In photography, painting, and other visual arts, middle gray or middle grey is a tone that is perceptually about halfway between black and white on a lightness scale; in photography, and printing, it is typically defined as 18% reflectance in visible light. This gray reflects exactly 1/5th the number of photons per square unit as compared to a reference white of 90% reflectance.
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Middle gray is the universal measurement standard in photographic cameras. To calibrate light meters, whether in a camera or hand held, the 18% gray card was conceived. It is assumed that the measurement taken by a meter gives the exposure for a shot so that some of the light reflected by the object measured is equivalent to middle gray. Because human perception adjusts to the overall brightness level (and is logarithmic rather than linear), the perceived middle gray is subjective to the observer. This must be kept in mind when using a camera with a built in light meter (which is not subjective nor logarithmic). Most scenes reflect just 12 % to 13 % of incident light falling upon them. Therefore, the camera light meter assumes a 18% gray level. This can easily be observed when one relies solely on the exposure given by a camera with a built in light meter when taking a snow scene - the image will come out dark. Using a 18% gray card as an expose guide will mitigate this error.
In the sRGB color space, CIELAB middle gray is equivalent to 46.6% brightness. In 24-bit color, this is rounded to RGB value (119,119,119) or #777777.
History
In the Zone System of Ansel Adams, middle gray is known as "Zone V" in the scale of 11 zones from Zone 0 (black) to Zone X (white).
As early as 1903, middle gray was defined as the geometric mean intensity between a white and a black intensity that are in a ratio of 60:1. That is equivalent to 12.9% of the white intensity.
Table of middle grays
Below are various "middle" grays as based on various criterion. In the center of the rendering of the "Absolute whiteness" middle gray, a small black and white checkered image has been included which, if viewed from a distance, should look like a gray with exactly 50% whiteness. On a correctly calibrated sRGB monitor, this should appear to be of equal brightness to rgb(188,188,188) or #BCBCBC.