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Cats' are masterful predators, capable of spotting prey from meters away in the dead of night. But place a treat in front of them, and they’re basically blind.

What’s up with that? How can such good hunters be so lousy at seeing what’s right in front of them? It turns out that their vision isn’t always amazing, since being able so see well at night comes with some drawbacks.

Cats are crepuscular, which means they’re most active at dawn and dusk. As a result, their eyes have evolved to see best when the light is low. For one thing, their eyes are huge.

They’re nearly as large as human eyes, even though their heads are less than half the size of ours. Their slit-shaped pupils also have a greater range of size. Human pupils can dilate to 15 times their smallest size; cats’ pupils can dilate by 135-fold, and get even wider than human pupils to let in a lot more light.

The parts of their eyes that let in and focus light —their corneas and lenses— are also proportionally larger, which ultimately means more light reaches the light-sensing tissue in the back, called the retina. And they have an extra reflective layer behind their retina called the tapetum lucidum , which reflects back any light that reaches it, giving the retina a second chance to catch what it’s missed. Special cells in the retina called photoreceptors are what actually sense light.

And like us, cats have two types of photoreceptors: rods and cones. Rods work best in low light but don’t sense color very well. Cones are used in color vision, but they only really work if there’s a good amount of light.

Cats have almost three times as many rods as we do, but 10 percent as many cones. That makes for killer night vision, but in bright light, those rods become overloaded and can switch off completely. That leaves the handful of cones to do the heavy lifting, which makes for pretty paltry daytime vision.

And those huge eyes? They’re really hard to focus. To zero in on a nearby object, you need to bend that hard part of your eyeball, called the lens, enough to reshape how light enters your eye.

You have muscles in your eyes that can do that quickly when you need to focus on something close up. But the massive lenses in cat eyes just aren’t as flexible, so they don’t bend as well as yours do. Ultimately, this all means that cats simply can’t focus on anything closer than about 25 cm to them.

So when you hold a treat in front of kitty’s face, all they see is a washed-out blur. Luckily, their sense of smell is twice as sharp as yours. So if you hold that treat long enough, they’re bound to sniff it out eventually.

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