Adjusting to Darkness: How Our Eyes See at Night

How Human Eyes Adapt to Light and Color

January 10, 2020
Vision Rods and Cones

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It’s escaped no one’s attention that this year’s name is also the term for sharp vision—2020. So let’s check out your vision in the sky! Plus, here are some fun facts about how long it takes for our eyes to adjust to darkness and whether your night vision is affected by your eye color.

The human eye is amazing and uses different modes to see during the daytime and to see at night, and can also 

Living in Full Color: Photopic Vision
People who move from a city into a rural area are often spooked by the darkness. City streetlights provide enough brightness to let our retina’s cone-shaped cells operate. This yields “photopic vision” which lets people see sharply, and in color.  

Seeing in the Dark: Scotopic Vision
But at night in the country, we only get to use our rod-shaped cells, which bestows scotopic vision. Scotopic kicks in when things are dim, but its not a great way to perceive the world. 

  • First off, rods are colorblind. Next, there’s not a single rod lurking in the middle one degree of vision; So in low light situations we suffer a one degree blind spot straight ahead, twice the size of the moon.  (There’s also a second, better known blind spot present in bright light. But this one’s off to the side, and we don’t usually notice it: If an object is hidden at the blind spot of one eye it will be seen by the other.)
  • Another quirk of rods is that they’re very slow-acting, which is why night sensitivity takes at least 5 minutes. When you first switch off your bedroom lights, you probably see nothing at all. After a few minutes, things in the room become obvious.
  • On top of all these failings, scotopic vision only delivers 20/200, ten times less sharp than photopic vision. You’ve always sensed the truth of this. Sharp details (like the creases in that shirt you tossed onto the chair), which are so obvious when the lights are on, now become a blur in the dim light. We’re so accustomed to it, we probably associate dimness with vagueness. But it’s those darn rods again.

This is why beginners who buy telescopes are sometimes appalled at how few details appear on galaxies and nebulae, on top of them being colorless. This is why astrophotography is so important: it brings out stuff the human eye would simply never see, even through the largest telescopes. 

Combining Both: Mesopic Vision
Photopic vision and scotopic vision combine in low but not quite dark lighting situations. A full Moon gives just enough light to slightly get the cones going, while rods are still operating. This is called mesopic vision—both. Here, the cones operate only at their place of peak sensitivity, which happens to be blue-green. That’s why the natural world in the country will appear that color under this month’s full moon.

Suddenly, the night makes sense.

A Few More Fun Facts about Night Vision

  • Can humans see in total darkness? Ever been in a cave when the lights are turned off? Now that’s dark! You can’t see anything—even your own finger in front of your face. Humans can see in the “dark” only if there is some starlight or, better, moonlight.
  • Does eye color affect night sky vision? According to some studies, there is a slight difference in vision capabilities based on eye color. Light-eyed people (with blue or green eyes) have slightly better night vision because they have less pigment in the iris, which which leaves the iris more translucent and lets more light into the eye. However, dark-eyed people tend to see better in bright sunlight and are less susceptible to glare, because darker irises act like a stronger filter for light.
  • How long does it take to adjust to darkness? It takes some time (20 to 45 minutes) for your eyes to adapt to the night sky or light-light conditions. Best conditions are on a night with no clouds and a full moon (try it!). When dark adapted, you can see only in black and white (no color). If light hits your face, the dyes in your eyes “bleach” and then have recover their dark-adapted vision. That’s why astronomers get annoyed when someone carelessly shines a white light in their eyes.

Avoid using a bright flashlight at a star party. Some amateur astronomers use red LED lights to view things without ruining their night vision. Of course, this means your eyes have already adpted to the darkness. 

Some star gazers will put on a pair of sunglasses at least 20 to 30 minutes before venturing in the dark to adjust quickly. (Being able to say, “I wear my sunglasses at night” is a bonus.)

Or, you could simply be patient. You’ll be amazed to watch the skies darken naturally while the stars come out to shine. 

About This Blog

Welcome to “This Week’s Amazing Sky,” the Almanac’s hub for everything stargazing and astronomy. Bob Berman, longtime and famous astronomer for The Old Farmer’s Almanac, will help bring alive the wonders of our universe. From the beautiful stars and planets to magical auroras and eclipses, he covers everything under the Sun (and Moon)! Bob, the world’s mostly widely read astronomer, also has a new weekly podcast, Astounding Universe