Contrast adjustments can turn a flat-looking photo into something entirely different (Figures 10.12a and 10.12b). Contrast adjustment works by enhancing the edges in your image.
Both Photoshop and Elements have an Auto Contrast setting that often does a very good job of adjusting your image to the right level. Simply select Image > Adjustments > Auto Contrast.
Figure 8.12a. Here's a typical flat image that needs a boost in contrast.
Figure 8.12b. After choosing Auto Contrast, the image has much better definition.
Although Auto Contrast works well in many cases, it isn't the perfect solution for every image. That would be way too easy. So, for those times that Auto Contrast doesn't produce the desired result, simply select Edit > Undo Auto Levels (Command+Z on the Mac, Ctrl+Z on Windows) and then select Layer > New Adjustment Layer > Brightness/Contrast to show the dialog in Figure 8.13. Moving the Brightness slider to the right increases the overall midtone brightness in the image exactly as the gamma slider does in Levels. Moving the Contrast slider to the right increases the contrast by making the dark tones darker and the light tones lighten in equal increments.
Figure 8.13. If Auto Contrast doesn't work the way you want, you can choose the Brightness/Contrast command to make the adjustment manually.
Although this can be a quick fix in many images, the Levels adjustment allows much more control by enabling you to move the Shadow and Highlight sliders independently of one another for a more customized contrast adjustment. In most cases, only a slight highlight increase is warranted to prevent the loss of highlight detail. A heftier increase in the shadows can then be applied without further affecting the highlights.
In summary, Brightness/Contrast can provide quick, wholesale adjustments, but Levels offers the most accurate control.