Why is 18% gray called middle gray in photography?
Asked 4/30/2015
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Photographers often refer to 18% gray as "middle gray," even though it does not reflect 50% of the light. Why was 18% adopted, and is it really halfway between black and white? Is this based on human visual perception, film and camera response, display gamma, or exposure latitude? I'm especially interested in whether 18% is a technical standard, a perceptual midpoint, or partly a historical convention.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
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The story goes that Ansel Adams came up with the "18% gray" figure. Back in the hay day of film photography he was developing the zone system and needed to define a "middle gray". It was a judgment call. Eventually, the idea caught on, but film and camera companies picked their own middle gray. It is a fun fact that your digital camera probably uses something more like 12% gray as middle gray.
Whatever the number, the idea behind middle gray is not that is "reflects 50% of the light". Or even that "it is half way between absorbing all light (pure black) and reflecting all light (pure white)". It has to do with your perception.
Your eyes are logarithmic detectors. That is, if a source gets brighter by a factor of 4, it will only seem brighter by a factor of 2 to you. If it increases by a factor of 32, it will only seem brighter by a factor of 5. If it increases in brightness by a factor of 128, it will only seem 7 times brighter to you.
The above are not the actual numbers. As you can imagine, measuring how bright things seem to people is very tricky, and varies from person to person. The important thing is that it is this weird logarithmic nature of your eyes that keeps middle gray from being 50%.
Originally by user25515. Source · Licensed CC BY-SA 4.0
user25515
11y ago
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18% gray is called “middle gray” mainly because photographic brightness is not treated linearly. It is not meant to be halfway in simple reflectance between black (0%) and white (100%).
Two ideas explain it:
- Human perception is roughly logarithmic, so we do not see brightness in a linear way. A tone that looks visually “midway” between black and white is much darker than 50% reflectance.
- Photographic systems are also non-linear in practice. Film, printing, and display gamma all map scene brightness to image brightness in ways that make a value around 18% appear near the visual middle. On a typical gamma 2.2 display, a 50% code value corresponds to only about 20% luminance output.
Historically, 18% gray was popularized in the film era and associated with the Zone System as a practical reference point for metering and exposure. It was partly a judgment call, but not an arbitrary one: it worked well as a useful midtone target. Film exposure latitude also made a darker midpoint practical, because placing the average tone around 18% left room for both highlights and shadows.
So 18% gray is best understood as a perceptual and practical photographic midpoint, not a literal 50% reflectance midpoint. Also, not all cameras meter to exactly 18%; some are closer to about 12%.
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