Open Wide! (What do you mean, my eyes?)

“I’ll do my dreaming with my eyes wide open, and I’ll do my looking back with my eyes closed.” –Tony Arata

Would you believe me if I told you that — in terms of your own eyes — you saw better at night than you did during the day?

It’s true! But it doesn’t have much to do with the amount of light available. When you’re looking out at distant objects, you’d love to be able to resolve them with your eyes.

For example, that object in the distance, down the road in the image above? Is it a motorcycle with one headlight, a car or truck with two, or (I hope not!) a train with three?

Well, with your eyes, there’s a limit to what size you can resolve. Either you’ve got to wait for the object to get closer — and that will help — or, well, you can open your eyes wider.

Not like that! I mean you need to let more light in to your eyes, and the way you do that is to dilate your pupils. You can accomplish this in a lot of ways, but the easiest way is simply to let it get as dark as possible, and your eyes will do this naturally.

Conversely, when your pupils constrict, they let light in, and you can’t see to as high a resolution. Amazingly, your eyes operate under the same principles as a telescope; the bigger the aperture, the better resolution you can get!

I can usually find illustrative images in English, but not in this case!

A young human can often dilate their eyes up to 9 mm in diameter, which is huge! (Some people can’t get more than about 4 mm when fully dilated.) The other thing that matters — again like with telescopes — is the wavelength of the light you’re looking at. Compare an optical telescope that you’re used to with, say, a very long wavelength, radio telescope.

In order to catch the same number of waves of a longer wavelength, you need a bigger aperture! Since your eye can only get to a maximum size, that means you can see the shorter-wavelength light at a higher resolution than you can the longer wavelengths.

Someone looking at short wavelength (blue) light can resolve things nearly twice as well as someone looking at long wavelength (red) light! In fact, animals with large eye pupils and good UV vision can beat even humans as far as resolution goes. This includes some of the largest rats (!), many reptiles, and perhaps the best example of ultraviolet vision combined with large pupils: the owl.

Put all the best things together — a fully dilated pupil, the shortest violet wavelength light still visible by humans, and a perfect eye — and you’ll get a person that can see down to resolutions of twelve arc seconds, or one three-hundredth of one degree!

That’s good enough to see the same things that a normal human would see through Galileo’s telescope, including Jupiter’s Moons, the phases of Venus, and craters on the Moon! Admittedly, the telescope is less expensive than the lasik and the pupil dilation, but your eye is an amazing tool. And if you want to see farther and at higher resolution?

Just turn down the lights.