Whilst the following was written with Nikon D-SLR’s (and D600 & D7000 specifically) in mind, most of the features and concepts described below apply across all the major manufacturers of modern cameras under perhaps slightly different names.
The Nikon D600 (FX) and D7000 (DX) share a similar auto focus system (which is also very similar to many other modern Nikon AF systems).
Specifically the D7000 uses the Nikon Multi-CAM 4800DX autofocus module with TTL phase detection, fine-tuning, 39 focus points (including 9 cross-type sensors) whilst the D600 uses the Nikon Multi-CAM 4800 autofocus sensor module with TTL phase detection, fine-tuning, 39 focus points (including 9 cross-type sensors; the center 33 points are available at apertures faster than or including f/5.6, while the center 7 focus points are available at f/8).
Most (Nikon) D-SLR’s (including the D600 and D7000) have three different Autofocus Modes to choose from, typically depending if your subject is still or moving. In addition they typically have multiple Autofocus Area Modes to choose how many of the AF points are active and how they track a moving object. You generally choose a particular combination of these two AF functions (Focus Modes and AF-Area Modes) to suit a particular shooting situation.
The following shows some specifics for the D600 and D7000 AF systems but the majority of the detail applies to all modern (Nikon) D-SLR’s.
Single-Servo AF (AF-S)
For stationary subjects or if the distance between you and the subject is not going to change between the time you lock focus (recompose) and take the shot. When using AF-S, you can select from two Autofocus Area Modes, either Single-Point AF where you select the AF point (recommended), or Auto-Area AF, where the camera selects the AF point(s) for you. By default the shutter can only be released when the in-focus indicator is active (focus priority for Custom Setting a2: AF-S priority selection).
Continuous-Servo AF (AF-C)
For moving subjects. If the subject is moving towards you or away from you, the camera will keep evaluating the focus distance, as long as the Shutter is half-pressed. You will need to use this in conjunction with the Autofocus Area Modes to determine if and how the camera tracks the subject laterally to the surrounding AF points, or if it will only track the subject if it remains at the initially selected AF point. If the subject is going to be difficult to follow or is moving across your field of view, set the AF-Area Mode to one of the Dynamic-Area AF modes or to the 3D-Tracking mode. Focus on the moving subject with the selected point if using Single-Point, one of the Dynamic Area Modes, or 3D-Tracking, or let the camera select the AF point in Auto-Area AF, and then as long as the shutter button remains half-pressed the camera will track the subject as it moves closer or farther in distance. Depending which AF Area Mode you are using, the camera may also maintain focus or track the subject to some or all of the surrounding focus points if it moves away from the initially selected point. By default the shutter can be released whether the subject is in focus or not (release priority for Custom Setting a1: AF-C priority selection).
Auto-Servo AF (AF-A)
A hybrid of the two other focus modes. The camera automatically selects Single-Servo AF (AF-S) mode if the subject is stationary, changes to Continuous-Servo AF (AF-C) mode if the subject is moving. Why wouldn’t you use this all the time? If you are focusing and then recomposing, your movement of the camera may fool it into thinking that the subject is moving and move your focus point.
Autofocus Area Modes
The Autofocus Area Modes are used to set if just a single AF point is active or else how many AF points surrounding your selected AF point will be used to maintain focus or to track a moving subject if you are using AF-C or AF-A Autofocus Modes.
The camera uses all 39 AF points to detect what it thinks is the subject and automatically choose the appropriate AF point(s). Generally the camera selects the nearest subject or a human in the frame, so it may not focus on what you actually want to focus on.
Only one AF point will be active, and surrounding AF points will not become active. Typically used with Single-Servo AF (AF-S) to focus on a stationary or still subject, or in a situation where you focus and recompose. It can also be used with accuracy with AF-S mode for moving subjects if the camera-to-subject distance does not change at all or very much in that period between locking focus and taking the photo. Use the Multi Selector to choose your active AF point, you can also hit the OK button to quickly select the center AF point. There is also a Custom Setting (a6 Number of focus points) which lets you choose whether all 39 AF points are available or a reduced number (11 points).
In the Dynamic-Area AF Modes you select an AF point to tell the camera where to start autofocus, if your subject briefly moves from that point to a neighboring point either by them or you moving (e.g. panning), the camera will use the surrounding AF points to help maintain focus on it. Select one of the Dynamic-Area AF options (below) when you are photographing moving or potentially moving subjects using Continuous-Servo AF (AF-C) or Auto-Servo AF (AF-A). These modes are ideal for a subject moving closer or further from the camera but which may also move laterally away from the selected AF point faster than you can react in order to keep it located at that point, or for when you are panning and following the subject and attempting to keep it located at the selected AF point. Note: you need to keep the shutter button half-pressed in order for the continuous focusing to occur. Note: Depending on your setting of Custom Function (a3 Focus tracking with lock-on) the camera can pick up and start tracking a new subject that falls under the selected AF point.
9-Point Dynamic-Area AF will use the immediate surrounding AF points to help maintain focus on a subject that briefly leaves the selected AF point. This can be used with predictably moving subjects like a car or a runner.
21-Point Dynamic-Area AF will use even more of the surrounding AF Points, more than half the total AF Points. This should be used for more unpredictably moving objects like sports players on a field.
39-Point Dynamic-Area AF will use all of the 39 AF points. It can be used for very quick and unpredictably moving subjects like animals, birds and wildlife.
The Dynamic-Area AF Modes are not used to track and maintain focus on a subject that is moving across the various AF points in the frame, but rather are used to stay focused on a moving subject that you attempt to keep located at your selected AF Point. To track a subject that is moving across the frame, intentionally passing from one AF point to the next, use 3D-Tracking.
This mode is used for subjects moving across the frame in any direction, or subjects moving erratically from side-to-side in the frame, and they are tracked by areas of colour (if the colour area you wish to track is too small or if it blends into the background you may get unpredictable results). Use this when you don’t wish to necessarily pan or follow the subject to keep it located in the same part of the frame, but rather when you wish to keep the camera relatively still as the subject moves across the frame.