Armor penetration
I know what it is you ignore some of a foes armor. Can someone explain it to me? Why and when to get it? Like why not get more damage and hit harder? how about assassins an ADC any differences ?
10 Comments
Armor reduces physical damage received by a percent. Beyond a certain point, penetrating the target's armor is more effective than buying more damage. As a really simplified example, let's say you're dealing 300 damage per auto against someone with 200 armor (67% reduction). So that results in 100 actual damage. Would you rather penetrate half their armor (100 armor, 50% reduction, 150 actual damage), or have an extra 100 AD (400 damage, 67% reduction, 133 actual damage)?
Penetration ignores Armor/Resistance
Reduction reduces Armor/Resistance (so your allies benefit from this too) [it is also possible to reduce armor/MR below 0, making it damage amplification]
Damage reduction by Armor/Resistance works like this:
100/(100+Armor/MR) if Armor/MR >= 0
2 - 100/(100+Armor/MR) if Armor/MR < 0
Most champs start with ~ 30 armor/30 MR, so all damage they receive are multiplied by this: 100/130 ~ 0.77
meaning 23% of your damage is lost. armor works against both autos and physical damage abilities.
1 chain vest will get your armor close to 100 which means 50% damage reduction.
All in all, penetration is efficient even against squishy enemies.
% penetration is more efficient against someone who's built at least 1 armor/MR item
flat penetration is more efficient against someone who's built no armor/MR item
(remember that even if you get more damage, that 23-50% damage reduction is still there)
Also 
and 
are very cost efficient and cheap and is usually rushed on all smart damage dealers.
On specific adcs like 
and 
they even get as a first/second item after 
or 
. On most adcs, they get right after 
to deal full crits.
usually get as their first/second item.
usually gets it second after 
tl;dr: ALWAYS get penetration >.>
Any one know of a good vid that that explains it?
I saw your question asking for "how much and when?". That will always depend on the champs you're playing with and against as well as how you and they are building. No easy answers!
For example, champs like 
have abilities that get stronger with more armor. They're probably stacking items like 
. You're going to need more armor pen for attacking a rammus with 300 armor and his W up. Probably a too since it ignores a % of his armor.
You also have to pay attention to champs on your team. Let's say you're playing adc and you have a support and a 
jungle. Wu's Q will shred the enemy armor (remove some of it for a short time) and he generally will build an item like 
that will reduce the armor of anyone hit by his autos and ultimate each second. Taric's shatter (his W) removes enemy armor in a similar fashion. So if those two champs are on your team, you can get by with buying less armor pen since they're doing it for you.
Also, if the enemy team hasn't bought any armor items (maybe it's a 
with 
) then you don't really need to buy armor pen. If you buy a to fight enemies with 10 armor, its passive is getting rid of 3.5 armor. If they have 100 armor, it's getting rid of 35 armor and the passive is worth more.
So it depends! See what I mean?
Most any AD's that have trouble with last-hitting or have lots of magic damage don't want it.
Some like Graves and Draven really like it, though, but again only so long as you are good at last-hitting.
Every AD assassin loves it. Some bruisers like it too, like Renekton. Brutalizer, however, isn't typically worth getting unless you're stacking it or all the damage you do comes from your abilities.
Marks are the most cost-effective form of flat armor pen. Last Whisper is the most cost-effective armor pen item if no one has much armor and is honestly still good if no one has any armor because you're still doing ~25% more physical damage to everybody. In short LW is ridiculous right now.
That's about it!
I want to know more about armor pen so i have what i need, but I don't want to over do it and not have the damage to burst squishes