Lens

Lens is an item in Pathologic. Using it will immediately upgrade the Bachelor's Plaguefinder, which is used to detect Plague clouds. Using a Lens will increase the distance at which you are able to detect these clouds.

To fully upgrade your Plaguefinder, 3 Lenses are required. These may be acquired in several ways, but through the main quests, you will have a fully assembled Sighting device by Day 7.

Quote
"Lenses like this can be set into an optical apparatus and used to identify dense clusters of infected particles. Different lens sets allow for early detection and identification of different types of infected clusters."