When is it legal for a runner to take off on a fly ball, the moment the ball touches the mitt of the catching player or till he has the full control of the ball?

Question
Does a runner have to wait for the ball to be in the glove before he can take off or the moment the ball touches the glove  and the catching person still do not have full control of the ball?

Answer
Hi Francis,

On a batted ball that will at some time be legally caught the runner can leave the base as soon as any defensive person touches it.

An extreme example would be a high line drive that F6 leaps for, defects off his glove into left field where F7 catches the ball for an out.  All runners can leave (legally tag up) as soon as F6 touched the ball.

Mark