Why 18% Gray

Does anybody know why 18% reflectance was chosen/adopted as the

standard for reflected light meters? I understand the need for the

standard, and I think I understand why standard reflectance cards are

gray: for colour neutrality. But why 18% reflectance and not 36% or

27.321%? Was there some reason for the choice?

As I understand it, if you could put all the outdoor daylight colors

in a blender, they would come out gray and reflect 18% of the light.

So, meters are calibrated for that blended, or average, color and

reflectance.

 

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Mel Brown

100% reflectance is a little less than 2.5 stops above 18%, 2.5 stops

below 18% is 3%, about as black as any surface one would encounter

outside an optics lab. So 18% is about in the middle of the 5 stop

reflectance range of some shadowless scene. Light sources in frame and

shadows expand this range.

Tim Brown's answer is absolutely correct.

 

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The dynamic range of film is approximately +/-32:1 or +/- 5 stops.

from full black to full white. Every stop doubles or halves the amount

of light. The mid point or average point is approximately 18%.

 

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Zone

I II III IV V VI VII VIII IX X

~1% 2.25% 4.5% 9% 18% 36% 72% >100%(White)

The "blender" answer is also kindof close (though not really the *reason* for the 18% gray), but not quite accurate... Ansel says that if you read a typical scene with an averaging meter, and the meter "blends" the scene into 18% gray (as meters tend to do), you'll more often than not get underexposed shadows. You actually need 1/3 stop more light, so a lot of meters are factory-set for something closer to 22% gray... The difference is called the K factor.

Actually, just to be technical here, there is no such thing as "18% gray", but in fact there is such a

thing as 18% reflectance. Its a fine point but can lead to confusion if not properly understood.