Can I set white using a non-neutral color?

Klorenzo

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Hi, I have several pictures of an event shot under a few different lights (natural, artificial, etc.). I did not take a picture of a gray card in all of these situations.

I know that I can use any white/gray element for an approximate WB but I already have a good starting point with the auto wb and I want to do something more precise and uniform across different pictures.

Now I'm wondering if I could use one specific color as a reference, like a red dress. Something like this:

- I "measure" the WB on red color in a picture shot with natural light and I get a "wrong" temperature like 2900K
- then I "measure" the same color in a picture with artificial light and I get 2500K

So now I know that I have a difference of 400K in the two pictures and I can take the temperature of the second picture and set it to the first minus 400. From a couple of tests I think this does not work.

Is there any trick like this that I could use? Or something to make a quick "color comparison" of the dress color in different photos?
 

barry13

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I'm not sure how it's done, but I read that Sports Illustrated knows all the colors of the NFL uniforms and uses that info to do color correction, so it must be possible.

Barry
 

OzRay

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You can use pretty much any colour, but you're not really setting 'white balance' but calibrating to a specific colour. If you are using Lightroom, you can copy the colour adjustments of one shot and paste that to all related shots and then do whatever you want with the other settings.
 

dwig

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...
Now I'm wondering if I could use one specific color as a reference, like a red dress. Something like this:

- I "measure" the WB on red color in a picture shot with natural light and I get a "wrong" temperature like 2900K
- then I "measure" the same color in a picture with artificial light and I get 2500K

So now I know that I have a difference of 400K in the two pictures and I can take the temperature of the second picture and set it to the first minus 400. From a couple of tests I think this does not work.

...
The Kelvin scale used in the White Balance control is not linear, its a geometric progression. A 400k shift in the neighborhood of 3000k is a vastly different shift than a 400k shift in the 5500k neighborhood. Also, choosing a strong color (e.g. red dress) is a very poor strategy for several reasons.

You should choose the most neutral color that is in both the "good" image and the "bad" image you are trying to correct. You need to sample the RGB values for that object in the "good" image and then adjust the color in the "bad" image so that you get the same sampled RGB values. In LR, you can sample the values by reading the numbers displayed below the Histogram while the cursor is over the chosen target. In PS, you can set several sample points and can control the size of the sample (number of pixels in the sampled array).
 

Klorenzo

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You should choose the most neutral color that is in both the "good" image and the "bad" image you are trying to correct. You need to sample the RGB values for that object in the "good" image and then adjust the color in the "bad" image so that you get the same sampled RGB values.
Is there a simple strategy to "convert" the RGB difference in a Kelvin difference? I do not expect a direct "mathematical" mapping, but maybe some "tricks".

Suppose I sample a white something in the "good" picture and it measures 220, 230, 240 and the second one it is 210, 220, 250. Now I can see the second one is shifted towards the blue, but how much? What about the green difference?
I think I could just try small corrections in WB and tint trying to make them reasonably close. It looks like a lot of work.
 

OzRay

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It becomes difficult to do because you are adjusting not just colour temperature, but colour hue as well. I had to do this with a wedding I once took in a church that had the most abominable lighting that I've ever encountered, arc lamps, daylight coming through huge multi-coloured leadlight windows and incandescent lighting as well. All I could really do was set white balance on the bride's dress and then spend hours doing fine adjustments trying to keep the dress white, but with detail, while trying not to make the groom's dark blue suit go completely black. I hate wedding photography, I really, really, hate wedding photography.
 

Ramsey

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there is a small tool avaliable in LR that can help you pick the neutral color (like grey) and it automatically sets the WB for the rest of the picture. That can be a good starting point.

Subscribe to see EXIF info for this image (if available)


But it is true you'd have to do it for each individual picture (or copy paste the settings for the pictures taken under same/similar light).
 
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