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It's real function is to replace the optical thickness of the colour balance filter which is removed to enhance the red response.
This allows auto focus function to work 100%
(The filter that is left behind....the front dust shake/ anti alias filter is actually a UV-IR cut filter! So if you don't really need 100% accurate auto focus, you don't need to replace the colour filter! I use my Canon with no replacement and mum and the kids can't tell the difference)
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