If you've done much study of Photoshop, you've probably repeatedly encountered the maxim that you should never use Brightness/Contrast to perform tonal corrections. Or perhaps you've tried using it on your own and been frustrated with the results.
The problem with Brightness/Contrast is that it's never been very intelligent about preserving the black and white points in your image. In CS2, for example, if you use Brightness/Contrast to apply a brightness adjustment to a grayscale ramp, it brightens everything, so you no longer have black tones (Figure 3.9).
Figure 3.9 If you use Brightness/Contrast in CS2 to brighten a gray ramp, you end up with a ramp that has no real black and blown-out whites.
Usually, when you brighten an image, you're concerned mostly about brightening the midtones. In CS3, Brightness/Contrast will do a more intelligent job of brightening your image without compromising your shadows and blowing out your highlights. The same edit applied in CS3 yields the result shown in Figure 3.10.
Figure 3.10 In CS3, Brightness/Contrast does a much better job of brightening an image without compromising its blacks.
Similarly, the Contrast slider in the CS2 Brightness/Contrast could wantonly wreck both your black and white points (Figure 3.11). In CS3, Adobe fixed this problem, so the same contrast adjustment yields a healthier result (Figure 3.12).
Figure 3.11 Lowering the contrast with Brightness/Contrast in CS2 moves both the white and black points.
Figure 3.12 In CS3, the same contrast-reduction operation reduces contrast without compromising black and white points.
The new Brightness/Contrast won't replace your current Levels or Curves use, but it's a handy tool to have around, especially for beginning users who aren't comfortable with those advanced options.