If you are using firmware version 1.1.0 or later, you can select [Mode 1] or [Mode 2] in [Movie Servo AF] to specify your preferred characteristics for AF operation.
[Mode 1]
Emphasizes fast, responsive focusing.
Suitable for responsive subject tracking, or if you will walk or move in other ways while recording.
Keeps subjects in focus, even when the shutter button is not pressed halfway.
[Mode 2]
Emphasizes natural focusing and smoothness in movie recording.
Suitable for landscapes or relatively still subjects.
Keeps subjects in focus, even when the shutter button is not pressed halfway.
Caution
When you select [Disable], the focus remains constant as movies are recorded.