Roll-Film Exposure Testing

A Much-Simplified Approach to the Zone System

 

By Doug Criner

The Zone System, popularized by Ansel Adams, begins with testing of black-and-white film, using your specific camera and personal darkroom idiosyncrasies. The prescribed testing procedure begins with determining your personal film speed by finding the exposure value that produces a film density of 0.10 (over the density of the film’s base plus inherent fog). This requires a densitometer, costing upwards of $1000.

I'll Stick With Manufacturer's Development Time

The next step is to find the development time that puts an exposure value of, say, two stops over the metered value, on Zone VII. This also requires a densitometer or a calibrated step tablet and a very good eye. Further, this implies that the photographer may be use some development time other than that recommended by the film’s manufacturer–with who-knows-what impact on mid-tones, grain, resolution, margin for error, etc. Unlike sheet-film users, who can develop each shot separately, this roll-film user does not like to monkey with development times.

A much simpler testing approach is to expose a roll of film in ½-stop increments (using a textured surface, such as carpet, as the target) with exposure values ranging from, say, -8 to +8, and develop normally as specified by the film manufacturer’s data sheet. Then, with a light table and loupe, evaluate each frame. In the table below are the results from a test of Tri-X film ("old" version), exposed in a Leica M6, and developed normally in HC-110 developer, Dilution B (7½ minutes in a reel tank at 20˚C).

From the data, it may be seen that there are 10 exposure stops from my evaluation of Zone 0 (first clear frame, without tone) through Zone X (first solid-black frame). This happens to be exactly what Ansel Adams would have wanted–but I wouldn’t have been heartbroken if the range had been a stop or so more or less than the Holy Grail of 10 since the exposure range of black-and-white film usually exceeds the range of values that are encountered in the real world.

Centering Film's Exposure Range

Anyway, based on my film test, what more could you ask? Well, there is just one thing:  we want the film’s exposure range to be centered on the metered exposure value (Stop 0),  i.e., there should be the same number of stops between Exposure Value 0 and the Exposure Values associated with the frames evaluated to be either Zone 0 or Zone X. This provides maximum maneuvering room above and below the indicated meter reading.  From the test data:

        Stops between Value 0 and Zone 0 = 5½ stops

        Stops between Value 0 and Zone X = 4½ stops

To get the range centered on a metered value of zero, we should increase the rated film speed by ½ stop. Because of the logarithmic nature of exposure, one-half stop speed increase corresponds to √2 times the film speed. Since Tri-X is rated at ASA 400, my personal film speed is:

        400 x 1.414 = 566 (say ASA 500, the nearest setting on the Leica)

Assume, for the sake of pedagogy, that the opposite situation were encountered:  i.e., there had been the 4½ stops down to Zone 0 and 5½ stops up to Zone X. Then I would have reduced my personal film speed by ½ stop: 400 / 1.414 = ASA 283.

Now, to make sure the arithmetic is understood, assume that we needed to increase the speed by 1½ stops. One stop increase is 400 x 2 = 800. Another half-stop is 800 x 1.414 = ASA 1131 (a rather unlikely result if everything is calibrated reasonably well, but you get the picture).

Likewise, if we wished to reduce the speed by 1½ stops: one stop decrease = 400 x ½ = 200. Another half-stop: 200 / 1.414 = ASA 141.

Watch the math carefully to avoid adjusting the speed in the wrong direction!

One other caution.  Evaluating the low exposure values is relatively easy, since the thresholds for tone and texture are readily apparent with a loupe. At the high end, however, the dark frames require very careful inspection to discern these thresholds.

In the above discussion, I conform to Ansel Adams’ nomenclature. Exposure values refer to the actual exposure, typically the number of stops, expressed in Arabic numerals, above or below the exposure indicated by the meter. Zones refer to the density of the negative, and are described with Roman numerals. The number of zones may vary depending upon the author. Ansel Adams finally settled on 11 zones (Zone 0 through Zone X), where Zones 0 and X are either pure white or pure black, depending upon whether the reference is to a negative or positive. Ansel’s habit was to put parentheses around "Zone 0" and "Zone X," while other writers may speak of just nine zones, I through IX. (See The Negative, by Ansel Adams, Little Brown and Company, 1981.)

TRI-X (ASA 400, Old) FILM EXPOSURE TEST

Leica M6 w/50mm Elmar 2.8 – Normal Development (7½ min. HC-110)