White Balance: How Does It Work, Really?

[Tom Voigt]

I am interested in delving into the specifics of white balance.

- Is it simply an adjustment of color temperature?

- In other words - does it affect individually the R, G, B signals or is it a simple continuum from blue to red?

- How does white balance differ from the Red-Green levels available in the GL-2 custom presets? Does one affect the other?

- I do mostly theatricals with stage lighting. How can I set a consistent and repeatable white balance without walking on stage with a white bedsheet during the performance?

- How does auto white balance work? Obviously the camera is not assuming that someone is waving a gray card in front of the lens. How does it decide on a white balance setting?

- On the Gl-2 does auto white balance stay constant for the duration of the shot? Most of my shots are over an hour long....

The genesis of all this is taming the Red Monster on the GL-2 and matching footage to my Optura Pi's. The Opturas deliver good color either on auto white balance or on the incandescent setting. Not so the GL-2. Incandescent settings deliver reddish or yellowish footage. Color correction with color wheels usually involves pulling down and too the right along the 315 line towards blue-cyan.

Thanks!

-Tom-

[Graham Bernard]

Tom, excellent question. I've asked exactly the same! "So how does my XM2 know what its looking at when AUto WBing? Come to that when I do manual WB how are the 'graphs' for each of the colours being adapted? " As I said I've asked the same question on more than one occasion. Almost been told, "Well it does .. now go away and film!" - I'm not very comforatable wth this response, but life had been too short to bother any further - just got on with the job! But, you askling this in the way you have has made me re-immerge wanting too to know how the circutry "deals" with the 3 filters.

And yes, we all know we can "fool" the 3 chips by placing warm or cool cards in front of it. If WE can do it what on Earth is happenning that allows the AUto to do it "correctly"?!?!

Best regards,

Grazie

[Robin Davies-Rollinson]

Graham,
You've started something now, you rascal...
Apert from doing my head in, I've spent ages Googling around for an answer as well.
It seems to be a better kept secret than Tony Blair's next election.
The closest I could get on a newsgroup was it "could" be that the image processor in the camera looks for the bightest pixels in the image and assumes that they are white. Using some complex algorithms, the processor balances all the other colours in accordance to the perceived white.
Perhaps someone should ask Stephen Hawking...

Robin

[Ken Tanaka]

Here's a fellow who's explored the matter a bit with a digital still camera.

[Graham Bernard]

Sorry I haven't answered until now . .been filming some actors in London today . . . excellent . .! It was a "Teach-in" by a "well-known" coach - NO I don't mean the Greyhound Bus company. .

So . . Manual White Balance ISN'T. It is not manual. It is at the desire and wishes of some preset thingy-wotsit-doofer-algyNON - yeah? My point completely .. okay my next bit of "Rascalism" is . .. well .. to make things "truly" manual, one SHOULD have a dial on each of the 3 chips and adjust accordingly . . ."I thank you . .. and good night!" Me case rests yer 'onour!!! . . see what I mean now? It isn't Manual!?!?! It is an affectation of Manual .. . it still allows the cammie to "make" a decision - yeah?

Best regards,

Grazie

[Tom Voigt]

That sounds like the ticket Graham.....

OK, press the manual white balance switch......Hmmm, Red 67, Green 71, Blue 56. Let's make it Red 60, Green 65, Blue 70!

If that's indeed how white balance really works.

There is an analogous process for movie film where you ask the lab to "Color time to Grey card at head." They then adjust the intensities of the RGB lights to both set exposure and balance the color.

-Tom-