III. Four AF Area Modes
The AF Area Modes determine which sensor(s) are active in initial focus acquisition and predictive focus tracking. The active sensor(s) can either be user-selected or camera-selected (in Closest Subject Priority).
AF Area Modes selector
Single Area AF: Only one of the AF sensors is used for AF. The sensor is user-selected. To allow predictive focus tracking to operate with moving subjects, the sensor must be kept positioned over the subject. If the sensor drops off the subject, the camera AF mechanism will start focusing on the area now positioned in front of the sensor. The camera may try to focus on areas in the foreground or background and in low contrast situations may hunt and search. The Nikon instruction manual recommends Single Area AF for relatively static subjects or with moving subjects that can be easily kept within active focus sensor.
Dynamic Area AF: Focusing begins with the user-selected AF sensor but the camera will switch focus to other sensors to follow a moving subject. If the subject moves onto an adjacent sensor, predictive focus will track its movement and AF activity will be transferred to sensor positioned over the subject. Nikon recommends Dynamic Area AF for erratically or rapid moving subjects—situations where it may be difficult to keep a single active sensor positioned on the subject.
Group Dynamic AF: Group Dynamic AF is the most complex of the AF Area Modes. Instead of selecting a single initial focusing sensor, the user selects a group of sensors. CSM a3 determines the number/shape of sensors in the group (the pattern) and how initial focus acquisition is achieved (center sensor or closest subject). Nikon recommends Group Dynamic AF for erratically or rapid moving subjects — in situations when one knows the general area within frame where the action will occur.
CSM a3 Options are Pattern 1/Center Sensor, Pattern 1/Closest Subject, Pattern 2/Center Sensor, and Pattern 2/Closest Subject
Pattern: Either a diamond/triangular pattern of four/five sensors (Pattern 1) or a row/triangular pattern of three/four sensors (Pattern 2) can be selected (see figures below).
Initial Focus Acquisition: Center Sensor or Closest Subject.
With Center Sensor selected by CSM a3, initial focus acquisition will be performed with the center sensor of the user-selected group but can automatically switch to other sensors in the group if the subject moves. Using center sensor does limit initial focus acquisition to only five of the eleven AF sensors (the center sensor of each group). Dynamic Area AF, on the other hand, allows any one of the eleven sensors to be selected for initial focus acquisition.
In Closest Subject Priority, the camera will analyze contrast and distance information in front of each sensor of the selected group and will attempt to focus on the closest subject. When one presses the focus button the selected group of sensors will light up in red briefly. In AF-S mode, the sensor that acquires focus will blink in red briefly once AF is achieved. In AF-C mode, the sensor used to set focus will blink in red briefly after photo is taken.