Ready-made reading glasses are sold by strength rather than by prescription, in fixed steps that usually run from +1.00 up to +3.50. If you have never bought a pair, the numbers are the first hurdle.
What the number is
Those figures are dioptres, the standard unit of lens power. The plus sign means the lens is convex: it converges light, which is what brings close work back into focus when your eyes have stopped doing it themselves. The higher the number, the stronger the magnification and the shorter the distance at which things are sharp.
That last point matters more than people expect. A stronger lens is not a better lens. Go too strong and your comfortable reading distance shrinks to something awkward, and you end up holding a book closer than you want to.
Why you need them at all
The lens inside your eye changes shape to focus on things nearby, a process called accommodation. From around the mid-forties it gradually stiffens and that range of adjustment shortens. This is presbyopia, it happens to essentially everyone, and it is a mechanical change rather than a disease or a sign of neglect.
The usual first symptom is not blurring but distance creep: you find yourself holding a menu further away, and eventually your arms run out.
Working out roughly where you sit
If you already wear readers, order what you have. If not, the rule of thumb most people start from is age-related, because presbyopia progresses fairly predictably:
- Early forties: often +1.00 to +1.25
- Mid to late forties: often +1.50 to +2.00
- Fifties: often +2.00 to +2.50
- Sixties and beyond: often +2.50 to +3.50
Treat that as a starting point rather than a result. It is a population average, and individuals sit either side of it.
The better test is practical. Hold ordinary printed text at the distance you would actually like to read at, usually around 35 to 40 centimetres. Try the lowest strength that makes it comfortably sharp at that distance. The correct pair is the weakest one that does the job, not the strongest one that makes the text biggest.
The mistake worth avoiding
Buying too strong is far more common than buying too weak, because a stronger lens feels dramatic in the shop. What it produces over a long session is eye strain and headaches, since your eyes are being asked to work against a correction they do not need.
When ready-made is the wrong answer
Ready-made readers apply the same power to both lenses. That is fine if your eyes are reasonably matched. It is not fine if they are not, and there are three situations where a single figure cannot help you:
- Your eyes need different strengths. Ready-made readers will always over-correct one and under-correct the other.
- You have astigmatism. That is a difference in the curvature of the eye, and correcting it needs a cylindrical component that a simple magnifying lens does not have.
- You also need distance correction. Readers do nothing for distance vision.
Persistent headaches, one eye that clearly works harder than the other, or vision that changes quickly are all reasons to see an optometrist rather than to try the next strength up. Readers are a convenience for close work; they are not an eye test.
