The Keen affinity enhances the Dexterity scaling of your weapon but lowers all other attribute scaling. It reduces the actual damage your weapon can deal while increasing the bonus you get from the scaling.
If you add Keen affinity to a 28 DEX weapon with 150+100 and C scaling in DEX, you’ll probably get 125+125 with B scaling. Basically, the Keen affinity makes your weapon scale more for DEX while sacrificing base attack power.
Ideally, you should add Keen to S-A scaling weapons combined with high DEX.
Ensure DEX is high enough before adding Keen
If you notice that you’re losing overall damage (base and scaling), this indicates your DEX is not high enough to make Keen worth it on that weapon.
When your DEX is high enough, the damage output will be higher. If your weapon is doing less than standard, level up your DEX until Keen becomes a good choice.
Conclusion
The Keen affinity makes your weapon base damage go down, while increasing scaling damage. Once DEX is high enough, your weapon will do more damage than standard. Make sure DEX is high enough before adding the Keen affinity to your weapon.
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