Let's start with a clarification. This isn’t a discussion about auto-focusing modes or auto-focusing in general. Though, at certain stages of this discussion we will make some impromptu references to the whole business of auto-focusing too. The topic of discussion is about AF-Area Modes and the veracity of each of the modes in terms of suitability in different situations.
What on earth are AF-Area modes?
Digital cameras have a bare minimum of 9 AF points. Some cameras like the D800, e.g., has 51 AF points to work with. These points look for contrast in the image and when they do find contrast they lock focus on the point. AF area mode basically gives you a range as to where the camera is likely to focus on.
Normally when a camera is working in AF mode it will look for contrast to lock on at any one or more of the active points. These points are spread across the frame. You can select how many AF points are active at any point. So you can switch between, 9, 21 or all of the 51 points, depending of course on the number of AF points available. This limits the area within which the camera looks for focus. These are known as AF area modes. Choosing the right AF area makes the whole AF process a bit more accurate and faster.
So how do one select the right AF-Area Mode for a scene?
This is where you need to have an understanding of how the AF modes on your camera works and the options that you have in terms of AF area modes. As you are aware, there are three AF modes on your camera. These are Auto-Focus Continuous Servo (AF-C), Auto-Focus Single Servo (AF-S) and Auto Servo Auto-Focus (AF-A).
The steps that we're going to detail here will be based on the Nikon system. Your camera system is likely to be slightly difficult, but the steps are more or less common for all systems.
Fast moving subject – Scenario 1
In a situation where you need to track a fast moving subject, choose the AF area mode that gives you the option of the maximum number of AF points, together with selecting the AF-C mode. In this case it is AF-Auto in which all the AF point are active. When you half-depress the shutter release the camera locks focus automatically on the point where it finds maximum contrast. Now, as the subject moves around the adjacent AF points will automatically acquire focus. All you need to do is keep the shutter release half-depressed. To make the shot fully depress the shutter release. In this mode however, you have no way to pinpoint a starting AF point. The camera selects the AF point on its own.
Fast moving subject – Scenario 2
3D tracking is a very interesting AF area mode that measures subject to camera distance for accurate focus locking. Additionally, this system uses a predictive focus tracking, anticipating where the subject will be when the shot is made and locking focus on that spot. This is necessitated from the fact that not all subjects move in a predictable path and that when the shutter is actually released, the inherent delay between when the shutter is released and the exposure made, the subject may have moved to a different location in the frame.
On Nikon systems you can select the starting point for tracking a subject in this mode, something that is unavailable in Auto AF mode. Almost all camera systems have the center AF point as the most sensitive, with some cameras having cross-type AF point at this position. This is the recommended AF point to start tracking a subject when it is moving fast and erratically.
Stationary subject – Scenario 3
For anything that is not moving about select the AF-S mode and single AF point. On Nikon systems you can select either all the points or any single point. The latter is recommended. The choice of the specific AF however depends on you. For entry level systems I would recommend using the center point as it is the best in terms of locking focus. Use the focus and recompose technique either by half-depressing the shutter button or pressing the AE-L / AF-L button to lock focus and then recompose to comply with composition rules such as the rule of thirds.