Middle gray
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Middle gray or middle grey is one of the basics of photography, a tone representing 18% reflectance in visible light.
At the time of developing norms for photographic media ( called DIN or ASA norm) one of the problems was to define the right amount of light to saturate a photographic film or photographic plate. Saturation was arbitrarily defined at 82% of the consumption of photographic emulsion because it was difficult to measure the exact point in exposition time when 100% was reached.
Naturally this number did not come as an inspiration in a dream but was caused by the typical five field sensitivity test made in the early days on films and papers. To do a five field test you place the photo-sensitive material under a controllable light source and cover up 4/5 of the material and start the light for a defined time. After that you cover 3/5 and so on. The result is that every field is exposed twice as long as the one before, or in mathematical terms : It is a 5 stop doubling progression. The geometric mean of a 5 stop doubling progression in between 0 and 100 happens to be 18.
The definition of the sensitivity by the 82% norm led to all photographic light meters being adjusted to show exact exposition at 82% saturation. To calibrate meters the 18 % reflecting card (NOTE: 18+82=100%) was conceived.
By using the reflective card it was possible to determine the point at which 100% saturation would occur on the film when recording pure white.
A broader use for the 18% card is nowadays to determine the color shift either by ambient light, lens aberration or quality shifts in films to reproduce true to life images.
Hence middle gray is the universal measurement standard in photographic cameras. It is assumed that the measurement taken by camera exposes the shot so that some of the light reflected by the object measured is equivalent to middle gray.
A zone system, developed by US photographer Ansel Adams, places middle gray in the middle of the scale from black to white therefore in the US 18% reflection is called middle grey.
