Small print, sharp again. Your exact reading glasses strength, measured.

Measure the closest distance at which small print stays sharp. 100 divided by that distance in centimetres is your focusing range in diopters. The weakest comfortable strength is 100 divided by your reading distance, minus half that range, rounded up to the next 0.25. The test above does the measuring and the arithmetic.

Hold print where you actually read it. If +1.00 keeps it sharp without effort for several minutes, +1.00 is enough. If the letters soften after a while or you push the page away, try the next step up. When two strengths both read well, the weaker one gives a wider sharp range.

The sharp range moves in close and gets shorter. Print has to be held nearer than is natural, and anything past arm's length blurs. A +2.50 pair, for example, is sharp only up to about 40 cm, so it works for a book and fails for a monitor.

Usually a weaker pair works better for a screen. A monitor sits at 55 to 70 cm, further than a book, so it needs less help. Many people end up with one pair for the phone and books and a lower one, often 0.75 to 1.00 weaker, for the desk.

The range of focus narrows slowly with age, so the closest sharp point drifts outward through the forties and fifties. Arms stop being long enough at some point, usually somewhere in the mid forties, and that is when people first reach for readers.

For ready-made readers with both eyes alike, a measured near point is a sound basis. See a licensed professional if one eye differs clearly from the other, lines look doubled or tilted, or the change came on suddenly or with pain, flashes or new floaters.