When is it more advantageous to choose magic penetration over ability power with my AP champions?
- Is it better to have a balance of the two stats, or is better to focus one more than the other?
Should I only get magic penetration when someone start stacking magic
resistence?- Is there a certain amount of ability power I need to achieve first before trying to gain more magic penetration?
@Michael Capobianco's answer is the closest one that I am looking for, but is not complete in my opinion:
If a champion is known to not scale well with AP items (E.g. Malphite,
Katarina), yet their abilities scale with level, then you should
consider building a heavier magic pen build.
Could someone make some math craft about it? On Wiki it's only make some examples about MR calculation. Not the importance over AP (of course, if you are making your enemies 0 MR you start stack AP).
Best Answer
Magic resistance stacks additively, and is calculated via this formula (assuming positive MR):
Magic penetration is applied in this order:
Magic penetration stacks multiplicatively. Void staff (40% magic pen) and the Arcane Knowledge mastery (10% magic pen) together equal 46% magic pen. source
We can see that the types of magic pen are very important, because buying flat magic pen (sorcerer's shoes) makes % magic pen (void staff) less effective. Unless the enemy has a lot of MR (ex: Galio), it's not worth it to buy multiple types of magic pen.
Compare some examples with a base MR of 30. With 0 magic pen, 100 magic damage does ~77 damage. With Arcane Knowledge, we ignore 10% of 30 MR (27 MR total) and deal ~79 damage. With sorcerer's shoes, we ignore 20 MR (10 MR total) for ~91 damage. With shoes and arcane knowledge, we ignore 20 MR + 10% of 10 MR (9 MR total), or ~92 damage.
We can see that for low values of MR, increasing Magic Pen provides very little gain. It also shows how effective flat MR is vs low amounts of MR, whereas percent reduction is not.
Now let's consider a large MR, say 200 and 500 magic damage. With 0 magic pen, that's ~167 magic damage. Arcane Knowledge reduces the MR to 180, or ~179 damage. Sorcerer shoe's are the exact same here. With shoes and arcane knowledge, we ignore 20 MR + 10% of 180 MR (162 MR total), or ~190 damage. If we add a void staff to these 2 (discounting it's bonus AP for simplicity), we get 200-20-72-11 (97 MR total), or ~254 damage.
It's safe to say at this point that unless the champion is stacking MR, you'll do more damage buying AP instead of Magic pen. A notable exception to this rule is Soraka, because her passive provides a substantial MR aura.
tl;dr: Runes, masteries, and sorcerer's shoes provide enough magic pen vs most champions. Only buy additional magic pen if the enemy champion has over 100 MR.