Another strange bug with D4, possibly D800!?!?

I think he understands that.  The point he is making, I think, is that the exposure changes when he lets go of the AF-ON button.  When he presses af-on the focus locks.  When he recomposes the exposure changes as he expects it will and as you have pointed out.  But when he lets go of AF-on the exposure calculation changes dramatically.  Am I understanding the OP correctly?

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Mike Dawson