Mastering contrast with Levels
Contrast, or more specifically, a lack of contrast, is often the most noticeable fault in many pictures. This is partly because cameras are designed to capture images with a slightly lower contrast than was actually present in reality – and in doing so, they capture a slightly wider range of tones than if they were recording higher contrast from the get-go.
JPEGs are processed in-camera—which is why, when compared with a RAW file, they will always appear slightly more colorful—but you can always extract more tonal range from a RAW file. It just needs a little more work.
The best tool to begin editing any non-RAW image (such as a JPG, TIFF, PNG, or PSD file) is Levels (Ctrl/Cmd + L or Enhance>Adjust Contrast>Levels).
Levels is used to adjust the tonal distribution in any image. You'll recognize this when you see the histogram – this is the same display that you'd see on your camera's LCD screen...
