For the exposure to be correct, there are three variables that must be balanced:
ISO speed : This is sensitivity to light of the sensor array. It defines the total amount of light that required. It is sometimes referred to as the film speed equivalent.
Aperture setting:This is the size of the opening in the lens through which the light passes.It defines the rate at witch light reaches the sensor array.
Shutter speed: This is the length of time the camera shutter is open. It defines how long light is allowed to reach the sensor array.
written by: John Ellis DIGITAL PHOTOGRAPHY