What i'm about to say some people will disagree others will agree.
Do not use autofocus when shooting video.
this link will give you the best instructions on how to shoot great video
Sure, the few times I've shot video, I focused manually. I understand video AF still lags far behind still AF, but really I am asking why it happens. It seems weird. If would be one thing were I stopped down, but I was shooting wide open. Pressing the AF ON button, the scene brightens dramatically, like more than a stop or two. What causes that to happen?