The LCD monitor on the back of the camera is a great tool to use when checking your images for focus and composition, but when it comes to determining proper exposure, using the LCD can be somewhat deceiving. Just like with computer monitors and TV screens, the brightness level on the LCD can be changed, so you may not be seeing the same exposure on the back of your camera that you'll see when you view the image on your computer. It's also possible that LCD brightness is defaulted to the Auto setting, in which case the brightness level will change even more, and will be inconsistent from one photograph to the next.
Now, don't get me wrongI use the image I see on the back of my camera all the time, to help determine the exposure for my photos. It's a really great tool, and I love having a nice big screen to view my photos, but there's one setting that I always make sure is enabled before I preview them: Highlight Alert (see Figure 3).
Figure 3 The Highlight Alert on the Canon 7D is located in the second playback menu tab.
The Highlight Alert setting will show you where the overexposed areas in your images are located (also referred to as being "blown out"). The image preview will blink in areas that are completely white (RGB 255, 255, 255) and have no detail. Sometimes you'll want to have blown-out portions in your photos. One example might be when you're photographing a person isolated on white in a studio setting. But in any other situation, you'll want to keep as much detail as possible in your images. The highlight alert makes you more aware of the exposures in your images, allowing you to adjust your settings accordingly.