5DIII & 6D Video-Rec goes dark during AF, Nikon D800 does not

[canikonvideo]

5DIII & 6D Video-Rec goes dark during AF, Nikon D800 does not.

Hi everyone.  I'm wondering what the issue is here: when I start a video-recording with 5DIII or 6D (have used STM & USM lenses to test) the screen goes dark (not black, just noticably darker) while I use AF-ON to autofocus during the actual recording.  The aperture is wide-open and I'm on manual exposure 1/50 with manual ISO.  This 'goes-dark' happens everytime I use the AF-ON to focus, even if the object is already in focus (just to test).  I'm indoors so the lighting isn't changing.  It's a drastic thing, completely unusable for any work as it's in the actual recording upon playback.

Interestingly, my Nikon D800's live-view only goes dark during AF-ON before recording.  Once I start the recording, I can still AF using AF-ON, but the exposure/display doesn't drastically change.

Have I missed an obvious setting somewhere?  Or is it something to do with Canon assigning AF-ON (& shutterbutton) as always including metering when you want AF, as in "metering and AF start", but Nikon assigns AF-ON as "AF only...no metering" ?

Thanks for any tips.  Also as an aside, does the 6DII have the same issue? 

[canikonvideo]

Update: I managed to quickly test a 6DII in a shop and it doesn't do the darkening-exposure thing when using AF-ON during recording.  I wonder if it's a limitation of the live-view of 5DIII/6DI?  Or maybe it's a setting I've got on both cameras causing this.  I've always shot with MF lenses so never had this before.