Reading glasses, where twenty titles say dioptre and seven say which ones you can have

Reading glasses are the rare thing on this site where the seller names the number that matters. All twenty of these titles use the word dioptre. Only seven tell you which dioptres you can actually have, and not one of them mentions the limit that decides whether a pair off a shelf can work for your eyes at all.

What a dioptre is, and roughly which one is yours

A dioptre is the power of a lens, and on reading glasses it is always a plus number. The lens does the focusing that the lens inside your eye used to do on its own and gradually stopped doing somewhere after forty. The bigger the number, the closer the comfortable reading distance.

As a starting point, and it is only a starting point:

• mid forties, +1.00 to +1.50
• around fifty, +1.50 to +2.00
• mid fifties, +2.00 to +2.50
• sixty and beyond, +2.50 to +3.00

Buy near the bottom of your bracket rather than the top. A pair that is too strong gives you a sharp page at an uncomfortably short distance and an ache after an hour, and it is the more common of the two mistakes.

Seven titles tell you the steps, ten tell you the ends

This is the split that decides how much use a listing is before you open it.

Seven enumerate: +1.0, +1.5, +2.0, +2.5, +3.0, +3.5, +4.0, written out so you can see your strength on the list before you buy. One writes the same seven as 100 to 400, which is the identical scale with the decimal point dropped, a convention borrowed from the optical trade.

Ten give the two ends only, as +1.0 to +4.0 or 0 to +4.0. A range is not a list. It tells you where the dropdown starts and stops and leaves you to hope the step you want is inside it, which for a half dioptre it usually is and for a quarter almost never.

Two give no number at all. They say dioptre and stop, which on a product whose entire specification is one number is a strange thing to leave out.

One sells a single strength, +1.50, and nothing else. It is the sports frame, and if +1.50 is not your number it is not your pair.

Four titles stop short of +4.0: one at +3.5, two at +3.0, and one at +2.0. If you are at the strong end of the range, a quarter of this page cannot serve you whatever the rest of the listing says.

The quarter steps, and the one pair that has them

Nineteen of these twenty climb in half dioptres at best.

For most people that is fine, because most eyes are content within a quarter dioptre either way. For the rest it is the whole problem. If an eye test has said +1.25, the choice on nineteen of these is to go under and hold the page further out, or go over and hold it closer than is comfortable.

One title lists +0.5, +0.75, +1.25, +2.25, +3.25 and +3.75. It is the only pair here that can hand you the strength in the middle, and if you already know yours is an odd quarter it is the only one on the page worth ordering.

The same lens in both eyes

This is the section to read if you read no other.

A pair of off-the-shelf readers has one strength and it goes in front of both eyes. No title here says so, because it is so basic to the product that sellers do not think to. It matters because eyes are rarely a matched pair.

If your two eyes need different help, a single strength has to be wrong for one of them, and you feel that as tiredness rather than as blur. If you have astigmatism, which is the eye being shaped slightly more like a rugby ball than a football, no amount of plus power corrects it, because that correction has to sit at a particular angle and a reader has no angle.

Neither of those is a reason to avoid readers. Plenty of people have two near-enough eyes and no astigmatism, and for them a three pound pair does the same job as a forty pound one. It is a reason to have had an eye test recently enough to know which sort you are. An eye test also looks at the health of the eye, which is the part no magnifying lens has an opinion about.

Anti blue light, on ten titles

Ten of the twenty say they block blue light, in one wording or another.

The idea is that screens put out more short wavelength light than paper does and that filtering it reduces eye strain. It has been tested properly. A 2023 review gathered seventeen randomised trials of blue light filtering lenses and found no clear effect on eye strain from computer use. The lenses do no harm and the faint tint is hard to see. There is simply no good reason to choose a pair for it or to pay more because a listing says it.

If your eyes ache at a screen, the things with evidence behind them are duller and free: a bigger typeface, a screen no brighter than the wall behind it, and looking at something far away every so often, because the ache is mostly the focusing muscle holding one distance for an hour.

What none of them says

The lens material, on seventeen of twenty. Resin on two, crystal glass on one, silence on the rest. Resin is the light plastic most readers use. Glass is heavier, survives scratches far better and can break.

The pupillary distance, on all twenty. Lenses should be centred over your pupils. Readers are made to an average, and at low strengths the error is too small to feel. From about +2.5 up it starts to pull your eyes in a direction they are not pointing, and a pair that is the right strength can still feel wrong for that reason alone.

Which strength is in the box. Every one of these is a dropdown, and the strength is a variant chosen at checkout rather than a property of the listing. The price shown is usually for the weakest option. Check what you have selected before you pay, and check the parcel when it arrives, because the wrong dioptre in the right frame is the commonest complaint on any listing of this kind.