Lens

Lenses are a common drop from Demon Eyes. However, they are not dropped by their Hardmode counterpart, the Wandering Eye.

Another form of lens, called the Black Lens, can be used to craft Sunglasses. Black Lenses have a much lower drop rate than the standard Lenses.

Making money
A much better way to get money from lenses, rather than simply selling them, is to stockpile them until you have 6, at which point you can craft a Suspicious Looking Eye, kill the Eye of Cthulhu and sell the reward.

History
Lentille