Improve Images With IPhoto's Histogram
Straightening, cropping, and removing red-eye are all fairly easy fixes to make to a photograph. Getting good tones, on the other hand, can be a more challenging problem. iPhoto's Enhance button tries to offer a one-click solution to this problem--when selected, the feature analyzes your image's tones and makes its best guess at fixing them. But while this feature can often get you started in the right direction, it seldom solves the problem completely. You'll usually get better results by making adjustments yourself. To do that, you'll need to open the Adjust palette.
Read the Histogram
The key to correcting an image's tone problems lies in understanding the histogram that appears at the top of the Adjust palette. 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.
By learning to interpret what the 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. 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.
Go Manual with the Levels Sliders
iPhoto offers a couple of tools that will adjust your image's tones. However, you'll get the best results from the Levels sliders, located below the histogram. There are three sliders: a black slider on the left, a gray slider in the center, and a white slider on the right. By adjusting these, you can alter an image's highlights, midtones, and shadows independently. This gives you much more control over the image's tones.
Most shots taken under normal lighting conditions look best if the black-point and white-point markers sit right at the point where the histogram data begins. For example, say a photo looks washed out. You look at the histogram and notice that while the bars representing the light tones stretch all the way to the right edge of the histogram, the dark tones fall short of the mark on the left side--indicating there's no true black.
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.
If you've corrected a 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 midtones, represented by the gray slider in the center. This lets you adjust the midpoint of your photo's data without affecting the white and black points. By moving the midtone slider to the right, for example, you can lighten up an image's middle tones without damaging the brightest highlights.
Use the Shadows Slider
If you've adjusted your levels but parts of your image still appear too dark--for example, if your subject's face is lost in shadows--you can often improve the situation by dragging the Shadows slider to the right. Likewise, if you adjusted your image, but then washed out the lightest parts of your image, you may be able to bring back some of details by adjusting the Highlights slider. But be careful not to overdo the effect. The Highlights slider in particular has a tendency to emphasize texture such as wrinkles--your loved ones won't thank you for that.
Improve Contrast with the Definition Slider
Sometimes a little boost in contrast can add visual interest, especially if an image's histogram falls short of the mark on both ends of the scale. But rather than reaching for the Contrast slider, try the Definition slider. This new addition to iPhoto adjusts contrast in a more refined manner than the Contrast slider. It can also make a slightly soft image appear sharper.