Aperture

Aperture.

Most lenses contain a diaphragm, a thin light-blocking plate or interleaving set of adjustable plates. The diaphragm contains a small hole, the aperture, which is adjustable in size and allows the photographer to control the amount of light entering the camera. Apertures are indicated by the f stop value, which is a relative value and does not indicate the actual size of the aperture hole.

f stop.

Also “f number.“ The relative aperture of a lens, or the numbers used to indicate lens apertures - the amount of light that a lens lets in. These numbers are a relative number and are equivalent to the focal length of the lens divided by the size of the lens aperture.
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. 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 the same amount of light through at the same f stop value regardless of focal length.
The usual f stop range on 35mm cameras is:

1.0  1.4  2  2.8  4  5.6  8  11  16  22

So 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 remember the series. 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)
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. (see numeric ratio printed on lens.)

f stop ratios.

Numeric ratios printed on lenses indicate the largest (maximum) f-stop of which the lens is capable.
For example, if a lens has 1:1.8 printed on it then you know you can open up all the way to f/1.8 if you like. The smallest (minimum) f-stop is not commonly printed on lenses, but is usually f/22 on most 35mm lenses.
If the lens has a range printed on it, such as 1:3.5-4.5, then the lens is a zoom lens which has a maximum aperture which varies across the zoom range. (ie: it’s a variable aperture zoom lens) The first number refers to the maximum aperture of the lens at the shortest focal length of the lens, and the second number refers to the maximum aperture of the lens at the longest focal length of the lens. For example, if you have a 28-105mm lens which has 1:3.5-4.5 printed on it then you know at 28mm you will be able to open up to f/3.5. However at 105mm you will only be able to open up to f/4.5. In between those two focal lengths the lens will change maximum apertures, usually in increments of a stop or so in the case of modern computerized lenses. Whether the lens keeps a large maximum aperture through much of the range or quickly goes down depends entirely on the design of the lens.

Prime lenses only have one maximum aperture. But not all zoom lenses have different maximum apertures across the focal length range. Some lenses, primarily expensive professional lenses, are constant aperture zoom lenses, which means that they have fixed maximum apertures. If a lens has a fairly small constant aperture then you know it’s probably a high quality lens. A 70-200mm 1:2.8 lens, for example, is likely to be a large and heavy pro lens.