Each ommatidium is an independent sensor, with its own light-sensitive cells and often with its own lens and cornea.
From the en.wikipedia.org
Each small eye, known as an ommatidium, consists of a corneal lens, a crystalline cone, and a light sensitive organ at the base.
From the sciencedaily.com
The typical apposition eye has a lens focusing light from one direction on the rhabdom, while light from other directions is absorbed by the dark wall of the ommatidium.