On the rear panel of Nikon’s high-end DSLR bodies, including the D300, D700 and D3 series cameras, you’ll find a little button marked “AF-On.” It seems kind of redundant to use this button for focusing when a half-press of the shutter release does the same thing. Actually, I’ve found that setting the AF-On button to be the only way to activate AF is the best way to operate my Nikon’s AF system, but it requires a few set up steps and a little practice.
Why should you use the AF-On Only Technique with your Nikon DSLR?
The idea behind setting the AF-On button to be the only way to activate your Nikon’s autofocus system is simple. Doing so allows you to set the camera to continuous-servo AF (AF-C) mode permanently, while still being able to get the benefit of focus-lock like you do in single-servo (AF-S) mode. This means that at any time, you can switch between a focus/recompose/shoot style of photography (portraits and landscapes) and continuous subject tracking (sports & wildlife) without having to change camera switches or menu settings.
Also, with this technique, you decouple VR activation (half-press of shutter release) from AF activation. That means you can be tracking a subject with AF and only engage VR when you want to. This technique can save battery life in your camera!
How to set up y0ur Nikon DSLR to use AF-On as the focusing mechanism
- Verify that your camera has an AF-On button. If not, you’ll need to set up the AE/AF-lock button in the custom menus to use it as the AF-On button. In the Nikon D90, this is custom setting f4.
- Set the camera’s AF servo mode to Continuous (AF-C). This is done through the switch next to your lens mount, or via a custom menu setting. D90 users: hold down the AF button on the top of your camera, and turn the Main Command Dial until AF-C is displayed in the top LCD panel.
- In the camera menus, go to submenu “a” (Autofocus)
- Set custom setting a1 (Continuous Release Mode) to Release Priority (in the D90, this is already set for you when you choose continuous servo AF mode)
- Find the custom setting for AF Activation (a5 on the D3s) and set it to AF-On Only. This step is not necessary for the D90 and other cameras, as it is already set up by custom setting f4.
Now you’re all set up and ready to go.
How to use the AF-On technique in the field
To emulate single-servo mode (focus/recompose/shoot)
- Place the active AF point on your subject
- Press the AF-On button to acquire focus
- Release the AF-On button to lock focus
- Recompose and shoot
To focus continuously on a moving subject
- Place the active AF point on the subject
- Press the AF-On button
- Keep the AF-On button pressed to track focus while simultaneously pressing the shutter release
- Remember to initiate the VR system (if your lens supports it) by half-pressing the shutter button prior to releasing the shutter. Remember, VR takes about a half-second to stabilize, so you’ll want to anticipate your subject.
Now that you understand how to use the technique, you’ll want to spend some time practicing. It usually takes about a day of shooting in the field to get used to the new technique. Once you know how to use it, the AF-On only method of focusing will help you get more “keeper” shots.