Moving the focus point
How Do Glasses Help You See?
Glasses bend light before it enters your eye, so the picture lands in the right place.
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Step 1 of 5
Your eye focuses light onto its back wall.
The lens bends incoming light so it converges to a sharp picture on the retina. Sharp vision means landing exactly there.
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Step 2 of 5
Short sight focuses too early.
A single millimetre of extra eyeball length is worth about three dioptres. If the eyeball is a little too long, light converges before reaching the retina and spreads again. Distant things blur; close things are fine.
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Step 3 of 5
Long sight focuses too late.
Long sight is the same fault as short sight, in the other direction. A slightly short eyeball means light would focus behind the retina. Distance is fine, but close work is a struggle.
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Step 4 of 5
A lens pre-bends the light.
A concave lens spreads light slightly before it enters, pushing the focus back. A convex lens converges it, pulling the focus forward.
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Step 5 of 5
The prescription number is the strength needed.
A −3 lens bends more than a −1. It is a measurement of exactly how far your focus has to be moved, which is why prescriptions are personal.
Almost everyone needs reading glasses eventually — the eye's lens stiffens with age and stops being able to refocus.
The short version
Sharp vision means light landing exactly on the retina, and a single millimetre of extra eyeball is worth about three dioptres of blur. A concave lens pushes the focus back, a convex one pulls it forward, and the number is how far.
Try it yourself
Look through someone's glasses at arm's length. If the view shrinks they are short-sighted; if it magnifies, long-sighted.