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This answer describes how Lightroom measures white-balance, primarily using a "Temp" slider and secondarily using a "Tint" slider.

My Sony NEX-5R seems to use a different system for white balance settings. In addition to the temperature, there's a second component called "A-B", and a third one called "G-M":

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First, where are there three components? Two should be enough to represent any color.

Second, How do I map these to Lightroom? In other words, how should I set Lightroom's Temp and Tint sliders to correspond to the same white balance settings as 5500K, A-B: A5, G-M: M4, in this example? I've set LR's Temp slider to 5500K, but what do I set its Tint slider to?

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The overall answer to your question: The system used by most digital cameras is based on the way color was fine-tuned in the film era. The color temperature setting corresponds to the film selected, and the AB and GM settings correspond to bias introduced by color conversion filters used during shooting or color compensating filters used during printing to fine tune the color of a particular film to a specific light source.

Since the color gamut your camera is capable of reproducing is two dimensional one might tend to thing you would only need two axes to express a particular wavelength and intensity of light.

But consider that most light your camera records is not a single, pure color at only one wavelength. It is rather a combination of many wavelengths. Since the human eye is particularly sensitive to three colors of light (red, green, and blue) and our brains use the comparative intensity of those three colors to express the color of everything we see, cameras have typically followed suit. Both color film and digital photography technologies almost exclusively use the combined intensity of red, green, and blue light in a particular light source to designate the color of the light.

The A <--> B axis on your camera is a fine tune version of the Color Temperature control. Say you want to use a color temperature setting of exactly 5550K. Your camera only allows you to select either 5500K or 5600K via the color temperature control. You can either select 5500K and then dial in some "A" (amber) to warm it slightly or you could select 5600K and dial in a little "B" to cool it slightly. The G <--> M axis on your camera corresponds to the tint control in Lightroom.

So exactly what color temperature and how much tint you would need to set in LR to correspond to an in camera setting of 5500K, +5 (AB), -4 (GM)? Since there is no precise standard amount of bias for each unit you would need to experiment to find the closest match. The +/- 1 amount of bias applied in most of my Canon cameras is loosely based on the amount of bias provided by a 5 mired color conversion filter or color compensating filter. These were filters used in the days of film to fine tune a particular film to a specific lighting source.

In reality I don't see a particular need to know the exact correspondence between the in-camera and Lightroom settings:

  • If you shoot RAW files, the in-camera setting is pretty much immaterial since LR ignores those in-camera settings when it opens and renders a RAW file.
  • If you are shooting JPEG files, the in-camera setting is the crucial one since information in the RAW data from the sensor is discarded during the in-camera conversion to JPEG, and in LR you'll only be able to alter the intensity of what was not discarded. You won't be able to recover any of the information from the RAW data that was discarded in the conversion to JPEG.
  • Thanks, Michael. This answers my question, except for the part about not needing to know the exact correspondence between the in-camera and LR settings: I shoot RAW, and a photo ended up badly color-balanced. So I went back to the scene with a gray card and set the camera's custom color balance function, and pointed it at the gray card to capture the color balance settings, which turned out to be 5500K, A5, M4. I now need to figure out how to apply these settings to the original photo. Just wanted to explain the need to know the correspondence. But thanks for letting me know there isn't one. – Vaddadi Kartick Sep 14 '14 at 6:42
  • That is one reason I don't use LR for most of my RAW conversion work. I prefer to use Canon's Digital Photo Professional that does apply the in-camera settings at the time the photo was taken with a Canon camera when it is first opened in DPP. – Michael C Sep 14 '14 at 8:19
  • Although you can't emulate it exactly, you should be able to get fairly close. Set the CT in LR to 5500K, then move the tint towards magenta until it looks close to the in camera result. If you have a way to also fine tune the AB axis then do that as well in amounts slightly higher than the GM axis. – Michael C Sep 14 '14 at 8:22
  • You should also be able to take a photo of your white card in the conditions the photo was taken and then use the photo itself from within LR to set a custom WB in LR. Use the picker tool to tell LR the shot of your gray card is white and then apply that to the other photo. You won't know the exact numbers LR is applying, but you should get results that are very close to what you could get in camera. – Michael C Sep 14 '14 at 8:26
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"A-B" is "amber-blue", or warmth bias; "G-M" is "green-magenta", or tint. The A-B setting is essentially a bias you can control around the colour temperature, so that if your camera would ordinarily have set the colour temperature to 5500K, a "A" filter setting will chose a slightly higher colour temperature so that the picture is rendered a little warmer (more yellow/amber), while a "B" setting will choose a slightly lower setting to render the picture cooler (more blue).

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How do I map these to Lightroom

Lr calculates colour temperature and tint based on white balance coefficients in the raw file, not based on the camera settings for colour temperature and colour bias (AB/MG). When the camera settings for colour temperature, amber-blue (those are in fact fine adjustments of colour temperature; so they make for 1 parameter together with colour temperature, not 2), and/or magenta-green are changed, new white balance coefficients are calculated in the camera. Lr takes those WB coefficients and calculate from their values.

The scales between Lr and camera are usually different. If a camera is set to some colour temperature for white balance, Lr reads it slightly differently. Same with the tint (amber-blue), and there is no guarantee that changing AB setting in the camera results in a proportional change in the tint in Lr.

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