The aperture is the hole through which light enters the camera, also called the iris. Apertures are measured using a relative scale, and designated by 'f numbers' such as f4, f5.6 or f8. The smaller numbers refer to larger holes to let in light. (More detail on this in the glossary - see box at top right.)
The larger the aperture (smaller f number) the less time the camera needs to take a picture. For any particular light level and film speed (or digital equivalent) there are a range of 'equivalent exposures' which will produce the same exposure on film, for example:
|1/500 f4||1/250 f5.6||1/125 f11||1/60 f16|
All these settings will produce the same exposure, but the results will often look rather different. If you are photographing a moving car, 1/500 f4 will probably give a fairly sharp result, but the slower shutter speed of 1/60 at f16 will produce a blurred result.
For any particular focal length, 1/500 at f4 will give a relatively shallow depth of field, perhaps useful to isolate a single figure from a background, but probably inadequate for a landscape subject with much depth. For this the much greater depth of field given by 1/60, f16 might be more suitable.
Many simpler cameras do not allow you to control the aperture, but use a 'programmed exposure' that chooses a combination of shutter speed and aperture. Sometimes there may be more than one program available, with a 'sports' program give faster shutter speeds and a 'landscape' program giving smaller apertures for any given light level.
With such cameras - and many digital cameras - the only way to control the aperture at all is to use a faster film speed, which will result in a smaller aperture being set.