A fly has two compound eyes and each eye is made up between 3,000 and 6,000 simple eyes. These eyes create small pictures that allow the fly to see movements from the left, right, front and above.
Flies do not see in one solid view or focus on a particular object like humans do. Instead each simple eye makes a view and they join together like a mosaic.
Flies also have three simple eyes in between the compound eyes called ocelli. The ocelli act like a compass to let the fly know which way is up. They work by keeping track of sunlight, helping the fly move towards it.