A cat's pupil is a slit, and the shape is a mechanical answer to one problem: how to keep a light-sensitive retina usable across a 100,000-fold change in brightness. In full sun the slit closes to under 1 mm and blocks up to 90% of the light that would otherwise enter. In near darkness it opens to 9-10 mm and covers roughly 90% of the visible eye. Between those two states the pupil changes area by a factor near 135, against about 15 in a human eye.
A slit cuts 90% of the light, and does it faster
Two muscles run the iris: a sphincter that rings the pupil and a dilator that fans out from it. In a slit pupil the sphincter fibres are arranged in two opposed arcs, so contraction pulls the opening into a line rather than a circle. Closing a slit removes area from the sides first, which is exactly where the cornea focuses stray light; the result is a 90% cut in luminance with the centre of the visual field still open.
Speed matters as much as depth. A cat's pupil can go from 8 mm to 2 mm in about 0.3 seconds in a sudden light change, and the slit shape lets the edges travel a shorter distance for the same area change. That is why a cat can track a bird against bright sky, then drop into shadow without a 2-second blind interval.
135 times the area range, against 15 in a human
A cat iris moves between roughly 0.5 mm and 10 mm across, so pupil area spans a factor near 135. A human iris moves between 2 mm and 8 mm, a factor of 16. A reflective tapetum lucidum behind the retina returns light for a second pass; the figure most texts quote is that a cat needs about one sixth of the light a person needs to see the same shape.
The cost is resolution. A dilated cat eye has a shallow depth of field and heavy spherical aberration, so a cat in dim light sees movement and shape well and fine detail poorly. Bright light plus a slit restores depth of field, which is why a hunting cat in daylight holds a narrow pupil while tracking a mouse at 2 m.
Pupil size against light level
| Light level | Pupil width | Share of eye opening | What you see |
|---|---|---|---|
| 0.1-1 lux (night room) | 8-10 mm | 85-90% | Round, iris almost invisible |
| 10 lux (lamp-lit room) | 5-7 mm | 60-75% | Oval, slight slit |
| 100 lux (overcast indoor) | 3-4 mm | 35-50% | Clear vertical oval |
| 1,000 lux (bright room) | 2-3 mm | 20-30% | Narrow slit |
| 10,000 lux (shade outdoors) | 1-2 mm | 10-15% | Thin line |
| 50,000 lux (full sun) | Under 1 mm | Under 10% | Hairline, sometimes shut |
Read the table as a baseline, not a diagnosis. A cat in a 100 lux room with an 8 mm pupil is either frightened, in pain, or has a problem with the iris. A cat in full sun with a 4 mm pupil is not relaxed; something is holding the pupil open.
A 200-degree field, with 120 degrees of overlap
A cat sees about 200 degrees horizontally, against roughly 180 in a person. The eyes sit forward enough for binocular overlap of about 120 degrees, which is where depth perception lives, while the outer 40 degrees on each side is monocular. Vertical field is about 120 degrees, helped by a globe that can rotate rather than staying square to the ground.
Compare that with a rabbit at 360 degrees and a dog at 250. The cat sits between predator and prey geometry, and the price of the 200-degree field is a blind zone of 5-10 cm directly under the nose, which is why a cat loses a toy dropped at its chest and then finds it by touch.
The third eyelid: normal for 1 second, not for 1 hour
The nictitating membrane sits in the inner corner and sweeps across the eye in about 0.1-0.2 seconds. A healthy cat shows a 2-3 mm sliver of it at rest. It rises to cover 30-50% of the eye in sleep, during vomiting, and after a dose of sedative — all normal, all brief.
What is not normal is a membrane that stays across 50% or more of the eye for over an hour, or one that appears on one side only. Persistent protrusion points to pain, dehydration, weight loss around the eye socket, or Horner's syndrome. In a cat that has lost 10% of body weight, the fat pad behind the eye shrinks and the membrane slides forward with no eye disease at all.
Horner's syndrome: four signs on one side
A cat with Horner's syndrome shows four things at once, all on the same side: a 1-3 mm pupil that will not dilate in dim light, a drooping upper lid covering 20-30% of the eye, the eyeball sunk 2-3 mm back into the socket, and the third eyelid up over 30% of the cornea. The cause sits anywhere along a 3-neuron chain from the brain to the eye, and in cats the middle ear is a common site, so an ear examination matters as much as an eye examination.
Most cases improve over 4-12 weeks once the cause is treated, but the pupil can stay 1-2 mm smaller permanently. Photograph both eyes in the same light at the same moment, every 3 days; the series tells the vet more than a single look.
When unequal pupils are an emergency
A difference of 1 mm or less, with both pupils reacting to light, is usually benign and often just angle. A difference of 2 mm or more, a pupil fixed and dilated, or a pupil that fails to constrict when a phone light crosses it is a same-day problem. Add any of these and go now: a head tilt, circling, a drooping lip, or a cat that stops eating.
One eye blown wide and unresponsive after a knock to the head is a neurological emergency, not an eye problem. One eye constricted with a cloudy cornea after a cat fight is likely uveitis.
- Check pupils at the same distance, in the same light, at the same time of day.
- Photograph both eyes every 3 days if you see a difference under 1 mm.
- Book same-day care for a 2 mm difference, a fixed pupil, or a third eyelid that stays up over an hour.
- Never use human eye drops; a steroid drop on a scratched cornea can cost the eye in 48 hours.
- Ask for an ear exam whenever Horner's signs appear on one side.
Related pages: cat eye discharge separates the watery, the purulent and the blood-tinged, and eye prolapse after trauma covers the 30-minute window that decides whether the eye is saved. Pupil response also slows with age, which is one reason the cat age calculator puts cats over 12 in a different exam schedule.
Frequently asked
Why do cats have slit pupils instead of round ones?
A slit closes further and faster than a round pupil of the same size, cutting up to 90% of incoming light while keeping the centre of the field open. It also extends depth of field in bright light, which matters for a predator that ambushes at 1 to 2 m.
How much better is cat night vision than mine?
A cat needs roughly one sixth of the light a person needs to resolve the same shape, helped by a 9-10 mm pupil and a reflective tapetum behind the retina. The gain is in sensitivity, not detail: in dim light a cat reads movement well and fine detail poorly.
My cat has one small pupil and one large one. Is that urgent?
A difference under 1 mm with both pupils reacting is usually benign. A difference of 2 mm or more, a pupil fixed wide, or a third eyelid that stays up for over an hour needs same-day veterinary care, especially with a head tilt or a drooping lip.