You can increase the protective effects of a spell you cast.
Benefit: Only spells that grant a bonus to armor class or saving throws can be prepared as a defensive spell. A defensive spell grants a bonus to armor class or saving throws 1 greater than a normal spell of the same type. Thus a defensive shield grants a +4 shield bonus to AC, and a defensive prayer gives its targets a +2 luck bonus to saving throws. A defensive spell uses up a spell slot one level higher than the spell’s actual level.
The Genius Guide To: Feats of Metamagic