Cats have a tapetum lucidum, which is a reflective layer behind the retina that sends light that passes through the retina back into the eye.
While this improves the ability to see in darkness and enables cats to see using roughly one-sixth the amount of light that people need, it appears to reduce net visual acuity, thus detracting when light is abundant. A cat’s visual acuity is anywhere from 20/100 to 20/200, which means a cat has to be at 20 metres to see what an average human can see at 100 or 200 metres.
Cats seem to be nearsighted, which means they cannot see far objects as well. The ability to see close objects would be well-suited for hunting and capturing prey. In very bright light, the slit-like pupil closes very narrowly over the eye, reducing the amount of light on the sensitive retina, and improving depth of field.
Big cats have pupils that contract to a round point. The tapetum and other mechanisms give the cat a minimum light detection threshold up to seven times lower than that of humans. Variation in color of cats’ eyes in flash photographs is largely due to the reflection of the flash by the tapetum.
A closeup of a cat’s eye
Cats have a visual field of view of 200° compared with 180° in humans, but a binocular field (overlap in the images from each eye) narrower than that of humans. As with most predators, their eyes face forward, affording depth perception at the expense of field of view.
Field of view is largely dependent upon the placement of the eyes, but may also be related to the eye’s construction. Instead of the fovea, which gives humans sharp central vision, cats have a central band known as the visual streak.
Cats can see some colors and can tell the difference between red, blue and yellow lights, as well as between red and green lights. Cats are able to distinguish between blues and violets better than between colors near the red end of the spectrum. But cats cannot see the same richness of hues and saturation of colors that humans can. A 2014 study found that, along with several other mammals, cats lenses transmit significant amounts of ultraviolet (UVA 315–400 nm) light, which suggests that they possess sensitivity to this part of the spectrum.
Cats have a third eyelid, the nictitating membrane, which is a thin cover that closes from the side and appears when the cat’s eyelid opens. This membrane partially closes if the cat is sick, although in a sleepy state this membrane is often visible.
Cats often sleep during the day so they can hunt at night. Unlike humans, cats do not need to blink their eyes on a regular basis to keep their eyes lubricated (with tears). Unblinking eyes are probably an advantage when hunting. Cats will, however, squint their eyes, usually as a form of communication expressing affection and ease around another cat or human.