Lockdown has made me revisit some technical features of my camera that I was unaware of, and this included “back button autofocus” which many middle to high end cameras have as a specific feature. Cameras that do not have this feature can often through the menu system enable another button eg AE to be reassigned to become an autofocus button.
Before I continue just to remind ourselves that normally using the shutter button on the front of the camera locks autofocus and the shutter, so using the shutter button to autofocus on the subject requires holding the button halfway to lock focus and then to depress fully for the image to be captured. If the subject moves distance this requires refocusing on the subject and locking again. Having locked does allow you to reframe the subject.
By changing to a back button focus system this separates the camera from linking focus and the shutter, the shutter button just takes the image capture, the back button is for autofocus. There are many circumstances that can benefit this alternative system of image capture.
To set the camera up several changes have to be made:
1 set to continuous autofocus
2 Set to spot focus
3 Disable the shutter button so that it does not autofocus but does retain exposure.
4 enable back button autofocus
The process will become clearer if you use the links below to demonstrate back button autofocus which can be used in viewfinder or live view mode. I have only looked at DSLR systems but I assume it will work for mirror less cameras.
pentax K3 ( my camera)