Why Compensate the Exposure? There are many situations where we need to compensate the exposure by our SLR Camera. The current cameras have a high accuracy in measuring exposure, exposure can be controlled by the compensation button correcting stop or third stop from underexpose or overexpose.
The question that you will be coming into your mind is: Why should I underexpose or overexpose an accurate measurement made by my camera?
To this question I give you the example of a scenario snowy, our reflex is expecting an average of values that allow to have an exposure based on 18% gray (gray on which all cameras are calibrated) and therefore with the scene is completely white for our camera, then after shooting (suitably exposed by our reflex) the picture will be very dark. We have this issue because for our Camera the entire environment snowy is like a big white sheet, being white above 18% gray to avoid burning the exposure our Camera shift the exposure values to medium gray (gray 18 %) underexposing the scene.
At this point we setting our camera to overexpose the measurement until we come to a histogram pretty neat. As you can see in the photo of the snowy mountain, it took us almost 2 stops (+2 EV) of overexposure to have a scene completely natural.