What are the equivalent RGB values when controlling color temperature?

[David]

Hi all,

If you shoot in RAW, you can adjust the color temperature, say in Photoshop's Camera RAW to make it warmer or cooler.

However, suppose your image is not in the RAW format, or you don't wish to go back to Camera RAW to readjust the color temperature, what are the equivalent Red, Green and Blue values (using Curves or Levels) that need to be adjusted to achieve the same effect?

I suppose the Green adjustment can be left out because that is separately adjusted under the Tint (Green-Magenta) slider.

To make the image warmer or cooler, we probably have to adjust the Blue and Red values together. Leaving one out may cause a color cast to appear.

But how do we know what is the proportion of each color to adjust? That is, how much Blue/Red to add or subtract to make the image nicely cooler or warmer?

I find that this can be achieved very easily in Camera RAW's Color Temperature slider, but in Photoshop itself, getting the right Blue-Red combination needs some careful adjustment. I can't always add or subtract them in equal proportion to make the image look correctly warmer or cooler.

Thanks v much for any input!

 

[ellery]

Dude use either lightroom or aperture for this - they work better than CS2 at least no idea if things got better in bridge for cs3 or cs4. Use the appropriate tool for the task (at least at level below professional DI artist)

 

[kandinsky]

But how do we know what is the proportion of each color to adjust? That is, how much Blue/Red to add or subtract to make the image nicely cooler or warmer?

If you expand the histogram, it will be easier to see what changes as you adjust the values.

 

[viix]

It helps if you have a grey/white reference in the photo. What I do is select the eyedropper tool, shift click the reference point (a few if I have to) and make sure my Info tab is open in Photoshop.

When I run a levels adjustment, I have the RGB values displayed in the info tab that tells me how much shift there is in those reference area.

It's should not be too hard to judge since you understand the histogram's RGB values

This helps when you want to match color temperature to all your other pictures as well..

A tip, you can control click on the reference points to define the sampling area size. I usually use the 51x51 average