theSuede
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 p.2 #9 · Anyone here profile their cameras? Is it worth it? | |
South:
No, you're not "off-base"... :-) But there's quite a lot to "add up" even before any light hits the sensor surface! Maybe I should have written "sensor measurement result" in stead.
Sensors measure RGB values that are "post-projected". This means the raw data you end up with on your memory card is a result of multiplying several arrays of wavelength information curves together - illuminant spectrum, spectral reflectance of the target or "target colour", and the colour filters' transmittance. You basically have three arrays of values multiplied/integrated together into the raw values.
Change the illuminant spectrum and the raw values internal [R, G, B] distribution changes. WB by "light-temperature" implies that the light has a maximum power at "this" wavelength, and less at others - both above and below. The filters are constant. The target is constant.
Now what happens if we take nice, even, target reflectance (pure green?) and light it with very "warm" coloured light as 2800K? The green in itself has a response curve that looks like a symmetrical rounded peak centered at say 540nm, and the light has an intensity curve that that slopes down. Everything on the "warmer" colour side of the green peak will be amplified, and everything on the "colder" colour side of the peak will be attenuated. The peak is no longer symmetrical - it has a shallow slope at one side, and a steep slope at the other side.
Consider what happens to an orange target (mid between green and red) when the slopes behave like this. Raw "equal green and red" means one wavelength, one colour, under one lighting condition - but asymmetrical slopes will move the colour that gives "equal red and green" further towards either red or green. The profile has to correct for this. 50Red,50Green, 0Blue means one target colour in front of the camera in one lighting condition, and another with different light (even after WB).
This is what the temperature dependent LUT calibration is about. You not only have to account for different total strength of the R,G,B channels, but also their effect on the colour in between the primaries. And this only works when the light is "well behaved", like a light bulb that glows at a steady 2800K, or any other light with a black-body radiance spectrum like halogen and to a certain level, speedlights. Fluorescents are VERY asymmetrical, and "spiky" in their spectral emittance, so they're usually the hardest to profile as [light spectrum] x [target reflectance] can vary WILDY in very short distances - 550nm might be 3x stronger than 560nm...
So the sensor/system behind is indeed very "dumb", but it has to make some very intelligent decisions to get you the right colour.
Most dcp profiles that still incorporate the original Adobe temperature dependent initial correction tend to be usable over quite a large range of WB values, when I calibrate for D55 light that profile usually shows very small deviations in any lighting between 4500K and 7000K.
redcrown:
I don't have the profiles here, but I'll try to remember posting a link later tonight.
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