How to read your glasses prescription (SPH, CYL, AXIS, ADD, PD)

The layout: one row per eye, one column per value

Prescriptions are written as a small table. Each row is one eye. OD (oculus dexter) is the right eye, OS (oculus sinister) is the left, and OU means both. Many UAE clinics simply write R and L. The columns are the optical values: sphere, cylinder, axis, addition, occasionally prism, and sometimes your pupillary distance.

The right eye comes first almost everywhere. That matters because online order forms follow the same order, and swapping the eyes is the single most common copying error we see.

A worked example

Read across the top row: the right eye needs −2.25 dioptres of sphere, with −0.75 dioptres of cylinder set at 170 degrees, and a +1.50 addition for reading. The left eye needs −2.00 dioptres of sphere and no cylinder, which the prescriber shows as DS, with the same +1.50 addition. The pupillary distance is 63 mm in total, 31.5 mm for each eye measured from the centre of the nose.

SPH: the main power

Sphere is the principal power of the lens, written in dioptres (D) in steps of 0.25. A minus sign means you are short-sighted (myopic): distant objects blur first. A plus sign means you are long-sighted (hyperopic): near work tires the eyes first. The larger the number, the stronger the lens. −0.50 is mild; −6.00 is strong enough that lens thickness and index become a genuine decision, which our lens index guide covers.

PL or Plano means zero sphere. Always type the sign. A form that defaults to plus will quietly make the wrong lens if you leave the minus off.

CYL and AXIS: astigmatism

Cylinder corrects astigmatism, where the eye focuses differently along two directions. CYL is the extra power, also in 0.25 steps, and AXIS is the direction it is applied, from 1 to 180 degrees. They are a pair. If there is a cylinder there must be an axis, and an axis without a cylinder is meaningless.

Prescribers write astigmatism in two conventions. Most optometrists use minus cylinder (−0.75 × 170 in the example). Some ophthalmologists use plus cylinder. Both describe exactly the same lens. To convert, or transpose, from one to the other: add the cylinder to the sphere to get the new sphere, flip the sign of the cylinder, and turn the axis by 90 degrees (add 90 if the axis is 90 or less, subtract 90 if it is above 90). So +2.00 +1.00 × 180 is the same lens as +3.00 −1.00 × 90. Most online forms accept either convention, but enter every value in one convention; never mix them.

DS (dioptres sphere) or Sph after a sphere value means no cylinder for that eye. Leave CYL and AXIS blank, or type 0.00 and leave axis empty, depending on what the form allows.

ADD: the near addition

ADD is the extra plus power for near work used in bifocals, progressive (varifocal) lenses and dedicated reading glasses. It is almost always the same for both eyes and usually runs from about +0.75 in the early forties to +2.50 or +3.00 later in life.

If you are ordering single-vision distance glasses, ignore it. If you are ordering reading glasses only, the laboratory adds it to the sphere: the example right eye becomes −0.75 −0.75 × 170 for reading. For progressives you enter the ADD separately and the lens design handles the gradient. Some prescriptions write a full second row of near-vision (NV) powers instead of an ADD; a good form accepts either.

PD, prism and the small print

PD is the distance between your pupils in millimetres, used to place the optical centre of each lens in front of each eye. It appears as a single number (63) or as two monocular values (31.5 / 31.5). A near PD, about 3 mm less, is sometimes written for reading glasses. Many prescriptions in the UAE leave PD blank because the dispensing optician measures it at the counter. If yours is blank, our guide to measuring pupillary distance at home shows how to get a reliable figure.

Prism and base appear on a minority of prescriptions, for eye-alignment problems. Prism is measured in prism dioptres and base gives the direction: in, out, up or down. If you have prism, order only from a retailer that explicitly supports it.

BAL or balance against an eye means that eye has no useful vision; the lens is matched to the other for appearance. Finally, check the date, the prescriber’s name and licence details, and any expiry written on the sheet. Most practices treat a glasses prescription as valid for one to two years, and a retailer may ask for a fresh one if yours is older.

Typing it into an online order form