Adams also wrote: "The photographer who wishes to work toward a predetermined result must visualize the tonalities in which he wants certain important parts of the subject to be represented, and plan his exposure and subsequent treatment accordingly."
Sounds easy, doesn't it? In Adams' world of film, this meant understanding and controlling the relationship that real-world exposure values have with the reflective tones one hopes to achieve in a print. Adams also used various techniques in his "subsequent treatment" (read: film processing), allowing him to expand or compress the tones within that scale to complement his subject matter.
When identifying the tones that might be produced by a 10 ƒ-stop range in his Zone System, Adams came remarkably close to shades of gray as they occur in a gamma 2.2 space, using 10% increments in Lightroom's 0 to 100% scale (Fig. 4).
But don't think for a minute that 10 exposure values in a digital capture will map to these same tones at all! To determine exactly where actual ƒ-stop increments of light would fall in the RAW world, a little testing was in order. This meant dusting off my trusty spot meter and devising a gradient that would fall across a white board, giving me as many exposure values as possible in one capture (Fig. 5).
Just as it did in the Zone System, exposure comes first in any digital test. An optimal digital exposure places a value that you want to be pure white as high as possible in your exposure range. Adams called this bright value just above Zone IX "the maximum white value of the paper surface."
For the purposes of these tests, I'm adjusting my exposure to place my starting value at the highest measurable tone (99.8% to 99.9%) in the gradient that isn't clipping (100%). Of course, the clipping point that each RAW processor indicates will be different, but once you've made a commitment to a specific exposure, the rest of your tones flow from that.
As an example of how radically different two RAW processors can be, let's compare Lightroom 3's 2010 Process Version to the new 2012 Process Version. (All tests were shot with a Canon EOS 5D Mark II.) Starting with the 2010 Process Version and using a camera exposure (1⁄3 sec. at ƒ/5.6) that puts a measured exposure value of 10 at 99.8%, the subsequent exposure values fall on these tones (Fig. 6).