Sebastiaan Mathot: do you see better with large pupils?

Do you see better with large pupils?

The pupil is the small black circle in the centre of your eye. Well, actually it’s transparent; it just looks black because the inside of your eye is dark. The size of the pupil is constantly changing. It responds to light and dark, but also to emotion. For example, the pupil becomes larger when you feel strong emotions. But why? It is sometimes said that the pupil widens so that you can see better. But if that is true, why isn’t the pupil always at its largest?

Imagine being alone in the dark, afraid that the slightest movement is something dangerous and that all kinds of things are hidden in the shadows. In this situation, ‘being able to see better’ means that the eye can catch as much light as possible, so that you are as sensitive as possible to detect movements. That is why your pupil becomes larger in the dark, especially if you are also afraid.

But if you are reading a book in the sun, the situation is different. ‘Being able to see better’ now means being able to see as sharply as possible, so that you can distinguish the letters from each other. The pupil therefore narrows, because small pupils provide the sharpest image. You can compare this with the small aperture of a camera lens.

So, ‘being able to see better’ means something completely different in various situations. This knowledge is crucial for our understanding of visual perception. And it can even improve the design of artificial senses, such as retinal implants.