Armor Focus (Combat)Source Armor Master's Handbook pg. 16
Your familiarity with a specific type of armor increases the amount of protection you receive from it.
Prerequisites: Base attack bonus +1, proficiency with selected armor.
Benefit: Select one type of armor, such as chain shirt or splint mail. The AC bonus granted by the selected armor increases by 1.
Special: You can gain this feat multiple times. Its effects do not stack. Each time you take the feat, it applies to a new type of armor. The Armor Focus feat counts as the armor training class feature for the purpose of armor mastery feat prerequisites and determines what types of armor you can use with armor mastery feats.Source Armor Master's Handbook pg. 20 (Amazon)
You can spend 2 stamina points to increase the additional armor bonus granted by this feat to +2 for 1 round. (See page 16.)