Blown blue channel

[chrisleica]

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When trying to create a dng profile (using the x-rite colour checker), it fails because the blue channel is blown. I looked at the histogram for other pictures and I noticed in the majority of them, the blue channel was consistently blown.

 

Is this due to a problem with the sensor ?

 

Ideas/suggestions much appreciated

 

Cheers,

Chris

[adan]

Can you post a sample of a "blown" image and a sample (screen capture) of one of the histograms?

 

How are you white-balancing (and in what light did you shoot your ColorChecker test image)? How's your overall exposure (the black background of the CC can lead to slight overexposure if metered directly, although I wouldn't think enough to blow a color channel unless something else is also going on.)

 

It's not something I ever ran across with the M8, and with the M9 only due to user error in metering when I was still getting used to it.

 

A sensor problem would be low on my list of suspects - but anything is possible, I guess.

[chrisleica]

adan, thanks for responding to this thread.

 

I have the CC passport and used the grey card for white balance. I shall post the histogram from the CC this evening.

 

Cheers,

Chris

[chrisleica]

Here is a jpg version of the file and a screen capture in LR3 showing the histogram:

 

[ATTACH]226768[/ATTACH]

 

[ATTACH]226769[/ATTACH]

 

 

Thanks for any help.

 

Chris

[adan]

Great! - some comments.

 

One, the histogram is NOT showing a blown blue channel (or there would be a big blue spike on the right-hand side - lots of blue pixels hitting "255"). At least not as I define "blown" (overexposed to the point of no visible detail).

 

Two - however the CC itself looks a tiny bit overexposed - the lighter gray patches look white and the pastel tints in the middle of the upper half of the CC also look washed out. But the critical blue and cyan patches in the lower half do not look so overexposed as to be unusable for profiling (especially the really critical primary blue (third row, first patch) - perhaps the auto profiling software is pickier than I am...

 

I wonder about your lighting, though. Ideally, a base camera profile should be done with "normal full-spectrum daylight" - direct sunlight close to noon, no clouds. This looks like a combination of open shade and low (late or early) sunlight - I see a shadow on the red wagon, but no shadows in the top part of the image? I can't tell for sure which lighting the CC is getting, but probably not plain white sunlight. Which may have pushed the WB so far one way or the other that that is causing the error.

 

Also as regards lighting and white balance - putting the CC amidst a lot of foliage reflecting green light may also be skewing the WB.

 

Here's how my M9 calibration shot from a year ago looked: 1 pm-ish daylight (5200K), tone in all the gray-scale patches (whitest patch about 235, 235, 235) - with the histogram for the image during RAW conversion - the yellowish cement background is separating the red and green from the blue in the biggest "hump" - but the two darker spikes show just about perfectly color balance (red, green and blue spikes almost exactly overlaid).

 

What you may need is a reshoot in more neutral light and setting (never wear a colored shirt when shooting a calibration test ) And bracket a little to the underexposed side to be sure of getting a full tonal range throughout the gray patches.

[epand56]

Andy, extremely instructive post, thank you.

[chrisleica]

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adan,

 

Thanks for your comprehensive analysis.

 

As for the "blown" channel, this is what the CC passport software reported when trying to create the profile.

 

I shall choose a better lighting setup in the future.

 

Cheers,

Chris

[roguewave]

Andy, extremely instructive post, thank you.

 

I agree.

[Nicoleica]

I agree.

 

Me too. Thanks Andy.