You have more control over the effect of spells you cast.
Prerequisite: Able to cast 2nd level arcane spells, 6 ranks in spellcraft.
Benefit: You can cast spells at a lower caster level than the spell would otherwise allow, reduce the area or duration of a spell, let a spell fade over time instead of ending suddenly, or make some other alteration that only reduces its power. If you create magical items at a lower caster level, making them still costs the same in materials, XP, and time.
Also, you can alter spells so that they in operate in a slightly different way, such as changing the colour of a light spell, the tone of an alarm spell, or making Melf’s acid arrow travel in a parabola. Altering a spell like that takes a spellcraft check with a DC of 10 + twice the spell’s level. If you fail the check, the spell is ruined.