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Simply stated, exposure is the amount of light it takes to record a scene correctly. Let too much light into the camera and your pictures will look washed out, lacking detail in highlight (bright) areas. If too little light gets into the camera, pictures will look murky and dark. Digital cameras have extremely sophisticated light-metering and exposure systems, and in most ordinary situations the camera will provide a good exposure with minimal help from you (in fact, if you put the camera in the "Automatic" or green mode, you won't have to provide any help). Still, having some idea of how the camera works its exposure magic will help you get a higher percentage of good exposures.
In deciding how much light is needed for correct exposure, your camera employs a light meter that measures the amount of light reflecting from the subject. Taking into account the ISO speed that you've set (or that the camera has set in the automatic mode), the camera then sets the exposure controls for a perfect exposure.
The two basic controls that all cameras use to set exposure are the shutter speed (it determines how long the camera's sensor is exposed to light) and the lens aperture (the size of the lens opening that lets light into the camera). Shutter speeds are described as fractions of a second (or whole seconds for long exposures) and typically range from one or more full seconds up to 1/4000 second. The faster the shutter speed the less time light has to enter the camera; the longer the shutter is open, the more light reaches the sensor.
Lens apertures are described as numbered f/stops; the f/stops designated with smaller numbers (f/2.8, for example) let in the most light, and higher-numbered stops (f/22) let in the least amount of light. In combination, shutter speed and aperture are the gatekeepers that regulate the amount of light that gets to the sensor.
Understanding the significance of the numerical sequences of each control isn't important. It is important, however, to know that the two controls have an entirely reciprocal relationship. If you open the lens by one aperture stop to let in more light (going from f/11 to f/8, for example) and then speed up the shutter speed by one stop (from 1/125 to 1/250, for example), you will get exactly the same exposure. The only difference is that the faster shutter speed stops action better, and changes in aperture affect depth of field.
For a complete reference that covers the entire subject of exposure, see Jeff Wignall's book Exposure Photo Workshop (Wiley Publishing, 2008).Next: "Travel Hints"