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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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