- Written by Nadia O’Carroll
The vast majority of species have two eyes. Although the eyes of diverse species such as mammals, reptiles, birds and fish are structurally similar, they have evolved and adapted to fit the needs of each species. Most insect species also have two eyes, but instead of one lens per eye they have compound eyes, which may have up to 30,000 tiny lenses. There are a small number of species that have more than two eyes - spiders have eight eyes, scallops have 64 blue eyes, jellyfish have simple eyespots on each tentacle and giant clams have thousands of tiny photoreceptors.
The location of the eyes and shape of the face effect an animal’s field of vision. American Woodcocks can see 360 degrees because they have eyes set far back on their head, horses can see 215 degrees with eyes set on the sides of their head and humans can see 180 degrees with frontally set eyes.
Within this field of vision, a proportion is binocular, where the view of both eyes overlap, and a proportion is monocular where the view is seen by one eye only. For example humans have 140 degrees of binocular vision and 40 degrees of peripheral monocular vision, 20 degrees left and 20 degrees right.
One of the vital requirements of vision is the ability to perceive depth, and this is most effectively achieved by binocular vision. A process called stereopsis occurs when slightly different versions of the same image are transmitted from each eye to the brain, which then factors in the disparity or parallax, and produces a single, integrated three-dimensional mental image. Monocular vision produces a flat single image that does not discern depth. However visual cues such as motion parallax, lighting, perspective, relative size, distance fog, focus and texture gradients also provide depth perception, so animals with less binocular and more monocular vision can still perceive depth. In many species there is a trade-off between a wide field of vision and a higher proportion of binocular vision. Predators tend to have more binocular vision and prey a wider field of vision
It is the process of stereopsis, which is utilised to create 3-D films. Two synchronised films with different polarisations are simultaneously projected by two projectors onto the screen. If viewed without 3-D glasses the images look blurry. Each lens of the 3-D glasses has a different polarisation so only one image can pass through each lens into each eye, which is then transmitted to the brain. The two slightly disparate images are integrated by the brain into a single three-dimensional mental image.