Exposure is controlled by the aperture and the shutter speed settings. The aperture setting also influences depth of field, thus plays a major part in creative focusing decisions. Aperture settings are called f-numbers, and are expressed by "f/" followed by the number of the aperture set.
Aperture settings are indicated on a lens by a series of numbers; with some cameras and lenses there is no aperture scale on the lens barrel and the settings appear in the camera's viewfinder and/or LCD panel. A typical aperture scale might read: 1.4, 2, 2.8, 4, 5.6, 8, 11, 16. Each number indicates the ratio of the actual diaphragm opening to the focal length of the lens in use, thus any same aperture on any lens always allows in the same amount of light. As these are actually fractional numbers the smaller numbers signify larger openings. Thus, f/2 (or 1/2) represents a wider opening (or greater value) than f/16 (or 1/16).
As mentioned, f-numbers represent the same light value regardless of the lens or format in use. Thus, f/2 on a 50mm lens for a 35mm camera delivers the same amount of light through the lens as f/2 on a 200mm lens for a medium-format camera, even though the diameter of the openings themselves are different. If this wasn't the case the entire light control system in photography simply wouldn't work.
Each subsequent number in an aperture scale represents a halving or doubling of the amount of light that the aperture allows through the lens. (You can calculate the next higher number in any one-stop-step scale by multiplying the previous number by 1.4). Each step in the scale (say, from f/2 to f/2.8) is called a stop. Thus, every time you open up or close down the lens by one stop (opening means going to the next lower number, or wider opening; closing down means going to the next higher number, or narrower opening) you are changing the amount of light entering by the power of 2. A one stop change (say, f/8 to f/5.6) is a 2X difference; a two-stop change (sat f/8 to f/4) is a 4X difference; and so forth.
The best way to see how aperture settings effect light transmission is to take the lens off the camera, hold it up to the light and click through the aperture settings. (Note: Some lenses do not allow for aperture changes on the lens itself, but rely on the camera to change apertures,) You'll see that the maximum aperture, say f/2, is the widest opening. As you click through the scale, you'll see the diaphragm in the lens getting smaller. Think of water as it flows through a pipe. Given that the water will always fill the pipe, a larger diameter pipe will allow more water through. This can be applied to light flow and the aperture diameter.
Most lenses and cameras today allow for partial stops, like older lenses with click stops, or detents, between the aperture settings. On older lenses these are half-stops, and though not marked indicate a halfway point between the two aperture numbers on the ring. On newer lenses you can have 1/3, 1/4 or other fractional spreads. These step-less aperture settings means any value can be set, such as f/9.7 or f/11.3. These values are indicated in the camera viewfinder and/or the LCD panel on the camera.
Aperture rings are usually inscribed with all the available full stop settings on the lens, from the maximum, or widest, to minimum, or narrowest lens opening. The range of the aperture scale may differ depending upon the construction of the lens and its focal length. One scale may read f/2, 2.8, 4, 5.6, 8, 11, 16, while another may read f/4, 5.6,8, 11, 16, 22, 32 (the latter is more typical of zoom or telephoto lenses.) The lowest number in the scale (thus the widest opening) is called the maximum aperture; the highest number on the scale (thus the narrowest opening) is called the minimum aperture.
Photo and text copyright George Schaub 2010. Aperture settings allow you to control the depth of field, what appears sharp and unsharp in your photo. To get focus from foreground to background here an f/16 setting was used.
