Recognizing Posterization

When you look at an updated histogram, you might see wide gaps in the histogram—this indicates posterization (Figure 5.35). 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.

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).

To see an updated histogram after adjusting the image, you must first apply the adjustment, and then reopen the Levels dialog box. You can also choose Window > Histogram to see before and after histograms overlaid on each other. I don't use this technique on every image, just on those that have extremely noticeable posterization. If you don't have the time or patience to apply the Eliminating Posterization technique mentioned here, then consider choosing Filter>Noise>Add Noise and use a setting of 3 or 4. That can help to reduce posterization, but will not be able to help in cases of extreme posterization.