How to read your glasses prescription: SPH, CYL, axis and PD

At first glance, a glasses prescription looks like a code: abbreviations, plus and minus signs, numbers with two decimal places. The values describe not your eye itself but the lens that offsets its refractive error – separately for right and left. These are labelled R and L or, as the American Academy of Ophthalmology (AAO) explains, with the Latin abbreviations OD (oculus dexter, right) and OS (oculus sinister, left).

The abbreviations at a glance

The dioptre is the reciprocal of the focal length in metres: a 2-dioptre lens has a focal length of 50 cm, as an optics review from the University of Iowa works out.

Sphere: what plus and minus mean

A minus sign stands for a diverging lens and therefore short-sightedness; a plus sign stands for a converging lens and long-sightedness.

For short-sighted eyes, there is a rule of thumb: 1 divided by the dioptre value gives roughly the distance in metres up to which your uncorrected eye sees sharply (the far point). At −2 D it lies at about 50 cm; at −5 D, at only around 20 cm. In this simple form, the rule applies to prescriptions with no cylinder.

Where does short-sightedness begin? In 2019, the International Myopia Institute (IMI) proposed standard thresholds. What counts is not the sphere alone but the spherical equivalent – sphere plus half the cylinder (more on the cylinder below): myopia at −0.50 D or less, high myopia at −6.00 D or less, each with near focusing relaxed. A prescription of SPH −0.25 and CYL −1.00 therefore works out at −0.75 D and meets the definition, even though the sphere on its own falls short of the threshold. A meta-analysis in the Journal of Current Ophthalmology (2018) shows how common refractive errors are: by its estimate, around 27 per cent of adults over 30 worldwide were short-sighted and around 31 per cent long-sighted (above +0.5 D).

What happens optically inside the eye

For a sharp image, light has to be focused precisely on the retina; the simplified model of a normal eye needs around 60 D for this. The bulk of it comes not from the lens but from the cornea: according to a review in the Indian Journal of Ophthalmology (2018), the cornea contributes about 40 to 44 D, roughly 70 per cent. The lens supplies the rest and can also become more curved to focus up close (accommodation).

• Short-sightedness (myopia): light comes to a focus in front of the retina. The eye is too long, or the cornea or lens refracts too strongly. In the standard eye model, just 1 mm of extra eye length corresponds to roughly 3 D of myopia.
• Long-sightedness (hyperopia): the focal point would lie behind the retina because the eye is too short or refracts too weakly. Young eyes often compensate by accommodating: with +2 D of hyperopia and 4 D of accommodation, distance vision stays sharp, but near vision only from about 50 cm. Over the years, this compensation can gradually disappear.
• Astigmatism: often the cornea isn’t evenly curved like a football but more like a rugby ball: steeper in one direction. Instead of a single focal point, two focal lines form, and the image blurs. An unevenly shaped lens can contribute too; the cylinder on your prescription corrects the total.

Reading cylinder and axis correctly

The cylinder tells you how strongly astigmatism is corrected; the axis tells you in which direction. The axis is an angle between 1 and 180 degrees, often written with three digits (090 instead of 90) – so a high axis number does not mean more astigmatism.

A cylinder is nothing unusual: the meta-analysis mentioned above estimated that around 40 per cent of adults over 30 have a cylinder above 0.5 D.

The same correction can be written with minus or plus cylinder, depending on the practitioner’s convention. To convert: add sphere and cylinder (this gives the new sphere), flip the sign of the cylinder and rotate the axis by 90 degrees. So +3.00 / −2.00 / 060 and +1.00 / +2.00 / 150 describe the same lens.

Addition, prism and PD

The addition is extra plus power for near vision, for example in varifocal, bifocal or reading glasses. It becomes necessary when the lens loses elasticity and near focusing weakens; the AAO gives the example of people over 40 who don’t want a second pair of glasses for reading. You can read more in our guide to presbyopia.

A prism appears on only some prescriptions, mainly for double vision: it redirects light so that both eyes once again produce a single shared image. The base gives its direction, for example in, out, up or down. Important: according to the UK’s National Health Service (NHS), double vision that starts suddenly should be checked by a doctor urgently. If it comes with a severe headache or an enlarged pupil, or follows a head injury, it is an emergency.

The PD (pupillary distance) is the distance between the pupils in millimetres, often given separately for each eye, such as 32 / 30. It determines where the optical centre of each lens sits. If the centre is off, an unwanted prism effect results – by Prentice’s rule, a 6 D lens and a 2 mm deviation already give about 1.2 prism dioptres. For near vision the PD is smaller, because your eyes turn inwards when reading – at a reading distance of 40 cm, the standard calculation gives just under 2 mm per eye.

Glasses or contact lenses: why the numbers differ

Glasses sit about 12 to 14 mm in front of the cornea; a contact lens sits directly on it. This gap is called the vertex distance. Because a lens acts differently depending on its distance from the eye, short-sighted people need less minus in a contact lens and long-sighted people more plus than in their glasses. With low powers this barely matters; according to continuing-education material from the US National Academy of Opticianry, contact lens powers above ±4 D are converted. With spectacle lenses above roughly ±7 D, even a change in how the frame sits can shift the effect enough that professionals compensate for it.

Calculated for a 12 mm vertex distance, for example:

The table is no substitute for a contact lens fitting: a contact lens prescription also includes details such as base curve (BC) and diameter.

A surprising consequence: short-sighted people have to accommodate slightly more at near with contact lenses than with glasses. Especially with higher prescriptions, someone may therefore still read comfortably in their glasses yet need reading help with contact lenses. For long-sighted people, it works the other way round.

What your prescription doesn’t tell you

The numbers describe only the optical correction. They say nothing about the health of your retina, optic nerve or eye pressure; an eye examination by an ophthalmologist covers that. This is particularly relevant with strong short-sightedness: according to the US National Eye Institute (NEI), it increases the risk of retinal detachment.

Warning signs: many new spots or floaters in your vision, flashes of light, or a dark shadow or curtain across your field of vision are an emergency, according to the NEI. Go to an eye doctor or an emergency department straight away. Find out more in our guide to flashes and floaters.

If you’re short-sighted and want a first rough indication, you can try the online eye test – an estimate, not a measurement. Whether laser eye surgery or another refractive procedure could suit you can’t be read from your prescription alone. You can have this clarified at My Laser Vision in a free pre-assessment lasting around 30 minutes.