Exposure is the amount of light a sensor or piece of film receives and the resulting brightness or darkness of an image. When the shutter button is pressed, the shutter opens for a specified amount of time (the shutter speed.) The amount of light that hits the sensor at this time determines the exposure. There are three variables that are at play in determining whether an image will be under, over, or correctly exposed. They are aperture, shutter speed, and ISO. You may hear these three variables called ‘the exposure triangle.’