Exposure
Any
time we take a picture our camera allows light from the outside
to be projected on the image sensor inside the camera. The
sensor (most are of the CCD type)
requires a certain amount of light to make a perfect exposure. If
there is too little light the picture is underexposed (the picture
is too dark), if there is too much the picture is overexposed (the
picture is too light). The camera has two ways to regulate the
amount of light that gets to the sensor.
First, the camera can change the amount
of time the sensor is exposed to light. This is done with a shutter
that opens and closes like a water valve. Typical shutter
speeds range from 1/1000 of a second to 1/30 of a second. The
longer the exposure the more chance there is that either the subject
or the camera will move, blurring the picture. As you might expect,
1/30 of a second lets in twice as much light as 1/60 second, etc. Exposures
longer than approximately 1/30 second might require a tripod to steady
the camera.
The second way the camera can regulate
the amount of light that gets to the sensor is by varying the size
of the opening the light can pass through before it gets there. The
wider the opening the more light gets through. The part of the camera
that does this is called the diaphragm.
The opening in the diaphragm is called the aperture.
The way aperture sizes are described is with the f-stop
number . While not critical to know, the f-stop number is
the ratio between the distance from approximately the center
of the lens to the sensor divided by the diameter of the aperture
opening. The f-stop numbers tend to range from about f/11, which
would be a small opening and would let little light in, to about
f/2.8 which would let in quite a bit of light. Remember, a large
f-stop number lets in less light than a smaller number. Sometimes
you might hear a photographer saying they are "going to
stop their lens down." This means that they are going to make
the size of the aperture go down, not the f-stop number, which
actually gets larger. Camera lenses vary in their minimum and maximum
f-stop values.
After you press the shutter release the camera measures
the amount of light in the location you have pointed it and sets the
shutter speed and f/stop accordingly to make a perfect exposure.
A third way digital cameras can regulate
exposure is by changing the sensitivity of the sensor. When the sensor is set
to be less sensitive the picture has more detail and has a smoother tonal
range. When the sensor is set to be more sensitive the pictures
tend to less detailed and can have visible “noise.” The
sensitivity number is called the ISO number
and range from about 100 at low sensitivity to about 1600 or more for
high sensitivity. This number was originally applied to film speeds,
but it is also used today to refer to sensor sensitivity in digital
cameras. Cameras
that have larger sensors, like digital
single lens reflex (SLR) cameras produce less noisy pictures at a
given ISO than do cameras with smaller sensors, like the typical point
and shoot. Cell phone cameras have really small sensors
and produce horrible pictures, though many find this quality charming.
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