David Pogue's Digital Photography: The Missing Manual

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iPhoto and Picasa are astonishingly alike. They have pretty much the same features, just tucked in different places and sometimes called different things.Each program does, however, have some editing strengths of its own.

iPhoto: The Histogram

Plenty of photos are ready for prime time after only a single click on the Enhance button. The beauty of iPhoto's Adjust panel, though, is that it permits gradations of the changes that that button makes. If a photo looks too dark and murky, you can bring details out of the shadows without blowing out the highlights. If snow looks too bluish, you can de-blue it.

Best of all, iPhoto offers a true histogram, a self-updating visual representation of the dark and light tones that make up your photograph, just like the one on an SLR (see The Histogram). It's a tool that's beloved by photographers. (Picasa has a histogram, too—click the tiny beanie icon in the lower-right corner of the editing window. But it's for display only; it doesn't have any adjustment handles.)

Note

The iPhoto histogram displays three superimposed graphs at once. These layers—red, green, and blue—represent the three "channels" of a color photo. When you make adjustments to a photo's brightness values—for example, when you drag the Exposure slider—you'll see the graphs of all three channels move in unison. When you make color adjustments using, say, the Temperature slider, those individual channels move in different directions.

If the mountains of your graph ...