How did an optometrist’s machine measure my eye prescription just from me looking at a hot air balloon?

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I just had an eye checkup, and it’s the first time they did this test on me. It was a desktop machine that measures my eye pressure with a puff of air, and then I was told to look into the hot air balloon image to ‘measure my prescription’.
The machine auto-focused the image for me, but how does it know when it’s clear for me?

In: Physics

Anonymous 0 Comments

If your eyesight is bad, your eye is straining to see things. There are muscles in your eye that flex your lens to focus. When the prescription is right, your eye relaxes. The machine is supposed to be able to see when your eye relaxes, so it knows it got it right.

They don’t work very well. You’ll want a real optometrist.