Relation Between Aperture, Shutter Speed and Exposure

Aperture Priority, sometimes referred to as AV or Aperture Value, is used when you need to control which parts of your image are in focus. Aperture controls depth of field in your images by changing the size of the diaphragm in the lens to control light. Smaller numbers, like 4.0, 3.5, and 2.8, mean a larger opening which allows more light to hit the sensor in less time. This also gives you less depth of field, or range of focus.

Each change in aperture, or f-stop, doubles or halves the exposure time, depending on whether the number goes up or down. As you go up in aperture and close the diaphragm, you reduce the amount of light hitting the sensor, and the shutter compensates by leaving the shutter open longer so that roughly the same amount of light ends up hitting the sensor. As an example, if a setting of f/4 gives you an exposure of 1/60 second, changing to f/5.6, the next higher f-stop, doubles the exposure time to 1/30 second.