Middle gray

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;[1] in photography, and printing, it is typically defined as 18% reflectance in visible light.[2] This gray reflects exactly 1/5th the number of photons per square unit as compared to a reference white of 90% reflectance.[3]

50% CIELAB lightness in sRGB color space. (Y = 18.4/100.)
rgba(119,119,119,1)

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.[4] 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 neither subjective nor logarithmic). Most scenes reflect just 12% to 13% of incident light falling upon them. Therefore, the camera light meter assumes an 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 an 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.[5] In 24-bit color, this is rounded to RGB value (119,119,119) or #777777.[6]

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).[7]

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.[8] That is equivalent to 12.9% of the white intensity.

Table of middle grays

Below are various "middle" grays as based on various criteria. 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.

Middle gray as defined byRelative whiteness (≙ CIEXYZ luminance)sRGB brightnessCIELAB lightnessgamma correctionRGB value for sRGB monitorsAppearance if viewed in sRGB[note 1]
Geomean of 60:112.91%39.46%42.63%2.95rgb(101,101,101) or #656565
L*a*b*18.42%46.63%50.00%2.44rgb(119,119,119) or #777777
18% gray card20.00%48.45%51.84%2.32rgb(124,124,124) or #7C7C7C
sRGB21.40%50.00%53.39%2.22rgb(128,128,128) or #808080
Mac, pre-OS X 10.628.72%57.23%60.53%1.80rgb(146,146,146) or #929292
Absolute whiteness50.00%73.54%76.07%1.00rgb(188,188,188) or #BCBCBC
  1. LCD screens, even when correctly calibrated, often have a brightness that varies considerably depending on the viewing angle. Try stepping back and changing your position until the checkered image in the center of the absolute middle gray (50% relative whiteness) appears to dissolve into the background. If the image does not appear to be of the same brightness, then the "middle grays" rendered in the table are NOT correctly displayed on your screen. (Also take care to make sure your browser window is not zoomed since any magnification may distort the brightness depending on how your browser adjusts for gamma when blending the pixels, e.g. rendering the zoomed image at sRGB middle gray, or 21% whiteness, instead of 50%.).

References
  1. Stephen Quiller (1999). Painter's Guide to Color: Includes the New Quiller Color Wheel. Watson-Guptill. ISBN 0-8230-3913-7.
  2. Blain Brown (2002). Cinematography: Theory and Practice : Imagemaking for Cinematographers, Directors, and Videographers. Focal Press. ISBN 0-240-80500-3.
  3. Woods, Mark. How to Effectively Use the Gray Card Archived 2014-11-29 at the Wayback Machine. cameraguild.com
  4. Steven Barclay (1999). The Motion Picture Image: From Film to Digital. Focal Press. ISBN 0-240-80390-6.
  5. Geffert, Scott (2008). Adopting ISO Standards for Museum Imaging (PDF) (Technical report). imagingetc.com, Inc.
  6. http://www.brucelindbloom.com/index.html?ColorCalculator.html
  7. Jonathan Spaulding (1998). Ansel Adams and the American Landscape: A Biography. University of California Press. ISBN 0-520-21663-6.
  8. Daniel Coit Gilman; Harry Thurston Peck & Frank Moore Colby (1903). The New International Encyclopædia. Dodd, Mead and Company.
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