How perception works: why what you see is your brain's best guess
The fovea: why your eyes see sharp only where you look
Only a tiny spot at the centre of each eye, the fovea, sees sharp. Yet the whole room looks sharp to you. Try to read the letters at the edge of a page without moving your eyes.
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1
Miva's next card is full of letters, with a cross in the middle. The rule: look at the cross, and name the letters at the far ends without moving your eyes. Rono cracks his knuckles.
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2
Try it now, both eyes open: look at the cross and name the letters at the left and right ends, without moving your eyes.
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3
You can tell there are letters out there, but you can hardly say which. Only the two right beside the cross are sharp. Rono's eyes flick to the end of the card, and Miva makes him start again.
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4
The back of your eye has one small spot where the light cells are packed tight: the fovea. Only light that lands there gives a sharp picture. Hold your arm out and look at your thumbnail: that is about how much the fovea takes in.
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5
So your eyes jump. About three times a second they move the fovea onto a new spot, and wherever you look, it is sharp. These jumps are called saccades. Look at Miva's face, and your eyes jump from eye to eye to mouth.
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6
So your eye sends a hole, one small sharp spot and three jumps a second. Yet what you see is one sharp, still room, with nothing missing. Where does that room come from?
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Quick check
When you read, your eyes move along each line in small jumps. Why can they not stay still in the middle of the page?
Show the answer
Only the fovea gives sharp letters, and it covers a few letters at a time. So the eyes must jump along the line to bring each group of letters onto it.
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Go further
Around the sharp spot, and where your eyes land.