Hello. been using darktable for a month now and I am generally pleased. However. today when I was editing an picture with high iso ( 25000 iso) I noticed an circular pattern in the noise after I applied lens correction to the file.
I am using an Leica q2 monochrome and that particular model is not included in the list of cameras. But a regular Leica q2 should have an identical lens as far as I’m aware
I also tried to edit the file in Art as close the how I did it in Darktable and no pattern was visible there when I applied lens correction
Is there a way to solve this? (other than start to use art, witch I rather not do, it lacks a lot of features from darktable I like)
edited:
uploaded the original dng file. And as apostel338 said, It might be there in art to just a bit more hidden. L1001045.DNG (86.2 MB)
I don’t have a satisfying solution, but am able to cover it up a bit with these profiled nr settings. They may be tweaked better. Feel free to experiment.
Normally, profiled nr is not that nice if your cam doesn’t have a profile (it can then still be useful for color noise), but it seems to help with these patterns.
Also, the effect seems to look stronger in DT than in the exported photo.
I think it is normal for lens correction to affect noise like this. I have seen it in images from DxO.
The lens correction is moving and blending pixels by different amounts and this affects the noise. For example, if the lens correction requires a pixel to be moved by half a pixel, then the result is a blending of the pixels. Other pixels may need to be moved by a whole pixel, so no blending is required.
What is the order of the processing? Noise reduction before or after lens correction?
Noise affects sensor elements, so logically denoising should be performed on the image before lens correction. For interactive performance, perhaps it is being done after lens correction, creating the moiré pattern.
I created an image of pure noise and then applied a lens distortion to it. It shows that when noise is present and a lens correction is applied, patterns will emerge. The patterns are a general problem that is not specific to Darktable.
So you are right, denoising should be done before lens correction or any geometric adjustments.
I see what you mean, thanks. I had thought the effect might be caused by lens correction followed by denoising, but, as you say, it can be just lens correction of a noisy image.
