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Author Topic: Just curious about color bit depth  (Read 2591 times)
hjulenissen
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« Reply #20 on: November 05, 2013, 03:07:21 PM »
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Its funny - I didn't manage to find any data about the spectral response of the camera channels - though it should be easy to create one for every camera by photographing a spectrally defined colorset and doing some math. (I think principal component analysis for every color channel should do the job).
This seems to be one recording of the spectral response of the Canon 40D. There are many other hits on google, but I think it is going to be hard to compare them as long as details about recording conditions (and even y-axis) tends to be scarce.


If you have a variable frequency monochromatic light source, there should be no need for PCA.

I have the Colorchecker passport. Do you know if its reflectance vs wavelength is defined anywhere? Anyways, I think you'd need a fairly large set of patches (and/or just the right kind of orthogonality) in order to get a detailed spectral response estimate?

-h
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digitaldog
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« Reply #21 on: November 05, 2013, 03:12:50 PM »
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When the RAW file leaves the camera: Has the mapping of the RGB values to a usable colorspace already been done, or is it done in the raw developer?
In the raw developer (if you feed it raw).
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Andrew Rodney
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Christoph C. Feldhaim
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« Reply #22 on: November 05, 2013, 03:16:20 PM »
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If you have a variable frequency monochromatic light source, there should be no need for PCA.

This kind of laser is quite an expensive toy, though they do exist.

But knowing the Spectrum of your light source and the reflective spectrum of your patches you could work it out.
You'd also need to take the filtering done by your lens into account.
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hjulenissen
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« Reply #23 on: November 05, 2013, 03:25:35 PM »
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You'd also need to take the filtering done by your lens into account.
I have tried to make camera profiles using my various lenses, and found that it was not worth the effort for my applications and eyes.

Granted, a lens profile might be something simpler than a full spectral characterization of your camera.

Basically, I have one "shaded daylight" and one "flash" profile for each camera, and those seems to do the trick for me and my humble needs.

-h
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ErikKaffehr
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« Reply #24 on: November 05, 2013, 03:41:16 PM »
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Hi,

I would think that a simple white balance on a gray card takes care of color shift in lens. Where profiles are really needed is where light has noncontinuos spectrum that cannot be approximated by a black body spectrum.

Best regards
Erik


I have tried to make camera profiles using my various lenses, and found that it was not worth the effort for my applications and eyes.

Granted, a lens profile might be something simpler than a full spectral characterization of your camera.

Basically, I have one "shaded daylight" and one "flash" profile for each camera, and those seems to do the trick for me and my humble needs.

-h
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hjulenissen
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« Reply #25 on: November 05, 2013, 03:49:25 PM »
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I would think that a simple white balance on a gray card takes care of color shift in lens. Where profiles are really needed is where light has noncontinuos spectrum that cannot be approximated by a black body spectrum.
The most significant difference (I dare not say "gain" as color is a minefield) was switching from Adobe-supplied profiles to my own. The second largest difference was profiling different illuminants. The difference when profiling different lenses was too minute for me to observe anything worthwhile, thus I happily stopped doing that.

So what you are saying is that spectral properties of lenses ought to be "smooth"? That was my gut-feeling as well.

-h
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Vladimirovich
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« Reply #26 on: November 05, 2013, 03:52:21 PM »
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Each raw converter has to make some assumption of source "color space" here to render into whatever RGB processing color space is to be used (in ACR that is ProPhoto RGB primaries).

to cieXYZ/D50 then to ProPhoto primaries

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Vladimirovich
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« Reply #27 on: November 05, 2013, 04:00:15 PM »
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I would think that a simple white balance on a gray card takes care of color shift in lens. Where profiles are really needed is where light has noncontinuos spectrum that cannot be approximated by a black body spectrum.

for ACR/LR users Eric Chan said (one of many quotes from many years ago) :

"...So to answer the question of WB and it's relevance to building & using DNG profiles: in principle, the applicability of a profile with a given WB has to do with how close the spectrum of the scene illuminant (used to photograph your real image) is to the spectrum of the illuminant used to build the profile. The closer they are, the better the results. Many flavors of natural daylight are spectrally similar (weighted differently), so this is why a daylight profile tends to work well in many flavors of daylight regardless of the actual CCT (e.g., 4700 K thru 7500 K). So even if you built your own profile under real daylight that ended up being around 6200 K, you should not hesitate to use such a profile under other similar daylight conditions, even if the CCT values vary a lot. But if you end up using a very different type of light (e.g., a fluorescent tube) then you should effect quite different, possibly unpleasant results, even if the CCT measures the same (e.g., a 6000 K office fluorescent tube)..."

so profile building (using 24 patch target and Adobe PE) for natural illumination is a snake oil or placebo... if you don't like how Adobe renders - then just replace/modify the LUTs (or get rid of them altogether with dcptool) with Adobe PE(or QP Card software) w/o actually shooting a target.

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ErikKaffehr
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« Reply #28 on: November 05, 2013, 04:01:56 PM »
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Thanks for making that clear.

Best regards
Erik

to cieXYZ/D50 then to ProPhoto primaries


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Vladimirovich
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« Reply #29 on: November 05, 2013, 04:10:35 PM »
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Thanks for making that clear.

Best regards
Erik


not me, that was again from Eric Chan.
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