6.10. Exposure

6.9. Introduction to the Histogram

Learning to use the Adjust panel effectively involves learning about its histogram, the colorful little graph at the bottom of the panel.

The histogram is the heart of the Adjust palette. It's a self-updating visual representation of the dark and light tones that make up your photograph. If you've never encountered a histogram before, this all may sound a little complicated. But the Adjust palette's histogram is a terrific tool, and it'll make more sense the more you work with it.

Within each of the superimposed graphs (red, blue, green), the scheme is the same: The amount of the photo's darker shades appears toward the left side of the graph; the lighter tones are graphed on the right side.

Therefore, in a very dark photograph ”a coal mine at midnight, say ”you'll see big mountain peaks at the left side of the graph, trailing off to nothing toward the right.

A shot of a brilliantly sunny snowscape, on the other hand, will show lots of information on the right, and probably very little on the left.

The best-balanced pictures have some data spread across the entire histogram, with a few mountain-shaped peaks here and there. Those peaks and valleys represent the really dark spots (like the background of a flash photo) and bright spots (a closeup face in that flash picture). Those mountains are fine, as long as you have some visual information in other parts of the histogram, too.

The histogram for a bad photo, on the other hand ”a severely under- or overexposed one ”has mountains all bunched at one end or the other. Rescuing those pictures involves spreading the mountains across the entire spectrum, which is what the Adjust palette is all about.

6.9.1. Three Channels

As noted on the previous page, the histogram actually 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 just above the histogram ”you'll see the graphs in all three channels move in unison . Despite changing shape, they essentially stick together. Later, when you make color adjustments using, say, the Temperature slider, you'll see those individual channels move in different directions.