Free Photography Tips for Beginners
Exposure is controlled by the aperture and the shutter speed . The size of the aperture determines the strength of the light reaching the digital sensor . The shutter speed determines how long the light is on the digital sensor. Most cameras today come with built in exposure meters which display the appropriate shutter and aperture settings (most of the time).
Every combination of f-stop and shutter speed that the meter shows has settings of equal value that you can use to get the same exposure. To work out an equivalent setting just remember that the standard numbers for f-stops (aperture sizes) and shutter speeds are in steps that double or halve the exposure . For example going from f11 to f8 or 1/500 to 1/250 is doubling the amount of light reaching the digital sensor in each case. If you take your aperture back to F11 or shutter back 1/500 the amount of light reaching the digital sensor has been halved in each case.
Therefore if you make the shutter speed slower by one stop eg 1/500 to 1/250 you have to make the aperture 1 stop smaller eg f8 to f11 to keep the exposure the same. This is known as exposure reciprocity. The decision on what combination of shutter speed and exposure to use depends on the results you are trying to achieve. If you are trying to freeze action use a fast shutter speed. Conversely if you want to blur action use a slow shutter speed. If you want your image to have a greater depth of field, say for a landscape use a small aperture (high f-stop like f22). On the other hand if you want a shallow depth of field , say for a portrait use a large aperture like f4.