The amount of light that is captured by the film or CCD sensor is called exposure. Recording mediums vary in their sensitivity to light, but to record an image of normal exposure, all need a specific amount of light at a given sensitivity (cf. the ISO numbers of conventional film). Too little light will lead to underexposure, while too much will yield a lighter than normal or overexposed image.
The required exposure value (EV) (value given to a single exposure) is determined by the combination of the sensitivity or ISO speed of the film or CCD, the aperture of the lens and the shutter speed used.
A perfect exposure of a scene shows details in both highlights and shadows.
Most digital cameras automatically calculate the best exposure for a well-balanced image but it is important to realize how this is determined, so as to know when to override the camera's results. Exposure meters measure the overall amount of reflected light from the subject. This level of light is then rendered as 18% gray in the picture. It will give a perfectly exposed image for scenes that contain mainly average gray tones. When darker or lighter areas dominate a scene, overexposure or underexposure may occur. The photographer will then have to decide which is the most important part of the image and change the exposure accordingly.