Camera's light meter bases all of its exposure settings on 18% gray, an "average" tone somewhere in the middle between light and darkness. This is great for a majority of picture taking situations and is the reason the light meter is calibrated in this fashion. Understanding this gives us the ability to make an educated decision about whether to go with our camera's suggested exposure or to override it.
Subjects that are lighter or darker than "average" fool cameras and ultimately lead to exposure errors that can ruin even the most well composed photograph. Always consider the tone of the subject!
Exposure Strategies
Look for an average tone
If you find you find an average tone that is in the same light as your subject, take a meter reading from it and lock that reading in your camera then compensate the shot to include your subject. Fortunately for the outdoor photographer average tones abound: tree trunk, most rocks, green foliage and even most skin tones qualify.
Use a gray card
Available at most camera store, the gray card is just that, a card that is coated with a calibrated 18% gray tone often available in a variety of sizes. Take your meter reading off of this card but be sure that the card is in the same light as your subject. For obvious reasons, this is not always an option when photographing wildlife or other elusive subjects.
Use exposure compensation
Most cameras have this feature which allows you to adjust the camera's built-in light meter reading. For subjects lighter than average; exposure by setting the dial to plus 1/2 or plus 1 stop (or more depending on the subject). Darker than average subjects require a minus setting (some cameras have a chart that goes from -2 stops to +2 stops with intervals of a 1/2 or 1/3 stop).
Bracket
Extra insurance for tricky lighting situations and subjects. Bracketing your exposure means shooting over and under the exposure setting recommended by the camera (or the one you set manually). This can be crucial when shooting slide film where even minor exposure errors can result in an unusable image.
Extracted from an article written by Justin W. Moore
