Talk:Magic penetration/@comment-72.71.210.48-20130112075217/@comment-122.108.145.9-20130112090756

Penetration and Reduction are different.

Penetration is only in effect when the target has positive MR. It does not change the target's MR, but treats it as lower for whoever is dealing the damage.

ex1: Katarina has Sorc boots (20 mpen) against a target with 30 MR. Whenever Kat deals magic dmg against that target, her damage calculations are treated as if the target had 10 mpen. Anyone else dealing magic damage, but with no mpen still use the 30 MR value.

ex2: Katarina has Sorc boots and Haunting guise (20 + 15 = 35 mpen) against a target with 30 MR. Although 35 > 30, the mpen cannot bring someone's effective MR below 0. Katarina deals damage as if the target has 0 MR (i.e. true damage)

Reduction changes the actual value of the MR, and benefits everyone.

ex3: Amumu AAs someone while holding an Abyssal sceptre (35 + 20 = 55 MR reduction). against a target with 30 MR. The target now has -25 MR. All players who deal magic damage against that target use -25 MR for their damage calculations. However, Kat who has 20 mpen deals no more damage than someone with no mpen, because mpen does not come into effect <0 MR.