Computer glasses vs reading glasses: why the distance matters

Reading glasses and computer glasses do the same job at different distances. Readers are set for print held about 14 to 16 inches (35 to 40 cm) from your eyes. A desktop screen usually sits farther away, around 20 to 28 inches (50 to 70 cm), so glasses for the computer are typically weaker than readers. Wear full-strength readers at a monitor and the screen tends to blur, and without noticing you start leaning in toward it.

In my years behind the counter of an optical shop, people in their late forties and fifties often brought in readers that were perfect for the newspaper and hopeless at the office. Nothing was wrong with the glasses. They were set for a different distance.

Why distance changes the strength you need

Inside each eye a clear lens changes shape to bring near things into focus. From around the forties it stiffens, and the closest point you can focus on drifts farther away. This is presbyopia. Reading glasses add the focusing power the eye can no longer supply.

How much power depends on the distance. A simple rule from optics says that focusing on something d centimeters away takes 100 divided by d, in diopters (the unit lens powers are written in). The closer the object, the bigger the demand:

The same rule explains why readers blur a screen. For someone with no distance prescription and very little focusing left, the farthest point that stays sharp through readers is roughly 100 divided by the lens power, in centimeters. A +2.50 pair tops out at around 16 inches (40 cm), and a +2.00 pair at around 20 inches (50 cm). Put a monitor at 26 inches and it sits beyond that point. Whatever focusing you still have lets you see a little closer than that limit, never farther. So the body does the obvious thing and moves the head forward until the text sharpens, and an afternoon of that is how a lot of stiff necks begin.

What people mean by "computer glasses"

The phrase covers three things that have little in common, which is half the confusion.

A prescription set for the screen

An optometrist or ophthalmologist can write a prescription for intermediate distance, the zone between reading and across the room. It usually starts from your reading prescription and is adjusted for how far away your screen sits, so measure that distance at home and bring the number to the exam. These single-vision lenses give a clear zone at the screen and a little closer, enough for a keyboard and a document stand. Across the room will be blurry, which is expected.

Office or occupational lenses

These are a kind of progressive lens designed for indoor work. They give up most or all of the distance zone in exchange for a wide middle area for the screen and a reading area below it. They suit people who move between monitor, paperwork and the person across the desk all day. They are not for driving. The column on reading glasses and progressives compares them with ordinary progressives and desk lenses.

Lenses sold for blue light

Many glasses marketed as "computer glasses" are really about blue light: a filter or a faint yellow tint, often with no focusing power at all. That is a separate question from distance. The American Academy of Ophthalmology says blue light from screens has not been shown to damage the eyes and does not recommend blue-light glasses for computer use. A 2023 review that pooled the trials found no good evidence that these lenses reduce eye strain from screen work, at least over the short periods studied. If your screen blurs because of presbyopia, a blue filter will not fix it; the right focusing power for the distance will.

Working out a screen strength with ready-made readers

Ready-made readers have the same power in both lenses and no correction for astigmatism. If an eye exam has shown that your eyes are well matched and you need no distance correction, a weaker pair kept at the desk can work well. Otherwise, skip ahead to the section on when to get an exam.

Start by measuring. Sit the way you normally work and have someone measure from your eyes to the middle of the screen, or hold a tape measure from your cheekbone. Then put that distance into the reading glasses strength tool, which gives a starting range by age and distance. It is a rule of thumb from optical practice, not a prescription.

Take someone aged 58. At a book distance of 16 inches (40 cm) the tool suggests +1.75 to +2.25, with +2.00 to try first. Change the distance to 24 inches, a typical desktop setup, and the range drops to +1.00 to +1.50, with +1.25 to try first. At 26 inches it falls again, to +0.75 to +1.25, starting at +1.00. A laptop at 20 inches lands in between: +1.25 to +1.75, starting at +1.50. The pattern holds at any age: the farther the screen, the bigger the step down from your book strength.

Then test at the desk itself. Sit in your usual position and read ordinary text on screen for a few minutes. If you catch yourself leaning forward, try a quarter step weaker; if the text looks soft even close up, a quarter step stronger. Choose the weakest pair that keeps the screen clear and comfortable.

Readers come in one power per pair, so people who settle on two strengths often keep each where it gets used: the stronger pair by the bed and in the kitchen, the weaker pair at the desk. A six-pack of ready-made readers comes in a single strength that you choose when ordering, so it suits one distance well; someone using two strengths would need a set for each.

Whatever the strength, readers blur anything beyond their range, so take them off to walk around and never drive in them.

Progressives at a desk: watch your chin

General-purpose progressives put distance at the top of the lens, reading at the bottom, and a narrow channel for middle distances in between. At a monitor set at eye level, the only way to see the screen through that middle channel is to tip your head back. Chin up, neck compressed, for hours. When I fitted progressives, the sore neck was the complaint I heard most from desk workers, and it usually had nothing to do with the prescription.

A few changes help:

• Lower the monitor so the top of the screen sits at or a little below eye level. Progressive wearers often find it comfortable lower still.
• Bring the screen slightly closer if the text needs your reading zone rather than the middle one.
• Turn your head to look at the edges of the screen instead of sliding your eyes; the sides of the lens are where the blur sits.
• Ask your optician to check the fitting height. A frame that has slipped down your nose moves every zone with it.

If you still spend the day with your chin raised, a separate office or intermediate pair for the desk is often more comfortable than asking one pair of progressives to do everything. Bifocals run into the same trouble, because the reading segment is set for close work and the screen falls in the gap above it.

Glasses are only part of screen comfort. People blink less while they concentrate on a screen, which dries the eyes, and long stretches of close focus add to the tiredness. The 20-20-20 habit helps: every 20 minutes, look at something about 20 feet (6 m) away for 20 seconds. The screen break timer gives a gentle reminder. Light matters too, and the column on autumn reading and tired eyes covers lamps and glare, something I notice myself in late October in Kyoto, when the evenings close in early.

When to see an eye doctor instead of buying readers

Readers are a reasonable choice for many people, but they are a poor guess for others. Book an eye exam with an optometrist or ophthalmologist first if any of these apply:

• Your two eyes need different strengths, or one eye seems clearly worse.
• You have astigmatism, or things look smeared or doubled rather than just blurry.
• You already wear glasses or contacts for distance.
• You get headaches, eye strain or double vision at the screen, with or without readers.
• You are under 40 and close work has started to blur. Presbyopia rarely needs correcting that early, and other causes, such as farsightedness or astigmatism, are worth finding.
• You need readers for the first time. 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. Early glaucoma has no symptoms, so this check matters even if you feel fine.

Some changes need help quickly. See an eye doctor the same day if 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 of light, a sudden shower of floaters or a shadow spreading across your vision. 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 around lights, headache or nausea. Eye emergencies and first aid explains what to do on the way, and finding an eye doctor helps with booking a routine visit.

Questions people ask

Can I use my reading glasses for the computer?

You can, but if the screen sits farther than about 20 inches (50 cm), full-strength readers will often blur it and pull you forward. Moving the screen closer helps a little. A weaker pair for the desk, or a prescription made for that distance, usually works better.

What strength should computer glasses be?

For people with presbyopia, typically weaker than their reading strength. How much weaker depends on the distance: in the tool's example, a 58-year-old goes from +2.00 for a book to +1.25 for a screen at 24 inches. An eye exam gives the exact figure, and that matters most if your two eyes differ.

Sources

• American Academy of Ophthalmology patient pages on presbyopia, computer use and blue light (aao.org/eye-health)
• Basic optics of focusing: the demand in diopters equals 100 divided by the distance in centimeters