To shoot the assignment, I drove to a local hydroelectric power station, strolled inside (you could do that in those days) and set up my brand-new Calumet 4×5 view camera (Fig. 1). Practicing the Zone System more or less requires the use of a 1° spot meter, and using one gave me the feeling of great precision in my black-and-white photography. Reading a 1° spot in the darkest shadows at the end of the turbine facing the camera, I remember placing that value on Zone II, thus determining the base exposure. Then, reading a highlight in the clerestory of the roof, I recorded a value in my notes to determine the development for Zone VIII, knowing full well that would put the overhead lights into specular (Zone X) territory. I no longer have those notes from this early effort, unfortunately, but I distinctly remember the range of the scene was great enough that it required pulling the development by several stops to bring the very bright white of the clerestory down to Zone VIII.
By then I had done enough darkroom testing to know what negative densities could be produced on PLUS-X sheet film with various development times, temperatures and agitations. Despite all that, pulling this first assignment out of the soup was still a revelation. The tones were magnificent (Fig. 2).
Based upon a deceptively simple idea, the Zone System mapped 10 exposure values (basically, ƒ-stops) found on the Weston meter (Weston called them light values) to 10 evenly spaced shades of gray in a print. Using this number of tones came conveniently close to the dynamic range of many black-and-white films at the time, so the system gave photographers very direct control over how and where they represented tones from a given scene. It only required that you "previsualize" your final output—which in those days meant a print. Ranging from a tone "slightly lighter than black to a tone just perceptibly darker than the paper base," the 10 tones appeared roughly spaced like this in reproductions overseen by Adams in his own books (Fig. 3).
Ansel Adams wrote: "A gray scale of 10 steps seems to be most convenient. With solid black and pure white as the extremes, there are 8 discrete shades of gray in between—not too many to visualize, and yet enough to symbolize various values in the subject."