In photography, exposure is the amount of light allowed to fall on each area unit of a photographic medium(photographic film or image sensor) during the process of taking a photograph. A long exposure showing stars rotating around the southern northern celestial poles. A photograph of the sea after sunset with an exposure time of 15 seconds. A properly exposed photo combines a number of elements- Particularly:
Aperture
Shutter Speed
And ISO (or film speed)
ISO: the measure of a digital camera sensor's sensitivity to light
Aperture: the size of the opening in the lens when a picture is taken
Shutter Speed: the amount of time the shutter is open.