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ELI5 Yahaaa

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.

Without glasses Image focuses in thewrong place — blurryWith glasses Image lands on theretina — sharp
Not magnifying. Relocating the focus.

  1. Step 1 of 5

    Your eye focuses light onto its back wall.

    1 Light enters2 The eye's lensbends it3 A sharp imageon the retina

    The lens bends incoming light so it converges to a sharp picture on the retina. Sharp vision means landing exactly there.

  2. Step 2 of 5

    Short sight focuses too early.

    Close objects Focus lands correctlyDistant objects?Focus falls short —blurry

    If the eyeball is a little too long, light converges before reaching the retina and spreads again. Distant things blur; close things are fine.

  3. Step 3 of 5

    Long sight focuses too late.

    Distance ClearReading?Blurry

    A slightly short eyeball means light would focus behind the retina. Distance is fine, but close work is a struggle.

  4. Step 4 of 5

    A lens pre-bends the light.

    For short sight Concave — spreadslight, pushes focusbackFor long sight Convex — convergeslight, pulls focusforward

    A concave lens spreads light slightly before it enters, pushing the focus back. A convex lens converges it, pulling the focus forward.

  5. Step 5 of 5

    The prescription number is the strength needed.

    −4 prescriptionstrongcorrection−2moderate−0.5slight

    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

Glasses bend light before it reaches your eye so the image lands exactly on the retina.

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.

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