What Is An F-Stop?

Aperture Diagram Icon Photography School

An a F-Stop refers to a camera setting corresponding to a particular f-number. A stop in photography is a unit used to quantify ratios of light or exposure, with each added stop meaning a factor of two, and each subtracted stop meaning a factor of one-half. A one-stop unit is also known as an EV (exposure value) unit.

On a camera, the aperture setting is usually adjusted in individual steps known as f-stops. Each "stop" is marked with its corresponding f-number, and represents the halving of light intensity from the previous stop. Beyond that each change in stop corresponds to an decrease or increase of the pupil and aperture diameters by a factor of 1√2 or about 0.7071, and hence a halving of the pupil area.

Here is a fairly standard f-stop scale:

f/1.4
f/2
f/2.8
f/4
f/5.6
f/8
f/11
f/16
f/22
f/32

This set of numbers corresponding to the sequence of the powers of the square root of 2. In this sequence each f-stop is one stop lower than the preceding unit to its left and one stop higher than the unit to it's right. These numbers are not exact as they're rounded off to conventional numbers. This makes them easier to write down and easier to remember.

This video explains f-stops quite nicely:

video by PhotoProTips

This video goes beyond just f-stops:

video by PhotoProTips

More photography resources.
More photography articles.
Photography Glossary.