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Exposure

The Exposure slider is really a white-clipping control, even though it affects the whole tonal range. The big difference between the Exposure slider and the Brightness slider is that the former lets you change the white clipping point, whereas the latter does not. With positive values, the Exposure slider behaves very much like the white input slider in Photoshop's Levels command, clipping levels to white. But since it's operating on linear-gamma data, it tends to be gentler on the midtones and shadows than white clipping in Photoshop on a gamma-corrected image, and it offers much finer control over the white clipping than does the white input slider in Levels on a gamma-corrected image.

With negative values on the Exposure slider, the story is very different. Unlike most raw converters, Camera Raw offers “highlight recovery.” Most raw converters treat all pixels where one channel has clipped at the highlights as white, since they lack complete color information, but Camera Raw can recover a surprising amount of highlight detail even when it exists only in a single channel. It does, however, maintain pure white (that is, clipped in all channels) pixels as white, unlike some other converters that let you turn clipped pixels gray, and then it lowers the gamma to darken the rest of the image, using special algorithms to maintain the color of the non-white pixels. See the sidebar “How Much Highlight Detail Can I Recover?” for more technical details, and see Figure 2-8 for a real-world example of highlight recovery.


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