This is the look of a typical negative under common lighting conditions. You can see density on the negative throughout the image, with no area so dense that it is impossible—or even very difficult—to print.
Figure 8-1. Negative of Stairway to Cittá Alta, Bergamo, Italy
In the 1940s, Fred Archer and Ansel Adams first developed the zone system of exposure as a means of making an exposure in a scientifically accurate manner. It is fully applicable to both black-and-white and color. The explanation that follows is expressed in terms of black-and-white tonalities; for readers interested exclusively in color, please think in terms of how these tonalities translate into colors on the color sphere (Chapter 6). After the explanation is complete in terms of black-and-white, the chapter concludes with specifics on how the zone system applies directly to color.
I’ll explain the first part of the zone system—exposure of the film—by meshing two concepts: first, how film reacts to incoming light when the shutter is open; and second, how light meters give an exposure reading.
A Brief Overview
There are only two things that can be done with a negative or transparency. First, it can be exposed; second, it can be developed. That’s it! There’s nothing more to it.
When you expose film, you can use greater or lesser amounts of exposure. That’s the limit of your control (except for changes brought about by filtration, which can alter the relative brightness of objects). In the next chapter, you’ll see that when you develop black-and-white film, you can reduce the scene’s inherent contrast (by reducing the normal development time); maintain it (by developing for the normal development time); or increase it (by extending the normal development time). Development of color film will not be explored, since it does not allow significant changes in contrast.
Film’s Response to Light: Building the Zone System
What happens to a negative when it is exposed to light? The camera lens focuses light from the scene onto the film, and it is sensitized in proportion to the amount of light that hits it. Those areas receiving the most light are sensitized to the greatest extent; those receiving the least light are sensitized to the least extent. Later, when the negative is developed, the areas sensitized to the greatest extent become the darkest, or “densest”, portions of the negative; the areas least sensitized become the lightest, or “thinnest”, portions of the negative.
Some portions of the negative may receive so little light that they are not sensitized at all. The unexposed edges of the film fall into this category, as do all portions of the film within the exposed area that are “below threshold”, i.e., areas that show no density in the developed negative except for the negative material itself, the so-called “film base fog” density. We will call all such subthreshold areas Zone 0.
There may be an area within the image space receiving just enough light to produce visible density in the developed negative, the so-called “threshold level”. We will call this developed density Zone 1. Zone 1 is an easily perceptible density, yet very thin. If you look at a portion of a negative and have to scratch your chin wondering whether or not there is density, we won’t consider it Zone 1. Only if the density is obvious, although light (thin), will we call it Zone 1.
If we were to double that exposure, the same area would receive twice the sensitization and the developed negative would show greater density. In a “normally” developed negative, we will call the density of that area Zone 2. (This is the temporary gap in the explanation referred to at the start of this chapter; the exact definition of “normal development” will be explained in Chapter 9. Here, it’s not critical to be specific about the term, so please read on.)
Now, if we again double the exposure, the Zone 2 area would receive double the sensitization, which we call Zone 3 sensitization, followed by greater density in the developed negative. Continuing to double the exposure, we obtain Zones 4, 5, 6, 7, and onward. Each successive zone represents a doubling of exposure from the previous zone and a corresponding increase in developed density (Figure 8-1).
Of course, since each zone above the Zone 1 threshold represents a doubling of exposure, those zones can be reached by successively doubling the amount of light within the scene while using the same exposure. Thus, we can obtain progressively higher zones either by doubling exposures via the lens apertures (the f/stops) or shutter speed, or by doubling the ambient light levels.
Zone 1 density—the threshold density—is the same on all negatives. It is always thin but easily perceived. Since all higher zones are obtained by successively doubling either exposure or light levels, Zone 2 density is the same on all negatives, as is Zone 3 density, Zone 4 density, Zone 5 density, and so on. Keep this in mind: Zone densities are the same for all negatives.
You can continue doubling the exposure, thereby increasing the negative density of the developed negative—but there is a limit. Eventually you’ll reach a density level that cannot be increased; the negative will “max out.” It turns out that on nearly all black-and-white films, the maximum density usually goes up to Zone 16 or even 18! Most photographers think it goes only to Zone 10, but in fact, it goes well beyond that. Zone 10 is just above the halfway mark on the density scale. When you fully comprehend how much the negative can encompass and learn how to control that immense range (which will become clear in the next two chapters), you can then appreciate how extraordinarily flexible and powerful photography can be.
Translating Negative Densities to Print Tonalities
Negative densities mean nothing until a print is made. What happens to those densities when they are printed, when the negative is placed in the enlarger and exposed onto a normal grade enlarging paper?
We begin this determination by defining a “standard exposure”. A standard exposure is the minimum length of exposure time through the enlarger required to achieve a maximum black from the Zone 0 portion of the negative. Suppose we require a 20-second exposure of the negative under the enlarger to achieve a maximum black on your enlarging paper from Zone 0 (i.e., the clearest portion of the negative). Any exposure greater than 20 seconds will produce a black that is no blacker; anything less fails to achieve maximum black. At best, any exposure less than 20 seconds will yield a very dark gray, but not black. Using the 20-second exposure on a normal contrast paper (generally a grade 2 paper or a mid-contrast level on variable contrast paper), you get the table shown below.
Note
On nearly all black-and-white films, the maximum density is achieved high into the teens: usually it goes up to about Zone 16 or even 18! Most photographers think it goes only to about Zone 10, but in fact, it goes well beyond that.
At first glance the table seems to indicate that Zone 1 has no real meaning, since it is essentially as black as Zone 0 in the print. But remember, Zone 1 represents the amount of light needed to sensitize the film just enough to produce visible density when the negative is developed, i.e., the threshold exposure. From this level of exposure, we begin the doubling of exposures (or light levels) to obtain the successive zones. For this reason, Zone 1 is especially important—though it is essentially unimportant for printing purposes. Because it has so little density, it blocks almost none of the light coming from the enlarger, so it prints virtually as black as Zone 0.
Surprisingly, the print reaches pure white by Zone 9, although the negative continues to show separations up to Zones 15, 16, 17, and even 18 (beyond the table)! These extremely dense zones will print as pure white in a “straight print,” but they are very usable. (A straight print is one with a single exposure; no additional exposure, known as “burning”, is used on any portion of the image.) In Chapter 9 you’ll see how those zones can be controlled in development, and in Chapter 10 you’ll see further methods of making the higher zones visible, meaningful, and essential in the print. They should not be excluded from consideration in exposure. Those zones are part of the zone system, and you’ll soon see how to fully use them. At that point, your artistic and creative potential will rocket upward immeasurably (Figure 8-1).