David Pogue's Digital Photography: The Missing Manual

Boosting Contrast

Contrast is the difference between the darkest and lightest tones in your picture. If you could see the photo's histogram (The Histogram), you'd see that increasing the contrast stretches out the histogram's shape, creating darker blacks and brighter whites. When you decrease the contrast, you scrunch the histogram inward, shortening the distance between the dark and light endpoints. Since the image data now resides in the middle area of the graph, the overall tones in the picture are duller. Photographers might call this look "flat" or "muddy."

• iPhoto. Open a photo for editing. Click the Adjust button on the toolbar (or press the letter A key) to make the Adjust panel appear. Drag the Contrast slider and watch the effect on the photo.

  Tip

  iPhoto Tip: You can create a similar effect by dragging the endpoints of the histogram (Juicy Bonus Features) inward.

  Which tool is better for fixing contrast? If the histogram data is centered in the middle of the graph, then Contrast is the easier adjustment because it pushes the data outward evenly. But if the histogram data is skewed to one side or the other, then Levels is the better choice, because you can adjust the highlights and shadow areas independently.

• Picasa. Open a photo for editing. Click the Tuning tab on the Editing panel.

  In this case, you can adjust both sides of the contrast equation independently: The Highlights slider makes the bright parts brighter, and the Shadow slider makes the dark parts darker.