my sensei says to keep my attention on a triangle formed by the opponents forehead and shoulders. he has not said anything about "soft focus", or "eagle vision" as my tai chi instructor called it, but keeping an eye on all points of the triangle at the same time forces this eagle vision in a way. and I do recall my sensei saying that most movements can first be spotted on the shoulders.
in real situations I always look a potential troublemaker straight in the eye so I can see his mind change before he (or she) makes a choice to either attack or withdraw. in the few cases where I've been assaulted I honestly cant remember where I was looking, only that the adrenaline sharpenes every sense there is