Reading your spectacle prescription

A spectacle prescription is a short table of numbers that most people never have explained to them. It is yours, you are entitled to a copy, and it is worth being able to read.

The columns

OD and OS, or R and L — right eye and left eye. From the Latin oculus dexter and oculus sinister.

SPH (sphere) is the main lens power, in dioptres. A minus figure corrects short-sightedness: distant objects are blurred. A plus figure corrects long-sightedness. The further from zero, the stronger the lens. Anything under about ±0.50 is a small correction.

CYL (cylinder) corrects astigmatism — where the eye's surface is curved more steeply in one direction than another, like the back of a spoon rather than a football, so light focuses at two different points instead of one. It blurs both distance and near, and it is very common. A blank or zero means none.

AXIS accompanies CYL and gives the orientation of that correction in degrees, from 1 to 180. It is a direction, not a strength; 180 is not stronger than 10. A small error here is more noticeable than a small error in power, which is why the measurement matters.

ADD is the additional plus power for reading, in a bifocal, varifocal or reading pair. It appears once presbyopia has set in, usually after forty, and it is the same in both eyes.

PRISM and BASE appear only if the eyes need optical help to align with one another. Most prescriptions have neither.

PD (pupillary distance) is the distance between the centres of your pupils in millimetres. It is not part of the refraction but is needed to make the spectacles, so that the optical centre of each lens sits in front of each pupil. Getting it wrong introduces unwanted prism and causes eye strain — a common reason cheaply made spectacles feel uncomfortable without looking wrong.

What the numbers do not tell you

A prescription is not a measure of eye health. Someone with a very strong correction may have perfectly healthy eyes; someone with no correction at all may have early glaucoma. The prescription is the optical part of the examination, and it is the part that produces a product — which is why it gets disproportionate attention.

Two prescriptions written a few months apart can differ slightly without either being wrong. Refraction has a margin, and the eye's focusing state varies with fatigue and with the time of day.

The extras, honestly

Anti-reflection coating. Worth it for most people. It cuts reflections off the lens surfaces, which improves night driving noticeably and makes your eyes visible to others rather than hidden behind glare. It also makes smudges more obvious, which is the trade-off.

Scratch-resistant coating. Standard on plastic lenses now and worth having. No coating makes a lens scratch-proof.

High-index (thinner) lenses. Genuinely useful at stronger prescriptions, where a standard lens would be thick and heavy. Below about ±3.00 the difference is marginal and the cost is not.

Photochromic lenses, which darken in sunlight. Convenient, and they provide UV protection. They darken less inside a car, because windscreens block much of the UV that triggers them.

Polycarbonate or Trivex. Impact-resistant materials. Strongly recommended for children, for sports, and for anyone with only one functional eye. This is a safety choice rather than an optical one.

Blue-light filtering. Trials have not shown it reduces eye strain relative to ordinary lenses. If it is offered as a remedy for screen fatigue, the honest answer is that breaks, blinking and an accurate prescription do more.

Buying elsewhere

You can take your prescription anywhere. If you do, make sure the PD goes with it, be cautious about varifocals ordered without a proper fitting — they need measurements taken in the actual frame, on your face — and remember that the prescription has a sensible lifespan. A two-year-old prescription may no longer be right, and buying to it is buying the wrong spectacles accurately.