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I'm not really qualified to answer this since I've done only a bit of video with my 5100 and I've never really read the manual...but nobody's stepping in here
I don't believe there is any setting to make it AF continuously like a video camera - you would need to zoom and press the shutter button to refocus after zooming.
Really you're not going to get great results, especially at shalllow depth of field, unless you switch to manual focus and practice quite a bit. I don't think any of the WOW videos you might see online use AF.