5.1 HOW YOUR METER WORKS

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All light meters, including the one built into your digital camera, operate on the same general principles. A light-sensitive photocell regulates the amount of electricity flowing in the metering system. As the intensity of the light reflected from the subject changes, the amount of electricity flowing through the photocell’s circuits changes and is used by the autoexposure system to calculate and set the shutter speed and aperture.

Your camera’s meter measures light reflecting from the part of the scene shown in the viewfinder or on the LCD panel. The coverage of the meter (the amount of the scene that it includes in its reading) changes just as your viewfinder image changes, when you change your distance relative to the scene or when you zoom the lens. Suppose you move close or zoom in and see in your viewfinder only a detail in the scene, one that is darker or lighter than other objects nearby: the suggested aperture and shutter speed settings will be different than if you meter the scene overall from farther away.

Meter Averaging and Middle Gray

Your exposure meter doesn’t "see" a scene the same way you see it. Its view is much like yours would be if you were looking through a piece of frosted glass.

Your meter sees scenes as if it were looking at them through a piece of frosted glass. It doesn’t see details, just averages.

Where you see a black and white checkerboard (left), your camera sees only an average gray (right).

Every scene you photograph is something like a checker board (left), but even more complex. Portions of it are pure black, pure white, and every possible tone in between. Regardless of the elements making up the scene, you camera’s meter can average and measure brightness only.

The exposure meter and exposure control system in an automatic camera can’t think. They do exactly what they are designed to do and they are designed to do only one thing. Regardless of the scene, its subject matter, color, brightness, or composition, the meter measures the overall amount of light reflecting from the scene. Since the light meter measures only brightness (how light or dark the scene is) and not color, the automatic exposure system then calculates and sets the aperture and the shutter speed to render this level of light as "middle gray" in the photograph. Most of the time this works very well because most scenes have an overall reflectance that average out to middle gray. But some scenes and situations don’t average out to middle gray and that’s when autoexposure will lead you astray.

A continuous spectrum of tones, ranging from pure black at one end to pure white at the other is contained in most scenes. In simple terms, this continuous scale can be thought of as dividing into a series of individual tones called a gray scale. Each of the tones in this scale has received 1 stop more exposure than the next darkest tone in the series, and one stop less exposure than the next lightest tone. The tone in the middle is called middle gray. A subject uniformly of this tone reflects exactly 18% of the light falling on it.

The gray scale is a series of steps reflecting different levels of brightness.

When you photograph a subject with an overall tone of middle gray, your camera’s autoexposure system will set an exposure so that the subject will appear in the final image as middle gray. When you photograph subjects that have an overall tone lighter or darker then middle gray they will also be middle gray in the final image and therefore look too light or dark. As a result, if you photograph first a white card, then a gray card, and third a black card, and each completely fills the viewfinder frame when the exposure is calculated, each of the cards will be middle gray in the captured image.

To make scenes that don’t average out to middle gray appear in an image the way they appear in real life, you have to use exposure compensation or some other form of exposure control to lighten or darken the picture.

Types of Metering

All parts of a scene are usually not equally important when determining the best exposure to use. In a landscape, for instance, the exposure of the foreground is usually more important than the exposure of the sky. For this reason some cameras offer more than one metering method. The choices might include the following:

• Matrix metering divides the image area into a grid and compares the measurements against a library of typical compositions to select the best possible exposure for the scene.
• Center-weighted meters the entire scene but assigns the most importance to the center quarter of the frame where the most important objects usually are located.
• Bottom-weighted meters the entire scene but assigns the most importance to the bottom of the frame where the most important objects usually are located.
• Spot evaluates only the area within a small area in the middle of the viewfinder. This allows you to meter just a specific part of the scene instead of relying on an average reading. This mode is ideal when photographing a subject against a bright or dark background.

Meter weighting can cause a few problems. For instance, a dark object located off center against a very light background may not be exposed properly because it is not located in the area the meter is emphasizing. Or, in some cases, holding the camera vertically may give undue emphasis to one side of the scene. These occasions are uncommon, but when they occur you can ensure accurate readings and exposure settings by metering the subject from close-up. The camera settings can then be overridden if necessary to produce a well-exposed photograph.

In this image, automatic exposure worked well because the scene averages out to middle gray.