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I am stacking a still scene I took with my Canon EOS 6D in Photoshop to reduce noise. I've opened up raw files and stacked them with a smart object/median filter. However I've noticed something odd at the final result: there is some black square-ish pattern all over the image. Have a look at this 100% crop from the final result:

enter image description here

While the effect is not very strong, it is easily noticeable. I do image stacks and this is the first time I've ever seen such effect. Why does it happen and how do I get rid of it?

UPDATE: Here is an "increased-contrast" version of the same image, just to make the grid more clear:

enter image description here

  • I don't see what you describe. Do you mean the Hermann grid illusion? – Aganju Jul 24 '16 at 13:57
  • @Aganju nope it's not illusion. the grid is clearly there. I've updated the question with a higher contrast version. it's much easier to see now. – Can Poyrazoğlu Jul 24 '16 at 20:11
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    I think this is called fixed-pattern noise. It is a consequence of how your sensor came out of manufacturing. – Nayuki Jul 25 '16 at 21:37
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It is a specific noise produced by the sensor.

Some models has a better low light performance and noise than others.

I have the feeling you are using canon. Some sensors has this pattern when boosting the dark shadows. For esample take a look at this video: https://www.youtube.com/watch?v=68Y5kCf9AVQ

At the end depends on the model and generation of the sensor. It could be an issue if you are pushing a lot the editing of the image, for example underexposing photos and correcting them again in post.

  • Oh I see. so are there any workarounds on this? or will I just accept it as it is? it's not really THAT important anyway, I just wondered the reason behind it. – Can Poyrazoğlu Jul 25 '16 at 17:15
  • @CanPoyrazoğlu The algorithm used for image stacking doesn't look optimized to deal with this problem to me. When working with raw files to do the stacking, it should be quite easy to eliminate the fixed pattern noise. – Count Iblis Jul 26 '16 at 18:19
  • @CountIblis I was actually working with raw files – Can Poyrazoğlu Jul 27 '16 at 14:47
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    @CanPoyrazoğlu Yes, but it's not clear how photoshop is handling these raw files. Given a reasonable number of raw files in the stack, you can reduce quite some number of problems by exploiting the small misalignments. Not just the (pattern noise), but also demosaicing artifacts, if the noise isn't all that large then one can enhance the resolution etc. etc. – Count Iblis Jul 27 '16 at 20:36

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