Back-button Autofocus
The Tip Worth Its Weight In Gold

     If you could only take one idea, tip or piece of advice from my entire collection of articles on this web site, then take this one. That's how much I think of this one tip.

     According to Canon, all digital EOS SLRs (except the original EOS Digital Rebel) have a Custom Function for moving Autofocus from the shutter button to a back-button. I'm a Canon guy so I'll stick to what I know here. You Nikon folks will need to look up the details from Nikon sources for your cameras. But no matter whether you're a Canon shooter or a Nikon shooter, the back-button autofocus technique still applies.

     When I first heard of this technique, I already had my hands full just learning the camera, and menus, and general operation of a serious DSLR. I certainly didn't want to start fiddling with "custom funtions" and mess everything up. But my shots of birds in flight never seemed to measure up with what I was seeing others accomplish, so I decided I was going to do whatever it took to improve my results. I got better lenses, better tripods and heads, and better DSLR bodies - spend, spend, spend. Sure, it helped, but I was still not where I wanted to be. After finally purchasing a 1DmarkIV, I decided I had to make the most of it's capabilities, or else I would have wasted a lot of money. I got serious about learning about back-button autofocus and what it could do for me. I began reading about it in earnest, trying to understand exactly what it was, and how it might help me. It took a lot of web searching and reading, and frustrated head scratching, until finally the concept began to sink into my thick skull.

What is "back-button autofocus"?

     Simply put, it is a way to remove the focus function from the shutter button. As you already know, when you press the shutter button half way down on a DSLR, two things happen:
#1 - the camera focuses the lens, usually signaled by a beep and/or flash of some sort in the viewfinder, and
#2 - the camera reads the light and decides what the exposure should be.
Then when you press the shutter button the rest of the way down, the exposure at that instant is set and the shutter clicks, taking the photo.

But when you have back-button autofocus set up, item #1 (autofocus) no longer happens when pressing the shutter button half way. With back-button autofocus you must now also press another different button to activate the focus. On the face of it having to press two buttons sounds rather dumb, but the fact is this actually makes things simpler and easier.

How does "back-button autofocus" work?
     First, you go into your menus and choose the appropriate custom function setting for your model camera and assign the duty of activating autofocus to a button other than the shutter button. Generally this is assigned to the "AF-On" button on the back of the camera, in the upper right near your thumb, next to the "AE-Lock" (asterisk) button -OR - to the "AE Lock" (asterisk) button itself. It doesn't matter if your camera has no "AF-On" button though, since most all bodies allow using the "AE-Lock" (asterisk) button instead. For most folks, the "asterisk" button is actually a little easier to reach with the thumb than the "AF-On" button. But to get the most effective use of this change, you must also set your autofocus mode to "AI Servo" instead of "One Shot". In fact, you will leave it in "AI Servo" mode and never have to switch to "One Shot" again. As an added bonus, you will never again have to change your lens focus switch between "AF" (autofocus) and "MF" (manual focus) either. It can stay on "AF" from now on.

      Now that you're all set up, here's how you use it:
     • Moving Subjects (like birds in flight) - To follow focus with moving subjects you press "AF-On" and hold it down for "AI Servo" focusing. Your camera continually focuses on the moving subject. To take a photo, just press the shutter button when ready to shoot.
     • Stationary Subjects - If your subject is stationary, simply press the "AF-On" button once to set the focus and let go, in which case it now acts like "One Shot" focus. Then just press the shutter whenever you're ready to shoot because the shutter button will no longer mess up your preset focus. You can also "bump focus", by following your subject and occasionally tapping the "AF-On" button to keep resetting focus as if in "One Shot" mode until you're ready to shoot.
     • Manual Focus - For manual focus situations, forget fiddling with the "AF/MF" switch on the lens ever again. Because Canon's lenses have full-time manual focus override, you simply forget pressing the "AF-On" button and just focus manually. The camera is not going to autofocus when you press the shutter button, so there is no need to switch to "MF" focus.

     As you see, the task of focusing has been greatly simplified. No more switches or settings to change. You can start and stop focus at any time without fear of accidently pressing the shutter too hard and taking unintentional shots. You can essentially switch between "AI Servo" focus and "One Shot" focus just by holding down the "AF-On" button, or by momentarily pressing it. You now have the best of all possible worlds when it comes to focusing.

     To be honest, I was initially afraid it would be difficult to overcome years of habit and that I would continually forget to press the "AF-On" button, but that was not the case at all. The truth is that for me, using the back-button focus method very quickly became perfectly natural. The only thing I had trouble with was remembering to shift my grip so I could more easily reach the "AF-On" button with my thumb. Using the "AE-Lock" button would have been an easier reach for my small hands, but I was determined to not confuse myself further by swapping the functions of the "AE-Lock" and "AF-On" buttons, while at the same time getting used to not focusing with the shutter button. I now automatically adjust my grip to begin with, which actually gives me a better grip on the camera. That's one more sloppy habit broken.

     Setting up back-button autofocusing and switching to "AI Servo" focus is only the first step though. There are other custom functions and settings that interact with the "AI Servo" mode and your focusing sensors that affect how well this works. Things like Spot AF, AF tracking sensitivity, AI Servo AF Tracking Method, AF Point Expansion and others all dictate how your camera focuses, so getting those settings right for your style of shooting will be critical as well. Reading through the custom function and focusing sections of this white paper on the 1DmarkIV (also in the reading list below) will help you understand what these settings do and how they affect the AI Servo mode. You Nikon folks will have different terminology, but you also need to read about these types of settings for your camera.

     What stronger testimony can I give except to say now that I'm using back-button autofocus, I will never go back to focusing with the shutter button. It has made focusing and re-framing shots, following action shots, and focusing in general much simpler. No more stopping to change focus modes, or fiddling with the focus switch on the lens. Bird photography keeps me busy enough without those hassles.

     I hope that this has, at the minimum, put you to thinking how this can improve your shooting, whether for action shots, or even for macro work. The ease of bump focusing can be a great benefit for macro work, so don't overlook it. For more information and better, more detailed explanations of the subject, I recommend the reading in the links below. While not all this will apply exactly to your model camera, the basics will help you understand the concepts and terminology that can then be applied to your camera. Read through YOUR camera's manual and guides and see how this stuff applies to your model. It made my head hurt for awhile, but finally it sank in. Give it a go and see what you can gain from it.

Reading list:

Other photographers explaining what Custom Function auto focus settings they use and why... good info-