Hello friendlies

I do following but it has two problem, first it happens on any collider and I want that only when player trigger it, and more the change is instant and I want it to be smooth over time, how to fix it?

function OnTriggerEnter (other : Collider) {
var go = GameObject.FindWithTag("MainCamera");
go.GetComponent("SmoothFollow").distance = Mathf.SmoothStep(6, 20, 5);
}

Sorry for my English I use google translated