Field-of-view:

Images: Field-of-view for six taxa (human, deer, hawk, songbird, frog, and fish). The total arc of vision is represented in light orange while the binocular overlap is represented in solid, dark orange. Figure by A.Z. Andis with animal sketches by Bayla Arietta.

How an animal’s eyes are oriented largely determines its field-of-view and can tell you a lot about its ecology. Humans have forward-facing eyes which allows a large portion of the field-of-view to overlap and produce highly binocular vision across roughly 40 degrees of the entire 180 degrees of our field-of-view. The eyes of a bullfrog, on the other hand, are positioned near the top of the head and face outward. For a frog, this produces a 360 degree field-of-view in almost the entire visual arc but more limited binocular overlap.

In general, there is a trade-off between wide field-of-view and accurate depth perception. Since humans evolved to hunt and live in groups, good depth perception was more advantageous than a wide field of view. Frogs, on the other hand, faced selection pressure from pond-side predators, so detecting movement from any direction, even if fuzzy, was most advantageous.

In this lesson, field-of-view is represented with a black vignette layer. For instance, in the slider below, a 180 degree field-of-view is compared to a 340 degree field-of-view.

Depth-of-field:

As light enters the vertebrate eye, it first passes through the cornea--a clear membrane at the surface of the eye (if you wear contacts, you place your lenses directly onto your cornea). As light passes through the cornea, it bends (refracts) as it transitions from travelling through the outside medium (air or water) to the cornea. Next, the light hits the lens of the eye. The purpose of the lens is to redirect and project the light onto the retina at the back of the eye. Without the lens, diffraction in the cornea would make it impossible to focus.

When we view objects at different distances from the eye, the light from closer objects refracts differently from those far away. In response, the eye must change shape to bend the light into focus, a process called accommodation. To test this out on yourself, close one eye and look out at an object in the distance. Now, hold one finger up about a foot from your nose. When you switch focus from your finger to the distant object, pay particular attention to the sensation in your eye. You are feeling the shape of your eye change through focal accommodation.

Since the cornea and water have very similar refractive properties, there is not much bending of the light in the eye of a fish. However, in terrestrial animals, the light bends quite a bit as it transitions from air through the cornea. So, the lens of terrestrial animals must correct for more refraction. In aquatic vertebrates, the lens is spherical and accommodation is achieved by moving the lens closer to the retina. As mentioned, more refraction occurs in the cornea of terrestrial vertebrates and the lens can be thinner and more oval shaped. Accommodation in terrestrial eyes is generally achieved by deforming the shape of the lens itself.

The range of distances over which animals can focus on objects is determined by the range of accommodation made possible by the eye anatomy. We call this the depth-of-field. We say that animals have shallow depth-of-field if the range of accommodation is narrow or deep if the range is broad. 

Even within the human species, people have different ranges of accommodation. Folks who wear glasses tend to have a shallower depth-of-field than average. Of those bespectacled folks, some people are nearsighted, meaning that they can focus only on close objects, while others are farsighted and can only focus clearly on distant objects. The same is true for animal species. Those species that live in open habitat or must see predators from far off are generally hyperoptic (farsighted) while those that live in close habitat and hunt for food within reach tend to be myopic (nearsighted).

Check out the slide below to compare deep depth of field to both hyperopic and myopic shallow depth-of-field.

Focal field:

After passing through the lens, focused light falls on the retina which contains high densities of photoreceptor cells. Given plenty of light, the limiting factor that determines the sharpness of an image is how many photoreceptors can be packed into the retinal plane. In most species, high densities of photoreceptors are clustered near the middle of the retina to provide the sharpest image at the center of the field-of-view. This area of high-density reception is called the fovea.

In human eyes, the fovea is small and restricts the clearest vision to just the central 5-10 degrees of our field of view. This is why it is extremely difficult to read unless you look directly at a line of text.

As a test, lock your eye onto the "A" at the beginning of this sentence, then without letting your eye move, try to read the rest of the sentence. While you can see that the entire line of text falls within your vision, you probably had a lard time making out even one or two words past "As".  Such a restricted field of clarity is no problem for humans since we can easily swivel our head and eyes, but is more of a consideration for animals with relatively fixed heads and eyes like snakes and frogs.

In this lesson, focal field is represented as a blurred vignette, like in the slider below.

Spectral sensitivity:

Image: Diagram of the electromagnetic spectrum, including human visible light. Figure by Philip Ronan, usage under Creative Commons.

Vertebrates have two types of photoreceptors: rods, that cannot discern color but can function in low light (although blurrily), and cones, that react to narrow spectral frequencies of light, and allow for color vision if two or more cone-types are present.

Photoreceptors respond to different frequencies of light depending on the pigment they contain. There are five types of pigments employed in vertebrate eyes, four color cone pigments and the pigment in rods which is sensitive to blue-green wavelengths. Cone pigments are sensitive to long, red wavelengths; medium, green wavelengths; short, blue wavelengths; or super-short, violet and ultraviolet wavelengths.

Image: A rudbeckia flower as seen in the human visible spectrum (left) and in ultra-violet frequencies (right). Notice that patterns visible in UV light are not apparent in other frequencies. Photos by A.Z. Andis.

It should be no surprise that animals have evolved to see some wavelengths of light better than others. The sun is the primary source of light energy for all animals. We humans are most intimate with the relatively narrow band of the electromagnetic spectrum from the sun between about 380 to 760 nm. These wavelengths encompasses our visual range across the rainbow from violet to red. Other species have evolved to see shorter wavelengths, in ultraviolet, and longer wavelengths, in infrared. Other species only see limited portions of the human visible spectrum. So, why did evolution result in the restricted frequencies of some species? And why are all species more or less restricted to such a narrow sliver of the entire electromagnetic spectrum?

Shorter wavelengths in the blue and ultraviolet spectrum are easily absorbed and scattered by water and air particles (that is why sunsets turn red and orange as the light travels through more atmosphere at the horizon). At the longer end of the spectrum, infrared light tends to dissipate too quickly and does not have enough energy to be useful for sight, especially over distance. Thus, due simply to physics, most animal vision is tuned within the Goldilocks zone from infrared to ultraviolet.

Within the Goldilocks frequency range, evolution has tuned animals into wavelengths most useful for their ecological situation. In water, ultraviolet light only penetrates about 50 meters, making it useful to species at shallow depths but useless for deep-swimming species. Similarly, yellow and red light cannot penetrate water far beyond 50-100 meters. So, fish tend to have evolved the highest sensitivity to the light available at their preferred depth.

In addition, the high energy of short wavelengths can be damaging to cells (which is why we wear sunscreen at the beach to block UV). In many cases, animals that are active during the daytime have evolved ways to shelter their cells from these frequencies rather than absorb them.

It is difficult to pack lots of color cones into the retina. Color cones are not usually distributed evenly across the retina. Many animals, including humans, pack most of the color-sensitive cones in the center of the field-of-view and sacrifice color vision around the periphery. In this lesson, the effective cone density is represented with a desaturation vignette, as demonstrated in the slider below.

While the vertebrate eye is remarkable, it is also a lesson in evolutionary missteps. Due to a quirk of evolutionary history and embryonic development, the retina of our mammalian eye is oriented so that the sensitive rods and cones are pointed backward and buried inside of the retina. This means that light must first pass through the neurons connected to the photoreceptor cells and the base of the cells themselves before reaching the cones and rods. Nevertheless, other species have evolved ways to work around this problem. For instance, in humans, the layer of cells and blood vessels that would block trace amounts of light are shifted to the side around the fovea in order to maximize light in this critical field of vision. Some animals, especially those adapted to noctural activity, have evolved a tapetum lucidem (this sounds exotic, but literally translates to “shining carpet”). It sits behind the retina and reflects light back toward the sensitive ends of the cones and rods. The tapetum lucidem causes the eyeshine that you see when bright lights are reflected back from the eyes of, for instance, deer and cats.

Next: Deer Vision