You probably think that because a fly "sees" your hand coming to swat it that a flies eyesight is really good. In fact the opposite is the truth. Compare to what would be prefect vision in a human (20/20 vision) a fly has only 20/1200 vision. It does have the benefit though of having around 4,000 lenses or ommatidia per eye. Each ommatidium or "lens" performs independently so is like an eye in its own right. Every ommatidium is like a long cone with the lens on the tip and bearing its own distinct nerve fibre which connects to the flies main optic nerve.