Any time you adjust an image, you run the risk of introducing artifacts, so let's take a look at what can happen to your image after applying Levels. But don't worry—remember, there is usually at least one "fix" for every artifact.
Low Contrast Onscreen Appearance
If you've adjusted an image that will eventually be reproduced on a commercial printing press, your results will most likely look rather flat onscreen (lacking contrast). This problem is a temporary one since the image will gain contrast when it's printed on press (dark areas become darker and bright areas become brighter). You're welcome to adjust the top three sliders in Levels to get an acceptable image and then hold off on adjusting the bottom two sliders until you're done working on the image in Photoshop. That way, the image will have good contrast for the vast majority of the time you work on it and then the bottom two sliders can be adjusted right before saving the image so that it's ready to be reproduced on press.
When you look at an updated histogram, you might see wide gaps in the histogram—this indicates posterization (Figure 4.31). Posterization is when you should have a smooth transition between areas and instead you see a drastic jump between a bright and dark area. Some call this banding or stair-stepping. As long as the gaps in the histogram are smaller than three pixels wide, you probably won't notice it at all in the image.
Figure 4.31 Gaps in a histogram indicate posterization.
Adjusting the image usually causes these gaps. As you adjust the image, the bars on the histogram spread out and gaps start to appear (remember that Slinky). The more extreme the adjustment you make, the wider the gaps. And if you see those huge gaps in the histogram, it'll probably mean that the posterization is noticeable enough that you'll want to fix it (it usually shows up in the dark areas of the image).
Here's a trick that can minimize the posterization. I should warn you that you have to apply this technique manually to each area that is posterized. Although it might take you a little bit of time, the results will be worth it.
To begin, select the Magic Wand tool, set the Tolerance to 0, and click on an area that looks posterized. Next, choose Select > Modify > Border, and use a setting of 2 for slight posterization or 4 for a moderate amount of posterization. Now apply Filter > Blur > Gaussian Blur until the area looks smooth (Figures 4.32 and 4.33). Repeat this process on all of the posterized areas until you're satisfied with the results.
Figure 4.32 Turn off the Preview check box to see the edges of the posterized area.
Figure 4.33 With the Preview check box turned on, increase the Radius setting until the posterized area appears smooth.
If you find that a large number of your images end up with post-scan posterization, you might want to look into getting a scanner that's capable of delivering 16-bit images to Photoshop. A typical grayscale image contains no more than 256 shades of gray, which is technically known as an 8-bit image. That's sufficient for most images, but extreme adjustments will cause posterization. One way to avoid posterization is to use a scanner that can produce images that contain thousands of shades of gray, which is technically known as a 16-bit image. Most scanners are capable of capturing more than 256 shades of gray from a photograph, but few are capable of actually delivering all those shades to Photoshop. So, the next time you shop for a scanner, be sure to ask if it is capable of delivering 16-bit images to Photoshop.