When a camera—or an eye—focuses on something close to it (see The Workings of an Eye), other objects slightly nearer or farther away appear a little blurry. You can see this by closing one eye, holding two fingers at different distances from your face, and focusing on one finger and then the other. This effect, called blur, helps a viewer judge the distance to a nearby object.
Banks and his colleagues found that a vertical slit aperture enhances the effect of blur. In the case of an animal’s eyes, that means vertical pupils probably help a cat or other predator determine exactly how far away its prey is, so it can pounce just the right distance. The pupil shape is an adaptation, a trait that helps the animal survive in its environment.
Banks’s team also noticed that animals with vertical pupils usually have eyes on the front of their face rather than on the sides of their head. This allows predators to focus both eyes on a single object, a capability called binocular vision. Each eye sees a slightly different overlapping image. The brain combines the two images to create the perception of depth. Like blur, binocular vision helps viewers judge the distance to an object—and it's even more powerful in vertical pupils.