A more technical approach recognises that a photographic film (or sensor) has a physically limited useful exposure range,Ray, S.F. et al. 2000 "The Manual of Photography" Focal Press, , p.230 sometimes called its dynamic range.Ray, S.F. et al. 2000 "The Manual of Photography" Focal Press, , p.121 and p.245 If, for any part of the photograph, the actual exposure is outside this range, the film cannot record it accurately. In a very simple model, for example, out-of-range values would be recorded as "black" (underexposed) or "white" (overexposed) rather than the precisely graduated shades of colour and tone required to describe "detail". Therefore, the purpose of exposure adjustment (and/or lighting adjustment) is 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.
