F-number
The ratio of a lens's focal length to the diameter of its entrance pupil, written f/2.8, f/8 and so on, and the standard way of stating aperture.
Formula / rule
N = f / D
Measured in
Dimensionless ratio
In depth
The classic full-stop series — f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22 — progresses by factors of √2, because light passing through scales with the area of a circle rather than its diameter. A larger f-number means a smaller opening, which is the single most common source of confusion for beginners.
Examples in practice
Moving from f/4 to f/5.6 halves the light because the pupil area halves, even though the number only rose by a factor of about 1.41.