F-stop (aperture) is a number that represents the size of an opening inside the lens. This size is controlled either by the camera or by the photographer. The larger the f-stop number, the smaller the opening.
The larger this opening is (the smaller the f-stop number), the more light will pass through the lens in a given amount of time. Thus the aperture is one means of controlling exposure. In the article on exposure, the aperture or f-stop is referred to as one of the four factors of exposure. The other three factors are light, sensitivity, and time.
Depth of Field
As stated, aperture affects exposure. However a change in exposure is not the only effect of a change in aperture. Aperture also affects depth of field. Depth of field is the distance between the nearest an object can be to the camera while appearing to be in focus and the most far away an object can be from the camera while appearing to be in focus. If that doesn't make sense, refer to the article on focus. There is a section there on depth of field, and the definition is given in a different way. Or simply look at the photos below:
Duck asleep on pier. Aperture = 5.6
Duck asleep on pier. Aperture = 11
The duck is well focused in both photos, but examine the railing several feet in front of and behind the duck. In the first photo, the railing is only sharp very near the duck. In the second photo, the railing still shows detail several feet away from the duck in both directions. The difference is due to two different aperture values used. The first photo was taken at f5.6, and the second was taken at f11. Earlier we learned that larger f-stop numbers are associated with smaller apertures. Clearly, smaller apertures (larger f-stop numbers) result in greater depth of field. Conventional photographic wisdom states that a large amount of depth of field is desirable only in certain situations, while 'shallow' depth of field is desirable in other situations.
The f-stop Scale
When operating your camera in a fully automatic mode, the aperture will be selected for you. Even so, you may want to know in advance what aperture value the camera intends to use. You may want to override that value. If you use an SLR, the aperture and shutter speed are generally displayed in the viewfinder and/or on the LCD. To understand the number you are seeing, it is necessary to be familiar with the f-stop scale.
There are a series of numbers that are universally associated with the f-stop. The values that are available to you will depend on the lens that you are using. Some lenses have a much wider range of aperture values available, so don't be surprised if some of the numbers don't show up on your camera, especially the lower ones. The following numbers are what you would see with a very fast prime lens: 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, and 32. Each of the numbers in that list differs from its immediate neighbors by one stop. For a discussion of the term stop, see the article on exposure. Your camera may allow you to select intermediate values if it is calibrated in half-stops.
Note that in the scale above, each stop differs by a factor of approximately 1.41 (or the square root of 2). As a result, we see that the value has doubled every other f-stop.
Aperture Priority Mode
Once you are familiar with aperture and its effects as well as its possible values, you have the option to select the aperture value yourself. The easiest way to do this is with the aperture priority mode of your SLR. Aperture priority is an automatic mode in which the photographer selects the aperture and the camera selects the shutter speed so that correct exposure is achieved. It is still advisable to check the shutter speed before taking the photo. If the shutter speed in seconds is slower than the reciprocal of the focal length in use, a tripod should be used. Image stabilization or vibration reduction will give you an extra stop. If you are not clear on this, please see the article on shutter speed.
There is something you should keep in mind relating to aperture. In your efforts to either minimize or maximize depth of field, it's a good idea to stay away from extreme aperture values whenever possible. On the wide end of the aperture scale, optical quality tends to suffer because more light is gathered through the outer areas of the lens elements. The outer, or rim, area of an optical element is more difficult to optimize than the central area.
On the other side of the coin, narrow apertures (above about f16) result in a form of optical degradation called diffraction. So once again, everything in moderation. Never max anything out.