Adjusting Exposure and Levels

iPhoto's Enhance button often does a good job of punching up a photo, but it's a "my way or the highway" feature: you either like its results or you undo.

The Adjust panel is a more accommodating place to fix brightness and contrast problems. And not by using the Brightness and Contrast sliders, either: they're blunt instruments that lack the precision you need.

For improving a photo's exposure and contrast, use the Levels sliders and the Exposure slider. By adjusting themwhile keeping a close eye on the photo's histogramyou can often make dramatic improvements in a photo's appearance.

Which tools should you use? It depends on the photo. Some photos respond better to the Exposure slider, while others benefit from levels adjustments. Still other photos benefit from both approaches: do some initial tweaks with the Exposure slider, then fine-tune the levels.

Reading a Histogram

A histogram is a bar graph that shows how much black, white, and mid-tone data a photograph has. Pure black is on the left, pure white is on the right, and the mid-tones are in between. iPhoto displays a color histogram that breaks this information down into an image's three primary-color channels: red, green, and blue.

Reading a histogram can help you discern problems in an image. And adjusting the Levels and Exposure sliders can often fix those problems.

A Sampling of Histograms

This properly exposed shot has a good distribution of dark, bright, and mid-tone areas. Notice that the histogram shows a lot of bright blue data: the ocean and sky.

This overexposed shot has very little data in the blacks; everything is bunched up toward the right sidethe white side of the histogram.

This photo's histogram shows a similar amount of red, green, and blue data around the mid-tonesexactly what you'd expect from photo of a predominantly white flower. But the whites could be a bit whiter; notice the absence of data at the right end of the histogram.

Using the Levels Sliders

Beneath the histogram display is a pair of sliders that let you change what iPhoto considers to be pure black or pure white. By dragging these sliders, you can often improve a photo's brightness and contrast.

Before

This photo of a rainy Paris lacks contrast and looks a bit flat. Its histogram tells the tale: there's little data in the darkest blacks and brightest whites.

After

The photo's brightness and contrast are improved, and its histogram shows a broader tonal range.

Using the Exposure Slider

The Adjust panel's Exposure slider makes a photo brighter or darker.

I rescued this overexposed photo by dragging the Exposure slider to the left. Notice that its histogram is much broader than the original (see opposite page). The sky is still blown out to pure white, thoughiPhoto can't create image data where none existed in the original photo.

For details on how the Exposure slider works, see page 157.

How it Works

When you drag the black- or white-point sliders, you tell iPhoto to stretch the photo's existing tonal values to cover a broader tonal range. Oversimplified, when you change the black point, you tell iPhoto, "See this grayish black? I want you to treat it as a darker black and adjust everything else accordingly".

Going too far. The Levels sliders can often work wonders, but they can't work miracles. If a photo has an extremely narrow contrast range, you may see visible banding after adjusting levels. You're telling iPhoto to stretch a molehill into a mountain, and there may not be enough data to allow for smooth gradations in shading and color.

Avoid clipping. Similarly, if you drag the black- or white-point sliders beyond the histogram's shoulders into the image data, you'll throw away tonal detail: areas that were almost black will become pure black, and areas that were almost white will become pure white. This is called clipping, and unless you're looking to create a special effect, you should avoid it.

You can use a histogram to spot clipping: if the histogram has a tall bar at its leftmost or rightmost edge, its blacks (or whites) have been clipped.