Spell Focus

Spell Focus item enchantment
Spell Focus item enchantments, including the Spell Focus Mastery, also increase DC of the spell and stack with the feat.

+1 versions
Can be added to items with Cannith Crafting. Can be found on randomly-generated weapons, bracers (and not rings) and various named items. Can also be slotted in and such on epic items.
 * Abjuration Focus
 * Conjuration Focus
 * (Divination Focus doesn't exist)
 * Enchantment Focus
 * Evocation Focus
 * Illusion Focus
 * Necromancy Focus
 * Transmutation Focus

Minimum level:
 * Weapons (scepters and such)
 * level 4+
 * level 8+ (foci in two schools, in one item)
 * Bracers
 * level 7+
 * level 11+ (foci in two schools, in one item)

+2 versions (Greater Focus)
Can be added to items with Cannith Crafting. Greater Evocation, Transmutation, Conjuration, and Enchantment Focus can be found on the third tier of Alchemical Crafting. Also found on various named items:
 * Greater Enchantment Focus
 * Dreamspitter
 * Xachosian Eardweller
 * Staff of the Petitioner
 * Epic Spectacles of Spirit Sight
 * Epic Elder's Cap
 * Scepter of Mad Trickery
 * Shard of Xoriat
 * Greater Evocation Focus
 * Black Dragonscale Robe
 * Upgraded Regalia of the Phoenix
 * Epic Robe of Fire
 * Epic Shaman's Beads
 * Greater Illusion Focus
 * Scepter of Mad Trickery
 * Epic Illusionist's Garb
 * Greater Necromancy Focus
 * Staff of the Petitioner
 * Death's Touch (upgraded)
 * Epic Spectacles of Spirit Sight
 * Greater Transmutation Focus
 * Scepter of the Fleshweaver

+3 versions (Epic Focus)
Can only be found on epic items.
 * Epic Conjuration Focus
 * Epic Diabolist's Robe
 * Epic Diabolist's Docent