A domesticated cat with complete heterochromia. Cats have excellent night vision and can see at only one-sixth the light level required for human vision. This is partly the result of cat eyes having a tapetum lucidum, which reflects any light that passes through the retina back into the eye, thereby increasing the eye’s sensitivity to dim light. Another adaptation to dim light is the large pupils of cats’ eyes.
Unlike some big cats, such as tigers, domestic cats have slit pupils. These slit pupils can focus bright light without chromatic aberration, and are needed since the domestic cat’s pupils are much larger, relative to their eyes, than the pupils of the big cats. Indeed, at low light levels a cat’s pupils will expand to cover most of the exposed surface of its eyes. However, domestic cats have rather poor color vision and can only see two colors: blue and green, and are less able to distinguish between red and green, although they can achieve this in some conditions.