In Median XL, direct damage spells gain a damage bonus from energy. This bonus has diminishing returns: the damage gained per additional point of energy is lower if you already have lots of energy to begin with.
The spell focus bonus provides an additional benefit regardless of your energy, and does not have diminishing returns. For every 10 spell focus, the base damage of your spells is increased by 1%, up to a maximum of 100%.
The formula used to calculate the bonus from energy and spell focus to the base damage of your spells is: