When you take a picture with a modern digital camera you are ‘exposing’ the digital image sensor to the light entering the camera. The exposure can be defined as the total amount of light allowed to fall on the sensor during the completion of one shutter cycle (the amount of time the shutter is open).
What defines exposure
The total light hitting the sensor is the total of the amount of light through the ‘Aperture’ and how long the camera shutter is open. Therefore:
Exposure = 'Light Intesity' x Time
For any given exposure value the shutter opening time and the aperture can vary. It is possible for the total amount of light hitting the sensor in 100th of a second to be the same amount of light hitting the sensor in 200th of a second – if the aperture has been opened up to allow more light to enter. To achieve a given exposure (at a fixed ISO) you can have a wide open aperture and a short shutter time OR a long shutter time and a small aperture.
Two exposures of the same scene and exposure value will not necessarily create the same picture. While aperture varies the intensity of light it also controls depth of field. And, while the shutter controls how long the sensor is exposed to the light it also determines the amount of motion blur. A wide aperture and short shutter time will give a shallow depth of field and tend to freeze any movement. Alternatively, a long shutter time and a small aperture will give a deeper depth of field but introduce motion blur.
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