I've had a chance to play around a bit now with the Nikon D3100 video. Specifically, I was interested to see how the AF-F continous video auto focus (full-time-servo AF) stacks up.
The short answer is, not so good.
It's choppy and intermittent. It struggles to maintain focus, often losing focus and having to hunt to regain it. Basically, I consider it close to useless.
That said, these results are from indoor tests, not dark but somewhat subdued lighting. To quantify, we're talking normal indoor lighting that my Canon HF200 camcorder has zero problems with. The D31oo doesn't even come close to performing on the level of my HF200 for auto focus.
Perhaps the Nikon D3100 AF-F is better outdoors in bright light, but indoors, I won't be using the AF-F auto focus. It stinks.
The question now that comes to mind is whether the Nikon D7000 will have better AF-F full-time-servo auto focus???
It's likely that Nikon is not equipping their low end DSLR with the best of the best of their new AF-F auto focus. Perhaps the D7000 will shine in this area. Let's hope so!
So, Nikon, I love ya, and while my D3100 is a fantastic little DSLR, the AF-F just doesn't cut it. I'm giving you an F in this department. Hopefully you'll blow me away when my new Nikon D7000 arrives.
Nikon D3100 Auto Focus First Impressions
Nikon D3100 at ISO 3200 with Bounce Flash Dialed Down
Nikon D3100 Comparison of ISO 1600, 3200, 6400, & 12,800
Nikon D3100 High ISO Wedding Images
Nikon D3100 Unboxed
Nikon D3100 vs Nikon D90