Automatic Exposure can be useful in situations such as those that happen too quickly to use the manual controls. However, automatic exposure should not be relied upon for all shooting situations. Some lighting conditions can lead to improper exposure. In most cases proper exposure will provide detail in both the highlights and shadows with pure white only in the specular reflections and pure black only in the deepest shadows.
Your camera’s light meter will measure how light or dark the overall scene is and calculate the exposure assuming everything should reflect a middle gray. This middle gray can be described as an18% gray. The reason that the automatic light metering works at all is that many scenes have a wide range of values that can average somewhere close to a middle gray. However if shooting very light scenes, such as snow covered mountains, the automatic exposure will adjust to render the snow as middle gray. A very dark scene, such as a dark colored car on black pavement, will be captured too bright because the camera will adjust to bring the overall dark values to an average of 18% gray.
One option for metering is to use a hand-held incident light meter. This type of light meter is placed at the subject and pointed toward the camera. It will measure the light that is falling on the subject. This avoids the concern of averaging all of the different levels of reflected light that can be affected but the lightness or darkness of the subject itself.
Another option is to use an 18% grey card with your camera’s light meter. To do so, hold the card in the same light as your scene. Fill the frame of your viewfinder, and hold the shutter button halfway down to measure the light. Use your camera’s exposure lock or manually set the camera to the proper exposure for the card and use this exposure setting for all shots taken in this environment. X-Rite makes the 4x7 inch pocket size Munsell ColorChecker 18% Grey target for this use. This card is produced the same scientifically engineered process that is used to produce all of the color checker targets to provide a precise uniform surface that is also spectrally neutral.