Today, we are going to touch on back button focus, otherwise known as AF lock. The AF lock button is on the back of most modern SLR and mirrorless camera bodies, like on my Fuji XT-1. Image compliments of Fujifilm.
So, what is AF lock / back button focus? I will explain it’s basic function and purpose without pixel peeper lingo (which is why I don’t do many photography articles anymore, and that seems to be the norm lately, lol) If you hit the AF lock button with your thumb, it’s the same thing as holding the shutter button down halfway to set your focus point. But it separates the Auto focus activation from the shutter button, by locking the focus point you select. This means that if you are taking possible multiple photographs of that subject, you can act in those decisive moments by only recomposing if needed, and not having to hunt for a new focus point. Simply hit the shutter button when ready to take the shot. Also it can be more accurate in the sense of another person entering the frame while your focus point is locked, the camera won’t move it’s focus point to the “photobomber” .
It’s an especially useful feature during moments of action, low light settings when the camera may hunt for a focus point (this is real talk with mirrorless especially, not that it didn’t happen with my Canons too). Or in moments of anticipation….you can fire in that split second because your focus point is already locked. Those are things it is for.
What AF lock is not designed for- It can most definitely track motion with the same subject the focus point is locked on. But it is not for panning moving subjects as distance to camera changes, that depth of field will not compensate for. That’s a common misconception. I have used it for aisle shots, if I am moving with them backwards, but am careful about my depth of field. You really have to know what you’re doing with AF lock to use shallow depth of field / large apertures with it. If you are panning a moving subject, use AI servo / Continuous focus. Using AF lock with that will also make the camera focus on your intended subject, and not hunt for a different depth / point of contrast. It’s not super complicated, a few hours of playing (I recommend street photography) it will come clear. Feel free to question or comment 😉