|This woman's eye is a complex sensory organ|
|Compound eyes are very different|
|Scallops have many simple eyes|
|The steps of camera eye evolution|
Following the development of the cup eye, the development of the cup into a pit with a small opening would allow more clarity of the image. This smaller opening of this pinhole eye allows the light to focus better onto the light sensing cells. However, this would still only allow for very limited vision. The nautilus has an eye that is of this type. If a covering developed over the opening to the pit, this would allow for more focusing power. Hardening of this covering into a distinct lens would clarify it even more. This sort of structure is what is seen in eyes like our own. Darwin himself theorized that just such a stepwise process must have been involved in the origins of the camera eye.
Interestingly enough, the camera eye evolved on at least three separate occasions. Similar, yet still quite distinct camera eyes are seen in vertebrates like ourselves, cephalopods such as the octopus, and, believe it or not, box jellyfishes. One interesting point of comparison comes up when people claim that the human eye is perfect. Though incredibly complex, the eye is, in fact, not perfect. Some aspects of the eye actually get in the way of the ability to see. For example, due to the structure of the nerves and receptors in the eye, there is a blind spot near the center of where light focuses on the back of the retina. In addition, the nerves that transmit signals from the retina to the brain actually sit in front of the retina so that light must make it past the nerves before hitting the retina itself. In contrast, in cephalopods such as squid, the nerves are behind the retina.
Here are two interesting videos on eye evolution, including one narrated by the ever-wonderful David Attenborough.
Source is Evolutionary Analysis. Images are from Wikimedia Commons and are either under Creative Commons licenses or are copyright free: one, two, three, four. Videos are from YouTube.com: one, two.