

A metal plate or diaphragm that controls the amount of light entering a lens barrel. In most cases, the diaphragm is iris shaped. The larger the hole in the diaphragm, the smaller the f/stop number. For instance, f/1.4 is wide open, whereas f/16 is the smallest hole, or fully stopped down. The reasons to use f/stops are: (1) to get a sharper picture (the smaller the f/stop the sharper the image); (2) to get greater depth of field (the smaller the aperture, the greater the depth of field); and (3) to control the exposure (the smaller the aperture, the less light gets in.

A fraction which indicates the actual diameter of the aperture: the "f" represents the lens focal length, the slash means "divided by," and the word "stop" is a particular f-number; for example, with a 50mm f/1.4 lens, the actual diameter of its maximum aperture is 50mm divided by 1.4 or 35.7mm; at f/2, the diameter becomes 50mm/2 or 25mm; at f/2.8, the aperture is 50mm/2.8 or 17.9mm across; as the numerical value of the f/stop increases, the aperture decreases in size.