The eyes of the cat are designed for the life they live - nighttime hunter
s. They are able to see at light levels 1/6th that humans can see.
The pupil of a cat opens about three times as wide as that of a person. At maximum dilation, a cats eye may be up to one centimeter across. This is in combination with a large cornea. For those who are familiar with photography (see f-stop for more information on this particular area) consider the maximum aperture:
The number after the 'f/' is the ratio of the focal length of the lens to the diameter
of the pupil
). The smaller the number, the more light that gets in.
The slit shape of the cat pupil is more effective at blocking bright light than that of the round pupil (as in the human and owl) - it can close down further. By being able to close down the pupil to a small slit it avoids over stimulation of a sensitive retina. The amount of light can then be further reduced by squinting. Because the pupil is not circular this does mean that the vision is not as clear when the pupil is closed (slit) as when it is wide open compared to animals that form a single pinhole when the pupil is closed down.
The retina of a cat has many times the number of rods as a human does. Rod cells in the retina are responsible for detecting light levels and motion (rather than color as the cones do). The ratio of rods to cones in a human is about 4:1 - a cat is 25:1. Cats may see some blues and greens but are deficient in red (which would look gray). This is likely due to the prey they hunt and the time of day when they are being hunted - seeing reds at night isn't that useful.
Furthermore, the back of the retina has a layer called the tapetum ludium. This layer reflects light back again into the retina (and out of the eye). This layer is what makes the cat's eyes shine at night. The eyesight is compromised by being slightly blurrier (the light has traveled a bit further and is not focus). This layer also shifts the wavelength of light toward the maximum sensitivity of the light detecting layer. This causes the dusk sky to be brighter increasing contrast between the sky and things in front of it.