An f-stop is kind of a combination of two terms. First off, f/N is generally the notation used to indicate the size of the diaphragm opening, or aperture, in a camera. Let me give a little detail about how that notation came about, before I go on to explain the meaning of a stop.

Aperture Values and f/Stops

Aperture openings are measured as fractions of the focal length of a lens. That is what the 'f' stands for in the aperture rating, 'focal length'. Assuming we have the epitome of lenses, the 50mm, with an aperture of f/2.8, we can determine the actual diameter of the aperture opening like so:

50mm / 2.8 = 17.85mm

If we open the aperture up to its maximum of, say, 1.4, we can measure that as well:

50mm / 1.4 = 35.71mm

The difference between an aperture of f/2.8 and an aperture of f/1.4 is a difference of four times as much light...or two stops. We know this because the area of the aperture opening itself is four times as large at f/1.4 (1001.54 mm2) as it is at f/2.8 (250.25 mm2). A stop in photography nomenclature means a difference of one exposure value, which is the doubling, or halving, of the amount of light reaching the sensor. There are a few standard "full stops" that f-numbers are rated in:

1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45, 64

These aperture settings all differ by one full exposure value, or one full "stop", and create the full f-stop scale. When you close down your 50mm f/1.4 lens from its maximum aperture of f/1.4 to an aperture of f/2.8, you are "stopping down" by two full stops.

It should be noted that most cameras these days offer a two additional f-stop scales beyond the standard full stop scale: a half-stop scale and a third-stop scale. Most cameras default to a fractional scale rather than the full stop scale, so it is important to learn and memorize the full stop scale so that you are making the proper adjustments when you change your aperture setting on your camera.

Half-stop Aperture Value Scale

1, 1.2, 1.4, 1.7, 2, 2.4, 2.8, 3.3, 4, 4.8, 5.6, 6.7, 8, 9.5, 11, 13, 16, 19, 22

Third-stop Aperture Value Scale

1, 1.1, 1.2, 1.4, 1.6, 1.8, 2, 2.2, 2.5, 2.8, 3.2, 3.5, 4, 4.5, 5.0, 5.6, 6.3, 7.1, 8, 9, 10, 11, 13, 14, 16, 18, 20, 22

Relationship with Shutter Speed

An important relationship exists between aperture and shutter speed. Both are rated in stops. While aperture differences are often denoted in 'f/stops', shutter speed changes are usually simply called 'stops', or possibly exposure values.

Back to our example with the 50mm lens. Assuming we are shooting on a bright sunny day, with an ISO of 100. We have the aperture set to f/16, and the shutter speed set to 1/100th. (This is called the "Sunny 16" setting, as photographic theory indicates that an f/16 aperture, with a shutter speed matching the ISO speed, will produce a proper exposure in bright midday sunlight.)

Assuming we need to shoot something that is moving very fast, and we need a higher shutter speed. We can easily calculate the proper aperture value, assuming we know how many stops of additional shutter speed we need. If we increase our shutter speed to 1/200th, that is a difference of one whole stop. Shutter speed and aperture are inverses of each other, so if we increase shutter speed by one stop, we must open the aperture by one f/stop, to f/11. Despite the difference from the original settings, the new settings will produce the same exposure. The same applies if you are using a half- or third-stop scale...any half or third stop adjustment of one setting requires an similar inverse adjustment of the other.