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set white balance with a grey card?



 
 
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Old May 17th 11, 01:35 AM posted to rec.photo.digital
David Ross
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Posts: 9
Default set white balance with a grey card?

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David Dyer-Bennet wrote:

On Wednesday, May 11, 2011 9:36:03 PM UTC-5, David Ross wrote:
Hi all

Although I am a beginner, I think I have a basic understanding about the
need to have white balance set to suit the prevailing light when taking
pics with my 60D.

However, i bought a pretty helpful book on the Canon 60D that included a
grey card to use to set white balance ???

Yep, a GREY card to set WHITE balance!!!

Obviously this must be right, but the book doesn't explain how setting
the grey balance is the same as setting the white balance, and I have
not (so far) read anything about setting the grey balance in anything I
have read.

So I am mystified - can anyone explain this?


Any "neutral" color card will work; you have to expose it properly.

What's needed is an exposure where all three color channels have
data -- no channel is "clipped" (off the top or bottom). White balance
is all about the relative exposure of the three channels.

So, white will work, grey will work. So long as they're precisely
neutral tones. Black would work, if the light was bright enough. They
all come out grey with suitable exposure.

I'm not sure why they prefer grey to white (the choice of card
with that book). If you fill the frame with the card, the AE system
will expose them all the same, so it doesn't matter. If you put the
card as one small item in the frame, though, a white card is more likely
to be exposed so at least one channel clips, whereas a grey card is
less likely; that could be the reason. Or maybe they want to use the
same card for other uses, like local exposure measurements; grey is
the standard for that.


Thanks

David
 




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