Using back-button focus (BBF) on your Canon camera
There are two ways to initiate (and lock) focus on your Canon DSLR
– using the shutter button, and / or
– using the AF-ON button on the back of the camera, near your thumb.
The AF-ON button can be set to be the only way to initiate focus, disallowing the shutter button from doing so. Depending on how you program your camera, the AF-ON button could allow you to trip your camera’s shutter independently of your focusing. Whether this is useful to you, (or perhaps even cause problems for you), depends on:
– your style of shooting, and
– the focusing mode that you use on your camera.
At some point, photographers who become frustrated with soft images, will look at solutions online on how to get crisply focused images. One of the pieces of advice that is often given, is to use “back-button focus”, rather than using the shutter button to initiate and lock focus. This may, or may not solve the problems they have.
As mentioned, you can initiate and lock focus with the:
– shutter button, and / or
– AF-ON button
How they affect you, will also depend on whether you tend to shoot in:
– Single mode, or
– Servo mode
Servo + AF-ON
Sport and action photographers will predominantly shoot in Servo mode. It makes sense because they most often shoot moving subjects. Then while they keep the AF-ON button pressed in, and while in Servo mode, the camera will attempt to track focus. In other words, the camera will continually refocus the lens to keep the subject in focus.
Now, the beauty of working this way, is that you can lock focus simply by not pressing the AF-ON button. If you’re doing a static portrait, you’d focus on your subject’s eyes, and then let go of the AF-ON button. Your focus is now locked.
This works beautifully for Sport photographers – they keep their camera in Servo mode, and then use the AF-ON button to track focus. The moment they photograph a static subject (or want to lock focus), they let go of the AF-ON button.
Servo mode is “Release Priority”. The camera’s shutter will fire, regardless of whether your subject is in focus or not.
There’s a downside to shooting in Servo mode and using the AF-ON button … your flash won’t emit the red AF-assist beam to help your camera focus. This is a real problem in low light events where you use flash. Your camera “thinks” it is in focus, and because it is release priority, the shutter will fire. And this is done in low light without the benefit of the AF-assist beam. This is a recipe for a lot of mis-focused images.
With Servo + AF-ON, the camera might have a tendency to keep hunting for focus in low contrast scenarios, often enough resulting in slightly mis-focused images when you trip the shutter while the camera isn’t accurately focused.
Single + AF-ON
This works in a similar way to Servo + AF-ON, with this exception: In Servo mode, the camera will continue to refocus while the AF-ON button is pressed. With Single + AF-ON, the camera will focus, and then once it is in focus, stop refocusing.
Now, if you let go of the AF-ON button, it will essentially lock focus. (The shutter button now doesn’t refocus the camera.)
In Single + AF-ON mode, the camera will make sure to lock focus before confirming that it is focused via the green dot in the viewfinder. (With Servo + AF-ON, the camera could keep refocusing, potentially giving you slightly mis-focused images.
With Single + AF-ON, the speedlite will emit the red AF-assist beam, giving you a better rate of success when focusing in low light.
Back-Button Focus and me
After decades of shooting with the shutter button as the way to acquire focus, I haven’t been able adapt to using the AF-ON button. I therefore use the shutter button to acquire and lock focus.
Ultimately, if you’re exploring the BBF option, you have to see which method works for you. If you’re a sport or action photographer, then using the BBF method with Servo mode, makes a lot of sense.
About the choice of using Servo or Single mode – since I mostly shoot events and use flash, Single focus mode is what works best for me. I need that AF-assist light from the speedlite. I need that.
How to set BBF on the Canon 5D mark II.
(It is very similar on all the Canon D-SLRs)
Custom Function IV – 1
Options 0 and 1, will allow the shutter button to initiate AF.
Options 2n and 3, will not allow the shutter button to initiate AF – so this now means only the AF-ON button can start the AF.
Custom Function IV – 2
This allows the * button (AE lock), to swap function with the AF-ON button. Whichever is more ergonomic for you.