Hi!
1 - Does anyone know what are those dark, irregular blotches on the kid’s hair?
2 - Does anyone know how to remove them? I’m trying to apply noise reduction, general noise is reduced, but the blotches remain.
Hi Rei,
To me, it looks like ISO 3200 noise…
Have fun!
Claes in Lund, Sweden
In general, splotches are hard to fix and detail recovery is even harder. I would try to select the detail layer in question and lower the contrast. @rawfiner’s tutorial is pretty amazing.
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Your profile is not public, hence I cannot PM to. Can you PM to me, I will tell you my email addy, then I’d like you to send me that raw e.g. via smash.cim , I wanna try before I speak 
Strange, I don’t remember making it secret.
Thanks for helping. This one will do it, the same dark blotches when I start denoising one of the channels:
Licensed by CC-NC
_MG_5113.cr2 (13.6 MB)
Yeah, I was exactly applying it, but when I start denoising one of the channels, as shown in the image, noise reduces in general, but the dark blotches remain and become more evident.
Maybe you’re right, thanks.
Experimental: you could try inpainting the dark spots; e.g., heal tool, G’MIC, …
@afre Couldn’t find the setting to make it public 
The only thing I explicitly made secret was my e-mail (I read “Never shown to the public.”)
Hum, right, but for now I’m lazy 
Thanks for the idea anyway
Could you send the xmp for above file pls!
To mee it looks like, I had to boost exposure by 1 and still my histogram looks diff than yours, also strange, your preview shows 100% but this seems to be a 200% zoom
+1EV means, we are at ISO6400. Unfortunately for your camera, there is no such denoise profile. Acc. to Harry Durgin, it helps to change in that “found match for…” on top left of denoise_profiled.
You used channel mixer to B/W, so do I and I fiddled around with the different options. On purpose I did not compress my history stack. To me "soft skin ever looks not nice, hence I used Kodak T-Max…
I tried the monochrome module and it produces a bit splotchier result.
This is, what I achieved and the efforts is similar for what I do with my pics. Around step 15 or so, I would have stopped… Later I will check, what my D750 says…
_MG_5113.cr2.xmp (12,5 KB)
what screen resolution do you run at???
@gadolf And that might be one reason… hence I am looking for your xmp of the file you provided…
symptoms fighting? Also way too soft in my eyes 
These dark blotches are, I think, due to photon noise (Shot noise - Wikipedia)
Basically, few photons hit that pixel, so the pixel is completely black.
If the pixel is black only on one of the 3 channels, the denoiseprofile can lead to ok results.
But denoiseprofile is generally struggling at lot at removing pixels that are black on all 3 channels.
My advice here is to use denoise bilateral, which smooths more dark areas than bright ones. It can be used combined with denoiseprofile or alone.
If you are using darktable dev build, you can also try putting an instance of denoise bilateral before denoise profile, it works quite well.
Also if you are using darktable dev build, for denoiseprofile, you can try using non local means with a big patch size (the parameter value can take values in [0,10] with right clic), setting the scattering parameter so that few chroma noise remain visible, and setting the details parameter at the end to get back some details 
Hope this will help 
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now I see…
- you boosted exposure by 2EV that equals to ISO12800!!!
- go to channel mixer, set it to grey and reduce red from 1 down to .5 or so and lift green and blue ~.25 or do all 1/3. You will see a much better result!
In fact!!
The point is that I don’t want to do a monochrome edit. The images I sent were both monochrome because I was in the middle of @rawfiner’s tutorial, where you temporarily switch to monochrome to be able to denoise each rgb channel separately (as previously mentioned by @afre, here)
I’ll keep experimenting with all suggestions and will later state my findings.
EDIT: @AxelG given that I want a color edit, I’m stuck at you advice to tweak the channel mixer channels, regardless the fact that it does what I want in monochrome.
This part was not clear to me…
I thought you where aiming B/W
Than I have to see again in color…
I am in a hurry now, so sorry for quickies…
Try this xmp
- zoom as you did
- switch off chromatic aberations, hot pixles and 3 instance of denoise-profile (denoise_2 is with L-mask)
- switch amaze back to ppg
- do a snapshot
- switch all those corrections on again (and play with ppg/amaze)
- move the snapshot slider and you will see…
it is in your sensor data (indeed) and has a very hard saturation (figured while I wanted to mask it), even at 99 of upper right input slider of “S” those pixls which makes your splotches, are out of the mask, right away)
Usually amaze (implemented here with two times smoothing) helps a lot but also costs colour…
try to borrow another cam and shot the same scene with both cams ( or three, one as yours and one different one, e.g. the Nikon D750, which is an ISO monster)
_MG_5113.cr2.xmp (32,9 KB)
The more I look, I feel, it is your sensor, sorry to say
that is a 400% zoom. on the left
_MG_5113_02.cr2.xmp (53,9 KB)
on the right, I switched off color channel and all denoising and than move the snapshot slider
you see, that those spots, black on the left, they are also black on the right (I know, above I talked about over saturated. This part I didn’t bring together yet, as I gotta go now…
I find you could get somewhat get better results when increasing brightness by using a tone curve instaed of the exposure module. The crop below shows the red channel using the biliteral denoise module.
_MG_5113.cr2.xmp (2.2 KB)