Read the prescription nobody explained to you

Why prescriptions are written the way they are

A spectacle prescription is a manufacturing instruction. It was written for a lens lab, not for you, which is why it reads like a parts order. Every figure on it has a precise meaning, and once you know them the document stops being opaque and starts being useful — particularly when you are buying online and nobody is standing there to interpret it.

OD and OS

Oculus dexter, the right eye, and oculus sinister, the left. Right is conventionally written first. Some practices use RE and LE, and OU means both eyes together. If only one row is filled in, only one eye needs correction.

Sphere

The main correction, in dioptres. A minus sphere corrects myopia: the eye focuses light slightly in front of the retina, so distance blurs while close work stays sharp. A plus sphere corrects hyperopia, where the eye must work to focus and close work tires first.

A dioptre is the reciprocal of a focal length in metres, which gives the minus numbers a physical meaning people find genuinely useful: an uncorrected eye at −2.00 D focuses at about 50 cm, and one at −4.00 D at about 25 cm. That is the furthest point that eye can hold sharp without help.

Cylinder and axis

Cylinder corrects astigmatism, where the cornea is curved slightly more in one meridian than another — closer to a rugby ball than a football — so the eye focuses at two different distances at once. Axis says which direction that extra curvature runs, from 1 to 180 degrees.

Axis is a direction, not a strength. An axis of 175 is not a worse prescription than an axis of 10. This confuses people constantly, because it is the largest number on the page.

The transposition trap

Here is the one that causes real worry. The same lens can be written two ways:

−2.00 −1.50 × 180 and −3.50 +1.50 × 90 are the same lens. Identical. Ground from the same blank, worn with the same result. One is minus-cylinder notation, the other plus-cylinder, and converting between them means adding the cylinder to the sphere, flipping the cylinder's sign, and rotating the axis by ninety degrees.

Optometrists generally write minus cylinder; many ophthalmologists write plus. So someone who sees both, or who moves country, gets two prescriptions that look alarmingly different and concludes their eyes have deteriorated. This tool prints both forms for exactly that reason.

Addition

Extra plus power for close work, added to the distance sphere in the reading part of the lens. It appears from the mid-forties onward as the eye's own focusing range narrows, which is presbyopia: a normal change, not a disease, and one that happens to essentially everyone. An ADD of +2.00 on a sphere of −1.00 means a reading power of +1.00.

Spherical equivalent, and why it decides the price

Sphere plus half the cylinder. This single number, not the sphere alone, governs how thick the lens will be and therefore which index is worth paying for.

Higher index materials are thinner, and almost always optically worse. The tradeoff is measured by the Abbe number: lower Abbe means more chromatic dispersion, seen as colour fringing toward the edge of the lens. Standard CR-39 at index 1.50 has an Abbe of 58, the best in common use. Polycarbonate is around 30. So a thin lens is not automatically a better lens, and at low powers paying for high index buys you very little while costing you clarity.

Frame choice matters here too, and more than people expect. A smaller lens opening and a frame PD close to your own both reduce edge thickness, sometimes as much as moving up an index would. That is why the frame size arithmetic is worth doing before you pay for the thinnest lens on the menu.

Contact lenses are a different prescription

A glasses prescription cannot be used to order contacts, for two separate reasons.

The first is optical. A lens sitting twelve millimetres in front of your cornea does not have the same effect as one sitting on it. Minus powers weaken at the corneal plane and plus powers strengthen, following F′ = F ÷ (1 − dF). Below about four dioptres the difference is smaller than the quarter-dioptre step lenses are made in, so it vanishes. At −8.00 it is three quarters of a dioptre, which is very much not nothing.

The second reason is the important one. A contact lens prescription also specifies base curve, diameter and a specific product, all of which come from a fitting where a clinician looks at how the lens actually sits and moves on your eye. A lens with the right power and the wrong curve is a corneal abrasion waiting to happen. The corneal-plane figure shown above exists so the numbers make sense to you, not so you can order from them.

What this does not do

It explains a document. It does not measure your eyes, and it deliberately never will.

There are tools on the web that will produce a refraction from your phone camera by having you find the distance at which text blurs. The arithmetic behind them is real, and the output looks plausible. The problem is not the arithmetic. It is that a sight test is not only a measurement of focus: it is the appointment at which glaucoma, retinal detachment, macular changes, cataract and the ocular signs of diabetes and hypertension are found, usually in people who had no symptoms and went in expecting a new pair of glasses. A number from a phone gives you no reason to book that appointment, and that is the harm.

This tool performs arithmetic on numbers you supply and explains standard notation. It is not an eye examination, does not diagnose anything, and is not a substitute for professional advice. Prescriptions expire for a reason. If you are experiencing changes in your vision, see an optometrist or ophthalmologist.

Questions people ask

What do OD and OS mean on my prescription?

They are Latin abbreviations: oculus dexter is the right eye and oculus sinister is the left. Some practices write RE and LE instead, and a few write OU for both eyes together. The right eye is conventionally listed first.

Does a bigger minus number mean worse eyes?

It means stronger correction is needed, not that anything is wrong beyond focus. Below about three dioptres is usually described as mild, three to six as moderate, and above six as high. The number describes where your eye focuses light, and a high number corrected properly gives the same sharp vision as a low one.

Why does my prescription look completely different from my last one when my eyes feel the same?

Most often because it has been transposed. The same lens can be written in minus-cylinder or plus-cylinder notation, and the two look nothing alike: -2.00 -1.50 x 180 is exactly the same lens as -3.50 +1.50 x 90. Optometrists generally write minus cylinder and ophthalmologists often write plus. This tool shows both forms so you can compare like with like.

Why is my PD not on the prescription?

Pupillary distance is classed as a dispensing measurement rather than part of the sight test, so in the UK and most of Canada there is no obligation to include it. You can ask for it, or measure it yourself.

Can I use my glasses prescription to order contact lenses?

No. A contact lens prescription additionally specifies base curve, diameter and the specific lens product, all of which come from a fitting where a clinician looks at how the lens sits on your eye. Above about four dioptres the power itself also differs, because a lens on the cornea is not the same as one sitting twelve millimetres in front of it. This tool shows the corneal-plane equivalent so the numbers make sense to you, not so you can order from it.

What does ADD mean?

Additional plus power for close work, added to the distance sphere in the reading portion of the lens. It appears once the eye's own focusing range narrows with age, which is presbyopia and happens to essentially everyone from the mid-forties onward. An ADD of +2.00 on a distance sphere of -1.00 gives a reading power of +1.00.