Disclaimer: this post was made before I understood all the mechanics fully. Abyssal may still be a better item in some circumstances, but I can understand why Void staff isn't useless.
Originally Posted by EasymodeX
Your analysis is fundamentally flawed because of 3 points:
1. In games where your spell damage matters and it is your primarily damage output, and you are successful enough that this discussion is relevant, opponents will have approximately 100 MR (Merc, Banshee, base MR).
2. Mag Pen augments the entire damage of your spell -- both the base damage, and the AP bonus damage. You do not appear to have accounted for the base damage in your analysis, which comprises the majority of the spell damage, marginalizing the benefits of AP to a certain extent.
3. AP bonuses in terms of alternative cost is additionally reduced due to most spells in various situations having a <1.0 AP ratio.
So, as an example,
Assumptions: Base spell damage of 250. AP ratio of 0.8. Target with an MR of ~80. Base AP of 100. 28.5 flat magic pen (Sorc boots + MPen Marks). 15% magic pen (mastery).
Base damage: 330 minus whatever 39.5 MR does.
+Void staff: 386 minus whatever 7.5 MR does.
+70 AP + 33.5 AP: 413 minus whatever 39.5 MR does.
I'm guessing that reducing the enemy MR from 40 to 8 will make a much larger difference than increasing your base spell damage from 386 to 413, tbh.
From what I've heard and seen, penetration is calculated like this:
1) Flat penetration from items
2) Percentage penetration from items
3) Penetration from runes & masteries.
So, let's assume they have 80 MR.
You have 20 pen from sorc boots. (down to 60 MR)
You choose void staff because youre an ignorant, selfish monkey. (down to 36 MR)
You aren't completely ignorant, and have 8.5 pen in runes (down to 27.5 MR)
Congrats, you took them from 80 MR to 27.5 for all of your abilites. Your allies still face their 80 MR.
Now let's assume the same scenario, but instead of buying Void Staff like a twit, you get Abyssal Scepter (Did I mention it gives you magic resist to? Bonus bonus bonus for being a teamplayer.)
Now they are at a comparable 31.5 MR for you, and down to 60 MR for the rest of your team.
Just for those of you who really care, Abyssal is better for YOU as well if their MR after your other items is 50 or less. Remember that it also helps your teammates! Void staff is for greedy barstuds who will more often than not get nuked over and over because they skip the nice magic resist that comes with Abyssal's aura. Be a teamplayer! Get Abyssal! Help yourself as well!
P.S. If you're a caster, you shouldnt be focusing the tank anyways. Nobody else should be stacking tons of MR. 'Nuff said.