Today’s Question: Can you explain how posterization can occur in the first place for a preview image versus a “real” image, as you explained in a recent Ask Tim Grey email?
Tim’s Quick Answer: Posterization is generally caused by applying very strong adjustments to a photo, which reduces the range of tonal and color values available and reduces the smoothness of gradations in a photo. Posterization is most common with images that have a low bit depth, which is why JPEG images are more susceptible to posterization than high-bit RAW captures.
More Detail: Posterization is the loss of smooth gradations of tone or colors in a photo, so in some respects you could say that posterization is the result of adjustments that reduce the total number of tonal or color values.
For example, if you increase contrast in an extreme way, you are compressing the range of tonal values, leaving fewer shades available. A simple example would involve a black and white image of a relatively clear sky. That sky would likely transition from a moderately dark shade of gray to a relatively light shade of gray, for example. Increasing contrast significantly would reduce the number of shades of gray that could be used within that sky, so that the gradation would no longer be smooth.
In some cases a strong adjustment might not be necessary to produce posterization in an image. For example, with some extreme situations simply converting from a high-bit image (such as a RAW capture) to an 8-bit image (such as a JPEG) could result in some posterization.
In particular, if you save a JPEG image (which is always only 8-bits per channel) at a relatively low Quality setting, there is a higher risk of posterization. In the context of Lightroom, for example, if you use a low Quality setting for the previews (this setting can be found in the Catalog Settings dialog) there is a higher risk of posterization in your JPEG previews displayed in the Library module. But chances are it is only the preview that is affected, and the Develop module will show you the actual quality of the image.