back button AF - AI servo

Back button AF – do you love it or loathe it?

Back button AF is a polarising capability – some photographers, me included; love it. Others just can’t see the value or get over the initial awkwardness.

What is back button AF?

In short back button AF moves the control of when a camera is actually autofocussing to a button on the back of the camera.  Most often the AF-ON or exposure lock [*] buttons. In parallel the shutter or “front button” is often reassigned to not actuate AF at all.

As a result you can always use AI Servo tracking focus AF and if you shoot static subjects just release the back button when the subject is in focus. Then you can recompose the frame as you like.

Back button confusion

If your camera is used only by you, or another photographer familiar with back button AF then do the change and persevere until you get it.

If you are one of the photographers that sometimes need to pass their camera to a friend or family member who has a different level of photographic knowledge this is where you run in to problems. A camera configured with back button AF almost always results in out of focus pictures when used by inexperienced users.

EOS 7D Mark II = back button AF Mark II

The EOS 7D Mark II has implemented what can only be called Back button AF Mark II!

Back button AF settings for EOS 7D Mark II

With the EOS 7D Mark II the level of adaptation possible with the camera controls settings is the most I have ever known:

EOS 7D Mark II is more flexible than an EOS-1D X

Firstly, you can configure the characteristics of the AF-ON and exposure lock buttons [*] totally differently, not just one is AF and the other is AE-LOCK. Each button can have different back button AF characteristics. This is what makes it so powerful.

For the AF-ON and the AE-LOCK [*] buttons you can individually set the AF start position, the AI Servo AF characteristics, AF Operation and AF area selection mode.

AF start position

EOS 7D Mark II back button AF

When you press the back button the camera can be set to keep using the currently selected AF point, or can jump to the registered AF point.

I leave it to stay on the selected AF point, but it does remind me I need to write about registered AF points soon too.

AI Servo AF characteristics

EOS 7D Mark II back button AF

The AI Servo characteristics option opens the opportunity to switch to a different AF Case quickly or stay with the one you have already chosen.

AF cases are important to understand and adapt to the subjects you photograph. Each AF case has three tuning parameters associated with it.

AF operation

EOS 7D Mark II back button AF

This is the key for me – pressing the AF-ON or [*] button can be the trigger to switch to AI Servo AF. If you use back button focus with Servo AF then set the buttons to do that all the time. You will then avoid the nightmare of thinking you are doing back button AF with AI Servo only to find that you set the camera to ONE SHOT and your back button AF is now working with one-shot and not tracking.

I’ll own up and say “been there done that…” but I never will again with making sure my back buttons also force the camera to AI Servo mode.

AF areas selection mode

EOS 7D Mark II back button AF

Yes you can also have the press of the back button switch to a preselected AF pattern. Depending on your manual and mental dexterity you could set the AF-ON button to have one AF pattern and the [*] button to select another as a way to quickly change from one setting to another.

It’s certainly less clicking than even moving the AF area direct selection lever on the back of the camera. I still haven’t trained my thumb to find that control reliably, plus it means taking my thumb off the AF-ON button.

Still reading?

EOS 7D Mark II back button AF

Wow you made it this far; thanks. This is the section you will love if you ever need to ask someone else to take a photo with your camera.

You can set the camera to ONE-SHOT AF mode as the default, and you can make the shutter button activate AF. The camera will autofocus and lock the focus just by pressing the shutter.

However when you the super trained experienced photographer pick the camera up and use the back buttons to control the AF it switches to AI Servo AF and back button focus.

This is ideal if tracking a subject is what you need – but I found a small “wrinkle”. If you use back button focus to focus on a subject, lift your thumb off the back button and recompose, when you press the shutter the camera will focus on the subject in one-shot again… Maybe it’s better to leave the camera in AI Servo too.

Getting in some practice

I’ve been out with the EOS 7D Mark II just once with the intention of working through my understanding of the AF and the practicalities. I’m finding that the auto point selection in area or wide area AF area modes is pretty good at “reading my mind”. I was walking back to the car and saw something in a field across from me, raised the camera to my eye hit the back button and pressed the shutter. The bird is a big one, but a long way off and small in the frame. The AF identified the subject and focussed on it, even when easier more contrasty subjects existed in the frame.


Screenshot of DPP showing the AF point automatically selected by the EOS 7D Mark II

If you don’t have an EOS 7D Mark II then get one 🙂

I don’t have an EOS 7D Mark II how do I set up back button AF?

You need to read these articles for the EOS 7D, EOS 5D Mark III and EOS 650D/700D or this one for older cameras like the EOS 5D Mark II and EOS-1D Mark III and EOS-1D Mark IV.