Damage in eve is not of a singular type. It comes in 4 flavours.
Some weapon systems can deal all of these, some can only deal 1 kind of damage. And some can deal mixes (More on that here: Ammo Types).
- Missiles, with missile types for each of the damage types
- Lasers, they do mostly EM damage and may do some Thermal.
- Hybrids, which only deal Kinetic and Thermal
- Projectiles, similar to missiles in that they can deal all types, but in mixed ratios of mostly 2 types at once.
Every ship has a Shield, armour and structure defence built in. Each of these three will have an inbuilt resistance against the four damage types.
For a T1 hull base, this usually this pans out like this:
- Shields have a very low resist against EM. growing up in strength from EM -> Therm -> Kinetic -> Explosive.
- Armour will have a low resist against Explosive. Growing up in strength vice versa Explosive -> Kinetic -> Thermal -> EM
- Structure will have zero resist to all 4 damage types.
Now to put it in a couple of examples:
Say someone is attacking your brand new frigate with 100 points of EM damage on your shields. Say your ship has 0% resist against EM. The damage that will be taken off your shields will be the full 100 points.
Now this person switches to 100 points of Explosive damage, which your ship has 50% resist against, will make your shields go down 50 points.
Adding Resist ModulesEdit
You can add modules that increase either 1 type of resist (or all of them at once bit with less gain for each). And it would be handy to take no damage at all, by adding resist modules till you reach a full 100%.
However, it doesn't quite work like that. Your resist % will keep creeping toward 100% but never quite reach it.
The mechanic works in a way that a resist module will only diminish the remaining resist hole that is still left.
Say your ship has 40% Resist in Explosive. You fit a module that has a 50% damage resist bonus to Explo. Your ship has 60% in resist open. on that the module will take away 50%, which is 30%.
The final total Explosive resist will be 40+30 gives 70%.
(100-Current Resist) * Module Resist + Current Resist (100-40) * 0.50 + 40 = 70%
Eve being eve, there is some extra depth to it.
Certain modules have what is called a stacking penalty. Meaning if you put more then one of them they function not at optimum.
Resist modules are one of those.
Continuing on the previous example: we have a ship that has 1 Explosive resist mod fitted, and has as a total 70% resist to that.
Now, we add 3 more Explosive resist mods.
(100-70) * 0.50 + 70 = 85%
(100-85) * 0.50 + 85 = 93%
(100-93) * 0.50 + 93 = 96% with the fourth added.
(100-70) * (0.50 * 0.86) + 70 = 83%
(100-83) * (0.50 * 0.57) + 83 = 88%
(100-88) * (0.50 * 0.28) + 88 = 89% with the fourth added
(100-Current Resist) * (Module Resist * Stacking Penalty) + Current Resist
Far less then without stacking penalties.
Note, stacking penalties work on modules that work on the same attribute. Say you have 1 explosive hardener, and one that resists all 4 types, you will still get a stacking penalty on the Explosive resist.
Its a bad idea, in general, to fit more then 3 mods that have a stacking penalty. The 4th will only add you 28% of the slots potential. At that point a different module that works at full potential is far preferred. there are many small details that work itself into stacking penalties, but this is the general gist of it.
A Deceptive Small IncreaseEdit
Now that 5% jump in resist we got from adding the third module may seem like a small jump. but consider it like this:
With 2 modules we had 17% of the damage incoming. of 100 points only 17 come in. with 3 that jumped to 12% with 100 points, yea. just 12 come in.
(1-12/17) * 100 makes that still 30% more increase of damage resisted. That is still a quite large jump from what only seems to be a 5% increase.