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From Terraria Wiki
Black Lenses are crafting materials dropped by Demon Eyes and Wandering Eyes with a 1% (1/100) chance.
- Blood moons and new moons are useful for collecting these, as Demon Eyes spawn more frequently. A common strategy is to make a tunnel under a block of material and then cover these blocks in enough lava to kill Demon Eyes, but not enough to burn the drops (a coin can be used to test for the correct depth). Water Candles and Battle Potions will further increase the spawn of Demon Eyes.