You got my +1 vote... I really think that both AP and MagPen are of similar value, cause one can't work well without the other. But it all goes down to the champion and his/her abilities!!! I even use MagPen on Nasus(haunting Guise to be exact) cause he does a lot of magic damage! Please, to all Nasus players: check out the damage stats after the match and you wil see what I'm talking about!!!

Your math is a little off. Also you're wrong about less than 0 MR being useless. The devs confirmed on the forums that negative magic resist, and armor, does amplify damage at the same rate the corresponding positive value would reduce damage.

If your target has 30 MR and you have Abyssal Scepter and Sorcerer's Shoes, the scepter will first reduce their MR to 10 and then the shoes will penetrate to -10 MR. (30 - 20) - 20 = -10

The reason Abyssal Scepter isn't just added into the total magic penetration is that it reduces the MR for everyone. So in the same situation above your target will have -10 MR for you and 10 MR for your teammates if they have no magic penetration themselves. Also the reduction from the scepter doesn't stack so it's good to coordinate with your team. If one person gets the scepter it would be better for other teammates to get Void Staff; or Haunting Guise if the enemy has low MR and you're more worried about penetration than you are about the AP you'd get from void staff.

Basic rule of thumb is magic penetration from Void Staff is more worthwhile than a flat 20 magic pen, when the enemy has 50 resist AFTER calculating your flat penetrations. Prime example for this is Ryze who doesn't really need the AP. If your runes and boots take them down to 49 resist or less you're better off getting the cheaper Haunting Guise than a Void Staff.

+vote! Thank you for going through this and explaining it all very well.
I redirect people from my Veigar to this guide to make them understand the ideal combination of AP with MagPen.

Abyssal Mask will reduce their magic resistance first, it will take affect before magic penetration so a pair of Sorcerer's Shoes and an Abyssal Mask will not give you -10 magic resist-- it will only reduce their magic resist to 0.

If however you have a Malady and an Abyssal Mask and you attack them 4 times- their magic resistance will be -10. This is the only way aside from abilities like Soraka's Starcall to reduce magic resist below 0 and this is applied to the champion not the damage source so that hero would take extra damage from any magic source.

Oh yea, and magic penetration is useless when they have less than 0 magic resist.

how can this only be 71% some people out there are heavy AP *****s who don't want to comment to make themselves look stupid so they just Down vote. Let the flaming begin!

A great guide that needs to be known. I dislike people who build pure ap. it's just easily countered :\.

By the way, I suggest not making a gigantic block of white text. I was half asleep when I first saw this, and i went, oh hell no. Not reading this. Now after a nap, everything is comprehensible.

-1. UMADBRO? Naw jk. +1.

Voted up. Overall solid information. Much of this is situational, depending on your team comp, enemy comp, etc. But as for general AP damage, this makes a lot of sense.

This "build" (I wouldn't really call it that) makes a lot of good points, but one thing I don't like is that you make it sound like Void Staff is completely useless, when it's not. Void Staff is not for squishies, that I can agree with you on.

But when I'm carrying 21-2 on Katarina and the enemy tank starts stacking Magic Resist, that Void Staff is gonna get me a lot further than any flat Magic Pen, because if you're exclusively stacking one defense you can easily get it into the 200s.

That's the only point I'd like to make. Voted up.

Pretty nice man, sad to think you only have a 70% or so rating... well thumbs up from me :D

"In general you can see, that there is never a point in going for pure AP, even if your opponent had only that theoretical 20 MR!"
Without doing the math, I can pretty surely hypothesize that this is incorrect. You haven't taken abilities into account. The magic resist deduction aura on Fiddlesticks might not make much of a difference, but Ryze's Spell Flux does. At skill level 5, it reduces magic resist by 24, and can hit 3 times. A loss of 72 magic resist is pretty huge. Bear in mind that this is not the same thing as magic penetration. Losing magic resist can take you into negatives, while magic penetration cannot. What I don't know is which is applied first. If they have 20 magic resist, and an aura takes them down to 0, but you also have 20 magic pen, will they go into negatives? Even if it did, it doesn't seem likely that building any magic penetration on Ryze would be a good idea, but I haven't done the math on that one.

EDIT: Any excess magic penetration is indeed lost. Ryze should never have any magic penetration whatsoever, as far as I can tell.