So, I beat all the mech bosses, I had a would where I stored a black lens, but deleted it... and, the black lenses have a 1% drop rate, but every night, about 3 demon eyes spawn, and about 2 wandering eyes, and I have been killing them repeatedly for about 5 hours, but still no black lens. Is there a way to get more to spawn/ farm them?
I do believe there are two items that allow for increased spawn:
- Water Candle, found on shelves and tables in Dungeons.
- Battle Potion, which can be found in chests and pots.
And I'm sure you know this already, but Blood Moon events increase monster spawn rates (for eyes that is) as well, and they have a 1/9 chance every non-New Moon night, so make sure to be on the surface when they occur.
I apologize for any mistakes in my answer, as I haven't played since just before 1.2
Source: Terraria Wiki Spawn page