Autofocus
In the D3100, Nikon sticks with the tried-and-tested 11-point AF found in the D3000, with options of single-point, dynamic-area, auto-area and 3D tracking using the Multi Cam 1000 AF processor module. The 11 AF points are fairly central in the frame, so focusing for the edge of the frame can be tricky. In single-point mode, any of these 11 points can be selected. Using the 18-55mm kit lens I found that generally autofocus is rapid and responsive, but slowed down slightly in low-light conditions.
When using the tracking autofocus mode with Live View the focus area tracks moving objects, as demonstrated by a green box. This is useful for focusing when panning with a subject, but I experienced mixed results in this mode because at times the tracking area lost its subject in the frame. Generally, I opted for single-point or dynamic-area focusing. In Live View and video modes four AF options are available: normal, wide, subject tracking and face detection, although manual focus is a better option.
