In certain lighting situations, your camera’s auto-exposure capabilities will be limited. To put it bluntly, in some scenarios your camera is going to get it wrong.
One of the main offenders in the “fool your camera’s exposure meter” arena is backlighting. These are situations where the light behind the subject is much brighter than the frontal light on the subject. Common backlighting situations include shooting toward a window or another bright light source indoors, or when outside shooting in the direction of the sun. Any situation where the backlighting overpowers the light from the front is going to fool your camera’s meter. And it’s going to create an image that’s correctly exposed for the bright background, with the subject in the foreground a total silhouette.
To try to compensate, you could change your metering mode to centerweighted or, even better, spot metering. The latter will ideally meter from the silhouetted subject alone and better ignore the bright background. Still, though, depending on the size and position of the dark subject against the bright background, it may be impossible to make an automatic exposure that’s right on. That’s where exposure compensation comes in.
To use exposure compensation, look for the Auto Exposure Compensation dial on the camera menu. Here you can activate exposure compensation and dictate whether the camera over- or underexposes, and by how many stops. Better still, though, is that on many cameras you can adjust the exposure compensation without entering the menus. On many cameras, the top-of-body LCD will display a small exposure meter, and by turning one of the dials (on my Canon EOS 5D Mark III, it’s the thumb dial on the back of the camera), you can set the exposure compensation to over- or underexpose. It’s a quick and easy way to account for the environment in which you’re shooting to ensure that, no matter how strong the backlight, your exposures will be more accurate.
For a typical backlighting situation, I would adjust the auto-exposure compensation to +2. That way the camera will automatically overexpose by two stops from whatever it determines to be the appropriate exposure. If this amount of compensation is too much or too little, a quick glance at the LCD and another spin of the dial will help to hone in on exactly how much compensation is necessary in a given lighting scenario.
Another option is to try combining auto-exposure bracketing with exposure compensation. Let’s say you’re unsure of what the correct exposure will be for a given lighting scenario. Take your best guess—let’s say it’s 1.5 stops—and then apply a half-stop increment to the auto-exposure bracketing system of many cameras. You would first dial in exposure compensation to -1.5. Then, using auto-exposure bracketing, you would program the camera to take three exposures for every press of the button: one at what it believes to be the correct exposure, as well as one exposure a half-stop over (for a total overexposure of two stops) and a half-stop under (for a total overexposure of just one stop). This way you can bracket quickly and efficiently to ensure you don’t miss a single frame.