iPhoto 5’s secret weapon

Although it’s not hard to spot a bad image, figuring out exactly what to do to fix it isn’t always so easy. That’s one reason iPhoto 5’s new Adjust palette is so exciting. With the help of its advanced controls, you can quickly diagnose and fix subtle image problems that were impossible to repair with previous versions of iPhoto.

The Adjust palette offers a collection of sliders that control basic image settings. Some—such as Sharpening and Rotation—are fairly obvious. But how can you tell whether an image requires a different temperature or exposure? Turn to the Levels histogram. This multicolored graph at the bottom of the Adjust palette is an easy-to-use guide to spotting and fixing common image problems. By learning to interpret what the Levels histogram is telling you, you can take much of the guesswork out of correcting bad photos and discover ways to make good photos even better.

Histogram Literacy

The histogram is a bar chart that shows the distribution of tones in an image. Black is on the far left edge, white is on the right, and everything else is in between.

Usually, you want photos with as broad a range of tones as you can get. Think about boxes of crayons: you can create a much more detailed image with 64 different crayons than you can with just eight. Likewise, if the bars in the histogram are crammed together in a narrow space, your image probably doesn’t have the resources it needs to depict subtle detail.

With a well-exposed image, the histogram’s bars will stretch across the full range of the graph. If your histogram’s bars are clumped on either the left or the right side, the image is probably underexposed (lacking good highlights) or overexposed (lacking good shadows), respectively. If the bars don’t stretch to either edge, then your image lacks contrast.

Fixing Dark or Light Photos

The Levels feature also includes easy-to-use controls that let you correct the problems your analysis reveals.

Say a photo looks a little washed out. You look at the histogram and notice that the bars representing the light tones stretch to the end of the graph, but your dark tones fall short of the mark—so there’s no real black (See first and second screenshots).

In previous versions of iPhoto, you might have solved this problem by increasing the Contrast setting. But the Contrast slider affects highlights and shadows. So you’d also brighten some of your highlights, which are already perfectly exposed—thus eliminating important detail.

For better results, use the Levels controls to adjust your photo’s highlights and shadows independently. Below the histogram are two sliders: a black slider on the left and a white slider on the right. Where these sliders are positioned determines which tones in your image correspond to black and white.

To give your washed-out image more zip, simply move the black slider to the point where the histogram drops off. iPhoto will then stretch and scale the image’s intermediate tones so that the overall distribution remains the same. But the image’s white point will remain untouched. The result is a photo with richer tones and stronger shadows. Likewise, you move the white slider to give an underexposed image better highlights.

Fixing Exposure

If you’ve corrected your photo’s white and black points but the image still looks a little too dark or too light, your next strategy is to adjust the exposure.

The Exposure slider (located just above the histogram) lets you adjust the midpoint of your photo’s data without affecting the white and black points. By moving the Exposure slider to the right, for example, you can lighten up an image’s middle tones without damaging the brightest highlights—something you can’t do with the Brightness slider.

Correcting Bad Color

One of the most challenging image problems to solve is bad color. It’s not always clear what needs to be done to remove unwanted colors. Here, too, the Levels histogram can help.

The image on your computer screen is made using three different color channels—red, green, and blue. When combined the right way, these three colors create all the other colors and grays you see. When combined at full strength, they produce white.

iPhoto’s Levels histogram displays three sets of colored bars—one for each color channel. By examining how these color channels overlap, you can get a sense of where color problems may exist.

Consider, for example, my picture of a tunnel (See third and fourth screenshots). If you look at the original’s histogram, you’ll notice that the three channels barely overlap—especially in the highlights. Since you need all three colors to produce a true white, the brightest areas of the image appear to be yellow.

To change the alignment of an image’s color channels, use the Temperature slider. Move the slider to the right to make your image warmer (shifted more towards red) or to the left to make it cooler (shifted more towards blue). In the case of the tunnel picture, I moved the slider to the blue side until all three channels ended at roughly the same place on the histogram—producing a more accurate white.

Once you have the channels lined up, you may discover additional color problems. In this case, the image became a little too blue overall. To fix this, I moved the Tint slider to warm up the image while preserving the tonal distribution (so whites stayed white). The result is much truer color and a shapelier histogram.

The Learning Curve

Like any well-engineered tool, iPhoto’s Adjust palette becomes easier to use as you gain practice. The histogram may be intimidating at first, but it offers a wealth of information to people who learn its secrets. And these skills will help you in many other areas—you’ll find similar histograms in programs such as Adobe Photoshop and After Effects.

Get It Right the First Time

If your image’s histogram bars run into the far edge before they have a chance to taper off, there’s a good chance that you’ve clipped your highlights or shadows. This results in large areas of white or black where there should be subtle shades of gray. While there’s not much you can do to correct this problem in iPhoto—you can’t improve detail where none exists—you may be able to prevent this problem from occurring in the future.

These days, most cameras can display histograms just like the ones in iPhoto 5, so you can check an image’s histogram immediately after shooting. If you find that either edge is clipped, adjust your camera’s exposure settings and shoot again.

[ When he’s not risking his life in subway tunnels, Ben Long writes books such as Complete Digital Photography, third edition (Charles River Books, 2004). ]

This image suffers from a lack of contrast. Notice that bars representing the image’s tones fall short of the black point.With a simple black-point adjustment, I improved the image’s contrast without blowing out its highlights.The original image suffers from a noticeable yellow cast.With the Temperature slider A, I put the image’s separate channels into better alignment, creating more-accurate color. After final Tint and Exposure adjustments, the image had better brightness and overall color.
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