Exposure denotes the amount of light reaching the film. It is controlled by shutter speed and aperture.
The shutter speed determines the duration of light and the aperture controls the volume of light that is allowed into the camera to expose the film. Various combinations of aperture and shutter speed will give the same or equivalent exposure value. To get the proper exposure of a given scene one also has to take film speed into account.
Film that is exposed to too much light results in overexposure, a lighter, washed-out image with loss of highlight detail. Too little light results in underexposure, a darker image with loss of shadow detail.
High contrast scenes that exceed the dynamic range of the film (the difference between the lightest light and darkest dark you can capture in one photo) will inevitably either have blown-out highlights or muddy dark areas indistinguishable from black.
Exposure compensation is a technique for adjusting the recommended exposure in consideration of factors that may result in a lighter or darker image. Exposure compensation may be required for predominantly light or dark scenes (high-key/low-key), for the use of lens filters, or for deliberate multiple exposures.
