Using the eyedropper tools in the Levels or Curves dialog box
Both the Levels (both Photoshop and ImageReady) and Curves (Photoshop) dialog boxes have eyedropper tools for setting the black point, setting the white point, and adjusting the color balance of an image. Simultaneously, using an eyedropper tool also applies a tonal correction to all the pixels in an image.
Note: The Set Gray Point Eyedropper tool 
is used primarily for color correction and is unavailable when working with grayscale images.
For the best results, don't use the eyedropper tools in images that require a huge adjustment to map a pixel to the maximum highlight or minimum shadow values. Because the eyedropper tools shift the tonal value of all pixels in an image in equal amounts higher or lower, some pixels may get clipped. It's best to make only small adjustments with the eyedropper tools.
Important: Using the eyedropper tools will undo any previous adjustment you've made in Levels or Curves (Photoshop). If you plan to use the eyedropper tools, it's best to use them first, and then tweak your adjustments with the Levels sliders or Curves points (Photoshop).
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