Fixing Contrast Problems with the Auto Contrast Command

The Auto Contrast command (EnhanceOAuto Contrast) sets the black- and white-point limit for you. Unfortunately, Elements doesn't offer any controls that enable you to change the Auto Contrast attributes. The command simply examines your file data and sets the black and white points based on the assumption that one-tenth of one percent of your image pixels (one in a thousand) should be pure white and the same proportion should be pure black.

It should really be called Auto Black and White Points because that's what actually happens when you use the command. It doesn't have a clue about the original overall image contrast of your file. It can, however, be a very useful edit for some files.

The Auto Contrast command does nothing on a file that already has some full white and black tones if the black and white pixels comprise more than one in a thousand of the total image pixels. Elements doesn't allow you to custom set the black-and-white clipping percentages like its big brother, Photoshop CS2, does. The default setting is a kind of best average, and you can't change it. The following example shows how it works.

To follow along in these steps, use the missouri.jpg file you can download from the Chapter 6 folder on the Wiley companion Web site.

To edit an image with the Auto Contrast command, follow these steps:

1. Open an image that appears flat and doesn't have true black or white pixels.

For this example, we use the photo shown in Figure 6-5. You can download this image from the Chapter 6 folder on the companion Web site if you don't have a photo with similar brightness and contrast problems. Because this photo has no white or black pixels, using Auto Contrast will set the black and white points by using the default settings.

Figure 6-5: This battleship is looking a little flat.

2. Press Ctrl+L (^+L for Macs) or select LayersONew Adjustment LayerOLevels to open the Levels dialog box and take a look at the histogram, shown in Figure 6-6.

After checking the histogram, click Cancel to close the dialog box.

3. Select EnhanceOAuto Contrast to run the Auto Contrast command.

4. Reopen the Levels dialog box and check the histogram again.

After using the Auto Contrast command, the histogram appears as shown in Figure 6-7. You can see the change in the remapping of the tone curve in this histogram, compared to Figure 6-6, as a result of the Auto Contrast adjustment. After the Auto Contrast edit, you can see confirmation that the black and white points have indeed been reset.

The Auto Contrast command got pretty close to the best setting in Figure 6-7. Black and white points were reset. But should you routinely use Auto Contrast for image editing? It depends. A custom setting of black and white points in Levels allows you to decide exactly where you want the black and white points to be for best possible results. Also, you can change image brightness by adjusting the Gamma slider in the Levels dialog box. Therefore, in many cases, the Auto Contrast feature can't give you the results you can get by using custom adjustments.

Figure 6-6: The Levels dialog box for the battleship image shows you where the data fall on the histogram.
Figure 6-7: The battleship's histogram after using Auto Contrast.

On the other hand, if you have 200 photos of a kid's birthday party to edit, you may find Auto Contrast a real time-saver. Doing 200 custom adjustments may just be impractical. As a matter of practice, you may want to manually correct images for brightness and contrast when time permits.

In the example in this section, a Levels adjustment took care of the brightness and the contrast, and it did a pretty good job. You won't always be that lucky, though. After adjusting the brightness in Levels, you may need to use other methods to either boost or lower contrast. Everything depends on your image data and the results you want. In this chapter, we stick to making adjustments on a single layer. You have more options for contrast control when you use additional layers, as we explain in Chapter 7.

Learn Photoshop Now

Learn Photoshop Now

This first volume will guide you through the basics of Photoshop. Well start at the beginning and slowly be working our way through to the more advanced stuff but dont worry its all aimed at the total newbie.

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