Getting the exposure right is at least as important when shooting digital as when shooting film
All digital SLRs and many advanced compact cameras on the market today use sophisticated metering systems that take light readings from many different points in the frame, but those automatic interpretations can sometimes give you unsatisfactory results. The information gathered by the built-in meter is sent to a microchip where it’s analyzed and compared to a large database, so the camera can make a recommendation of what it "thinks" the optimum exposure should be. Much of the time, these built-in metering systems deliver excellent results, but they have limitations.
In complex lighting situations, the built-in metering system can lead to a blown exposure. And no amount of Photoshop manipulation is going to rescue the poor tonal definition or lack of detail in the shadows and highlights.
Let’s say you’re photographing forest landscapes in winter and a fresh blanket of snow has fallen. You point your camera at the scene to get an exposure reading, and one of two things may happen.
If you favor the trees, the metering system likely will interpret all the dark foliage to mean the ambient light is low and a longer-than-necessary exposure is needed. The resulting photograph will be washed out, and the snow and the trees will lack the right tonalities. Remember, the image sensor of your camera is only capable of handling a certain range of brightness. Go past that in either direction, and the sensor won’t capture enough image detail in the original exposure to make a good print.
If you frame for the snow, the meter will interpret this as excessive light. This happens because the light meter of your camera is reading reflected light. Even though the snow and the trees are illuminated with the same sunlight, their reflective qualities differ greatly. The metering system doesn’t "know" this, so if you go with an ƒ-stop and shutter speed that the camera tells you is a correct exposure, your image will be very underexposed.