Photo Tip: The Zone System
in 415 Words
For some reason the Zone System for black and white photography conjures up trepidation among those who don't know what it is, as if it were some arcane and mysterious knowledge that only High Priests are privy to, beyond the ken of ordinary people. This is unfortunate, for it's really pretty simple, as I hope to make clear in a moment. Granted, the execution can take time and diligence, but the concept is straightforward.
Before I launch into my explanation, let me review a few things about black and white photography. When light hits film a reaction occurs in the photosensitive emulsion, such that when immersed in a developer the parts of film hit by light turn black. The more light that hits a patch of the film the blacker that patch will develop. The dark parts of the scene (the "shadows") leave the film relatively clear, white the bright parts of the scene (the "highlights") render the film dark. This tonal reversal of lights-are-dark and darks-are-light is why the developed film is called a "negative."
When making a print from a negative the process repeats itself, undoing the tonal reversal. The clearer parts of the negative -- the shadows -- let more enlarger light hit the photosensitive enlarging paper, which turns dark when developed. The shadows of the scene are restored to the shadows in the print. Similarly for the highlights: the dark parts of the negative block much of the enlarger light, leaving the paper relatively unexposed in those areas, resulting in light tones in the print.
AND NOW: THE ZONE SYSTEM
The development of film is a chemical reaction. The longer the film is in the developer, the more the reaction occurs. This means that the dark parts in the negative will continue to get darker the longer you leave it in the developer. In the relatively clear parts of the negative, however, the reaction soon goes essentially to completion -- the few exposed grains of film have mostly been developed, and longer immersion in the developer brings little additional change.
This means that the film density in the highlight areas -- the dark parts of the negative -- depends both on the original exposure in the camera and on how long one leaves it in the developer. The shadow density, however -- the clearer parts of the negative -- depends mostly on the original exposure, and very little on the development time.
This means that photographers may determine what shadow density they achieve in the negative by choice of exposure in the camera, irrespective of the development time the negative receives. They may independently determine a highlight density by choice of developing time, given the in-camera exposure. The photographer can therefore independently control two tonal values in the negative, one shadow and one highlight.
Put another way, since in-camera exposure and developing time affect the shadows and highlights differently, one has two controls at one's disposal and can therefore pin down any two tonal values in the negative. All the other tonal values will simply fall in place in accordance with the two that are specified. It's sort of like a two-band equalizer on a radio: you can set the bass and the treble the way you like them, and the midrange will be fixed by your bass and treble choices.
This freedom to independently specify the negative density of one shadow and one highlight lets the photographer tailor the exposure/development combination to the scene being photographed and to the look desired in the print. If it's a very high contrast scene -- say, dappled sunlight hitting the understory in a dim forest -- one can opt for less development to bring the highlights down inside a printable range. Or, in a very low contrast scene -- say, a rose in very soft light -- one can give extra development time to stretch highlights away from shadows, increasing contrast and adding more sparkle to the final print.
The idea can be summed up in the classic phrase, "Expose for the shadows, develop for the highlights."
AND NOW: THE DEVIL
That's it for the Zone System. Two controls, two tonal values pinned down. Period. End of story.
The concept is simple. But the devil, as they say, is in the details. Execution of the system takes time and discipline. One must run tests to find out what exposure results in the desired shadow detail, and how much development to give to arrive at a specified highlight value. These tests must be performed for each film/developer combination one wishes to use. There are special techniques, such as water bath development and compensating developers, that can fine-tune the control offered by development time. And since the essence of the technique is to adjust the negative development to match the scene at hand, the Zone System makes most sense when one can develop each negative separately -- in other words, it makes more sense for sheet film photography than for roll-film. (Although there are ways to adapt the system for roll film.)
WHERE FROM HERE?
From this brief introduction you won't know how to implement the Zone System. Entire books are written on implementation, lots of them. But hopefully you get the idea, and now will be able to learn the implementation from books or classes. As books go, I don't think you can do better than Ansel Adams' The Negative, part of his The Camera, The Negative, and The Print trilogy. (The 1981 version, not his earlier set.) Many people also like Phil Davis' Beyond the Zone System. For fundamentals on black and white photography try Henry Horenstein's Beyond Basic Photography.
Oh, and why is it called the "Zone System?" The brightness of the light reflected off a subject is called luminance. Convention has the continuous luminance range labeled by tonal "zones," the way inches label a ruler. Just as a ruler is marked at 0", 1", 2", 3", etc., the illuminance is marked by Zone 0, Zone I, Zone II, Zone III, etc. Each zone has exactly twice or half the illuminance of its neighboring zone. Zone 0 exposure/development results in pure black in the final print, and Zone X exposure/development gives pure white. (In practice one usually chooses an exposure that places an important shadow at Zone II or so.) Just as a distance need not be exactly 1" or 2" or 3" -- it could be 2.25", say -- an illuminance can be anything at all -- Zone II 1/4, for example. The Zone System is the approach that lets you translate subject zones into the print tones that you desire.
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