Your camera uses three variables to determine the correct exposure.
1. The amount of light let into your camera (aperture).
2. The length of time the light is let in for (shutter speed).
3. How sensitive the sensor in your camera is to light (iso speed).
They are all interactive, so for example, to maintain the correct exposure if you select a faster shutter speed, you either need to let more light in (increase the aperture) or increase the sensitivity (higher iso).
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