Pixiq

All Nikon autofocus (AF) SLR cameras (film and digital) have two principal AF modes: AF-S (single-servo) and AF-C (continuous-servo). At default settings the AF-S mode is intended for subjects that are stationary and where the camera-to-subject distance will not change in the time between when the AF system acquires focus and making the exposure. It works by locking the focus distance as soon as the camera has confirmed focus, and keeps the focus distance locked while the shutter release button is pressed down halfway.

By contrast the AF-C mode is designed for focusing on a moving subject, because once the camera has “locked” onto the subject the camera monitors the focus distance constantly and will adjust it whenever the camera-to-subject distance changes. As soon as the camera detects that the subject is moving it will engage Nikon’s Predictive Focus Tracking system automatically; Predictive Focus Tracking adjusts the focus point to take account of the distance the subject will move in the brief period during which the reflex mirror is lifted out of the light path and the shutter opens, a process that blocks light from reaching the AF sensor. It achieves this by analysing the focus information to estimate the speed of the subject and anticipating the subject’s likely position at the instant the shutter opens.

The two AF modes work well for their intended purpose but there are many shooting situations where it would be ideal to have the ability to switch seamlessly between them, without having to change camera settings, so at one moment you could lock the focus distance on a subject that is still, enabling you to recompose a picture at will, and then change instantly to have the camera adjust focus continuously, with Predictive Focus Tracking available, to follow the subject as it moves. Typical scenarios where such dual-functionality could be really useful include sport/action and wildlife photography.

Well it is possible to have the best of both these AF worlds! Start by setting the camera to its AF-C mode. Next comes the key step, which is to change the normal arrangement of camera operation by separating the activation of the AF system from the shutter release button. Many Nikon SLR models from mid-range upward have a dedicated AF-ON button to which activation of the AF system can be assigned in isolation, via the Custom Settings menu. Less well-specified models lack such a feature; however, it is possible for the AE-L/AF-L button of these cameras to perform the same function as the AF-ON button by setting the appropriate Custom Settings menu item accordingly. Finally, if the Nikkor lens you are using has an A/M (autofocus with manual override) mode select this as well. 

Since the AF-ON and AE-L/AF-L buttons are located on the rear panel of the camera to the right of the viewfinder eyepiece, you can switch autofocus on and off as required with your right thumb, so the camera deals with keeping the subject in focus, while using your right index finger to control the shutter release to time the shot, without any interference to focusing. 

Once your camera is configured this way AF-C is only active when you press the AF-ON, or AE-L/AF-L button. Cover a moving subject with your selected AF point, press and hold the button down, and as you track the subject in the viewfinder the autofocus system will follow it and maintain focus, while you operate the shutter with the shutter release button. If and when the subject stops you can lift your thumb off the AF-ON, or AE-L/AF-L button and focus will lock at the current camera-to-subject distance; you can now recompose the picture at will, since AF is no longer active, and with those lenses that have an A/M setting it is now also possible to focus manually by rotating its focus ring. To resume autofocus simply press the AF-ON, or AE-L/AF-L button again. I leave my cameras set up this way all the time, so I never have to worry about changing, or checking the AF mode.

Here are a few examples of the Custom Settings menu route for different Nikon D-SLR camera models to set AF activation: 

D3-series, D700, D300-series: Custom Setting a5 (AF Activation) > AF-ON only

D7000: Custom Setting f5 (Assign AE-L/AF-L) > AF-ON 

D5000: Custom Setting f2 (Assign AE-L/AF-L) > AF-ON 

Note: Pressing the AF-ON button, or the AE-L/AF-L button, when it is assigned the AF-ON function will not activate the Vibration Reduction (VR) feature available on some Nikkor lenses; VR will only become active when the shutter release button is pressed halfway, or down fully.

© Simon Stafford