iPhoto 6 for Mac OS X

The Levels slider under the histogram gives you two independent controls for adjusting the black and white points, which define the pixels that should be considered pure black and those that should be considered pure white.

To adjust the black and white points of a photo:

  • In the Adjust panel, drag the left Levels slider to the right to set the black point.

  • Drag the right Levels slider to the left to set the white point.

What the Levels sliders do

If, when you look at a photo's histogram, you see a blank space between the end of the mountain ranges and the black or white points (Figure 4.53), it's often safe to move those sliders toward the middle of the histogram, which is conceptually the same as grabbing the edges of the mountain ranges and pulling them out to the edges of the histogram. In Figure 4.54, I've moved the black point to the right to deepen the shadows; it's redefining what was a dark gray as total black. In Figure 4.55, I've moved the white point to the left to set a new value for what should be considered white, thus making the water glow a bit.

Figure 4.53. All the pixels in this image are huddled together in the middle of the histogram.

Figure 4.54. By redefining the black point, I give the picture a little more depth and darkness.

Figure 4.55. Then, resetting the white point brings up the luminosity a bit.

Tip

  • It's best to use the Exposure and Levels sliders in favor of the Brightness and Contrast sliders. Exposure can increase and decrease brightness without blowing out the image, as would happen with the Brightness slider. And the Levels sliders let you set the black point and the white point independently, which is impossible to do with the Contrast slider.

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