Kind of confusing question I know. But what I’m talking about is I could put my hand in front of my face and force my eyes to focus on it, which blurs out the background. I can also stop focusing on it and then it will get blurry while the stuff behind it becomes more clear. But how does that work? Do our retinas have some kind of adjustable focal length like cameras?
In: Biology
Exactly. Your eyes function very much like a camera. There is an adjustable opening, which can control how much light gets in, a lens which can literally change shape and allow you to focus on things as different distances, and a screen which captures the image.
– Opening: it controls how much light enters, in dark areas you want more light to enter so the opening expands. This is most visible in animals like cats, although it occurs in other animals too.
– Lens: The lens is eyes is very versatile and flexible. There are tiny muscles which can change the shape of the lens (by either pulling it slightly taut, or allowing it to slack and slightly thicken) in order to change its focal distance and focus on different distances. Your eyes’ focal distance can go from as close as 25cm to literally infinity, arbitrarily far away. How cool is that!
– Screen: This is called retina, its function is to record a snapshot of the image being projected and send that screenshot to the brain so that brain can make sense of what you are seeing.
All in all its a pretty amazing system.
You have two eyes. Your brain combines the image from your two eyes into one image.
In order to focus on something, both of your eyes need to have the object in the middle of their field of vision. Imagine drawing a triangle, where one point is on the object, and the other two points are on your two eyes. The angles between your eyes and the object will depend on whether it object is near or far.
When you focus on one object over another, you’re changing the angle of your eyes. Notice how if you close one eye, you lose your depth perception – because with only one eye, you can’t change the angle, so you can only tell how far away things are by how small they are.
Incidentally, this is how 3D movies work. They actually show a different picture to each eye, and you wear glasses to ensure that each eye only sees the picture that’s intended for that eye.
Your eyes have muscles that can slightly alter the shape of your eye. So yes, your eyes do have adjustable focal lengths.
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