Video on 6D: focusing without any changes to metering (exposure)?

Hi everyone,

When I shoot video on Canon 6d and press AF-ON button, it changes focus AND exposure is also changing for a second or so, picture becoming a little darker or brighter, and this looks very annoying on video. Is there any way to focus in video mode (in the process of shooting) without ANY changes to exposure? I'm using "P" for video.

AF-ON is set to "Metering and AF Start", but there is no just "AF Start" option for this button in Menu C.FN.III-5 settings. AF mode for video is FlexiMode now...

Thanks a lot in advance. I hope it's not too noob question.