As a photography instructor, one of the most common questions I see from students is about back-button focus. It's easy to gloss over it to keep things simple for new users, but doing so on the latest generation of mirrorless cameras leaves a lot of useful features on the table. It's worth learning.
I've seen a lot of explanations over the years, but Canadian wildlife photographer Simon d'Entremont has offered up the best explanation of this often overlooked feature of most mid-range cameras and up (and, hallelujah, even the entry level Canon Rebel T8i finally got a dedicated AF-On button, so no more using the auto-exposure lock (*) key to assign to what I consider a critical camera function).
I discovered the importance of back-button focus years ago when using the focus and recompose technique, mostly because the cameras I was using (the original Canon EOS 5D, then the 5D Mark II, and later the 6D) had lousy outer focus points, and so, I would hit the focus with the center cross-type autofocus point, then recompose by moving the camera slightly. This made it nigh-impossible to work with very shallow depth of field, because at something like f/1.4, you'd still slightly lose focus this way, but it was better than the camera missing focus entirely with every shutter button press. With the original 5D, I had to reassign the AEL button for this purpose, but Canon wisely added a dedicated AF-On button to later models. I also found it useful for more precisely controlling my focus for sports photography.
In fact, about the only time I ever turn off this function is when I'm shooting around friends and family and I know that I want to be in a photo. No one ever understands the concept of back-button focus, and so, it's easier to have one button do both focus and capture at once for the uninitiated.
All that said, even if you are a long-time user of back-button autofocus, you'll want to check out this video. As a long-time DSLR user, where eye-detection autofocus wasn't even a thing, when I purchased my Canon EOS R6 (and this applies to R5 users as well), I immediately set it up with back-button autofocus just like my 10-year-old 6D. These newer cameras have face- and eye-detection autofocus; however, I found it difficult to jump between these modes and traditional autofocus modes where I had more control over the area. I found myself using my shiny new camera much of the time without its advanced eye-tracking feature because it was too cumbersome to dive into the menu. d'Entremont shows you a way to configure your back-button autofocus to sidestep around this problem so that you can have your cake and eat it too, as in get that back-button focus you are used to while also quickly being able to access eye-detection features. I know my mind was blown when I saw this, so even if you've been shooting professionally forever, it's worth getting to the end of the video to check out d'Entremont's setup. He goes through setups for newer mirrorless models as well as older DSLRs.
Are you a fan of back-button autofocus? What do you use it for? Share your own tips and tricks in the comments below.