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White balance on White or Gray card?


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#1 william koon

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Posted 11 October 2011 - 11:48 PM

I am quite confused of the white balance on video camera. Normally white paper is used to execute the white balance but why Gray card can be used too? Does this mean that the gray colour will be neutralised to 'white' since a white paper with colour tint will have the colour tint neutralised during 'white balnce'? Please help explain in details. Thank you very much.
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#2 Patrick Cooper

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Posted 21 October 2011 - 07:23 AM

A grey card is white on the opposite side. Perhaps they are referring to white balancing off the white side of the card. I don't see the point of white balancing off grey - surely that would cause an unnatural colour cast.

Edited by Patrick Cooper, 21 October 2011 - 07:24 AM.

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#3 Matt Dennie

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Posted 10 November 2011 - 02:50 AM

I always use a grey card for white balance. Never had a problem. Of course, I'm coming from a photography background.
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#4 Chris Millar

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Posted 10 November 2011 - 07:27 AM

First and foremost define 'white'.
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#5 David Mullen ASC

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Posted 10 November 2011 - 08:40 AM

If the white or grey card are neutral (no color tints), it doesn't matter which you use to white balance -- white is just overexposed grey after all and grey is just underexposed white. The main reason some people say to use a grey card is that you can't have any color channels clipping when white-balancing, hence why often you have to stop down a bit when pointing at a white card in sunlight or white light that is very bright.
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#6 Chris Millar

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Posted 10 November 2011 - 02:58 PM

I was going to try to lead you to realising how it works by asking questions in a way that you'd discover the answer yourself ;)

David has given you some very large clues though !

Anyway, back to the questions - take a look at this image:

Posted Image


Ask yourself, how would a sensor interpret the greys in the boxes marked A and B ?

We interpret them as A being darker than B, but take the image into photoshop or something and check the RGB values...

It's kind of backwards to the topic at hand, but illuminating nevertheless Posted Image
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