Section 6.12. Exposure

6.12. Exposure

Most of the sliders in the Adjust palette affect the histogram in some way. But where do you begin?

Here's a general suggestion: Make exposure adjustments first. In the simplest terms, the Exposure slider makes your picture lighter when you move it to the right and darker when you move it to the left.

Its effects differ slightly depending on which file format a photo has:

• When you're editing JPEG graphics (that is, most photos from most cameras ), the Exposure slider primarily affects the middle tones of a photo (as opposed to the brightest highlights and darkest shadows). If you're used to advanced programs like Photoshop, you may recognize this effect as a relative of Photoshop's gamma controls. (Gamma refers to the middle tones in a picture.)

• When you're working with RAW files, however (Section 4.2.6), Exposure is even more interesting. It actually changes the way iPhoto interprets the dark and light information that your camera recorded when it took the picture. A photographer might say that it's like changing the ISO setting before snapping the pictureexcept that now you can make this kind of change long after you snapped the shutter.

  The Exposure slider demonstrates one of the advantages of the RAW format. In a RAW file, iPhoto has a lot more image information to work with than in a JPEG file. As a result, you can make exposure adjustments without sacrificing the overall quality of the photograph.

Watch the data on the histogram as you move the Exposure slider. Make sure you don't wind up shoving any of the "mountain peaks" beyond the edges of the Histogram box. If that happens, you're discarding precious image data; when you print, you'll see a loss of detail in the darks and lights.

The first step in fine-tuning a photo, then, is to drag the Exposure slider until the middle tones of the picture look acceptable to you (Figure 6-10). You can't add details that simply aren't there, but brightening a dark shadowy image, or deepening the contrast on a washed-out image, can coax out elements that were barely visible in the original photo.

If the dark and light areas aren't yet perfect, don't worry; you'll improve those areas next with the Levels control.