Image of a photographer taking a photograph

The action of exposing the sensor or film in a camera to light.

A correct exposure has detail in the highlights and shadows. Correct exposure means an image with maximum detail, that gives us the most options in editing.

Too much light leads to overexposure:
images that are too bright and have no detail in the light or highlight areas.

Too little lights leads to underexposure:
images that are too dark and have no detail in the dark or shadow areas.

The amount of light entering the camera is controlled by the aperture, shutter speed and  ISO (Sensitivity).