Compound eyes are essentially large clusters of simple eyes that are functionally capable of discerning images, whereas simple eyes only detect changes in light level. Compound eyes are found in many types of arthropods, and range from the relatively simple to the very complex. Regardless of their complexity, however, few arthropod eyes approach the acuity of vertebrate or cephalopod eyes, especially the keener vision of species, such as humans.
Each unit of a compound eye has a function similar to an entire simple eye. The unit, or simple eye, detects the presence or absence of light, along with its color in some species. Since each unit faces a slightly different direction, each conveys slightly different color and light level information. Different insects have different acuity of vision, with their acuity correlated to the number of units in their eyes.
Simple eyes are the only eyes of some arthropods, but even those with compound eyes usually have a few. In addition, many insect larvae have simple eyes in the same locations where they will eventually have compound eyes as adults. It is thought that these larval compound eyes are developed along a very separate pathway from the other compound eyes found in the adult forms.