For many years, photographers shooting transparency (slide) film were taught to “always expose for bright, detailed highlights.” This rule is even more important for images created using a digital camera. With digital cameras, the best exposure is always the one that records the brightest possible highlights with meaningful detail. Underexposed digital images exhibit far more camera noise and have far less tonal variation than images that are properly exposed.
The digital camera’s sensor does not see the world the way our eyes do. With our eyes, brightness and tonal variation are linear, but for the camera this scale is logarithmic. Basically, this means that the camera can record lots of tonal variation at the brightest end of the spectrum but it captures very little tonal variation at the darker end.
Now you have to be careful here because the line between empty, blown-out highlights and bright, detailed highlights is a very fine one. For maximum image quality you must expose so that your highlights are as bright as possible.
Image enhancement programs, like Adobe Photoshop Lightroom and Adobe Photoshop CS4, can do amazing things to polish up your images, but the old rule of “garbage in, garbage out” still applies.
Making tones darker with sophisticated image enhancement software is no problem, but brightening up an image will always make it appear to have more noise. There is simply no substitute for nailing the exposure while you are out in the field shooting.
I suggest reading Real World Camera Raw with Adobe Photoshop CS4 for a far more detailed explanation of this critical topic.
You can also find more information on the need for proper exposure with a digital camera in this online article: