Defenders Hand blocking opponent's vision

[ursamajor]

when i was young i was always coached to "get a hand in his face" when defending a shooter. If, after he has released the ball, i keep my hand in his face, am i committing a foul? i have found that a shooter's pct goes way down when i do this (pick up and playground ball) i do not put my hand in his face while he is shooting, only after the release, thanks