White balance question

[Greg D]

Okay, I must be missing something here - Are the numbers on the white balance slider in LR3 supposed to be Kelvin temps?  I would assume so, but they seem to be backwards; i.e. higher numbers look "redder-warmer", not "bluer-cooler" (intuitive but not corresponding to K temps as I understand).  And if I set a Kelvin white balance in my camera, what comes up in LR doesn't jive with that.  What am I missing?

Thanks

[FranciscoDisilvestro]

They are not backwards. They are related to the scene not the output, that's why you see the contrary effect when you move the slider.
For example if you shot a scene under tungsten light (3200 K) you need to increase the blue channel to get a neutral white, so that's why the output becomes more cooler when you move the slider toward a lower temperature.

[Greg D]

They are not backwards. They are related to the scene not the output, that's why you see the contrary effect when you move the slider.
For example if you shot a scene under tungsten light (3200 K) you need to increase the blue channel to get a neutral white, so that's why the output becomes more cooler when you move the slider toward a lower temperature.

Okay, I get it - you match the slider to the K temp of the scene as shot, and that should neutralize it, correct?

[digitaldog]

They are not really backwards if you think about it a bit. What you are doing is compensating for the current WB. You are telling the slider how to correct the WB for the appearance you want.

Even more important, the numbers are rather meaningless (they describe a large range of possible colors with a single value). You need to season to taste, not try for an exact numeric value.

[bjanes]

Okay, I get it - you match the slider to the K temp of the scene as shot, and that should neutralize it, correct?

That is an oversimplification. Look up correlated color temperature on Wikipedia and then look at the color temperature sliders in ACR or Lightroom (ACR shown below). The top slider (blue-yellow) moves the color temperature along the Planckian locus, which describes color emitted by a black body radiator.

The sun and incandescent bulbs closely follow the Planckian locus while many other light sources do not. For these, an additional adjustment in the green-magenta axis (the second slider) is needed. This slider adjusts along lines of correlated color temperature, which run perpendicular to the Planckian locus.

Regards,

Bill

[PeterAit]

The eyedropper color tool is very useful, assuming there's something in the image that should be a neutral color. Click on it and - voila - the color temp is adjusted. It might not be exactly what you want, but it's a good starting point for manual adjustments.

[Greg D]

That is an oversimplification. Look up correlated color temperature on Wikipedia and then look at the color temperature sliders in ACR or Lightroom (ACR shown below). The top slider (blue-yellow) moves the color temperature along the Planckian locus, which describes color emitted by a black body radiator.

Yes, I know - that's what my mathematically simple mind needs  .  But thanks for the explanations.  I haven't really had any trouble using the WB sliders (or numbers or presets) to get the white balance right in my photos, this was mainly just a curiosity question.  Good to actually understand what I'm doing, sort of..............

Thanks to all for the help.

[Peter_DL]

Look up correlated color temperature on Wikipedia and then look at the color temperature sliders in ACR or Lightroom (ACR shown below). The top slider (blue-yellow) moves the color temperature along the Planckian locus, which describes color emitted by a black body radiator.

Yes, Bill,
however, the OP is also right that the Kelvin scale in ACR/LR is inverse
compared to the colors and Kelvin marks along the Planckian locus in the CIE xy chromaticity diagram.

Bruce Fraser wrote:

>> 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. <<


Peter

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