The aperture
(Usually located in the lens) controls the intensity
or amount of light,
while
theshuttercontrols the time of the exposure.
Combined with thefilm
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.