PBS has a great video up about the evolution of camera eyes, from their documentary, Evolution: ‘Darwin’s Dangerous Idea’. Using a synthetic optical demonstration and examples from nature, Dan-Eric Nilsson describes some of the selectable gradations between a flat patch of photosensitive cells and a fully functional camera eye. Camera eyes, also called simple lens eyes, have a single chamber with a light-sensitive retina on one wall, across from a lens, through which light enters and is focused onto the retina.Camera eyes are found in most vertebrates; the only exceptions being species that have regressively lost their eyes due to low-light lifestyles. There are other examples of camera eyes in nature, however. The best know among these are cephalopods. Octopus, squid, and cuttlefish all have camera-style eyes that are structurally very similar to vertebrate eyes. This is a classic example of convergent evolution: Despite independent retinal derivation, vertebrates and cephalopods have both hit on a similar eye design solution for seeing their world. There are further examples of lens eyes in jellyfish, annelid worms, and gastropod mollusks; also typifying the concept of convergent evolution.
Less commonly known is that some arthropods have also discovered simple camera eyes. The majority of arthropods have compound eyes, composed of many independent optical units called ommatidia. Each ommatidial facet of a fly’s eye, for example, has its own lens and photoreceptor apparatus. Compound eyes work very well for most arthropods, however their maximum resolution is limited by their structure: The smaller the lens, the greater the light diffraction anomalies it produces; and compound eyes require thousands of very small lenses. Therefore, some arthropods have evolved camera-like simple eyes, with a single chamber in order to greatly increase resolution.
Arachnids, and specifically jumping spiders, possess the prime examples of simple eyes among the arthropods. They make use of a corneal simple eye. In a corneal simple eye there is no crystalline lens and the refractive focusing of light is carried out solely by the curved cornea. Using simple corneal eyes, jumping spiders achieve the greatest visual resolution among arthropods. In addition, some tiny copepod crustaceans have been found to have simple eyes with lenses. Among these, certain genera even have multiple lenses, producing telescoping optics.
Simple camera eyes are a spectacular example of convergent evolution. They can be found in at least five of the major animal phyla; Cnidaria (jellies), Arthropoda, Chordata (including vertebrates), Annelida (worms), and Mollusca. These phlya represent a tremendous diversity of creatures separated by vast eons of deep evolutionary time. This demonstrates the ability of a useful design to reveal itself through evolution in disparate lineages; built from whatever components are available.