Posterization, also called "color banding," occurs when a photograph loses detail in an area with a color gradient, resulting in a series of colored bands rather than a smooth, continuous gradient. Color banding can be caused by over-compression, underexposure or converting a photograph to a much lower colorspace (which refers to the number of colors that can be rendered in the image). Posterization can be intentionally applied for artistic effect--usually in posters, hence the name "posterization"--but is most often undesirable in photographs. Adobe Photoshop includes tools that can help you mask, remove and correct posterization.
Open the image in which you wish to fix posterization in Photoshop by clicking on the "File" menu and choosing "Open." Find the image file in the explorer window, highlight it and click "OK."
Select the "Magic Wand" tool and click in an area of the photograph suffering from posterization. If the tool does not automatically select the entire area of posterization, hold down the Shift key and click on another color band to add an additional area to your selection. Continue doing this until the entire posterized area is selected.
Access the "Image" menu and choose "Adjustment" and then "Hue/Saturate." Move the "Saturation" slider to the left to desaturate (remove some of the color from) the image and to make the color banding effect less pronounced. The amount of desaturation you need to apply will depend on the degree of posterization and the photo you are using. Manipulate the slider until you are satisfied with the result, and click "OK."
Open the "Filter" menu and then choose "Noise" and "Add Noise." Select the "Gaussian" ratio box and make sure the "Monochromatic" box is checked. Set the "Amount" value to a small amount, such as 2.0 percent, and click "OK." The noise will help to obscure and hide the color bands.
Access the "Filter" menu and choose "Blur" and "Gaussian Blur." This will smudge the color bands together to fix the posterization. The amount of blur you must add depends on the image, so continue to manipulate the sliders until the results are satisfactory. Keep the "Preview" box checked so that you can observe the edits in real time on your image.
Tips & Warnings
- Keep your image in a large colorspace until you are done editing it. An image's "colorspace" refers to the number of colors that are available to be used in the image. For example, an image in 8-bit colorspace would have 256 available colors, while an image in 24-bit colorspace (or "Truecolor") would have over 16 million available colors. Editing photographs in low colorspaces can lead to color banding because the wider variety of colors must be forced into a smaller color selection.
- Applying too many manipulations to a photograph can cause color banding. Be sparing in your edits to avoid causing posterization problems.
- Ron Bigelow: Posterization
- "Photoshop CS2 Before & After Makeovers;" Taz Tally; 2006
- Photo Credit Ciaran Griffin/Lifesize/Getty Images