I thought I understood how MR Reduction and MR Penetration worked, but after reading some of the item forums I'm wondering if everyone else is confused or just me. (If I'm right, the boots of Sorcery are AWESOME vs. high MR targets)

I posted this in another thread before I started having my doubts. Can someone confirm my math?

Abyssal Scepter reduces magic resistance by 20.

Void staff penetrates 40% of the target's magic resistance. (Boots of Sorc are in the same category for 15%)

If I understand this correctly, these are not applied in the same way.

- Say the target has 30 magic resist (normal naked champion). This means your magic damage is reduced by 23%. (target has 1.3X more effective health)

- If the target has 60 magic resist, your magic damage is reduced by 37.5%. (target has 1.6X more effective health)

- If the target has 90 magic resist, your magic damage is reduced by 47%. (target has 1.9X more effective health)

Let's examine the effects of magic resist reduction versus magic resist penetration on these 3 examples:

Magic Resist Reduction (Abyssal Scepter, -20)

-Target has 30 Magic Resist, now 10 means magic damage now reduced by 9%, not 23%. This represents 18% increase in damage with vs. without Abyssal Scepter.

-Target has 60 Magic Resist, now 40 means magic damage now reduced by 28.6%, not 37.5%. This represents 14% increase in damage with vs. without Abyssal Scepter.

-Target has 90 Magic Resist, now 70 means magic damage now reduced by 41%, not 47%. This represents 11.8% increase in damage with vs. without Abyssal Scepter.

From these examples, we can conclude that Abyssal Scepter works better versus lightly armored targets vs. heavily armored targets.

This is NOT true of Magic Resist Penetration.

Magic Resist Penetration (Void Staff -40%)

-Target has 30 Magic Resist, now 18 means magic damage now reduced by 15%, not 23%. This represents 10% increase in damage with vs. without Void Staff.

-Target has 60 Magic Resist, now 36 means magic damage now reduced by 26.5%, not 37.5%. This represents 17.6% increase in damage with vs. without Void Staff.

-Target has 90 Magic Resist, now 54 means magic damage now reduced by 35%, not 47%. This represents 23.4% increase in damage with vs. without Void Staff.

Just for kicks,

-Target has 200 Magic Resist, now 120 means magic damage now reduced by 54.5%, not 66.7%. This represents 36.4% increase in damage with vs. without Void Staff.

As we can see, Magic Resistance Penetration actually does better the more MR the enemy stacks (versus just taking more +Ability Power, for example).

Magic Resistance Penetration does not make Magic Resist Negative. Only the Abyssal Scepter can do that, and since nobody starts with less than 30 MR I don't think negative values are likely / possible. Incidentally, if you have both the Abyssal Scepter and the Void Staff I think the Scepter gets applied first (unfortunately).

One more question: If I have the mastery (15%), the boots (15%) and the Void Staff (40%), does that mean I get to ignore 70% of the target's MR? If this is how these values stack, then I would say it is incredibly important to stack Magic Resist Penetration.