AF-On Button - A Better Way to Focus

Damn, my camera didn't focus where I wanted it to!  Has this ever happen to you? Automatic focus, no matter how sophisticated it is, can be both a blessing and a curse. When it works, it does a great job - until it doesn't. Here is a quick tip to help you get more in-focus images. It involves re-thinking and retraining how you focus. Sounds a lot worse than it actually is.

By default, most DSLR cameras come set up so that a half press on the shutter will enable VR, set the exposure, and acquire focus. Generally, this is way cool, but sometimes you want make some of those decisions yourself.

Focus reliability is my big pet peeve with such an arrangement, particularly when shooting with a lens that is wide open in less than optimum light, or when you want to isolate the subject from its background - and you need to be very precise with your focus selection.

On the more advanced Nikon camera bodies you will find a menu selection that allows you to disable focus on shutter press,

and another selection that allows you to assign the focus activation to a different button - usually the AF-On button.

Why bother?

Let's consider what happens when you are shooing people at an event. You frame the face and you use the focus aids in the viewfinder to get the eyes in focus, then you reframe the image to make a reasonable composition, at which point you press the shutter - and you get a perfectly focused torso. This happens because the camera will re-acquire focus.

You could use single servo focus, so that once you acquire focus it remains until the picture is taken. But I can't tell you how many times I left the camera on that setting, then started to shoot a moving subject, and ended up with a whole series of images that were out of focus, simply because I forgot to set the camera back to continous focus.

When you disengage the focus from the shutter release and assign it to the AF-On button, you have the freedom of activating focus only when you need to. A single press of the button will allow the camera to behave as if it were in single servo mode - acquiring focus only when pressed, and not re-acquiring focus again as you release the shutter. Yet, if I want to follow a subject that is moving around, I can hold the button and "track" the subject, and still have the flexibility to use it in "single servo" mode with the button press - without ever having to change the camera from single to continous or vice versa.

Another benefit is battery life. If you use VR lenses, with the default setting, each time you half press you are focusing and activating the VR, which really chews through batteries. Separating the two functions only engages VR when you press the shutter. You will get anywhere from 25% to 50% more shots by using the AF-On button to focus.

It does feel very awkward at first, especially if you are one of those photographers that also set the audible "beep" to let you know you are in focus. In order to best take advantage of AF-On button, you will have to train your thumb to focus, use your eye in the viewfinder to check the dot that shows you have acquired focus, and use your index finger to just release the shutter. Trust me, once you learn this new way, you will never go back to the old. In a matter of a couple of outings, you will be comfortable with the new method and your percentage of "keepers" will improve. There is a reason why sports photographers almost universally prefer this method to focus with over the default one.