The camera's metering system will sometimes determine the wrong exposure value needed to correctly expose the image. This can be corrected by the "EV Compensation" feature found in prosumer and professional cameras. Typically the EV compensation ranges from -2.0 EV to +2.0 EV with adjustments in steps of 0.5 or 0.3 EV. Some digital SLRs have wider EV compensation ranges, e.g. from -5.0 EV to +5.0 EV.
It is important to understand that increasing the EV compensation by 1 is equivalent to reducing EV by 1 and will therefore double the amount of light. For instance if the camera's automatic mode determined you should be using an aperture of f/8 and a shutterspeed of 1/125s at ISO 100 (13 EV) and the resulting image appears underexposed (e.g. by looking at the histogram), applying a +1.0 EV exposure compensation will cause the camera to use a shutterspeed of 1/60s or an aperture of f/5.6 to allow for more light (12 EV).
Of course, as you become more familiar with your camera's metering system, you can already apply an EV compensation before the shooting. For instance if your camera tends to clip highlights and you are shooting a scene with bright clouds, you may want to set the EV compensation to -0.3 or -0.7 EV.
This article is written by Vincent Bockaert,
author of The 123 of digital imaging Interactive Learning Suite
Click here to visit 123di.com