"Shield Focus (Combat): When an attack is made against you while you are using a shield, you can spend up to 2 stamina points. If you do, the shield's bonus to AC against that attack increases by an amount equal to the number of stamina points you spent."
When is this ability allowed to trigger?
When the attack is declared?
After the Result is known?
I have always played that unless an ability/spell/feat/class feature explicitly says you get to see the results of the roll before using it, you have to choose to use it before, but I don't know if that is expressly backed by the rules.