How is exposure controlled?

The aperture  (Usually located in the lens) controls the intensity or amount of light,

while the shutter controls the time of the exposure.

Combined with the film speed  (ISO or ASA) these two controls have the ability to produce a correctly exposed film or ccd. A small intensity of light for a long time can be equal to a large intensity of light for a short period of time.

A small flow for a long period  (f16 @ 30seconds) Move over image to see roll over image
A large flow for a short period  (f11 @ 15 seconds) Move over image to see roll over image

 If we use the analogy of filling a bucket of water that has to be filled to exactly the top to exposing our film to make the correct exposure, we can either fill it with a weak flow for a longer time or a large flow for a shorter time. In each case the correct exposure is when the water reaches the top.

If we had a fast subject like a Grand Prix racing car – to allow very fast shutter speeds to stop the action you need to have a fast shutter speed - but the shutter speed the aperture and the ISO all correlate to provide a correct exposure.

So we can have a fast shutter speed say 1/4000 sec with a fast ISO like 1600 and a wide aperture like f2.

However this can produce the same exposure as a slow ISO like 100 with a small aperture like f22 and the shutter speed would be ½ second. As you can imagine although the exposure is the same, the visual effects are quite different.