Source of the 18% grey reference.greenspun.com : LUSENET : Black_and_White_Photography : One Thread
Can anyone explain the source of the 18% reference for meter calibration. Over the last few years the magazines have proposed several sources none of which agreed with what I think is the correct answer. I have a vague recollection that I read somewhere that it is based on the newspaper printing industry and related to the amount af black ink needed to reduce the white to the precise amount of grey. I'd appreciate any pointers to a solution. Regards David.
-- Anonymous, June 05, 1997
I hope this is what you are asking. According to Ansel Adams "The Negative". Quote: "A calibrated middle gray value exists in the Kodak 18 percent reflectance neutral gray card, available at most camera stores or bound into several of Kodak's publications. The 18 percent reflectance value is mathematically a middle gray on a gometric scale from "black" to "white," and it is this value that the meter is calibrated to produce in the final print. This 18 percent reflectance is a fixed key reference point, and functions like the "A" of the musical scale as a universally recognized basic value."
-- Anonymous, June 06, 1997
I've wondered about this for some years now...
I find Adams answer unconvincing. Between 0% reflectance and 100% is an infinite number of stops. So what "average" is there in infinity?
The best answer I can come up with is that 18% is 2.5 stops down from 100%. (One stop up is 36%, two stops is 72%, 2.5 stops is 100%).
But why did someone choose 2.5 stops? I don't know. Perhaps empirically someone (in Kodak?) determined that "average" subjects reflected 2.5 stops down (ie 18%) of pure white.
-- Anonymous, August 01, 1997