If a camera's focus seems to going in-and-out (often described as popping or jumping), is is usually caused by Autofocus. As the subject moves, Autofocus will continuously try readjusting. If you are experiencing this issue, we recommend disabling Autofocus.
How to See if Autofocus is Enabled
To quickly see if Autofocus is on, look for the letters AF at the top of the Sources / Production pane. If There is a blue box around AF (shown in the screenshot below), then Autofocus is enabled.
To turn off Autofocus, tap the AF icon. Once it is disabled, there will be no blue box around the letters, and shown in the screenshot below.
If you use the Preview window, and you want to use Autofocus in Preview mode but disable it in the Live view, you should disable Autofocus as follows:
Go to the Camera Control tab (icon with two horizontal lines)
Tap the toggle bar next to Lock Autofocus When Live.