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12-19-2010, 03:00 PM   #1
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K20D blueish high iso noise

I've got my K20D for about a year now and I always wondered how to best get rid of the blueish noise at the top and the bottom of images taken at high iso. These artifacts get visible beginning with about iso 2000 and are really annoying for iso 3200 and above.

Today I did some testing and took dark frames at iso 6400 and analyzed them using dcraw and ImageMagick. What I found is that the fixed pattern read noise is not evenly distributed - the noise floor is much higher at the top and bottom of the frame, as expected. This is not only limited to the blue channel; red and green are affected, too.

Why then the blueish artifacts? I guess this is because of the multiplier for the blue channel, which is actually 2.0 compared to approx. 1.4 for the red channel and 1.0 for the green channel (according to dcraw output). As soon as the SNR gets too low this will result in a blueish color cast...

dcraw has the option to subtract a darkframe from images before demosaicing. I tried this using an averaged darkframe (about 10 single frames averaged using ImageMagick) and - tataa - The blueish color cast is gone and the reconstructed image seems to be having overall a bit more contrast.

Since I'm using Apple Aperture 3 for all of my image handling dcraw is not what I wanted to use for demosaicking. So I hacked the code a bit (many thanks to Dave Coffin for this really smart tool) and finally do have a tool now to "fix" K20D .DNG raw files, meaning .DNG + darkframe in and corrected .DNG out. I still have to automate this in the sense that taken high iso shots are automatically processed using this tool before being imported into Aperture, though.

Generating "working" dark frames is not a big issue. Take a few shots, average them, done. However, generating "good" darkframes is probably more of an issue. I still have to think about this for a while...

I know from a thread within this forum that this is a known issue with the K20D. Is anybody interested in the source code of the modified dcraw program?

Best,
Norbert

12-19-2010, 06:39 PM   #2
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You will need to post a sample.
12-20-2010, 09:01 AM   #3
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... and now after subtracting fixed pattern noise ...

And this is how these shots look like after I've subtracted the obvious fixed pattern noise using the .DNG to .DNG tool:

ISO 3200:
Name:  _IGP5250fixed.jpg
Views: 376
Size:  178.8 KB

ISO 6400:
Name:  _IGP5251fixed.jpg
Views: 325
Size:  212.5 KB

Both have been developed using the exact same settings as have been used for the originals (default settings of Aperture 3 - no additional noise reduction nor any other changes... )
12-20-2010, 10:56 AM   #4
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here the original shots showing the artifacts

Hmm, it seems my first post today has either been lost or is still waiting for a moderator to review it. In case it's been lost - here they are again: The unmodified shots taken at ISO 3200 and 6400...

ISO 3200:
Name:  _IGP5250.jpg
Views: 325
Size:  174.4 KB

ISO 6400:
Name:  _IGP5251.jpg
Views: 313
Size:  203.6 KB

Both show the blueish color cast at the top and bottom of the frame...

12-20-2010, 01:10 PM   #5
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Search for GordonBGood's tool... This is something I found nearly two years ago (except that I used the hidden borders of the DNG instead of subtracting a dark frame), and he kindly made an automated tool that will do this based on those hidden borders found in the DNG files...

Pretty neat trick, as it does not need the dark frame!

But all this shows that the K20 is still quite a capable camera...
12-20-2010, 03:02 PM   #6
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Many thanks for the link - what I found so far is a brief description of how the compensation works and some Windows .NET executables. I'm a Mac user and tried to run these using Mono, with no success.

For Mono 2.2 the application starts up and allows one to select input and output directories but dies immediately after the first image has been done but, sadly, before it is saved on disk. With Mono 2.8.1 it dies immediately when being started even before the GUI comes up...

This is definitely a neat trick, this is for sure!
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