Are you sure riot's formula applies magic/armor reduction before flat penetration? I heard it was applied last.

Reduction is applied before percent penetration which is applied before flat penetration.

Percent penetration of void staff also combines with 15% pen from offense tree and MPEN runes/items extremely well.

Yes. I think the rule of thumb is the stacking order is always in favor of the victim's survivability. Not that it's a bad thing for me as a tank, mind you.

With that said, I don't think you can compare MRpen to ARpen. Champions that requires MRpen usually do not benefit from MRpen, hence the items are generally mutually exclusive.

If you truly want to discuss their worth, you have to factor in burst potential, DPS potential over a period of time (which includes CD, etc), mana efficiency, etc. It's not worth comparing.

Pro tip:

Look at your enemies' items before you buy an item designed to counter certain items.

QED

The comparison to LW is interesting, but it doesn't help me make a decision. It is not as if, as a caster, I am deciding between LW and VS. A useful number would be one comparing how much damage you gain from VS vs other late game magic items, then finding the break even point on enemy Magic resist. Much more useful than basing it on an "average" since that doesn't tell you what to do in any particular situation. I know that I love to get merc treads, chalice and BV on plenty of toons when I'm facing decent casters. Is it worth it when someone has that set-up?

Yes, I agree with the numbers that Void staff is worth getting. But more to the point is the feel of the item in game against folks stacking MR.

Here's what I do in a game. I know about when I'm going to get Void Staff, so right at that point I check the enemy to see if they are stacking MR. If they are noobs and refuse to stack MR against me, I'll happily get a couple of Blasting Wands instead to work towards a Zhonya's (or maybe a Lich Bane). If I do see the MR, I always, always get MR. The fact of the matter is, the whole game is structured this way. If you do the math, you realize you want some magic pen, and some AP. There's optimization we can do to get more exact and it does depend upon your enemy's MR, but generally speaking the two work well together. Getting all AP or all magic pen is less effective. If you need a full numerical analysis, I can run the numbers but I've already worked it out in another post.

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.

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.

EDIT:
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.

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Debuffs are calculated first. Aka Spell Flux, Abyssal Scepter, etc.

Then, % reductions -- Void Staff, the mastery 15%.

Then, penetration -- Guise, Sorc Boots, runes.

Penetration is floored at 0.

Hmmmm... that's not what I've heard. I guess I'll have to test it unless you have some reliable sources?

http://www.leagueoflegends.com/board...ead.php?t=3287

This is a STICKY, for God's sake.

Thanks, I stand corrected. For some reason when I read that post, I was certain that reduction meant all flat reduction/penetration from items, and penetration meant all reduction from runes. My bad, I understand why void staff isn't worthless now. Although the OP might not >_>