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10-11-2011, 05:45 PM   #1
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K10d Issue at ISO100

Hi

I have had my K10 for a while now and am just getting back into photography more seriously.

I have recently taken some images (RAW, DNG, custom white balance off a white card on the back of the grey card) of a 18% grey card to build a zone ruler at ISOs 100, 200, 400 and 800. Everything behaves as expected apart from zone IX (+4 stops) and above where the image, instead of blowing to a complete white result, I get a huge magenta cast. This behaviour only appears at ISO 100. I looked at the colour channel histograms in camera and on PS, and both show a saturated red (i.e. 255) as expected, but the blue was about 240 and the green about 225. I have seen this in my photos before (from when I got it new) where the white highlights have blown, but just thought it was the angle of my screen or it's calibration - but this only appears at ISO 100 under a controlled test. I managed to get all the colours to saturate under ACR, but had to slam some of the sliders to the max, not really what I want to do!

Has anyone seen this before? is the sensor broken? have I got some setting(s) wrong?

Any advice would be greatly appreciated.

Cheers

MAtt

10-11-2011, 08:57 PM   #2
dms
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So your saying the green was blown (you see magenta)--but the numbers say otherwise?
10-11-2011, 09:09 PM   #3
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No, the red was saturated i.e. 255 the blue was around 240 and the green around 225.
10-11-2011, 09:11 PM   #4
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BTWI did what you are doing, but with my K20d and approaching clipping I did not see anything other white and then clipped. But when I do the same with a single color (green, then red, then blue) the green seems to clip earliest, then blue and then red.

10-11-2011, 09:14 PM   #5
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That's what I meant if I understand you. The numbers say the green is not clipped--but if you see magenta (red+blue) it suggest the green channel is clipped.
10-11-2011, 10:01 PM   #6
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Ok, I understand now - thank you. I would guess that both green and blue are clipped as if I push the exposure to +5 and +6 etc, the values don't change - it's just that blue is clipped at a higher value than green along with red saturation I guess that's why I see a magenta cast.

So the question remains, is this a sensor issue, a user issue, or a camera setting issue - comments and help appreciated.
10-15-2011, 08:43 AM   #7
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wondering.... the monitor you are looking at these settings, is it calibrated?
if not, it will have to be

let us know how you make out

cheers
10-15-2011, 03:36 PM   #8
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I haven't got a calibrated monitor. However, the colour channel histograms in camera are pretty much identical to the ones I see with ACR (i.e. Red at 255, blue around 240 and green around 220), tho the camera LCD does show it as white as expected. I will upload the 16MB DNG file if I can find out how, so that some kind person with a calibrated monitor can have a look at the file.

As I stated before this only happens at ISO 100, in camera, ISO 200 and above is no problem the histograms are as I would expect for a deliberately +4 stop over exposure.

Cheers

Matt

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