Two eyes is the short answer, and it’s the right one. But the way an octopus uses its two eyes — combined with the strange fact that it cannot see colour despite its famous chromatophore display, and the additional possibility that its skin itself may detect colour — makes this one of the most interesting topics in cephalopod biology. The closer you look at how an octopus sees, the less the question of how many eyes it has is the right question to be asking.

Two eyes — the camera-type design

An octopus eye is structurally similar to a human eye in nearly every important way. It is one of the cleanest examples of convergent evolution in the animal kingdom.

The octopus eye has all the parts a human eye does: a cornea on the outside that protects the eyeball and provides some refraction, an iris that controls how much light enters, a lens that focuses incoming light onto the back of the eye, and a retina covered with light-sensitive photoreceptors. The eye is set inside a socket, can rotate to track moving objects, and can adjust focus by moving the lens forward or backward (vertebrate eyes change focus by reshaping the lens; octopus eyes move the lens like the lens of a camera). The basic optical principle is identical. What makes the convergent evolution remarkable is that vertebrates and cephalopods last shared a common ancestor more than 500 million years ago, before either lineage had complex eyes. Each group independently developed the camera-type design from scratch, working from simple light-sensitive patches and gradually adding the components. The fact that they arrived at almost the same design suggests that the camera-type eye is one of a small number of optimal solutions to the problem of seeing well in water.

The differences from a human eye

Despite the structural similarities, two clean differences set the octopus eye apart from the vertebrate eye. Both are improvements over the human version, in their way.

The first difference is the orientation of the retina. In vertebrate eyes (including human eyes), the photoreceptor cells face away from the incoming light, with the nerve fibres and blood vessels running in front of them and then bundling together at a single point to leave the eye through the optic nerve. That single bundling point is the blind spot — a small region in each eye where there are no photoreceptors. Brains compensate for the blind spot by filling it in computationally, but the gap is real. In octopus eyes, the photoreceptors face toward the incoming light, with the nerve fibres running behind them. There is no need for a single exit point through the retina, so there is no blind spot. The octopus retina is “wired correctly,” while the vertebrate retina is wired backwards. The second difference is the pupil shape: octopus pupils are rectangular (typically a horizontal slit) rather than round, which optimises visual detail across a horizontal field rather than emphasising depth. This is well-suited to a benthic animal that mostly cares about what’s happening at substrate level.

The colour-blindness paradox

Octopuses are famous for instantaneous colour change. They can match almost any substrate, signal threat or courtship with vivid colour patterns, and ripple chromatic waves across their entire body in seconds. And yet, by all the standard genetic and physiological tests, they cannot see colour.

Vertebrate colour vision works by having multiple types of photoreceptor cells (cones) tuned to different wavelengths of light: red, green, blue. The brain compares the relative activation of these cones to perceive colour. Octopus retinas contain only one type of photoreceptor, tuned to a single peak wavelength. Genetically, they should be functionally colour-blind — able to detect differences in brightness but not in hue. Behavioural experiments have repeatedly confirmed this: trained octopuses can distinguish patterns and shades but cannot reliably distinguish two colours of equal brightness. This creates the famous paradox: how can an animal that cannot see colour produce such accurate colour camouflage? Two leading hypotheses are being researched. First, octopuses may infer colour from the brightness patterns produced by the rectangular pupil shape — different wavelengths focus at slightly different distances, and a strange pupil that distorts the focus on purpose might let the brain reconstruct colour from the resulting blurring patterns. Second, octopuses may detect colour through their skin directly, using opsin proteins (the same photoreceptive molecules that line the retina) that have been found in skin samples. Neither hypothesis is fully confirmed, but the second is gaining experimental support. The animal may, in effect, “see” colour with its skin.

Field of view and depth perception

With eyes on opposite sides of the head, an octopus has a wide field of view but limited binocular overlap. The compromise suits a benthic predator that needs to watch for danger from all directions.

The eyes are mounted on the sides of the octopus’s head, giving each eye a roughly 170° field of view. Combined, the animal can see roughly 340 degrees around itself with very little overlap between the two visual fields. This means almost no part of the surrounding water is invisible to the animal — predators approaching from behind are seen as well as prey ahead. The cost is reduced binocular vision, the kind humans and most predatory vertebrates use to perceive depth. An octopus has only a small overlap zone where both eyes see the same region simultaneously, and depth perception in that zone is poor compared to vertebrate standards. Octopuses compensate by using head movement to triangulate depth (moving the body side to side and comparing the resulting visual shifts) and by relying on the chemo-tactile suckers of the arms for close-range work, which essentially substitute touch for stereo vision. The eye placement, the rectangular pupil, the wide field of view, and the limited binocular overlap are all consistent with a body plan optimised for substrate-level ambush hunting rather than depth-aware pursuit predation.