Edit Specific Areas of an Image
One feature missing from iPhoto on the Mac is the ability to apply adjustments to specific areas of a photo, instead of adjusting the entire image. This option is helpful when you want to change just a portion of the shot; for example, lightening something in the foreground without blowing out the background exposure.
In iPhoto's editing mode, select the Brushes tool and tap the type of adjustment you want. Then simply drag to "paint" the areas you want to change. Try zooming in by pinching outward with two fingers, or dragging with two fingers to reposition the image onscreen. It's also helpful to turn on the Detect Edges feature to let iPhoto help keep your strokes limited to areas with distinct lines.
For a better sense of what's being affected as you edit, tap the Settings button (the gear icon) and enable the Show Strokes option (see Figure 4).
Figure 4 Show brush strokes to help see where the effect will be applied.