Please correct me if I am wrong:
According to Phil Davis's book Beyond the Zone System (pp. 112-113 of the 3rd Edition), the 18% gray card (and incident meters which give us an 18% reading) are off by 1 stop. 18% is too light: it's not middle gray. He states that a normal subject has a brightness range of 7 stops, which is log 2.1. The middle of that is log 1.05, which corresponds to 9% reflectance.
Therefore, according to Davis, using the manufacturer's box film speed and metering a normal subject in direct light (not the shadows) with a either standard gray card or incident meter, we under-expose by 1 f-stop.
The Zone System gets around this problem by determining that the "real" film speed is 1/2 the box speed. The result is that we give 1 f-stop more exposure for normal subjects, IE the correct exposure.
BTZS gets around this by using the box speed (or something rather close to it, determined by careful testing) but metering in open shadows. Open shadows are (normally) 1 f-stop darker than (real) middle-gray, so by metering them, we give 1 f-stop more exposure for normal subjects, IE the correct exposure.
According to some videos that I have seen, portrait photographers customarily place an incident meter "under the chin" of the subject, IE in the open shadows of normal lighting, and use that reading along with the manufacturer's box speed to determine exposure. Although they aren't necessarily BTZS adherents, they appear to arrive at the same basic destination with regards to exposure.
Note: I'm not claiming that these three approaches are equivalent. I'm just asking if what I stated - as far as it goes - is basically true.
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