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I'm correcting white balance issues for my photos in Adobe Lightroom 3. The white balance picker tool in the Develop module lets me to pick a light neutral grey spot in the picture from which Lightroom automatically calculates the needed data for white balance corrections. I didn't had a grey card.

  • Can I set the picker so I could choose a spot that should be white (or black) after white balance corrections?
  • Or should I be just fine and consider the chosen neutral grey spot as underexposed (or overexposed) white (or black); since the neutral grey is — well, neutral, and white balance corrects the white balance, not the exposure.

Compare to e.g. Photoshop's curves and exposure tools which both have this feature (choosing either supposedly neutral grey, white or a black spot).

I do have access to Photoshop and could correct WB with it, but I'd like to edit the photos non-destructively.


Edit: After a closer look, the white balance picker tool doesn't ask for a target neutral grey spot but just for a target neutral.

In short: you must pick a spot which should appear as neutral — ie. where R ≈ G ≈ B — after Lightroom's automatic adjustments.

So it could be white (255,255,255), grey (128,128,128) or black (0,0,0), or any other neutral between.

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2  
Correct, it only cares about what should be a neutral tone. Black, shades of gray, or white are all fine because the RGB values are balanced, so, for the amount that they differ from neutral in the photo Lightroom will pull or push the RGB values back to a medium tone. As for having a gray card, when shooting my pro-rodeo work I'd find a banner or sign with no colors, defocus, and shoot a frame, and use that to set a white balance. It always worked well as long as I was sure it was a pure white, not an ultra-white, which has blue added to make it appear even whiter. –  Greg Jan 24 '11 at 5:50

5 Answers 5

up vote 4 down vote accepted

I think the key point with setting (just) white balance is that you pick something where the colour channels should have, more or less, the same intensity. Therefore, picking something that is supposed to be white should also have the desired effect, although it's best to stay away from the extreme neutral greys (white and definitely black). Using a specific tool to pick a white or black point is usually about more than just white balance.

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3  
I don't think picking an area that should be black (or too dark grey) will work, as a black object will be black regardless of the lighting. However I agree that anything from darkish grey to white should work as long as it has identical RGB values. –  Nick Miners Jan 23 '11 at 12:01
2  
Yeah, what Nick said. Basically, the closer to the middle the color is, the more data there is to work with. (Blown-out white is as useless as pure black.) –  mattdm Jan 23 '11 at 16:09
    
It actually was Lightroom's help file that got me confused as it states to choose a spot that should be neutral light gray. Now when looking at the white balance selector tool again, it actually asks for target neutral not for target neutral grey. Apparently I subconsciously appended "grey" where it shouldn't be. –  koiyu Jan 23 '11 at 17:18
    
Also, I did test varying the exposure and sampling the white balance from the exactly same pixel and - as expected - temperature and tint got calibrated in the exactly same manner. I can't remember what was so hard to understand when I asked this question, as it is all so clear now. –  koiyu Jan 23 '11 at 17:23
    
"black (or too dark grey)". Correct. It can't be RGB 0x0x0, there has to be some light, but a dark gray will work. –  Greg Jan 24 '11 at 5:52

You must take a sample from a place that should be neutral, not something that already is.

It can be anywhere you know to be perfectly white. The best is a white-reference card inserted into the photo but if you have fabric or other object known to be truly white, you can use that instead.

Lightroom will then compute how to make the selected color neutral white and adjust colors accordingly. Due to the way these things work internally, it will be more accurate if you choose something relatively bright but not close to being over-exposed.

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1  
"Lightroom will then compute how to make the selected neutral into white" It won't try to make it into white, it makes it into a neutral with equal RGB values. If it was RGB 70x7fx7f it would pull toward 7fx7fx7f, not ffxffxff. –  Greg Jan 24 '11 at 5:57
    
@Greg - Good catch! Corrected. That's what I actually meant. –  Itai Jan 24 '11 at 15:51
1  
It's very powerful and pretty smart. I had occasions when I fired well before my strobes were ready, so I was a couple stops below normal exposure and had ambient light from mercury-vapor floods tint the image. A single click on what I knew was a neutral tone poster or shirt kicked the white balance back to where it belonged. With some careful examination a lot of images turn up having usable neutrals and can be saved. –  Greg Jan 24 '11 at 23:45

The White Balance Picker tool in Lightroom 3 is designed to pick a neutral color, not a color of a specific tone. The general idea with neutral tones is that all three color channels have the same value. If you pick a near-white color of R255 G252 B253, Lightroom will assume those colors should be normalized, and average the individual color channel values out (the process is a bit more complicated, as it maintains perceptual neutrality, but thats the gist.). Lightroom can algorithmically determine how to adjust white balance based on the color shift to neutral of that one pixel, and will readjust the white balance of the entire photo.

Since the white balance picker tool works off of the basis that whatever color you pick should become neutral, regardless of tone. If you have a dark shade, middle shade, bright shade, or highlight tone that you believe should be without any color cast, then you can pick it and LR will readjust the white balance of the whole image. It should be noted that it is a lot easier to identify neutral shades in the middle tonal range, than it is in the shadows. Highlight tones can also be used, but it is a little more difficult to identify color casts to highlight tones. It is easy to pick a color that results in a radical undesired color shift, either in the temperature plane or the tint plane.

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I frequently use the eye dropper to choose white objects, and I can tell you, it works well. In theory, black could be used, but it will have more noise, which will make it more likely to have issues.

Also, remember that clothing usually isn't pure white, but most other white objects are pretty close, so... Still, it'll get you most of the way there.

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The white balance of a photo is determined by comparing a portion of an image to the neutral color (white/grey/black) with the same luminosity value. The difference between the neutral and the target area is then applied to the entire image, correcting the color cast.

In order to get the best result, a grey is preferred, simply because you have a larger margin of error.

For example, suppose that you have RGB values (on a 0-255 scale):

Selection             | Neutral               | Correction
--------------------------------------------------------------
128 x 118 x 138 (384) | 128 x 128 x 128 (384) |  0 x -10 x +10
255 x 254 x 250 (759) | 253 x 253 x 253 (759) | -2 x -1 x +3

If you notice the difference here, the 1st example (a middle grey) has a significant correction, and the correction of the white is less drastic. This could easily be incorrect in the second case though because the red value could be significantly higher, but since it is at the max value in the sample (255) the rest could be clipped.

For instance, if the Red value needed a -20 correction instead of a -2, but the point selected clipped the highlight, then the selected point would be corrected and the rest of the image would have a red cast leftover.

Summary

Any selected point can be used to fix an image's white balance, assuming that the point is exposed well enough to contain valid data. Since grey is centered in the exposure, a grey point will tend to be more accurate, and is therefore preferred for white balance corrections.

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