White Balance

[Wink]

Hi everyone,

I was watching a Lightroom tutorial by Matt Kloskowski about photo retouching and his first step was white balance correction.
He took the sample tool and found a grey area and the RGB scale shown in the target window were all about 82%. He went on about how that was as near to perfect as you could get.

This got me thinking...
I know a grey card is always referred to as a '18% grey card'.
I know pure white is 100% on the RGB scale.
I know pure black is 0% on the RGB scale.

So does that mean that the RGB value of the '18% grey card' is 82%? eg. 100 - 18 = 82.

If so it makes things much easier for correcting white balance in Lightroom if you don't have a grey card. Providing you've got a grey area close to that value in the photo.

Merry Christmas!

[ATJ]

Adam,

The important thing about white balance is to get a true grey object to appear as the same percentage of Red, Blue and Green. It doesn't matter (from a white balance perspective) if it is 82% across the board or 50% or 70%, etc. As long as a) the object is really grey and b) they are as close as possible to equal.

The first point is very important as while something may look grey, it may not be and if you use it your while balance may be off.

The actual percentage will depend on the exposure. When I shoot an 18% grey card where the card fills the whole frame and I let the camera choose the exposure, I get around 50% for Red, Blue and Green in Lightroom. This is to be expected as the camera is going for a neutral exposure which is 50%. If I had the grey card as a small part of the frame and the whole picture was exposed "correctly", I might expect a different percentage for the grey card, but RGB would be equal if the white balance was correct.

[Wink]

Cool.
Thanks for the explantion!

Dammit! Thought i was onto something there.

[surenj]

Thanks for this explanation Andrew. I am learning something new everyday.

[Matt. K]

If you can find a tone/colour in your image where the RGB readings are all the same...IE 12,12,12 or 45,45,45 or 82, 82 82, then that tone/colour is colour neutral...IE it has no colour and can be used to set the WB of your image. An 18% Grey card is equal to the 5th step in a 10 step black & white step wedge. It can be used as a reference in metering accuracy or to adjust or check White Balance. Used correctly it can be very useful. One way to use it is, say for instance you are doing a portrait under mixed lighting....you would include the grey card in the first shot and then discard it for subsequent shots. If you encountered a WB problem with that image you could use PhotoShops IMAGE/CURVES/GREY EYE DROPPER to click on the grey card and allow PhotoShop to correct the WB. You could then apply that curve correction to the other portrait images.

[Mr Darcy]

If you can find a tone/colour in your image where the RGB readings are all the same...IE 12,12,12 or 45,45,45 or 82, 82 82, then that tone/colour is colour neutral.

This won't work Matt. Think about it. If the colour of the image is off, any tone that reads nn,nn,nn will be different once the colour is corrected. If you already know a colour is neutral (e.g. a white door), then you can use the amount it seems to be off neutral to correct the image.

[Matt. K]

Oooops! You're right Greg. If you know a certain part of the image should be colour neutral then using the eye dropper in curves should correct the WB. After the correction the RGB values of that area should be the same...or close to it.