This post is about the AF-ON button or back button focus. If you already use this technique – go and make yourself a martini!
Before computers enabled auto-focus systems in cameras, the photographer had to manually focus their lenses and the process of taking a picture involved four steps: set the exposure, focus, compose and release the shutter. Today, it still requires these four steps but our modern digital SLR cameras have simplified it to two actions: compose and press the shutter button. However, behind the scenes, the four steps are still there, but just done by computers. And therein lies a big problem.
I’ve met many photographers who are not setting up their cameras to shoot sports that would maximise their opportunity get great shots. In fact, they’re using a $6000 camera like a $500 manual film camera. Well, maybe not that bad, but almost. In this post I’m going to write about one of these settings: why you should not use the standard “half press shutter” to auto-focus in sports photography.
Auto-focus out of the box
Out of the box, every digital-SLR is set up so that the shutter button does three things. Yes, three – count them: 1. When you press it half way down it activates the auto-focus function; 2. It starts the exposure meter and the exposure programming if you’re using an auto-exposure mode; and 3. When you press it all the way down, it fires the shutter and writes the images out to the memory card. You might ask what’s wrong with this? Well, remember that taking a photo before computers was four separate steps? The main problem with putting three steps into one is the loss of control in the two steps of focus and compose:
- Focus. In sports, your camera is almost aways set up to use continuous auto-focus (AF-C on Nikon and AI Servo AF on Canon). This allows the camera to track, or keep focus, on moving objects like an athlete or racing car. Of course, continuous auto-focus (I’ll use AF-C as a shorthand) has been one of the biggest breakthroughs in digital SLR technology and that’s why the best camera bodies cost $6000 or more, and sports photographers use this feature, but when you link AF-C with the half-press shutter it doesn’t work for a lot of shots. I’ll explain why and how to fix this below.
- Compose. Coupled with focusing, with the current AF-C systems it is either impossible or not practical to compose a photo where the subject is in focus and not positioned at the selected focus point when the half-press shutter button is used to activate auto-focusing. Again, more below.
Since 1989, all Canon EOS cameras have been able to be set up by the user so that a button on the back of the camera near where the right thumb usually rests is used to start the auto-focus function. Nikon also have this in their d-SLRs. According to Canon, this feature appeared through feedback from their professional sports photographers. This button is sometimes called “Back button AF” but on both camera systems the button is labeled “AF-ON”. This allows the photographer to start auto-focus by pressing the button, but equally important, to turn OFF the default setting of having the half-press of the shutter button starting the auto-focus. By turning off auto-focus on the shutter button means that the photographer can control when to stop and start focusing. Auto-focus starts with you press the AF-ON button, if you keep it pressed, and you’re in AF-C mode, the camera will keep focus on the object, and here’s the most important bit – the camera will stop auto-focusing when you release the AF-ON button and the focus will not change.
Practically, the situation where it’s most important for sports is when you’re tracking a moving object like a runner and out of the corner of your eye you see another object that’s going to cross in front of your object and your camera. This means that your camera is going to change the focusing from your runner to the interfering object. In this case, you just take your finger off the AF-ON button and keep shooting with your shutter button. If you hadn’t disabled auto-focus on the shutter button, all your shots will be out of focus because you wouldn’t have been able to tell the camera to stop auto-focusing and the interfering object will be focused instead of your object.
Here’s the magic setting to disable half-pressed shutter focus:
Function a5: AF Activation. Default is Shutter/AF-ON, change to AF-ON only
Custom function – C.Fn IV-1 Shutter button/AF-ON button. Default is 0, Metering and AF start. Use 2, Metering start for the shutter button and AF for the button. More info here.
Most sports photographers use the center focus point and when shooting fast moving sports you don’t have time to move the focus point around. What do you do when you want to compose your shot so that the object you want to be in focus isn’t in the middle of the photo (of behind the selected focus point)? Well, if you’re using half-press shutter button for auto-focus and you’re in AF-C mode you’re out of luck. You can’t, as when you recompose, the camera will just keep auto-focusing and focus on whatever is now in the middle of the photo or behind the focus point. But, if your using the AF-ON button and disabled auto-focus on the shutter button it’s super easy. Just tap the AF-ON button to focus – don’t hold the AF-ON button down. After focus has been achieved just recompose and press the shutter button. Pressing the shutter no longer tells the camera to activate auto-focus. In fact, this method makes the AF-S mode on a Nikon and One-Shot AF mode on a Canon redundant. The AF-ON button has duplicated this single shot auto focus mode. This technique is also useful for portraits and situations where you want to re-compose your shot after focusing and you don’t have time to set up the shot and manually move the focus points around.
Let me know how you go.
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