Originally Posted by ic-racer
It's interesting thing or coincidence that paper's 18% reflectivity is near 0.75 density which... Happens to be about old Zone system's middle gray density.
A long time ago, I got myself in the brainless strict variation of ZS, where the calibration was done by printing using only one print exposure time: The time that gives pure black from the film's base fog.
The calibration was real PITA (no wonder that I later abandoned such system). First had to find EI, it's easy part. Just the exposure value that gives just barely distinguishable black tone from pure black. It's still useful approach.
The calibrating middle and highlight tones was troublesome. With that one fixed enlarging exposure time, the print of negative where graycard (put on zone V) should gave equal tone as real gray card when compared each other.
That was hard and often required going for complete different developer and/or film.
Then came the highlights, where was two important things. First, the graycard exposed to zone IX should be barely darker than bright, unexposed paper white and rough gray paper/canvas exposed to VIII should gave some structure in print.
Not an easy task to get all these right! When concentrating only to zone I and VIII (or IX), it's quite easy and straight method to got calibrated zone system. But the weird requirement of getting subject's 18% reflectance as 18% reflective tone to print was.... insane.
(when I messed with that, I was young (still teenager) and could get better guides. There was no internet, ... And library had long queue for adam's books and then there was requirement for densitometer.. )
I don't know where the idea of getting zone V in print to match exactly reflectance of gray card come. It wasn't in the material I studied then. Perhaps it came from Minor White?
However that experience gave me strong idea of visualizing print tones before making exposure. And it gave the lesson that there's no sense to try to be too accurate with ZS stuff. There's always so much variables involved.