Nikon D6 - The AF-Mode Button; Choosing an AF Mode

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The AF-Mode Button
Choose the AF and AF-area modes. The AF mode determines
how the camera focuses in autofocus mode, the AF-area mode
how the camera selects the focus point for autofocus.
Choosing an AF Mode
Hold the AF-mode button and rotate the main command dial.
The options available vary with camera settings.
The AF-Mode Button