Pursued by the paparazzi? Watch out for those animals equipped with telephoto lenses...

Many animals benefit from high acuity vision, as this allows them to distinguish fine detail in the retinal image. The resolving powers of an eye generally depend on receptor density as well as image size, which is specified by the focal length of the optical apparatus. However, the wave nature of the light imposes constraints on receptor density, and the focal length is usually limited by the axial length of the eye. This means that in order to increase the focal length, the eye has to become larger, which is often impossible. Some animals have solved this problem in the most remarkable way…

Falconiform birds

The diurnal birds of prey (Falconiformes) famously possess a higher visual acuity than humans – the image quality of their eyes is at least twice as good as ours and they can detect point sources at much greater distances. However, the receptor density in their retina is only slightly higher than in human eyes and their head size is limited due to aerodynamic constraints, so how do they achieve this extraordinary resolution? The falconiform fovea (the depression in the retina where acuity is highest) is deep and its bottommost part has the form of a concave hemisphere. This hemisphere functions as a negative lens, meaning that light diverges rather than converges. In combination with cornea and lens, which both have positive power, this constitutes a telephoto system with extended focal length that magnifies the retinal image and hence increases resolution. The bird can thus resolve a superior retinal image in a localised region (albeit with a reduced field of view). In addition, the fovea functions as a sensitive, directional focus indicator, analogous to the autofocus of a camera.

Jumping spiders

Jumping spiders (Salticidae) are effective predators and well known for their astonishing visual capacities.In particular the large anterior median eyes are highly sophisticated, and their retina is very unusual indeed. It contains a characteristic pit, which is conical with a rounded apex and has (in at least some species) a refracting interface. Analogous to the fovea in falconiforms, it augments the corneal lens, thus increasing the focal length of the eye, sometimes beyond its axial length. This results in a magnification of the retinal image and increases the resolving power of the eye above that of an eye equipped with only a corneal lens. The power of the pit depends on its curvature, which varies considerably among species. In some spiders, it is probably too slight to provide significant image magnification, whereas the curved pit of Portia fimbriata increases focal length by about 1.5. Jumping spiders are small, and the axial length of their eyes is limited by their small body size, so this is an excellent way of increasing visual acuity without having to increase eye size. Thus, “both a group of vertebrates and an invertebrate have (…) adopted the same strategy to improve visual acuity despite a restricted cephalic space” (Williams and McIntyre 1980, Nature, vol. 288, p. 580). However, image magnification comes at a cost. It reduces the size of the visual field of the spider’s central retina, but this is compensated for by scanning movements of the retina.

Chameleons

The eyes of chameleons are very peculiar indeed, not only because they can move independently. Their lens is capable of considerable changes in shape – when accommodating, it has positive power, but when resting, its power becomes negative. This remarkable feature distinguishes chameleon lenses from those of all other vertebrates. The positioning of a negative lens behind a positive one (the cornea) provides a telephoto system, rendering the eye’s focal length longer than its physical length. For its eye size, the chameleon has the largest known retinal image of any vertebrate. Chameleons use fine detail in prey capture, and their accommodation needs to be fast and precise. Furthermore, they judge distance by means of their focussing mechanism (rather than by binocular triangulation as in humans). All this demands a high visual acuity, so it is easy to see how these reptiles benefit from telephoto eyes. The negative lens could have other advantages, too. It gives the eye a greater accommodative range and pushes its nodal point forward, close to the rather weird iris, which is formed by the eyelids and moves with the eye. This means that when the eye is rotated, objects at different distances move relative to each other. This differential motion parallax might help the animal to detect prey hidden amongst leaves without conspicuous movements of the head or body.

Sandlances

Sandlances (Ammodytidae) are small fish famous for a number of extraordinary eye-related convergences with chameleons. In a number of ways their eyes are highly unusual for a fish and are more similar to those of terrestrial vertebrates. They possess a flattened lens with greatly reduced power and a thick cornea with lens-like properties (normally, the cornea plays no role in refraction in an aquatic environment). As in chameleons, this design provides a telephoto arrangement and pushes the nodal point towards the front of the eye. Sandlances are also ambush predators, darting from their burrow to catch tiny copepods, and thus benefit from large image size and differential motion parallax.