Lens

“
Keep your eye on the prize, buy a lens!
— The Merchant

A Lens is a common crafting material that has a 33.33*1/3 (33.33%) chance to drop from Demon Eyes and Wandering Eyes.

Crafting[]

Result	Ingredients	Crafting station
Deer Thing	Flinx Fur3 Demonite Ore5 Lens	Demon Altar or Crimson Altar
Deer Thing	Flinx Fur3 Crimtane Ore5 Lens
Suspicious Looking Eye	Lens6
Mechanical Eye	Lens3 Any Iron Bar5 Soul of Light6	Mythril Anvil or Orichalcum Anvil
Optic Staff	Black Lens Lens2 Hallowed Bar12 Soul of Sight20	Mythril Anvil or Orichalcum Anvil
Archery Potion	Bottled Water Daybloom Lens	Placed Bottle or Alchemy Table
only: Goggles	Lens2	Work Bench
only: Goggles	Lens2	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.
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.

In some of his quotes, the Merchant claims to sell lenses, despite not actually selling them.

Entity	Qty.	Rate
~~Demon Eye~~ Demon Eye	1	33%
~~Wandering Eye~~ Wandering Eye	1	33%