Today’s Question: I know that posterization is bad, but I don’t know what it is! Can you explain what this term means?
Tim’s Quick Answer: Posterization refers to the loss of smooth gradations of tone and color in an image. As an example, a sky should generally appear as a very smooth gradation. If a photo that includes the sky becomes posterized, that sky will instead appear with obvious bands of color that don’t blend together very smoothly.
More Detail: More specifically, posterization refers to a situation where fewer color and tonal values are available to represent your image, creating “gaps” in the colors and tones that would normally fill in a gradation.
Under normal circumstances you are most likely to see posterization in an image that is in the 8-bit per channel mode, and when relatively strong adjustments are applied. Posterization is also much more likely to occur in black and white images, because a smaller number of overall pixel values are available when there are only shades of gray available.
Strong adjustments tend to reduce the total number of colors and tones available for an image. As an extreme example, consider a black and white image with an extreme increase in contrast applied to it. The most extreme version of a high-contrast black and white image would be one that only contains two tonal values: black and white, with no shades of gray in between. Higher contrast or saturation (among other adjustments) increases the risk of posterization.
To minimize the risk of posterization, I recommend always working with high-bit data if it is available. That generally means working with RAW captures, and converting those RAW captures as 16-bit per channel images rather than 8-bit per channel images.
As an example of the importance of bit depth, consider the number of tonal values available for a black and white image. In the 16-bit per channel mode you can have up to 65,536 shades of gray available for a black and white image. If that image is converted to the 8-bit per channel mode then only 256 shades of gray would be available, greatly increasing the risk of posterization.