Spell Focus

Spell Focus item enchantment
Spell Focus item enchantments, including the Spell Focus Mastery, also increase DC of the spell. They are Equipment bonuses, and thus stack with the feat.

On randomly generated items, school-specific Spell Focus is available on weapons, up to +5. It used to be available on bracers (up to +2).

On crafted items
 * You can use Cannith Crafting to craft school-specific Spell Focus +1 and +2. Not all schools are available (Abjuration only goes to +1, Divination cannot be crafted).
 * Alchemical Crafting (tier three) offers +2 bonus to selected schools (Evocation, Transmutation, Conjuration, or Enchantment)
 * Thunder-Forged weapons can offer school-specific Spell Focus +6, or universal Spell Focus +3.

This enchantment is also available on many named items.

Other stacking bonuses are available for example on yellow Augments (Insightful), Thunder-Forged armor (Profane) or two-handers (Exceptional).