Look up “Back Button AutoFocus” on Google – using the AF-ON button to focus your lens. You’ll find lots of articles extolling the virtue of separating AF and Shutter Release. That is one benefit, but not the big one, in my opinion. The really cool thing is instantly and instinctively switching between Continuous Focus and Focus Freeze. Allow me to explain.
Set up Back Button Focus, and be sure to disable Shutter Release button focus. Critical! Don’t forget the least part. Test it to be sure. (Repeat the Google search, adding your camera model number. You’ll get lots of instruction on how to accomplish this on YOUR camera. Oh, and if your camera doesn’t have an AF-ON button, set up the AF-L button to behave like one! Google!)
Put your camera in Continuous Focus mode (AFC or AI-Servo), not Single or One Shot Focus mode (AFS or Single Servo).
Focus by pressing and holding the back AF button. Follow your subject if it’s moving. The lens will continue focusing as you follow your subject.
If you lose the subject, and the lens starts hunting, release the button, put the point back on your subject, and press the back focus button again. Instant re-lock.
To stop focusing, lift your thumb off the button. The focus is trapped.
You may release the shutter at any time, while continuously focusing, or when focus is trapped because your thumb is off the button. Pressing the shutter release will NOT cause refocusing. Only your thumb controls focusing.
Super simple and quickly intuitive!
* Thumb on – it’s continuously focusing
* Thumb off – focus is frozen
If you are using live view, same thing! Move the focus point anywhere in the frame. Press the back focus button and hold until focus is achieved. Release the back focus button. Press the shutter release to make the image. Pressing the shutter release will not cause the camera to refocus.
Yep, it takes a few days to get over the old muscle memory and learn new muscle memory. But I’ve found the benefit well worth it. Give it a try, and don’t give up too easily. I’ll bet you’ll like it!