http://imaging.nikon.com/product ... /slr/d3100/spec.htm
[NEW]
Subject-tracking AF
D3100 ‘memorizes' a subject then automatically tracks it even if it moves. Should the subject momentarily leave the frame, the camera starts tracking again once it re-enters. By using this with AF-F (full-time-servo AF) you can also maintain focus on the subject while tracking. This is a great way to photograph active children and pets.
Note: Autofocus operation is interrupted while the camera is tracking the subject.
http://imaging.nikon.com/product ... 85mmf_14g/index.htm
