What does "Auto White Balance" Mean ?

[daveleo]

What does AWB mean?

It means one defers decisions about parameter optimization (in this case color temperature) to strangers who can not anticipate every possible circumstance required to optimize the parameter(s) of interest.

Often AWB can not succeed (regardless of camera brand). AWB does not work when a scene is lit by two or more strong light sources with very different color temperatures. A few common examples would be: blue color casts in shadows when a scene is lit by daylight; an interior where one are is lit by tungsten light and another is dominated by outdoor light from windows. AWB is impossible because a single set of color temperature parameters is insufficient.

I understood the impossibilty of getting mixed-light colors correct in complex light situations, by setting a single degK to establish the colors.
What surprised me is that a uniform color shift (caused by an 81A filter) in a single light-source image was not even "corrected" accurately, when set to "auto WB".

 

[daveleo]

Just to close the loop (for me), here are two pictures from today.
X-E1 & 35mm Fuji lens, set on Auto WB (no color adjustment, camera JPG output) w/wo an 81A filter.
Clearly (to me) the WB algorithm is not correcting very accurately . . . the image with the filter looks warmer than the one w/o the filter.
If I did my 81A color shift homework correctly, this means that the autoWB feature is inaccurate to more than 500K.
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[uhoh7]

You have fixed the crappy fuji auto WB!!

 

[willie_901]

So Dave, what if you used a filter that blocked 95% of the blue light?

At that point so much information (blue-light amplitude) is missing, the mathematical model used to compute the WB is no longer valid. The algorithm requires a model (equation) and all models depend on assumptions. In the case of auto WB, one of the the assumptions is the light is not being modified by a color filter.

How does/could the WB algorithm know the light you care about in the scene doesn't have a temperature shift equal to the filter's?

In your case you are filtering enough information to throw the WB calculation off by 500K. You empirically determined the filter's blue shift. I would be shocked if the camera could compute WB with this filter attached..

 

[Bille]

If I did my 81A color shift homework correctly, this means that the autoWB feature is inaccurate to more than 500K.

Auto White Balance is not going for "neutral" at all cost. The software is trying to guess the setting and apply a relative correction.

 

[judsonzhao]

The auto balance does not simply add/minus yellow to your image, however your filer only functions as such. The differences in between these two eventually leads to the results you've seen

 

[daveleo]

^ and ^^ .... I understand that. I am simply amazed that auto-WB does/can not "correct" for relatively minor (81A) uniform color shifts.

It produces pleasant results for casual shooting, but really if you need to nail those colors, you need another process (manual WB? RAF computer processing? whatever).

@willie_901 : I see your point. If it screws up an 81A correction by 500k, the error must become more absurd had I used a stronger filter (which sadly I gave them all away).

 

[willie_901]

Except for artistic effect, a high-quality, thin clear filter is the nest way to protect the front lens element and take advantage of auto WB.