Lens

Keep your eye on the prize, buy a lense! [sic]
— The Merchant

A Lens is a common crafting material that has a 33.3% (1/3) chance to drop from Demon Eyes and Wandering Eyes.

Crafting[edit | edit source]

Result	Ingredients	Crafting station
Suspicious Looking Eye	Lens (6)	Demon Altar or Crimson Altar
Mechanical Eye	Lens (3) Any Iron Bar (5) Soul of Light (6)	Mythril Anvil or Orichalcum Anvil
Optic Staff	Black Lens Lens (2) Hallowed Bar (12) Soul of Sight (20)	Mythril Anvil or Orichalcum Anvil
Archery Potion	Bottled Water Daybloom Lens	Placed Bottle or Alchemy Table
Goggles ()	Lens (2)	Work Bench
Goggles ( )	Lens (2)	Work Bench and Chair

A much better way to get money from lenses, rather than simply selling them, is to accumulate them to craft Suspicious Looking Eyes, kill the Eye of Cthulhu and sell the rewards. (The ore is best sold as bars.)
Later in the game, Mechanical Eyes can be crafted and used similarly, but even in the advanced game, The Twins remain a tougher opponent, and profiting off their drops is a rather more involved process.