November 12, 2007
Bracketing Your Exposure
Why would anyone want to dial in an over or under exposure on purpose?
Because there are certain situations where the light meter of your digital camera can be fooled.
As an example, say you are taking a scene where there is an abundance of light around your main subject (for example, at the beach on a sunny day, or surrounded by snow). In this case, using Weighted-Average metering or Multi-Pattern metering, your camera might be ‘deceived’ by the abundance of light and expose for it by closing down the aperture and/or using a faster shuter speed (assuming ISO is constant), with the result that the main subject is under-exposed. By dialing in a positive exposure compensation, you are making sure that your main subject is correctly exposed — though the surroundings would be overexposed.
Another example would be going the other extreme where the surrounding is too dark, and the camera exposes for the lack of light by either opening up the aperture and/or using a slower shutter speed (assuming ISO is constant), then the main subject is over-exposed. By dialing in a negative exposure compensation, you would in fact be under-exposing the surroundings, but properly exposing the main subject.
Bracketing The Exposure
These are only two of the many situations where dialing in a positive or negative exposure compensation makes sense. In fact, many photographers will bracket their exposure when taking an important shot by taking one shot using the camera’s measured exposure, a second one at positive exposure compensation, and a third one at negative exposure compensation. This is called "bracketing your exposure" or simply "bracketing" and is a good technique to use whenever you need to nail a particularly difficult-to-expose scene.
Automatic Exposure Bracketing (AEB)
Instead of manually bracketing and taking the 3 shots as above, many digital cameras now include Automatic Exposure Bracketing (AEB) where the camera will take the three shots with one press of the shutter release button: one at the camera measured exposure, a second at a negative exposure compensation (usually -1/3 EV, though some cameras allow you to specify the amount), and a third at a positive exposure compensation (usually +1/3 EV, though some cameras allow you to specify the amount).