Everyone knows that looking through the viewfinder and part-depressing the shutter release is how you start the process of making pictures. That first touch performs a number of important tasks, the most obvious one being to activate the AF mechanism. Top-of-the-range Professional cameras like the Nikon D3s have an extra button which also performs this focussing task. It is known colloquially as the “back button” and to most pro sports, wildlife and action shooters it is the preferred way to get the subject in sharp focus – especially when taking multiple shots of fast moving subjects in a series of bursts. But why is it used and what are the advantages of using it? If your camera does not have this button, you might be thinking “this is of no interest to me…” but you would be wrong; it is highly likely that your camera can be used in the same way by converting one of the other buttons on the rear of your camera to perform the same task. More about how to do this later…

To get started, you need to just first make sure your camera has right autofocus set up for continuous shooting. On pro cameras or older DSLR’s, make sure your camera is in “C” mode, or “Continuous Servo AF Mode” to give it the full title. This is done by a rocker switch on the side of the camera near the lens release button;

With the current generation of cameras, however, it has become a little more complex to perform this task. Instead of the options seen above, this same rocker switch will have two options “AF” and “M”. In all cases, “M” stands for manual focus but the “AF” option by default puts your camera in “Auto Area AF Mode {AF-A)” where the camera selects “Continuous Servo AF Mode” automatically if it detects that the subject you are trying to focus on is moving. This is fine for normal casual candid shooting but it unfortunately also puts your camera in charge of choosing what part of the scene to focus on in the first place. For action photography, you need to be able to select your focus point manually but still be in a position to exploit the full power of the autofocus system. The best way to do this is by selecting “Dynamic Area AF Mode”.

This is achieved in one of three ways;

With some cameras you will have to go into the Custom Menu or the Set-up Menu and make the choice there. On Pro cameras, there is a rocker switch with three symbols – the middle of which is for “Dynamic Area AF Mode”. With the most recent cameras, a third method that involves pressing the “AF Mode” button (on the side of the camera) and rotating the Sub-command (front) dial needs to be employed.
To understand how to master the “back button” professional autofocus technique, we will start by looking at a sequence of shots made using the shutter release to initiate AF in the normal way. Our camera is now in “Continuous AF” mode and “Dynamic Area AF” has been selected from the “AF-area Mode” options.

In the example above, we can see that an AF point off to the right side of the viewfinder has been selected. Depending upon the camera model you are using, when the camera achieves focus the AF point may light-up red or turn black. This is called “Lock on” and when shooting a moving subject, the camera will register both the subject distance and the direction it is traveling. In Dynamic Area AF mode, the camera will use this info to do “Predictive Focus Tracking” which means it anticipates where the subject is going to be and the new focus point will be selected automatically. Confirmation that the subject is locked in focus will also be given by a small green dot lighting up within the viewfinder.

As we move through the shots, pressing and releasing the shutter one frame at a time, there comes a moment as we can see in the third frame where the boy has moved away from coverage by the chosen AF point. Pressing the shutter release at this point will cause the AF mechanism to attempt to re-focus on the subject behind the red focus point and….

The result is pretty catastrophic – the AF mechanism tries to focus on the part of the scene where the chosen AF point is. Our running boy is no longer in focus.

However, most people would not follow a subject in this way taking one shot at a time. Instead, it is more likely that you would shoot in a continuous burst using either CL or CH shooting mode, with the shutter fully depressed throughout the sequence. In this mode, the camera’s AF mechanism would have locked on the subject and followed the boy using “Focus Tracking” , calling into play other AF points that cover the boy as he moves.

The AF points indicated in yellow in the illustration above are some of the ones used by the camera to check on the subject’s distance and in turn to provide instruction to the lens as to the new focus point. Please note; these points do not actually illuminate when they become active. Although autofocus is better when the camera is in continuous shooting mode, there is still potential for things to go wrong. For example, when shooting short bursts of three or four frames at a time and removing your finger from the shutter release button momentarily. The danger is that in these instances, autofocus can be lost. Restarting AF again by pressing the shutter release would, as in the single frame example above, give your camera a new instruction – and it would assume you want to focus on the AF point you originally selected.

To ensure that the subject remains in focus all through the sequence – even through the times when you are not taking pictures but just following the subject through the viewfinder – you need to keep autofocus working. You need it to perform focus tracking and continuously sending new instructions to the lens to keep up. To do this, you must separate the functions of releasing the shutter and initiating AF. This is the primary reason for using the back button. Initiating AF by pressing and holding down the Back Button allows you to shoot either in Single frame or Continuous shooting modes without the risk of sudden re-focussing taking place. The camera is receiving ongoing instructions from you to maintain focus as long as you keep your finger on the AF-ON button. In this way, the full technical power of the AF mechanism is brought into play. Here’s how to make the back button work effectively on those models that have a dedicated AF-ON button. First, a Custom Menu option is selected;

The camera’s default setting will be indicated in yellow. Select “AF-ON only” and press “OK” and you will have separated the two functions. Now part depressing the shutter release will no longer activate AF. Only by pressing the back button (AF-ON) will the subject be brought into sharp focus and the AF mechanism start to track your subject through the sequence. It will continue doing this as long as you keep pressing the button and the subject remains within the viewfinder AF frame area.

For those cameras like the Nikon D80/90, D3000, D5000 and D7000 that do not have a dedicated “AF-ON” button, it is still possible to separate the two functions and use one of the other back buttons to initiate AF.

Where there is only one, it is most likely to be the “AE-L/AF-L “ button which locks Auto Exposure (AE-L) and Auto Focus (AF-L). In your camera’s Custom Menu you will have an option under the section “f: Controls” (in some camera’s it is in the ‘Setup Menu: Buttons’ ). Here you will find the option to “Assign AE-L/AF-L Button” to “AF-ON”. Selecting this option will provide the same functionality as the purpose-designed AF-ON button found on professional camera models.

Putting this technique into practice can be a little difficult at first but if you persevere, it will pay dividends and improve your sports, wildlife or any kind of action photography. However, using the back button for static shots like portraits or landscapes can seem unnecessary, if not a little awkward, as it requires you to use two digits instead of one and think AF first and release second. But many pro shooters use it for all their photography because taking your finger off the back button will now lock the focus point. Normally, this requires you to press the AE-L /AF-L button. This is handy for preventing unwanted re-focussing and is especially useful when using telephoto lenses without a tripod.

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