Ensure stream recording at start of fast_up_down_up_autofocus

           might slightly hurt accuracy, but is unlikely to be a big issue.  Too big

           may cause you to overshoot, which is a problem.

        """

        with self.monitor_sharpness() as m:

        with self.monitor_sharpness() as m, self.microscope.camera.lock:

            # Ensure the MJPEG stream has started

            self.microscope.camera.start_stream_recording()

            df = dz #TODO: refactor so I actually use dz in the code below!

            if initial_move_up:

                m.focus_rel(df/2)