Basic Exposure
The more light strikes the film or sensor then the resulting image will be brighter, and vice versa.
When an image becomes too bright, the image is called over-exposure, while the images is too dark so it's called under-exposure.
Exposure is measured in lux seconds, and can be computed from exposure value (EV) and scene luminance in a specified region.
Exposure is the amount of light that you allow to hit an object or area in a photograph. This can convey a certain message or mood in one's picture. Exposure influenced by the speed ratio between Shutter Speed, Aperture and ISO sensitivity.
Aperture (f) describes how much light coming into the lens to burn an image sensor. Speed (s) describes how quickly the shutter is distribute the incoming light to burn the image sensor.
While the ISO sensitivity of the film is to explain the size of the lighting needs. The three components are referred as "Exposure Triangle"
Before we can achieve the correct exposure it is important to know what a correctly exposed image looks like.
A correctly exposed digital picture is a file that shows a full range of tones, from deep shadows to bright highlights, with detail across the entire image.
You should see some detail in the dark shadow areas while at the same time retaining detail in the brighter highlight areas.
Getting a good exposure is extremely important in photography, because if you try to correct the exposure too much in post processing, then you’ll lose a lot of detail on your images.
Based on the explanation above, the key to success in creating quality photographic work is largely determined with accuracy of the combination of aperture (f), speed (s) and the ISO sensitivity at the Exposure time.