Testing indicates that a cat's vision is superior at night in comparison to humans, and inferior in daylight. Cats, like dogs and many other animals, have a tapetum lucidum, which is a reflective layer behind the retina that reflects light that passes through the retina back into the eye. While this enhances the ability to see in low light, it appears to reduce net visual acuity, thus detracting when light is abundant. In very bright light, the slit-like iris 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.
Many cats have a visual field of view estimated at 200°, versus 180° in humans, with 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 colours near the red end 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, content cat this membrane is often visible. If a cat chronically shows the third eyelid, it should be taken to a veterinarian for evaluation.
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. Cat owners can often entice their pets to squint or even fully close their eyes just by talking to them in a soothing or pleasing manner. Many cats will also squint in response to seeing their owners squint. When a cat does blink, it is a slower blink than in humans, so it will typically blink one eye at a time so it can still see from the other eye. When the cat is looking at someone while doing this, it gives the impression that the cat is winking at them.
Cats have a wide variation in eye color, the most typical colors being golden, green and orange. Blue eyes are usually associated with the Siamese breed, but they are also found in white cats and kittens. It is a common misconception that all white cats with blue eyes are deaf. This is not true, as there are many blue-eyed cats with perfect hearing. However, white cats with blue eyes do have slightly higher incidences of genetic deafness than white cats of other eye colors. White cats having one blue and one other-colored eye are called "odd-eyed" and may be deaf on the same side as the blue eye. This is the result of the yellow iris pigmentation rising to the surface of only one eye, as blue eyes are normal at birth before the adult pigmentation has had a chance to express itself in the eye(s).