Reading glasses strength by age and distance

How to use it

Sometime in their forties most people find that small print needs more light, or that the phone has drifted out to arm's length. This is presbyopia, and ready-made reading glasses are often the first thing people try. The tool gives a starting point for that first try: a rule of thumb from optical practice, not a prescription.

Enter your age and your usual working distance: hold your book or phone where you naturally read it and measure from your eyes to the page. Printed reading is often around 40 cm (16 in); a desktop screen is usually farther, often 50 to 70 cm (20 to 28 in). Centimeters or inches both work.

You get a range, a strength to try first and the age band used. The tool starts at age 40, and results stay between 0 and +3.50.

How the numbers work

Inside each eye, a clear lens changes shape to focus on close things. With age it stiffens, so the nearest point you can focus on drifts farther away. The change begins early in life, usually becomes noticeable in the forties and keeps progressing until somewhere in the sixties. Readers add the focusing power the eye can no longer supply.

The tool starts from these typical ranges at about 40 cm (16 in):

For other distances it uses a basic rule of optics: focusing on something d centimeters away takes 100 ÷ d diopters, which is 2.50 at 40 cm. The tool adds the difference, (100 ÷ your distance) minus 2.50, to both ends of the range and rounds to the nearest 0.25. The strength to try first is the middle of the age range, adjusted the same way and rounded to the nearest 0.25, taking the lower step when it lands exactly halfway.

The tool opens at age 52 and 40 cm, in the 50 to 54 band, so the range is +1.50 to +1.75. The middle, +1.625, sits exactly between two steps, so the first pair to try is +1.50. Move the distance to 60 cm (about 24 in), a common screen distance, and the focusing demand falls from 2.50 to 1.67, about 0.83 less. The range becomes +0.75 to +1.00 (from 0.67 and 0.92, rounded), with +0.75 to try first. At 33 cm (13 in) it rises to +2.00 to +2.25. That is why readers that suit a paperback often feel too strong at a computer.

Trying readers in a shop

Take something you actually read, such as a paperback or your phone. The near-vision card on the printable eye chart page works too.

1. Start with the suggested strength and hold the page at your usual distance, both eyes open.
2. Read for a minute or two, then try a quarter step weaker and a quarter step stronger.
3. Choose the weakest pair that keeps ordinary print clear and comfortable. Too strong a pair pulls your focus point in close and narrows the range that stays sharp.
4. Check that the lenses sit centered in front of your eyes and the frame doesn't pinch.
5. Look across the room. Distance will blur, which is normal, so don't walk around or drive in readers.

Good light helps more than people expect.

Once you know the strength that works, spare pairs in the places you read save a lot of hunting, and a lamp you can aim at the page helps any strength do its job.

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Why ready-made readers don't suit everyone

Ready-made readers have the same power in both lenses, no astigmatism correction, and lens centers set for an average face. They tend not to work well if you have astigmatism, if your two eyes need different powers, if you are strongly nearsighted or farsighted, or if you already wear glasses for distance. An eye exam then gives a reading prescription, or an add for bifocals or progressives. The column on reading glasses and progressives compares the options. Headaches, eye strain or double vision with readers are a reason to stop and get checked.

What this tool can't tell you

It can't examine your eyes. Glaucoma, cataract, macular degeneration and diabetic eye disease all become more common from the forties on, and early glaucoma usually has no symptoms. The American Academy of Ophthalmology recommends a baseline comprehensive eye exam at 40 for adults without symptoms or risk factors, and earlier for people with diabetes, high blood pressure or a family history of eye disease. Needing your first readers is a natural time to book it; the column on glaucoma after 40 explains why.

Presbyopia comes on slowly and affects both eyes. Other changes point elsewhere:

• See an eye doctor the same day if your near vision changes suddenly or in one eye only, if straight lines look wavy or part of the page is missing, or if you notice new flashes, a shower of floaters or a curtain across your vision. The Amsler grid is a simple home check for wavy lines.
• Go to an emergency department, or call your local emergency number (such as 911, 999, 112, or 119 in Japan), for sudden loss of vision in one eye, even if it comes back, or severe eye pain with blurred vision, halos, headache or nausea.

Questions people ask

I'm 38 and small print is getting harder. Why is there no result?

Most people don't need readers before 40, so the tool doesn't guess. Earlier trouble can come from farsightedness, astigmatism, tired or dry eyes, or a change that needs checking. An eye exam will sort out which.

Will readers make my eyes weaker?

No. Presbyopia progresses at its own pace, with or without reading glasses. Going without doesn't train the eye; it just makes close work more tiring.

Can one pair do for my phone and my computer?

Often not. The phone is usually held closer than the screen, so a pair that suits one can feel wrong for the other. Run the tool at both distances. Many people end up with a second, weaker pair for the desk.

I'm nearsighted. Do I need readers?

Perhaps not. Many mildly nearsighted people read comfortably by taking their distance glasses off. If you wear glasses or contacts all day, the answer depends on your prescription, so ask at your next eye exam.

How often will I need a stronger pair?

Every few years through the forties and fifties is common, since presbyopia keeps progressing until it settles in the sixties. If you seem to be moving up quickly, or one eye seems worse, have an eye exam rather than buying the next strength.

Sources

• American Academy of Ophthalmology patient pages on presbyopia and its baseline eye exam recommendation (aao.org/eye-health)
• Optical-practice rules of thumb for reading adds by age (starting points, not prescriptions)
• Basic optics of focusing: the demand in diopters equals 100 divided by the distance in centimeters