Improved Shield Focus (Combat)Source Armor Master's Handbook pg. 18
Your mastery makes the most of your shield.
Prerequisites: Shield Focus, base attack bonus +6.
Benefit: You reduce the armor check penalty of any shield you are using by 1. You also use your character level in place of your base attack bonus for the purpose of shield mastery feat prerequisites.Source Armor Master's Handbook pg. 21 (Amazon)
You can spend 3 stamina points to negate your shield’s armor check penalty for one skill check. (See page 18.)