Have you ever wondered how you might appear through the compound eyes of a fly?
The best analogy to describe a fly’s vision is to compare it to a mosaic: thousands of tiny images convalesce to present one image. Each one of these images represents information from the fly’s individual ommatidium. The effect is much like how we see stippling or newspaper print: up close the image is a lot of tiny dots, but take a step back and it’s a complete image. The more ommatidia a compound eye contains, the clearer the image it creates.
There’s a reason why flies are especially jumpy creatures that take off at the slightest flinch: a fly’s vision is nowhere near as clear or effective as a human’s, but it’s especially good at picking up form and movement. As an object moves across the fly’s field of view the ommatidia fire and stop firing. This is called “flicker effect”. It’s similar to how a scrolling marquis works: like tiny lights turning on and off to give the illusion of motion. Because a fly can easily see motion and form, but not necessarily what the large moving object is, they are quick to flee, even if the moving object is harmless.