Using AF-ON Focusing Technique

Written for Nikon cameras, specifically the D3s, but similar settings can be found on most manufacturers bodies. Please see manual for details.
Pictures to illustrate this technique will be added later. Apologies for now for the amount of text.

One aspect of my photography that often causes interest is how I utilise the AF system to maximise my successful shots. One setting that I use that I find few others do is to only use the AF-ON button for focusing.

Why Use AF-ON Only Technique?
The general principle in setting the AF-ON button to be the only way to activate the autofocus system is simple, it allows you to set the camera permanently to continuous servo (AF-C) mode, whilst retaining the benefits of focus-lock as you would in single servo (AF-S) mode.

This means that at any time you can switch between a virtual AF-C and AF-S mode without the need to change any settings on the camera. This means that you save time fiddling with either buttons or menu settings, in the field these seconds can be the difference between getting the shot and missing it.

There are arguments over the benefits of not engaging the VR mechanism until it is actually needed, but I am not fully aware of the technicalities of this, but I have not been able to measure an advantage or disadvantage in my own work simply by observation.

Setting The Camera Up for AF-ON Shooting
The AF-ON button on the D3s is found on the rear of the camera, near the rear dial, the position in vertical orientation is not quite so nicely placed, but that is an entirely different conversation.
1. Set the camera body to AF-C mode
2. In the autofocus menu, setting a1, select RELEASE
3. In the autofocus menu, setting a5, select AF-ON only
Make sure that setting a9 is set to AF-ON and that the AF-ON button has not been re-programmed

That is the camera ready to go, so now practice is the key to mastering this technique. At first you will undoubtedly find the technique strange, if not counter-intuitive! Persevere as it will become second nature.

To emulate single servo mode
(To allow a focus point to be locked before recomposing)
1. Ensure that the active AF point is on your subject
2. Press the AF-ON button to acquire focus
3. Release the AF-ON button, effectively locking the focus
4. Recompose and shoot.
If you forgot the step to set the AF-ON button to be the only means of autofocusing then you will notice that pressing the
shutter button caused the camera to refocus, so check out the a5 setting.

To focus continuously
(To allow follow focusing of a moving subject)
1. Ensure that the focus point is on the subject
2. Press the AF-ON button to acquire initial focus
3. Keep the AF-ON button pressed whilst simultaneously pressing the shutter button.