The Nikon D2X has eleven AF sensors. Nine of the sensors are sensitive to both horizontal and vertical detail. The three sensors in the center row are crosshatched. The upper and lower rows consist of sensors that are T- shaped and inverted T-shaped, respectively.
Gray bars indicate approximate shape of actual sensors
The far right and left sensors are narrow lines that are sensitive to detail in the cross-axis direction only. It is important to note that the actual sensor shape does not correspond to that depicted by the viewfinder focus brackets. The sensors are narrower and longer than the viewfinder AF areas brackets (see figure below). For reliable accurate AF, it is best if the subject covers at least half of the sensor area.
Many Options, one at a time
I. Two Autofocus Activation Methods
These are controlled via Custom Settings Menu (CSM) a5, AF Activation.
a) "On Shutter/AF-ON" (Shutter Button/AF-On Button) is the default setting. Autofocus is initiated by either depressing the shutter button halfway or by pressing the AF-On button.
b) Selecting "OFF AF-On only", separates the focusing action from the shutter release. Autofocus can then only be initiated by pressing the AF-On button. Depressing the shutter button has no effect on autofocus action. However, the focus status can have a definite effect on shutter release. If camera Focus Mode (AF-S or AF-C) is in Focus priority, then the shutter will only release if the area in front of an active AF sensor is in focus.
*My personal preference is to set CSM a5 to "OFF AF-On only", to separate AF action from the shutter release button. I use the AF-On button exclusively for auto focusing.