This is a really nice picture of a cat, actually a cat with a big following on Twitter whose name is Curious Zelda, with wide open pupils because it was taken in low light conditions. You can click on it to see it in large format.
There is often a confusion between pupils and irises. The iris is the sphincter muscle which is pigmented and which opens and closes the aperture (the pupil) through which light passes to the retina via the lens.
I like good pictures of cats and this is one of them. When the iris contracts and the aperture becomes smaller it goes from round, as you see in the photograph, to an oval shape and then finally a slit. The slit is important because it allows the cat to use her eyelids to half close over the slit to make the pupil even smaller. This is important because the cat’s eye is sensitive to light. We know this because cats see well in low light conditions. It becomes black-and-white vision but relatively good vision thanks to an abundance of rod cells in the retina.
If you click on this link you can see how domestic cats see. It is interesting to see the world through cats’ eyes.