How do reading glasses work?

What reading glasses are

Reading glasses are simple magnifying lenses in a frame. Both lenses have the same plus power, from about +1.00 to +4.00. They are sold without a prescription in pharmacies, supermarkets and online.

What they do

To see something close, the lens inside your eye has to bulge to bend light more strongly. From about 40 the lens is too stiff to bulge enough, so close print focuses behind the retina and looks blurred. A plus lens in front of the eye bends the light part of the way before it enters. Your eye does the rest.

The strength is measured in diopters. The stiffer your lens has become, the more diopters you need.

What they do not do

• They do not correct astigmatism.
• They cannot give each eye a different strength.
• They do not help distance vision. Far things look blurred through them.
• They do not treat or slow presbyopia, and they do not make it worse.

Readers and prescription glasses

Prescription reading glasses are made for your eyes: a different power for each eye if needed, astigmatism correction, and lens centers set to the distance between your pupils. Store-bought readers are one size for everyone. They work well for people whose two eyes are similar and who see clearly in the distance.

How to choose reading glasses

1. Find your strength. Take the test or use the chart by age.
2. Try that strength and the one below it on something you really read.
3. Pick the lowest strength that makes print sharp and comfortable.
4. Choose a frame for how you will use them: half-frames to look over the top, full frames for long reading.