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The images were taken with a Canon EOS 5Ds R. All 3 exposures (JPEG) were merged with easyHDR Pro. We have discovered that the original images have this artifact - in blue skies, hard transitions are shown. As far as we know, it is not the HDR software.

What is causing this and what can I do to get rid of it?

example picture of stitched HDR

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    Why do you call them "sensor artifacts" if you don't don't what causes them? – Caleb May 27 '18 at 2:42
  • Because an analysis by the software company proved that they are not caused by the HDR software or process. They exist on all three RAW images. I have seen this on a T6i also. It is not visible in photos unless you have a clear sky. It occurs primarily in blue. I believed it was the software. – Ray Cunningham May 27 '18 at 2:47
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    @RayCunningham can you post a link to a copy of a raw file that exhibits this when you process it with easyHDR Pro? – Michael C May 27 '18 at 3:15
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    I edited the question (including title) so it reflects these comments. – flolilo May 27 '18 at 15:16
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Those artifacts look like the result of JPEG compression. Such compression artifacts are particularly likely to be visible in large flat areas with slowly changing intensity, like your blue sky.

If they are really sensor artifacts, then they would be in the raw files. Take a look. Make sure you are examining true raw files, not compressed raw files.

If this is due to compression, as it appears to be, compress less. At least don't compress until you make the final image, then chose the level of compression so that these artifacts are not visible. Any lossless compression is OK. Note that JPG has both lossless and lossy compression. The "quality" setting adjusts the tradeoff of how aggressive the lossy compression is versus the artifacts it causes. Only you can be the judge of what the right level is.

If you need really good quality, like to send out to have a large poster made, consider using only lossless compression. The forward differencing with LZW compression of TIFF files does a good job on flat areas like your sky, without introducing any artifacts at all.

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