The aperture of a lens is its second most important optical property after its focal length. Consider the human eye. It has a coloured iris with a pupil which can dilate or contract in order to let in more or less light, depending on ambient light conditions. When it’s dark the pupil opens up to let in as much light as possible. And when it’s sunny the pupil contracts to prevent the bright light from overwhelming the eye. Most camera lenses have a device analogous to the iris - a metal or plastic diaphragm which can be adjusted in size to control the amount of light entering the lens.

The variable-sized hole in the diaphragm is known as the aperture, is analogous to the pupil of the eye, and is indicated numerically by an f-stop or f-number value. This value, the relative aperture of a lens, describes the amount of light that a lens lets in. The value is relative because it is equivalent to the focal length of the lens divided by the size of the lens aperture, not the physical dimensions or anything.

For example, if you were to take a 50mm lens with a 6.25mm diameter aperture you’d have a lens set to f/8 (since 50/6.25 = 8). Generally each increase or decrease in f-stop value either doubles or halves the aperture size. Since f-stop values are relative to the focal length, each camera lens should let basically the same amount of light through at the same f-stop value regardless of focal length. (barring complex technical factors such as light loss from large numbers of elements and so on, but we won’t get into that here)

The usual f-stop range on 35mm and digital SLR camera bodies is:

1.0  1.4  2  2.8  4  5.6  8  11  16  22  32

though most camera lenses are only optically capable of a subset of that overall range.

The relationship between these values involves halving and doubling the amount of light. Going from f/2.8 to f/4, for example, involves a halving of the aperture size. Each number is approximately 1.4x more than its previous stop since 1.4 is the square root of 2 (to one decimal place), though since the specific numbers derive from tradition they are not always spot on. Lenses for larger camera systems such as large format cameras usually have even smaller apertures - going to f/64, for example.

This series of numbers may look difficult to work with, but in fact there’s a fairly simple way to recall it. Just remember that the first two values are 1.0 and 1.4 respectively. Each following value then doubles by every other value. So 1.0 becomes 2, then 4, then 8 and then 16. 1.4 becomes 2.8, then 5.6, 11 and 22. (the only minor glitch, of course, to this handy mnemonic scheme is between 5.6 and 11)

Confusingly enough, when the number is small (eg: f/2.8) then the lens diaphragm is open wider (“opened up”) and thus more light enters the lens. If the number is large (eg: f/22) then the lens diaphragm is closed smaller (“stopped down” or “closed down”) and thus less light enters the lens. In addition to altering exposure times, the aperture setting also affects depth of field.

The letter f is frequently italicized for good looks, and a slash is often placed between the letter f and the numerical f stop value to indicate that the f-stop value is a fraction of the focal length. eg: f/4 means that the aperture is a quarter of the focal length. The letter f stands for “focal,” “factor” or “focal length” depending on who you talk to, and the number is also often stated as a ratio. (eg: 1:2.8)

Note that not all lenses have adjustable diaphragms. Many types of lenses, though not usually those sold for use with EOS cameras, have fixed apertures. Mirror lenses, for example, fall into this category since they lack (adjustable) diaphragms. Really crummy cameras - disposable cameras being one example - also have fixed apertures. These two examples aside, however, nearly all lenses sold for use with EOS cameras have adjustable apertures.