Canon, Nikon and other camera manufacturers have the Back Button Focus. It allows to disengage the focus from the shutter button.

Why? By remove the focus from the shutter, you can control when you want the focus to happen. Often you want/need to focus when you press the shutter, but there are plenty of times when you don’t want to focus when pressing on the shutter. Like when you focus and re-frame, or when you want something out of focus on purpose…

Finally, Fuji with the X-PRO2 and the X-T2 almost has it.

  1. Set the Focus Mode Selector on the front of the camera to “C”
  2. Press on the AF-On button (or the AE-L if you have swapped the position)

The X-PRO2 and the X-T2 will continue to focus between the frames, as long as you keep pressing on the AF-On button.

With the X-T1/X-T10, the camera will do the continuous focus while you are pressing the she AF-On button, but as soon as you press on the shutter, the focus will lock.