Stats and Graphs about Magic Penetration impact on damage dealt, but without the graphs!
Just sharing some tests I made on how magic pen would affect the damage you deal. I found out that, unlike as I thought, it doesn't seem to be possible to drop enemy magic resist to -5, as I heard somewhere: enemies (jungle camps and dummies, at least) will only drop to 0 MR, where you deal 100% of your skill's damage. The only exceptions seem to be jungle camps, as they already have a negative base magic resist that cannot be lowered (and probably not raised either).
*All tests below were done by me inside Training mode, using Ahri's Charm (200 base damage, 0 AP, AD runes, no masteries).
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0 Magic Pen on Jungle Camps -30 MR monsters - 246 dmg - 123% of skill's damage 0 MR monsters - 200 dmg - 100% dmg 20 MR monsters - 166 dmg - 83,0% dmg 60 MR monsters - 125 dmg - 62,5% dmg
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15 Magic Pen on Jungle Camps -30 MR monsters - 246 dmg (+0%) (cap) (by cap I mean it reached 0 MR, as minimum) 0 MR monsters - 200 dmg (+0%) (cap) 20 MR monsters - 190 dmg (+14,5%) (14,5% is the amount of damage increase relative to the 0 Magic Pen's damage) 60 MR monsters - 137 dmg (+10,0%)
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30 Magic Pen on Jungle Camps -30 MR monsters - 246 dmg (+0%) (cap) 0 MR monsters - 200 dmg (+0%) (cap) 20 MR monsters - 200 dmg (+05,0%) (cap) (compared to 15 Magic Pen's damage, would be a bigger increase if it didn't reach cap) 60 MR monsters - 153 dmg (+11,5%)
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Dummy (100 MR) 0 MPen - 100 Dmg - 50,0% of total damage 15 MPen - 108 Dmg - 54,0% - +4% increase 30 MPen - 117 Dmg - 58,5% - +4,5% 45 MPen - 129 Dmg - 64,5% - +6% 60 MPen - 142 Dmg - 71,0% - +6,5% 75 MPen - 160 Dmg - 80,0% - +9% 90 MPen - 181 Dmg - 90,5% - +10,5% 75 Mpen + Void - 100 Dmg- 100% (cap)
The pattern on the damage increase against the dummy is very interesting, yet I cannot understand it. It sometimes increases by 0,5% (from 4 to 4,5%), then suddenly increases by 1,5% (4,5 to 6) then goes back down to 0,5% (6 to 6,5). Anyway, I hope this gives you guys some notion on how magic penetration will actually affect your damage, and help decide if it's worth it. Note that 90 MPen can be obtained by purchasing Liandry's and 5 Sorcerer Shoes, or 6 Sorcerer's Shoes. You can go up to 116 by using Runes and Masteries.
Another similar (but kinda random) tip that I can give is for when buying stuff like Cloak of Agility or Pickaxe: 20% crit will increase your average damage by 20%, and Pickaxe will increase it by flat 25. So, if 20% of your AD is higher than 25, you may prefer to buy the Cloak, otherwise Pickaxe. This does not always apply to champions that interact with crit, such as Caitlyn and Yasuo, and probably only makes sense when you have 0% crit.
Thanks for reading, correct me if I'm wrong on anything.
