I thought I’d write a post to discuss dealing with posterisation and the relationship with noise on shadow detail. I recently submitted a few of my photographs to Getty Images. They liked some of them and asked me to prepare them for sale. This mostly involves uploading a high resolution version to their servers and adding a suitable description etc. As the uploaded image is full resolution, every little flaw is visible on inspection at 100% so you really have to address all minor issues which are normally unnoticeable on the web.

One such issue is posterisation. Posterisation is when the tonalities of the image are stretched out and banding starts to occur in areas which should otherwise be smooth tones, clear blue skies are a good example. I normally work in ProPhotoRGB colour space, which occasionally results in banding issues even when working in 16-bit mode. Switching to AdobeRGB can help reduce this. However one of my favourite techniques for dealing with this issue is to apply some noise to mask the banding. Noise can also be used in areas of deep shadow to give the appearance of greater shadow detail, which is particularly useful if you do low light or night photography.

Illustrated below is before and after shots of dealing with banding issues in the image at the top of this post, by adding a new layer containing a genuine photographic grain and setting the blending mode to overlay. Adjust the opacity to taste.


No grain, banding/posterisation is clearly visible. Click for larger.


Grain applied. Banding is masked by the grain. Click for larger.

You can read more on this subject at The Online Photographer.

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