Adams presents it as a visualization tool first and foremost. He distinguishes between measured values and placed values, as a means of appropriately exposing the film to facilitate placing the tones where we want them in the print.
The whole densitometry and testing aspect becomes a set of tools for predictable and repeatedly achieving those tones in the negative and print, and those tools are mostly used for black and white. One might have to compress or expand the tonal range to fit measured tones to placed values. But measuring and placing tones, and the testing needed to connect them, applies equally to color.
For example, if you know that highlights three stops above middle gray will be clear on, say, Velvia, then values that measure three stops brighter than 18% gray (if targeted to Zone V) will fall on Zone X. And if you know that values three stops less bright than 18% gray become featureless black, the those will fall on Zone I.
So, through testing, you build this equivalence table between what you measure and the results you get. You automatically think that subject material a stop brighter than mid-gray will fall on Zone VII. Now, if the important part of the image measures a stop brighter than mid-gray, but you visualize the image such that it gets viewed or printed at mid-gray, you are “placing” that value at Zone V, and you must underexpose (relative to 18% gray) by a stop.
If in that same image, there is a value that measures a stop less than mid-gray, and you want to preserve that, you have a conflict between two placed values. In black and white, we would compress the tonal scale during development, but that isn’t available with Velvia. So, we either have to filter it (using graduated filters), light it differently, or let the darker value go darker and try to brighten it up in printing. But at least we are doing things on purpose, and that was Adams’s main motivation.
Rick “always a visualization tool” Denney
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