Exposure

•  is the amount of light per unit area reaching a photographic film or electronic image sensor, as determined by shutter speed, lens aperture and scene luminance.   
• is measured in lux seconds, and can be computed from exposure value(EV) and scene luminance in a specified region.
• is generally referring to a single shutter cycle.
• is classified into three types : 

  1) Optimum Exposure

        Correct exposure is an exposure that achieves the effect the photographer intended. Exposure adjustment is done to control the physical amount of  light from the subject that is allowed to fall on the film, so that significant areas of shadow and highlight detail do not exceed the film's useful exposure range. This ensures that no significant information is lost during capture. 
  

  2)  Overexposure

        A photo is overexposed when it has a loss of highlight detail, that is, when important bright parts of an image are "washed out" or effectively all white, known as "blowned-out highlights" or "clipped whites". 
  

  3)  Underexposure

         A photograph may be described as underexposed when it has a loss of shadow detail, that is, when important dark areas are "muddy" or indistinguishable from black, known as "blocked-up shadows" or sometimes "crushed shadows", "crushed blacks", or "clipped blacks".
   
  

  3 photos taken at the same location displaying three different exposures.

  

  Underexposure                   Optimum exposure                Overexposure