What a meter assumes
A light meter measures how bright something is and recommends an exposure. But it has no idea what it's looking at. A white wall and a black cat in the same light reflect very different amounts of it, and the meter can't tell a bright white thing from a gray thing in brighter light. So it makes an assumption: whatever it reads, it treats as a middle tone, and recommends the exposure that will make it come out middle gray. That's the tone of a standard gray card, which reflects about eighteen percent of the light falling on it.
Most of the time that's close enough, because most scenes, averaged, come out near middle gray. It goes wrong when the scene isn't average: a snowfield comes out gray and dingy, a black dog against a dark wall comes out gray and washed out. The meter isn't broken. It's answering the only question it can.
Three ways to read the light
- Averaging (reflected). The meter reads the whole scene and averages it. Fast and usually fine, easily fooled by big bright or dark areas.
- Spot. The meter reads one small area: a face, a rock, a patch of sky. Now you choose what the meter treats as middle gray, which is the basis of everything below.
- Incident. The meter faces the light, from the subject's position, and measures the light falling on it rather than reflected off it. A white shirt and a black coat get the same reading, which is what you want in a studio or for portraits.
GlassMeter does all three, and it knows which film is loaded, so its finder shows roughly what that film will make of the scene.
Eleven zones
Around 1940, at the Art Center School in Los Angeles, Ansel Adams and Fred Archer worked out a way to think about tones in a picture before you take it. They divided the range from black to white into eleven zones, numbered with Roman numerals from 0 to X. Each zone is one stop, a doubling of light, brighter than the one below it.
The zones, each a stop apart, as tones on a print.| Zone | What it looks like |
|---|---|
| 0, I | Pure black, and nearly black, with no texture |
| II | The first hint of texture in a deep shadow |
| III | Dark materials and shadows with full detail: the darkest tone you usually want to read clearly |
| IV | Dark foliage, dark stone, shadow on a face |
| V | Middle gray: the gray card, weathered wood, a clear northern sky |
| VI | Average light skin in sun, light stone |
| VII | Very light skin, light gray objects, snow in raking light: the brightest tone with full texture |
| VIII | White with only a little texture |
| IX, X | Near white, and the pure white of the paper |
Place one tone, and the rest fall
Here's the idea that makes the system useful. Spot-meter one important area and the meter puts it on Zone V. But you decide where it belongs. If it's a white shirt you want to keep textured, it belongs on Zone VII, so you give two stops more exposure than the meter says. That's called placing a tone.
Once one tone is placed, every other tone in the scene falls where it falls, at the same number of stops above or below it as it is in real life. So spot-meter the deepest shadow where you want detail, and the brightest highlight where you want texture, and count the stops between them. That's the scene's range.
Then compare it with your film. Color negative and black-and-white film hold a wide range, nine stops or more. Slide film holds only five or six, and its highlights burn out first. If the scene's range is wider than the film's, you can't keep everything: choose what matters, or change the light, or come back at a different hour.
With negative film, protect the shadows: give enough exposure that your darkest important tone lands on Zone III, and the highlights will usually look after themselves. With slide film it's the reverse: protect the highlights, because once they're clear film there's nothing to print. "Expose for the shadows" is the negative-film half of an old rule; the other half, "develop for the highlights", is about adjusting development time in the darkroom, which Adams also controlled zone by zone.
Long exposures
At very long exposures, of a second or more, film stops obeying the simple rule that doubling the time doubles the exposure. It gets less sensitive the longer the exposure runs, so you have to give extra time beyond what the meter says. This is called reciprocity failure, and every film has its own correction. GlassMeter allows for it with the film you've loaded, which is one less thing to look up at dusk.
On the bench, and in the field
The bench lessons in this module put each of these ideas into your hands: spot-metering the gray card, measuring the white and black cards on the zone ruler, placing the white card on Zone VII, switching film, and choosing which setting to lock. In the fieldwork you'll use them where they matter: on snow and granite in Yosemite, and on a still life with a full range of tones in the studio.
Check yourself: you spot-meter a sunlit snowbank and the meter says f/16 at 1/250. You want the snow white with texture. What do you set?
A scene's deepest important shadow and its brightest important highlight are eight stops apart. Which film can hold it all?
Sources
- Ansel Adams, The Negative (Little, Brown; New York Graphic Society, 1981), the standard account of the Zone System and its zone descriptions.
- The Ansel Adams Gallery and the Ansel Adams Publishing Rights Trust, biography: anseladams.com.