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I made some post-processing and came to the point to see the gradient in the sky turns into zones separated by sharp borders, which I believe to be classic example of posterisation.

Firstly I thought that will be resolved easily by local reduction of sharpness. However, that only somewhat blurred separation borders, the zones retained.

Than I went to removing all things I did, one at the time, but whatever I do, I cannot restore the gradient.

What should be the first to look for in these circumstances?

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    The main cause of posterization is creating or saving jpeg files with too high compression. But there is too little here to address your question. Please provide more details. Camera, file type, camera settings, application and a sample of the image posterisation. – doug Feb 14 at 0:17
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  • For the cause see this. This can often be mitigated by applying "spread noise" that blurs the limits. – xenoid Feb 14 at 10:50
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    @doug and channel clipping, exacerbated by the ETTR habits of the RAW tribe.... – rackandboneman Feb 14 at 15:37
  • @rackandboneman Agree. Blowing out the B channel is a frequent culprit for banding in skies. OP needs to provide more detail to zero in on the cause. There are also issues that can cause banding when using a working space larger than the monitor's gamut and when printing using relative colorimetric. – doug Feb 14 at 17:04
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Posterization was an intentional artistic enhancement. This was a darkroom technique used to create a relief image often used as an eye-catcher.

In modern time this term is applied to a defect also called banding. Banding occurs mainly in digital photography as bands of continuous tones in areas that should reproduce smoothly like blue sky etc. This artifact occurs mainly when the bit depth is insufficient to display a continuous gradation.

To save file space we often use file compression techniques. These work by casting out data in areas where the content is mundane (such as uniform sky or water).

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After lot of time spent on moving any button/slider available, I came to the surprising discovery that the culprit is - LR adjustment brush.

Posterisation in sky was solely caused by adjustment brush in this case. Even when each and every slider within brush is set to zero, posterisation still existed; the only way to get rid of it was to remove brush entirely.

I never had such occasion before.

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