<p>Lenny's explanation of the Zone System is good and simple. I was reminded of my first AA workshop in 1966; on the first day, Ansel introduced himself to the group and began to explain the <em>exposure formula</em> (which he explains in greater detail in "The Negative" book). Most of the eyes in the group began to glaze over quickly, but he said that with a little mental practice it becomes easy (i.e., easy for the nerd of nerds). Use of a spot meter is a huge help with the system. Many of today's spot meters are calibrated in EV units, but he describes in the book how to re-calibrate such meters so they are more useful with the <em>exposure formula</em> method (which allows for very quick exposure settings without having to read the dial on the meter.<br /> The method follows:<br /> The meter must read in candles per sq. ft. (c/ft<sup>2</sup>) or converted EV.<br /> The square root (rounded off) of the film’s box speed = key stop (aperture)<br /> e.g., ASA/ISO 125 = f/11<br /> The shutter speed, used in combination with the key stop, that produces Zone V in the metered area = the reciprocal of the c/ft<sup>2 </sup>reading.<br /> e.g., 60 c/ft<sup>2 </sup>reading means a shutter speed of 1/60s at f/11 would give Zone V, and if you wanted that area to give you Zone III then you’d use 1/250s at f/11 (or an equivalent combination).<br /> Important: In this example, if your film speed tests for .10 above film base + fog (fb+f) indicate, for example, a film speed one stop more exposure than box speed, you MUST apply that correction after determining the exposure of 1/250s at f/11 (or equivalent). Also any filter factor MUST be applied at the same time as the speed correction.<br /> AA used this method for the “Moonrise” exposure, as he didn’t have much time in the quickly fading light and could not find his meter. But he somehow remembered that the c/ft<sup>2</sup> luminance of the full moon at that zenith was 200 c/ft<sup>2</sup>, allowing him to place the moon on a preferred zone and pray for adequate shadow detail. It missed in the shadows, but he was able to apply some toning locally in the shadows at the bottom of the negative to boost the local contrast somewhat and save it.</p>