Exposure and the Histogram
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For me, one of the biggest benefits of digital photography is the instant feedback. Looking at the image on the camera's LCD screen after I have taken it lets me analyze the composition and the exposure. If the exposure is incorrect or the composition could be improved, I can take another photo.
Composition and exposure have another thing in common—it's important to get both right in-camera, as only limited adjustments are possible in post-processing. You may be under the impression that it's easy to adjust exposure in post-processing, and to an extent that's true. You can certainly brighten or darken the image, but it's best to think of this as a fine-tuning process rather than a fix for poor exposure.
Incorrect exposure creates problems in your image that can't be fixed in post-processing. If you overexpose the photo, you may "clip" the highlights. This term simply means that these pixels received too much light during the exposure and haven't recorded any detail. Instead, all you get is white. Because no detail was recorded in the first place, you can't bring it back in post-processing.
On the other hand, if you underexpose the image, you risk increasing the noise levels. Noise occurs in the parts of the photo that received the least light during the exposure—the dark tones. If you underexpose an image and then increase the brightness in post-processing, you increase noise levels in the dark tones. You may also lose detail in these areas.
Histograms
Whereas film photographers have to wait until the film is developed to see whether an exposure was accurate, digital photographers have the advantage of being able to use the histogram. The histogram is the only reliable way to tell whether your exposure was accurate. The camera's LCD screen is misleading; in bright light, the screen is difficult to see, and your photos will look dark. In low light, the screen appears quite bright, and makes your photos seem brighter than they are.
Displaying the histogram is simple. On my EOS 60D, all I have to do is press the playback button to show the last photo taken (as in Figure 1), and then press the Info button twice to display the luminance histogram (see Figure 2). Pressing the Info button again displays the luminance histogram alongside three color histograms for the red, green, and blue channels (as shown in Figure 3). You should check your instruction manual to see how to display the histograms on your camera. Older cameras will show only the luminance histogram—not the color histograms.
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Figure 1 The LCD screen of an EOS 60D. Pressing the playback button once displays the last photo taken.
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Figure 2 Pressing the Info button twice displays the luminance histogram, plus some extra information about the photo.
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Figure 3 Pressing the Info button once more displays the red, green, and blue histograms.