你们都是怎么在拍视频时拉近拉远啊？

安卓机用音量加减键就能拉远拉近，用小七怎么弄啊？是在拍摄过程中。。。拍更近点该怎么弄？
-- How long will I hold you?
as long as your father told you,
as long as you can.