Canon PowerShot G7 X Mark III, Advanced User'S Manual, Page: 138 / 329

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Capturing a white subject that is too bright or dark may prevent you from
setting the white balance correctly.
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Choose [Cancel] to choose a different image. Choose [OK] to use that
image for loading white data, but note that it may not produce a suitable
white balance.
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Instead of a white object, a gray chart or 18% gray reflector (commercially
available) can produce a more accurate white balance.
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The current white balance and related settings are disregarded when you
capture a white subject.
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