Square Sensor Artifacts in Smooth Sky Gradient?

[mcmorrison]

Hello,

I am working on a 5D MarkII raw image of a large, smooth, post sunset sky gradient. It appears to have slight artifacts, roughly in a checkerboard pattern, that are visible on screen and print. They are not dramatic, but are there. Various NR settings in LR3 (2010 process) do not remove it. There is some contrast added which strengthens the issue. The checkerboard appears as low-saturation magenta/green patches overlaid on the sky gradient.

My sense is that at root this is a sensor artifact, but my primary question is what might be an appropriate strategy for removing it. Metadata = iso 1000, f/5.6, 1/800 second, Zeiss Planar T 85mm f/1.4.

Thanks,

Michael

[DarkPenguin]

Sample?

[mcmorrison]

Hello,

See attached file!

Thanks,

Michael

[DarkPenguin]

To me it looks like noise.  What are your NR settings for that?

[mcmorrison]

Hello,

See the attached screen shot of the "Detail" panel.

There was a fair amount of noise in the image that was successfully removed with these settings. These "checkerboards" seem much coarser, and seem to be understood as real image data by the NR algorithms. . .

Thanks!

Michael

[John R Smith]

Michael

Those NR settings look very high by my standards, but maybe that's OK at 1000 ISO. However I would be suspicious of Masking set at 100 which in my experience can introduce artefacts of its own. The rest of sharpening seems entirely reasonable. Have you applied a grad to the sky in LR? This itself can really increase problems with noise. The other thing to try, just for interest, is processing the file in alternative RAW converters like Canon's own or C1, perhaps, to see if there is any difference. Alternative demosaic algorithms can give surprising results.

John

[Ken Bennett]

That sort of magenta/green blotchiness is pretty common in smooth dark tones at higher ISO values, in my experience. Color noise reduction can get rid of the smaller color pattern, but often leaves this larger pattern intact. I used to see this all the time in gray studio backgrounds, even at lower ISOs.

You could try other raw converters -- when I first started shooting digital, Capture 1 was the only raw converter that could fix this problem in the original EOS 1-D files. Mmmm, it made nice neutral gray smooth backgrounds. Haven't used it in a while, but it's worth a try.

Another option is to do a second conversion with a huge color noise reduction value, and blend that into the sky where you see the problem.

[mcmorrison]

Hello John and k. bennett,

Thanks! This is helpful!

Michael

[Lightsmith]

Whenever I see artifacts or banding from using Adobes RAW converter I find that if I use the Nikon or Canon converter instead to do the conversion and save it as a 16-bit TIFF the problem goes away. I do see a lot of noise in 5D Mark II files that people seem to overlook or feel is normal but is actually quite high when compared to most other cameras (30D, 40D, 1D Mark III, D300s, D3) at the same ISO setting.

[MGH]

Hi...Can I ask, did you use a NDGrad filter with this shot ?  I am haveing a similar problem, I´m seeing green and magenta banding in a RAW file in a grey sky. I can still see it even when printed and I can still see green and magenta even when in monochrome picture style in Canons DPP. Here´s a screen shot of the RAW file with monochrome picture style in DPP, I pushed exposure down to -2 so you can see it better.
   I think the problem is caused by the LEE NDGrad I used, but I may be wrong.

 
Dibujo by marcodemalaga, on Flickr

[madmanchan]

This is banding, probably due to lack of dithering during the conversion to 8-bit for display.