Photoshop users sometimes assume that the Adobe Camera Raw (ACR) sliders for Exposure and Brightness do pretty much the same thing. After all, they both lighten or darken the photograph.

• Exposure is used to adjust the clipping point for highlights. As you move the slider to the right, the photograph will brighten. As you move it to the left, the photograph will darken. What is happening is that Exposure determines where the lightest pixels will clip (any pixel that bright or brighter in the photograph will be changed to 255). All other tones relative to the new white point -- including the middle tones and shadows -- will be smoothly adjusted.

  

• Brightness focuses its attention on the middle tones. It works similar to Gamma slider (the middle input slider) on the Photoshop Levels command dialog window. When you make a positive Brightness adjustment, the brighter pixels are compressed and the darker tones are expanded. A negative Brightness adjustment reverses the effect on the photographs tones. The highlights are expanded and the shadows are compressed.

  

Exposure makes smoother adjustments to tones, since all of the tones between the new highlight clipping point and the deepest shadows are smoothly mapped. The tonal range is not compressed at one end and expanded at the other, as it is with Brightness setting. Brightness is more likely to damage a photograph, since compressing and expanding tonal ranges can easily lead to visible posterization effects and loss of contrast.

My recommended ACR workflow involves three steps:

1. Adjust Exposure first. Use Exposure to set how light or dark the photograph appears overall.
2. Adjust Recovery, if necessary. If you end up with some stark specular highlights or the photograph has some "burned out" detail, the Recovery slider can compensate.
3. Adjust Brightness, if necessary. Use the brightness slider to fine tune how light or dark the photograph appears.

If you use Exposure and Brightness properly, rather than treating them as interchangable, your photograph will have more "pop" because the Brightness slider is more likely to reduce contrast in either the highlights or the shadows.

When you compress tones in part of the tonal range, such as compressing the highlights with a positive Brightness setting, you lose contrast among those tones. The highlights will be brighter, but they will also have less contrast than they otherwise might if Exposure was used instead. Likewise, when you make a negative Brightness adjustment, the photograph will darken but your shadows will appear flatter than if you used the Exposure slider with a negative value.

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