Fine Focus: Choosing and Using AF Modes

One of the most important features of today's digital cameras is their autofocus operation. Sometimes it seems, though, that AF is also one of the most confusing features—and that may be because there are so many autofocus options available.

Compared to how autofocus worked when first introduced years ago, today's systems are amazingly fast, accurate and smart. But they require that we make some choices, and what those choices require is that we think about what we're shooting and how we want the autofocus to behave.

Autofocus options really boil down to this: do we want single or continuous operation? Single AF operation means that once the camera finds the focus for the chosen point, it locks that focus until the picture is taken. So single AF is a good choice for portraits or stationary objects. Continuous AF operation means the camera will continue to adjust focus based on the movement of the subject, which makes it a good choice for sports or photographing children.

In addition, continuous AF uses "predictive autofocus" to actually think ahead of the subject's movement, to anticipate it, because by the time you press the shutter button, a moving subject will have moved even further.

Additional Choices
Nikon cameras offer Dynamic AF, which means that if the chosen AF point is moved away from the subject (due to a change of composition or movement of the subject), the AF system will look to the other AF points to see if one of them has now picked up the subject, and it will hand off the focus to the new point.

Nikon also offers a mode called Closest Subject Priority, in which the AF system will focus on the closest object a focus point sees. That's handy if you just want to point and shoot. In fact, that's what many cameras default to when in full auto mode and in many of their scene modes.

It's important to remember that AF systems depend on contrast and light. The less you have of either, the harder it will be for the system to find focus. If you try to focus on a gray sky, for example, the AF system will likely get confused and keep searching. Likewise, if you try to focus in a dark room, the system will struggle, which is why some cameras shine a brief beam of light to help the system find the subject.

By default, autofocus systems are activated by the pressing of the shutter button. With digital SLRs, you can almost always assign that activation to a button on the back of the camera, a button you can press with your thumb. Many professional photographers use this method so they can tell the camera when they want to use AF and when they don't, regardless of whether they're pressing the shutter button or not.

Finally, as good as new autofocus systems are, they're not perfect. We have to understand how to use them, and we have to tell them how we want them to work. Check your camera's manual to learn about the focus options your camera offers and how to set your autofocus choices. Then pick up your camera and put those AF capabilities to work for you.