Today’s Question: I really enjoyed reading your article on the back button focus technique in the July issue of Pixology and I picked up a couple of neat tips. I’ve been using this technique for some time but have some confusion around using this option along with continuous focus/predictive autofocus. Specifically, how do I ensure that a moving subject will be in focus at the time the shutter releases? A great example of my question comes from viewing your “Parting Shot” at the end of the issue. In this picture you have a crop duster plane coming straight at the camera, at some speed I’m sure. How did you manage to keep the plane in such sharp focus? Were you using back button focus for the shot? If you held down the back focus button while pressing the shutter release, will the camera use predictive autofocus?
Tim’s Quick Answer: Back button focus will trigger predictive autofocus as long as your camera is otherwise configured for that purpose. In the case of the crop duster shot (which you can view here: https://500px.com/photo/112385163/crop-duster-spraying-in-palouse-by-tim-grey), I was indeed using back button focus, and that was triggering predictive autofocus.
More Detail: The key here is to configure your camera for continuous autofocus when you enable back button autofocus. You can then choose whether you want “one shot” versus “continuous” autofocus based on your use of the back button for focusing.
When the back button is pressed and held, the camera will focus based on the current settings. If you have the camera set to a “one shot” autofocus mode, then pressing and holding the back button for autofocus will only enable “one shot” autofocus.
If you have enabled continuous autofocus, pressing and holding the back button focus button will trigger continuous autofocus. And this is where one of the most powerful features of back button focus comes in.
By configuring your camera for continuous autofocus and enabling back button autofocus, you have the best of both worlds. When you want continuous autofocus you press and hold the back button focus button. When you want the equivalent of “one shot” autofocus you press the button until focus is established, then release the button.
It is worth noting that autofocus in general can become even more powerful by selecting a specific focusing point within the viewfinder that corresponds to the position of the key subject within the frame. That is exactly how I captured the crop duster photo referenced above.
If you’d like to get a free copy of the Pixology magazine issue that features the article on back button focus, you can sign up to get that issue for free here: