What is a diopter?

The definition

A diopter measures how strongly a lens bends light. It is one divided by the focal length in meters. A lens that focuses parallel light at 1 meter has 1 diopter. A lens that focuses at half a meter has 2 diopters. The stronger the lens, the shorter the distance.

What a diopter means in glasses

The number on a pair of glasses or a prescription is the lens power in diopters.

• Plus (+) lenses add focusing power. They are used for farsightedness and for reading glasses.
• Minus (−) lenses reduce focusing power. They are used for nearsightedness.

Glasses are made in steps of 0.25 diopters, which is about the smallest change most people notice.

Diopters and reading glasses

Reading at 14 to 16 inches takes about 2.5 to 2.8 diopters of focus. A young eye supplies that easily. An eye with presbyopia supplies less each year, and reading glasses make up the difference. That is why strengths rise with age, as the strength chart shows.

Is a diopter the same as magnification?

No. Magnifiers are sometimes labeled 2×, 3× and so on. A common rule converts diopters to magnification by dividing by four and adding one, so a +4.00 lens is about 2×. Reading glasses are chosen to bring print into focus, not to enlarge it.