What Is Exposure?
In photography exposure is the moment when the light strikes the film or sensor and the image is recorded. Three factors combine to determine the
correct exposure for a digital image: the amount of light in the scene that
strikes the image sensor (controlled by the lens aperture); the length of
time that the sensor is exposed to the light (controlled by the shutter speed); and the sensitivity of the sensor (determined by the ISO setting).
The importance of a good exposure cannot be overstated. If an image is overexposed, the highlights will be completely white without any tonal information
If an image is grossly underexposed, the image will be
dark and lack shadow detail
Although it is possible to improve moderate exposure mistakes using image editing software, no amount of software magic can save a picture that
is extremely overexposed or underexposed. Fortunately, camera light meters
are very sophisticated instruments and do an excellent job of determining
the settings for a proper exposure in most common photographic situations.
However, by changing any of the three main settings, you can gain control
over certain characteristics that can influence the look of the image.