How a Light Meter Works
Original Chinese version: 測光表的工作原理
# How a Light Meter Works
Light meters can be divided into two types: incident-light meters and reflected-light meters. Here, we will not discuss incident-light meters, which are usually designed as handheld instruments. A reflected-light meter works in a way that is similar to the human eye: it directly measures the intensity of light reflected from the area being measured. That may sound abstract, but it simply means this: when we see black, we say it is dark; when we see white, we say it is light.
But we need to look a little deeper at how a light meter defines brightness and darkness. Pick up the camera at hand and point it at a light-colored object that takes up at least one third of the viewfinder. The camera will give you an EV reading, expressed through a shutter speed and aperture combination. Then point the camera at a darker object that also takes up at least one third of the viewfinder. You will get another EV reading. If you are not sure what to choose, you can follow the two examples below and try it yourself.
Dark subject occupying a large portion of the frame
Light subject occupying a large portion of the frame
You should now have two EV readings: one higher, one lower. The light-colored subject gives a higher EV, while the dark-colored subject gives a lower EV. In other words, a lower EV means the scene is darker, and a higher EV means the scene is brighter. This is how a light meter tells the photographer about the light conditions of the scene.
When you step outside and choose a scene to photograph, that scene will usually contain a mixture of several EV values. For example, in the scene below:
Photo: Xiaopuzi
The tree shadow on the left gives one EV reading. The sky gives another. The tiled pavement gives another. The wall gives another, though it is less than half a stop away from the tiled pavement. The backlit brick wall gives another. The camera itself gives yet another, though it may be closer to the tree shadow than to the brick wall, because the camera body is darker in tone.
But this is not all a light meter does. Suppose we simplify the world into only three levels of brightness, with no continuous gradation between them: black, gray, and white. In that simplified world, the light meter has only one job. Given an EV reading, it exposes the area it sees as gray on film. So if the camera frames something white, and you shoot according to the EV given by the camera, the white will not look that white. If the camera frames something black, the black will not look that black.
For photographers, this is a problem. One purpose of photography is recording. And recording always carries some element of the reality that was present at the scene. This problem does not only appear on film; it happens with phone photography as well. Black pants can come out as gray pants. A fair-skinned princess can be rendered as a gray-skinned girl. This kind of mismatch can sometimes be corrected later, but it is much more convenient to use exposure directly, at the moment of shooting, to bring the photograph closer to the reality we remember and the atmosphere we imagine.
A simple method is exposure compensation. After metering from a dark subject, reduce the EV by one or two stops. After metering from a light subject, increase the EV by one or two stops. By manually adjusting the EV, we correct the camera’s judgment and bring the result back toward what we actually want.
Now let us make the model of the real world a little more complex. Instead of three levels of brightness, we divide brightness into ten levels, with each level separated by one stop. All light meters are designed to render the metered area as Zone V. The meter gives you an EV because it wants to photograph the area it measured as Zone V brightness. But if we believe that the measured area should actually appear as Zone III, then we need to reduce the camera’s EV by two stops. That will render the area as Zone III. This is the full purpose of exposure compensation: to control how light is rendered.
And if we want to control how light is rendered, we must first understand light. But speaking about light can be too abstract. In practice, photographic control is often closer to controlling shadows. How bright should the shadow areas be? That is the question we must answer after observing the scene.
Take the earlier photo of the TLR camera as an example. The twin-lens reflex camera is backlit. So how much non-black detail should remain in the shadowed body of the camera? That is the essential question about light at the moment of shooting. The solution to that question will be worked through in class.
The next article, “M, A, S, P Mode,” will be simpler. But please make sure you understand this article, “How a Light Meter Works.” If you do not understand it, ask. I can explain it in dozens of different ways, and that is necessary. Because if you do not grasp this, I will not be comfortable going straight into the core of the next lesson, and that will only delay the time when you can truly enjoy taking photographs.
