Why even with only one eye open, and therefore no depth perception, do we still have to focus on objects near and far?

Question

What happens in each eye individually when you cross them that makes them focus closer if it isn’t just simply two converging images?

In: Biology

Answer 1

Have a telescope handy? Or an SLR camera? Or your phone camera, really. Anything with lenses.

I assume your phone camera is probably the closest thing to hand. Cameras focus by using a lens. You’ve probably seen cameras like [this](https://i.imgur.com/Bhhgd2O.jpg) (SLR/DSLR cameras) before. These focus the image by changing the distance between the focusing lens and the camera’s sensor.

Notice something? Those things I listed (telescopes, cameras, phone cameras) all have only one lens (ignoring the new iPhones). Just like using only one eye! And yet if you’ve ever tried to use your phone camera to take a picture of something up close, you’ll have noticed that things in the background become blurry. So clearly the level of focus doesn’t actually have to do with the number of eyes we have open.

Having two eyes just allows us to determine how far away things are (due to, as you noted, depth perception)–this is why Portrait Mode is only available on iPhones with more than one lens. Focusing an image is a different thing entirely and it’s done independently in each eye. This is why people who need glasses can have different prescriptions for each eye; their eyes vary in effectiveness at focusing things. I, for example, am nearsighted, but my right eye is worse than my left eye–in other words, my right eye has more trouble focusing on faraway things than my left eye does.

Let’s get to the crux of the issue. How does focusing happen? Here’s an ELI5: think of your eye as a person looking through a camera. When the person looks through the camera at something nearby, they must focus it on things close to them. And when the person looks at something far away, they must refocus the camera so that faraway things are clear.

Well, on the inside of the back of your eye, you have a bunch of special cells that are sensitive to light–this is the person. At the front of your eye, you have a lens, which can change shape in order to change the way light passes through it–this is the camera. When you look at things that are close to you vs. far away, the lens in your eye changes shape in order to bring it into focus.