"I don't recall ever reading Phil making mention of any 'existing standards' as being incorrect, but would appreciate any references."
See Beyond the Zone System (pp. 112-113 of the 3rd Edition).
He provides 2 possible historical reasons why the 18% standard became the standard.
"The origin of the 18% reflectance gray card standard is unclear, but it appears likely that it was intended to be used with artificial lighting in the studio and that the original instructions suggested increasing the indicated exposure by 1/2 stop when using the card outdoors. Alternately, it may have originated in the graphic arts, where the luminance range of typical copy subjects is approximately 1.5 in log terms. For subjects of this range, the gray card is in fact, a middle gray because its 18% reflectance is approximately equal to a log value of 0.75.
The normal range of an ordinary photographic subject, however, is generally considered to be 7 stops, or 2.1 in log terms. The middle gray of this range is 1.05, which translates to a reflectance of about 9%. If the geometric center of the normal range is considered to be middle gray (which can be debated) the standard gray card is a full stop too light in value."
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