- "Not having eyes is probably a very unlikely event in animal evolution. Eyes seem to evolve so easily and must convey such advantages that it is hard to think of a reason to stop them evolving. All you need to start is some light sensitive tissue, some movement ability and the most basic of nervous systems, and you are off (unless there is total darkness)."
- ―Sigmund Nastrazzurro
Eyes are electromagnetic sensory organs typically designed on the pinhole principle. That is, an eye will consist of tissue arranged to allow light shine through a small aperture onto nerve-filled, light-sensitive tissue. This is a common method of sensing over a relatively long distance, as light travels quite far in many gaseous and liquid environments, and the small aperture allows the direction from which the light traveled to be assessed.
Field of View
Once an organism has invested in creating eyes, it costs relatively little to watch a wider area, so eyes will be placed where they confer the most benefit. In environments with gravity and little buoyancy, this typically means placing the eyes high up. A high placement takes advantage of mobility of an organism and offers a line of sight not blocked by terrain and other organisms. When height is not a concern, simply getting the eyes somewhat out away from the body can still be a concern.
In order to get a good view, eyes often 'stick out'; but at the same time, like any complex sensory organs eyes tend to need protection. It can take effort or special structures to keep eyes clean and transparent, as well as fending off physical damage. Ridges or bumps near an eye can fend off some blows, but must not obstruct too much view. When an organism has multiple eyes, protective structures may develop where the eyes are most redundant, so as to obstruct from one eye an area still viewable from another; however, having multiple eyes can make protecting them less important.A very different type of protection is shading from interfering light sources. Depending on the design of the eye, bright sources of light can have effects ranging from simply overwhelming the senses, to physically degrading the view (for example by creating a large halo, or scattering light through the lense material causing a washed-out look). This can be solved with a protective structure around the eye, as well as by ensuring the existing protective structures have dark coloration and do not reflect light into the eye; but another solution is to shape the pupil such that undesired light is kept out.
Various types of movement can replace these strategies. Movable eyes can potentially both enjoy a greater range of vision (though, without seeing everything at once), and move to protect themselves from injury. Protective structures may also move, so that they block vision when needed for protection but get out of the way at other times.
Like any body structure, eyes may come to be used for signaling when a species develops social habits. In species with movable eyes or movable eye protection, high-contrast coloration will often be used to make this movement more obvious. Exaggerated markings or structure may also be developed to make it easier to find the eye when the signaling is important. Of course, other senses may be used to signal, as in the case of eyes which make a particular scent when opened or a sound when swiveling.
Though a basic 'pinhole camera' design is simple and effective, just as there are other ways to design cameras, there are other ways to design eyes.
Rather than a large number of nerves measuring a single light-sensitive surface, each eye can have comparatively rudimentary measurements but eyes can be placed together in large groups, often with each eye facing outward in a slightly different direction to form a rough sphere. The brain then combines the field of view.
If a wide field of view is not a concern, a cluster of eyelets can be faced the same direction. Rather than each eye focusing light into a picture, a long tube can be used simply to try to filter out all light but that which comes from straight forward. In this way the structure creates an orthographic projection, an image in which distance does not change apparent size in the visual field. However, blur will still tend to increase with distance.
A plenoptic or light-field camera captures information about the three-dimensional structure of light entering the device; in engineered use this can be used to construct a 3D image from a single snapshot, or to allow arbitrary focusing of the image after capture. Nothing in principle prevents this from being used biologically.