A large aperture (e.g. f/4) lets more light through the lens than a small aperture (f/16).
The aperture is a hole created by the lens diaphragm.
The lens aperture is controlled by the lens diaphragm – a series of metal blades that move to make the aperture larger or smaller. The size of the aperture doesn't just affect the amount of light passing through the lens; it has another effect. The size of the aperture also affects the Depth-of-Field; the area of apparent sharpness in an image.
Depth-of-Field extends in front of and behind the focused distance. A wide lens aperture, such as f/2.8 or f/4, gives a narrow Depth-of-Field with only a short depth of focus around the focus plane. An example of this can be seen when taking a portrait using wide apertures where it is sometimes possible to have only part of the subject's face in focus. A small aperture, such as f/16 or f/22, has the opposite effect, giving a larger depth of focus so that close subjects as well as distant subjects are in focus, making much more of the image appear sharp. To maximize the Depth-of-Field, focus about a third of the way into the scene and select a smaller aperture such as f/16. To learn more about the technical aspects of Depth-of-Field click here.
With EOS cameras, you can set the aperture using Aperture-priority AE (Av) mode. You select the aperture and leave the camera to choose the shutter speed for correct exposure.
Walking the dog, © Enda McCormack 2011, Canon EOS 1000D
This photograph was taken using a very wide lens aperture. The focus is on the second bollard from the front. The Depth-of-Field is very narrow, putting most of the scene out-of-focus. The effect is reminiscent of an impressionistic painting.
Beauty and the beast, © Eric Dutcit 2011, Canon EOS 7D
A narrow Depth-of-Field is good when you want to concentrate attention on just one aspect of an image. Here, the subject is the small spider on the pink flower. A wide lens aperture has given the perfect result. A small lens aperture and a larger Depth-of-Field could have drawn attention to less interesting areas of the scene, particularly in the background.
Lie of the land, © Philip Escott 2011, Canon EOS 1000D
This landscape shows the extreme Depth-of-Field given by a small lens aperture. Everything from the foreground grass to the distant horizon appears sharp.