Article: Autofocus Areas

Where autofocus mode chooses when to autofocus, the Autofocus Area parameter tells your Nikon D3100 how to choose which of the 11 focus points in the viewfinder should be used to evaluate and lock in focus.

Ordinarily, your camera would like to be able to choose among the available AF points itself. In fact, that’s the default behavior, and when AF-Area mode for Viewfinder is set to Auto-Area, the D3100 chooses the focus point automatically in Auto, No-Flash, Portrait, Landscape, Night Portrait, and PAS (Program, Aperture-priority, and Shutter-priority) exposure modes. Giving the D3100 free rein in selecting a focus point works well much of the time, and you can use this default mode with confidence.

If you want to choose a focus point yourself, you must do two things.

1- When the focus point is unlocked, you can use the multi selector pad to shift the active point to any of the 11 focus points seen in the viewfinder. The available points are shown in Figure 1. The currently active focus point is highlighted in red.

Figure 1: There are 11 possible focus points shown in the viewfinder.

2- The second thing to do is to switch Focus Area mode in the Shooting menu from Autoarea (which always chooses the focus point automatically) to Single-point, Dynamic area, or 3D-tracking (11 Points). These modes change the D3100’s behavior as follows:

1. Single-point.
   You choose which of the 11 points are used, and the Nikon D3100 sticks with that focus bracket, no matter what.
   
   "This mode is best for stationary subjects, and is used automatically in Close-up scene mode."
   
   In this mode, you always select the focus point manually, using the multi selector button. The D3100 evaluates focus based solely on the point you select, making this a good choice for subjects that don’t move much.
   
   
2. Dynamic-area.
   You can select the focus point, but the D3100 can use other focus points as well.
   
   "You’d want to use this mode when photographing subjects that are moving unpredictably, but want the flexibility of being able to choose one of the 11 focus zones yourself."
   
   Once you’ve specified the focus bracket you want using the multi selector’s buttons, the D3100 will use that area exclusively in Single-servo autofocus mode (AF-S). If you’ve chosen Continuous-autofocus mode (AF-C) or Automatic-autofocus mode (AF-A), if the subject begins moving after autofocus is activated, the D3100 will focus based on information from one of the other focus zones.
   
   Well suited for sports photography, this mode is applied automatically with the Sports scene setting, and can be used with other types of moving subjects, such as active children.
   
   
3. Auto-area.
   This default mode chooses the focus point for you, and can use distance information when working with a G or D lens that supplies that data to the camera.
   
   
4. 3D-tracking (11 points).
   In this mode, you select the focus point using the multi selector, but if you subsequently reframe the picture slightly, the D3100 uses distance information when in AF-C (Continuous Autofocus) or AF-A (Automatic Autofocus) modes to refocus on the original subject if necessary.
   
   When using AFS (Single Autofocus), this mode functions the same as Single-point focus area mode.
   
   "This mode is useful if you need to reframe a relatively static subject from time to time." If your subject leaves the frame entirely, you’ll need to release the shutter button and refocus.