N5 + 50/1.8 steady focus in videos
The only way to do what you want is via focus tracking (tap the screen) and that's the best you'll get automatically with the 5n. Also, don't record @ F/1.8, record at something much smaller than that if you want continuous focus. But aside from that you're looking at some kind of steady shot system in manual focus mode with small apertures. Contrast detect isn't the fastest auto focus system out there and it'll never keep up with moving objects with wide apertures.