Exposure is the space of time in which sunlight falls on the light-sensitive element. During this period of time a certain amount of light succeeds in passing though the fixed diameter of the diaphragm and hits the image sector (which has its own light-sensitivity setting). Exposure is controlled by a camera’s shutter which can open and close at varying speeds.
Exposure time and lens diaphragm settings determine the overall exposure of the photograph. Exposure can be calculated using a light meter, with the help of a table or ones own practical experience according to the brightness of the object being shot and the light-sensitivity of the photographic material (eg. film) being used. Digital cameras come with a build-in light meter.
Increasing light-sensitivity is inversely proportional to exposure. If you increase the light-sensitivity by x2 then you must decrease the exposure time by x2. When using digital cameras it’s important not to forget about image sector “noise” at high ISO (light-sensitivity) settings.
Exposure is calculated in fractions of a second: for example, 1/30 s, 1/60 s, 1/125 s, 1/250 s. This is displayed on the screens of many cameras as simple – “60”, “125”, “250”. Often long exposures are displayed as numbers in inverted commas – 0″8, 2″5. There is also a standard range of exposures: 1, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000, 1/4000 s. Adjacent values differ by one degree and alter the exposure by a factor of x2.
As with the lens diaphragm in a camera it is possible to programme in exposure preferences or alter them manually. In simple compact cameras there may be no manual function.