Question about incident light meter

18% turned into Zone 5 of the Zone System. Zone 6, one step lighter, was chosen as it also approximated the gray of Caucasian skin tone when photographed in open north light (the light coming from the North without any direct influence from the sun itself) when printing a B&W print. What was trying to be accomplished was to give some “constant” from which to make calculations for the photographic process from shooting to the print.

Yeah.  I just touched on it in my reply above.  That's a whole approach unto itself.  I've used it for years and it's still very valuable today when shooting digital.

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