Metering and Middle Gray
Your autoexposure system can't think. It is designed to do only one thing. Regardless of the scene, its subject matter, color, brightness, or composition, the system calculates and sets the aperture and the shutter speed to render the metered part of it 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, especially in desktop photography, don't average out to middle gray and that's when autoexposure will lead you astray. Let's see why.
Many digital desktop setups contain a continuous spectrum of tones, ranging from pure black at one end to pure white at the other. This continuous scale can be thought of as dividing into a series of individual tones called a gray scale. In the gray scale shown here, each of the tones has received 1 stop more exposure than the next darkest tone, 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 having different levels of brightness.
When you photograph a subject with an overall tone of middle gray, your camera's autoexposure system sets the exposure so the subject appears in the final image as middle gray. This gives you an ideal exposure. The problem is that 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. For example, if you photograph a white card, a gray card, and 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 capture scenes that don't average out to middle gray so they appear the way they do in real life, you have to use exposure compensation or some other form of exposure control to lighten or darken the picture.
White, gray, and black cards will all photograph as gray cards unless you use exposure compensation or manual exposure mode.