All exposure systems, including the one built into your digital camera, operate on the same general principles. A meter continuously measures the light reflecting from the subject and uses this measurement when you press the shutter button halfway down 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 monitor. 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 to the scene or change the focal length of 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 for the detail than for the overall scene.
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.
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.
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 only brightness, or how light or dark the scene is. The automatic exposure system then calculates and sets the aperture and the shutter speed to render this level of brightness as "middle gray" in the photograph. Most of the time this works very well because most scenes have an overall brightness that averages 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. Let's see why.
Most scenes contain a continuous spectrum of tones, ranging from pure black at one end to pure white at the other—the gray scale. When shooting JPEGs there are 256 tones in the scale (28) and when shooting RAW images there are 65,536 (216). The tone in the middle of these ranges is middle gray and reflects exactly 18% of the light falling on it.
The gray scale captured
in an image is a range of tones from pure black to pure white.
When you photograph a subject, your camera's autoexposure system sets the exposure so the average brightness in the image is middle gray regardless of the scene's actual brightness. As a result, when you photograph a scene with an average brightness lighter or darker than middle gray, it will be too dark or light in the image. For example, if you photograph a white card, a gray card, and a black card, and each completely fills the viewfinder when the exposure is calculated, each of the cards will be middle gray in the captured image.
Because of the way your exposure system
works, if you photograph a white card, a gray card, and a black card, the exposure system sets
the camera to capture each as middle gray.
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 most common choices include the following:
- Matrix, sometimes called evaluative, metering divides the image area into a
grid and compares the measurements against a library of typical scenes to select the best possible exposure for the scene.
- Center-weighted meters the entire scene but assigns the most importance to
the center of the frame where the most important objects usually are located.
- Spot, or slightly larger partial metering, evaluates only a small area in the
middle of the scene. 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 use exposure lock or exposure compensation to get a good exposure.
When you fill the viewfinder with a gray card and press the shutter button halfway down, your camera will indicate the best exposure regardless of how light or dark the scene is.