How does the eye know when the image is in focus? There is distance measuring device, only light entering the eye. No outer feedback to be sure that focus is in fact focus not something the eye think is focus.

Question

Thank you all for your respons and upvotes.

I can now see and focus on the answer of my question 🙂

In: 3993

Answer 1

A lot of interesting answers here. Occurs to me that a person learns to focus the same way that a camera can auto focus. You just look for the focus position that provides the most contrast.

Answer 2

The eye doesn’t know when the image is in focus. It’s sole purpose is to absorb light through the lens and pupil until it hits the retina. Your retina is connected to nerves in the eye which all gather at the optic nerve at the back of the eye. The optic nerve sends that signal to the brain and your brain decides if the image is in focus or not. And if it’s not in focus, your brain will control the ciliary muscles which will bend or stretch the lens to focus that image. So in short, your eye doesn’t know. It’s just a peripheral tool. Your brain does the deciding and controlling.