Would A7 III owners like to help research into this and report their findings...
First of all find a bright red object, preferably a car in daylight.
Set white balance to auto / ambient priority white.
Set Picture Profile to Gamma HLG and Colour Mode to BT.2020
Black Level -15
Saturation +10
Now test two clips - one with Colour Mode BT.2020 and one with Rec.709 (both with HLG selected in the picture profile). Use HLG not HLG1,2,3, etc.
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What to do...