How can our eye change focus?

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A good example of this is when you’re looking at semi reflective glass windows, you can focus on the reflection you’re obviously seeing but the actual glass characteristics can be a little blurry, but then you can change your focus and the reflection becomes blurry but now you can see the glass characteristics such scratches on the surface for example.
How does the human eye do that?

In: Biology

Glass is an interesting example, let’s back up.

Your eyes focus on a specific distance, say 2 feet in front of your face, or 20 feet in front of your face. Muscles in your eye pull and bend a lens in the front of your eye which bends the light entering your eye and projects it on special cells in the rear of your eyeball which “see” the image being focused. A subconscious process in the brain triggers the muscles to bend the lens so that the image being projected is in focus.

Glass is tricky because it’s partially see through. Are you looking through the glass and out the window? In that case your eyes might be focusing 30 feet away to the deer that are eating your goddamn azaleas *again.* But you can also make your eyes refocus on the actual pane of glass in front of you, now you’re seeing the details on the surface of the glass itself.

Our eyes have a lens which focuses the light. The lens is flexible, so the muscles in our eyes can bend it and change its curvature, changing the focal length of the eye.

You have tiny muscles in your eye called “ciliary muscles” that change the shape of the lenses. Like a camera lens mechanism that lengthens and shortens, flattening or bending the lens in the eye changes how light hits the retina at the back of the eyeball.

The lens in our eye can change shape!

Focus is all about focal distance. The light reflected off an object passes through a lens, that focuses that light on a single point. If that point is on the receptors then the image is clear – if it isn’t, the image is blurry.

Since objects are different distances away, focal distance changes for each object. Cameras solve this problem by physically moving the lens forward and backwards to change the focal distance. Our eyes use a different trick – there are small mussels around the lens that push and pull on it, changing how the lens bends the light and thus changing the focal distance.

When we focus on a specific object, our brain automatically tells the mussels in the eye to change the shape of the lens until it gets a clear image.

The ciliary muscles adjust the shape of the eye lens for focusing on nearby or far objects. By changing the shape of the eye lens, the eye changes the focal length of the lens. This mechanism of the eye is called accommodation.

Do you know how the glass of magnifying glasses is curved? A round curved glass like this is called a lens, and we have the same thing inside our eyes – but better!
The lens inside our eye, right behind the center black dot, is very soft and malleable. It is attached by tiny strings of muscles which can pull it back and forth in order to stretch the lens or make it looser. That way, the lens can change shape (its curvature) to make sure your focus is always best for the distance you are looking at! It’s pretty neat.
(Note, this system doesn’t work if you focus on anything further than about 10-12 meters. Beyond this point, the muscles stretching the lens are completely relaxed and can’t loosen further.)

Think of your eyes like a monocular. You can zoom in or out to adjust the focus. Your eyes have muscles that do that. They squeeze and relax your eyeball to make it longer or shorter.