Aperture
The adjustable opening inside a lens that controls how much light passes through to the sensor, and with it the depth of field.
Formula / rule
N = f / D
Measured in
f-stops (EV)
In depth
Aperture is expressed as an f-number, the ratio of focal length to the diameter of the entrance pupil. Because it is a ratio, f/4 admits the same light density on a 24 mm lens as on a 200 mm lens, which is what makes the scale universal across every lens ever made. Opening up one full stop doubles the light; stopping down one full stop halves it.
Examples in practice
On a 50 mm lens, f/2 means the entrance pupil measures 25 mm across; on a 200 mm lens, f/2 would demand a 100 mm pupil — which is why fast telephotos are so large and expensive.