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 Author Topic: White balance in ACR  (Read 3259 times)
finnhaug
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 « on: October 24, 2006, 04:50:29 AM » Reply

Hello to all!

When correcting WB, there is 2 sliders, one for temperature and the other for tint.
Why is it so that when correcting the temperature we only use the yellow/blue axis in the color wheel, and not (also) the red/cyan? Is there a logical explanation to this?

Regard
Ken
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jani
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The short answer is that white balance adjustment is along two perpendicular axes.

By adjusting tint, you're adjusting the axis from a colour close to green to a colour close to magenta.

Red-cyan would not be a perpendicular axis to yellow-blue, so it wouldn't make much sense to adjust that.
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Jan
finnhaug
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Im aware of the two perpendicular axes, but im afraid im not following you from there...
I do not really understand why not all 3 axes are involved when correcting the WB, like for instance "Color Balance" in PS. It obviously work with these 2 axes in ACR, but I am curious what theory is behind this. Would for instance red/cyan work together with green/magenta? Hm....

Regards Ken
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bjanes
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Quote
Im aware of the two perpendicular axes, but im afraid im not following you from there...
I do not really understand why not all 3 axes are involved when correcting the WB, like for instance "Color Balance" in PS. It obviously work with these 2 axes in ACR, but I am curious what theory is behind this. Would for instance red/cyan work together with green/magenta? Hm....

Regards Ken
[{POST_SNAPBACK}][/a]

The diagram in the correlated color temperature of this link may be helpful:

[a href=\"http://en.wikipedia.org/wiki/Color_temperature]http://en.wikipedia.org/wiki/Color_temperature[/url]

Bill
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Graeme Nattress
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Basically, I think.....

Only True black body radiators have a "colour temperature". These are indicated on the curved line you see in the CIE diagram. When you change white balance, you're changing your point on this line. Tint moves you perpendicular to that line to correct the white balance of non-black body light sources. White balance is hideously more complex than thinking of colour as along two axes. I'm still trying to get my head around some of it's nuances.....

Graeme
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finnhaug
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The dive into physic seems quite a complex matter...
Thanks for responding guys, I think I got some understanding of the subject now, though Im still far away from fully see what WB is all about....
Regards
Ken
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PeterLange
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Supplementary, here’s a related quote by Bruce Fraser:
http://www.macworld.com/2005/03/secrets/marcreate/index.php

>> The Temperature slider indicates, in kelvins, the color of the light for which Camera Raw is trying to compensate. Moving the slider toward higher color temperatures (bluer light) results in a warmer, yellower image, while moving toward lower color temperatures (yellower light) results in a colder, bluer image. You can think of the Temperature slider as a blue-to-yellow control.

The Tint slider controls the axis that runs perpendicular to color temperature, so it’s essentially a green-magenta control—negative values add green, positive ones add magenta. <<

Color Temperature, Tint and Exposure can be seen as differently associated Levels’-highlights slider for R/G/B input and output.  They represent three degrees of freedom which are separated / composed of linear scaling per channel (aR/bG/cB; wherein CT and Tint control the ratio of multipliers a:b:c, thus changing colors in terms of R:G:B, whereas Exposure scales linear with a=b=c).  It’s finally a kind of maintenance of dimensions.  Further, these settings are applied on the native date before they get distorted by any tone curve.  It’s properly implemented with ACR as far as I can tell.

Peter

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Hermie
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Short version: http://www.etcetera.cc/pub/index.php/artic...cleview/12/1/5/
Long version: http://doug.kerr.home.att.net/pumpkin/Color_Temperature.pdf

Lots of info hidden here: http://www.nikoncafe.com/vforums/showthread.php?t=91217 (Iliah = Iliah Borg = RawMagick)
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PeterLange
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Quote
From page 8: >> In photography, we often compensate for the chromaticity of the ambient illumination in order to produce the “expected” color result in the delivered image. This may be done with the use of a filter over the lens, or (with a digital camera) by image processing either in the camera or in subsequent “post-processing” software. This is often spoken of as of obtaining proper “white balance”. <<

Brief comment:

Given that digital white-balance compares to the use of a correction filter, the obvious question is:  why should I want to white-balance an image, when I wouldn’t have used a correction filter in times of classic film.  Or in other words, said proper neutral white-balance can easily wipe out the mood of a luminous landscape, e.g. the warm late afternoon or cool shade look.

In such cases I wouldn’t miss simply to try the Daylight setting in ACR. Maybe the Tint will need some correction, because Adobe didn’t calibrate it for your camera + lenses.

My 2ct.

Peter

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