A camera shutter is a mechanical device which permits light to enter the camera body for a specified period of time. The longer the shutter is open the more light enters the camera.
Although modern digital Single Lens Reflex (SLR) cameras usually have a mechanical shutter, electronics can achieve the same effect by clearing the image sensor and then allowing the sensor to gather light for the required time. This type of ‘shutter’ is used on less expensive cameras.
Many digital cameras have an automatic setting to determine the appropriate shutter speed or it may be set manually for greater artistic control.
Shutter speed is always calculated in relation to ‘aperture’ – the size of the opening of the lens. These two settings are used in conjunction to control the amount of light that hits the image sensor.
Fast shutter speeds are used to ‘freeze’ action. Longer shutter speeds can be used in low light conditions and to allow moving objects to blur. This effect is often used when photographing running water.
Since a fast shutter speed lets in less light than a long shutter speed, it is usually combined with a larger aperture setting to allow sufficient light to reach the image sensor. Similarly, long shutter speeds are combined with small apertures.
Shutter speeds are measured in fractions of a second in an approximate 2:1 scale starting with 1 second. The scale runs 1 second, 1/2 second, 1/4 second etc. down to the fastest speed of 1/8000 second. In addition, many cameras have a ‘B’ shutter setting (the shutter stays open as long as the button is pushed) and a ‘T’ setting (the shutter stays open until the button is pushed again).
Aperture settings are also measured on the same 2:1 scale. This allows for the same light exposure by increasing shutter speed one notch while opening the aperture one notch. Light conditions for a given scene allow for a range of shutter speed/aperture combinations. There is no ‘correct’ combination – it depends on what kind of effect the photographer hopes to capture.
For example, if you wish to photograph a moving subject such as an athlete during a sports competition, you would normally use a fast shutter speed to freeze the action. This fast speed must be used with the correct aperture to expose the image correctly.
A slower shutter speed, however, could be used if you pan the camera to match the movement of the athlete. This can create a more dynamic effect as parts of the picture (especially the background) will be blurred. Slower shutter speeds must be compensated for with smaller aperture settings.
Another common photographic effect is controlling the depth of field to highlight the subject. Large apertures have a smaller depth of field which means that the foreground and background will be (pleasingly) out of focus. To achieve this effect you must combine the large aperture setting with a fast shutter speed.
Flash adds another dimension to the aperture/shutter speed equation. Interesting effects can be achieved by using flash to freeze the main motion while allowing some of the action to be blurred with a slower shutter speed.