>I have discovered that D4's Face-Priority Auto-focus in movie >mode is buggy. I recently did some oral history and was very >disappointed with the results. Despite the camera being >mounted on a tripod, the subject being stationary, and the >photographer not touching the camera or lens after starting >the recording, the D4 engages in persistent auto-focus >activity that: (a) moves the focal point slightly forward and >back, reducing the quality of the video recording: (b) makes >noise, reducing the quality of the audio recording; (c) >sometimes changes the exposure value, causing the image to >lighten and darken, reducing the quality of the video >recording. Similar problems are observed with other video >settings. > >The poor results can be seen at http://youtu.be/qotGpcy5kFo >
For this reason, in the situation you describe, try AF-S, focus once before starting filming, and then start the video recording. Works great for the focusing situation you describe above plus of course no focussing noise.
I haven't tried the AF-F mode on my D4 yet, but I plan to try it soon if only to see if it has gotten any better than my D7000.