A shieldsman focuses on mastering the use of his shield in battle for both defense and offense.
Combat Focus I
At 3rd level, you increase by 1 the bonus to AC and maximum Dex bonus, if any, of any shield you wield. You also gain a +1 bonus on attack rolls when making a shield bash attack.
Combat Focus II
Beginning at 5th level, you can use an immediate action to defend an adjacent ally, granting that ally your shield bonus to AC, including any bonuses granted by your combat focus. If the attack still hits defended the ally, you and the ally each take half damage from the attack, rounded up.
Combat Focus III
At 9th level, your ability to deflect attacks increases to the point that you can apply your shield bonus to your touch AC.
Combat Focus IV
At 13th level, your shield bonus to AC, maximum Dex bonus, and armor check penalty again improve by 1. The bonus on shield bash attack rolls increases to +2.