There are 2 controls on a
camera that effect exposure. The first is aperture. This
is an opening in the lens that allows light to pass
through the lens in a controllable manner. This control
is available because you can control the size of the
opening. The aperture size is expressed in f-stops. In
the water bucket scenario, think of this as a
water faucet. Where you control how far you open a
faucet to control water flow, you can also control how far you open the
aperture to control light flow.
second control is shutter speed. Shutter speed is how
long the shutter in front of the film or sensor is
allowed to stay open exposing the film or sensor to the
light passing through the aperture. The shutter speed is
expressed in a time value such as 1/60 or 1/125 of a
second. Shutter speed would be comparable to how long
you leave the water faucet open to fill the bucket.
controls plus the sensitivity of the film (size of the
bucket) or digital
camera sensor work together to achieve the proper
exposure or fill the bucket, if you will.
For any given film or sensor
sensitivity (ASA xxx) there is a given amount of light
that is required for a proper exposure. Your camera
meter will tell you what this is if your camera is set
to 'manual metering' or it will set the shutter speed
and aperture if it is in 'auto' mode, to give you the
right amount of light to represent a middle tone
properly at the given film speed or sensor sensitivity.
Now, to try and answer the
second part of the question. "How do I know what
settings to use"?
Like I said,
it depends. Let me explain. Let's say it is sunny
outside and your using ASA 100 film. Therefore, for a
middle toned subject, your cameras meter should give you
settings equivalent to an f16 aperture and an exposure
time of 1/125 of a second shutter speed. We know this
because of the
Sunny 16 rule. (click the
link to read what the rule is)
Lets say you look at your depth of field
preview and decide that too much is in sharp focus.
Assuming your camera is in manual mode, here is what you
would do. Open your aperture more to give you less depth
of field (area in sharp focus) by setting the aperture
to a smaller number. The next full stop aperture setting
is f11. This will allow one full stop (twice as much) of
light to reach the film causing it to be over exposed.
To compensate for this overexposure you cut your
exposure time in half by changing the shutter speed to
the next full stop of 1/250 of a second.