How to Read a Glasses Prescription: What Do SPH, CYL, AX, ADD and PD Mean?

The short answer to how you read a glasses prescription: it is made up of a few abbreviations that show the power and direction of the lens, on a separate line for each eye. The most common are SPH, CYL, AX, ADD and PD. Knowing these abbreviations helps you understand your prescription; interpreting the values is the ophthalmologist's job, and making the lenses that match the prescription is the optician's.

How do you read a glasses prescription?

A glasses prescription is read line by line: first check which eye the values belong to, then look at the SPH, CYL, AX and, if present, ADD columns in turn. The layout of prescriptions can vary from one clinic to another, but the fields mean the same thing. Printed forms have ready-made column headings; on handwritten prescriptions the values are usually written in the same order. The reading order is:

1. Find which eye the line belongs to: right (OD) or left (OS).
2. Look at the sign and the number in the SPH column.
3. If the CYL column is filled in, read it together with the AX value next to it.
4. If there is an ADD field, it shows the extra power for near vision.
5. If a PD is given, it is not a prescription power but a distance in millimetres.

All the abbreviations are summed up in one table:

A sample glasses prescription

The table below is a sample prescription that shows how the abbreviations appear on a prescription. The values have been chosen for illustration only and do not belong to a real person.

Sample PD: 63 mm

This sample prescription is read as follows:

1. The right eye shows SPH -2.00. The minus sign shows that this eye has a correction for short sight.
2. The right eye shows CYL -0.75 and AX 180. This means a 0.75 dioptre astigmatism correction is placed on the 180-degree axis.
3. The values for the left eye differ from the right: SPH -1.50, CYL -0.50, AX 10. It is common for the two eyes to have different values.
4. ADD +1.50 is the power added to the distance value for near vision, and in this example it is the same for both eyes.
5. PD 63 mm gives the distance between the two pupils.

Your real prescription may be shorter than this example: some prescriptions show only an SPH value, with the CYL, AX and ADD fields left empty.

Right and left eye: what do OD, OS and OU mean?

OD means the right eye, OS the left eye and OU both eyes together. These abbreviations come from Latin, and you may see them on printed prescription forms or on prescriptions written abroad. Turkish prescriptions usually say "Sağ" (right) and "Sol" (left).

Right and left refer to the right and left of the person who will wear the glasses, not of the person looking at the prescription. The values for the two eyes can differ, so it is important not to mix up the lines.

What does the SPH (spherical) value mean?

SPH is the value that shows the spherical power of the lens: a value with a minus sign corrects short sight (myopia), and a value with a plus sign corrects long sight (hyperopia). Short sight is known as difficulty seeing distant objects clearly, and long sight usually as difficulty seeing near objects clearly.

The SPH value is written in dioptres (D) in steps of 0.25: 0.25, 0.50, 0.75, 1.00 and so on. The further the number is from zero, the stronger the lens. The sign is the point to watch most carefully: the plus and minus versions of the same number are two different lenses that are the opposite of each other.

CYL and AX: how are astigmatism values read?

CYL shows the power of the astigmatism correction, and AX or AXIS shows the direction of that correction in the lens. With astigmatism, the eye does not bend light with the same power in every direction; a cylindrical lens corrects this difference in one specific direction only. That is why the two values are always read together.

• CYL is written in dioptres, in steps of 0.25.
• AX is in degrees and is a number between 0 and 180.
• AX is not a power. A higher axis value does not mean more astigmatism; it only gives the direction.
• If the CYL column is empty, no cylindrical correction has been prescribed for that eye, and there is no axis value either.

The cylinder value can be written with a minus or a plus sign. Because the same correction can be expressed in two different ways, do not interpret the sign of the cylinder on your own; the doctor who wrote the prescription will explain what it means.

What is ADD (addition), and which lenses use it?

ADD is the power added to the distance value for near vision and is prescribed for presbyopia. Presbyopia is the age-related difficulty the eye has focusing on near objects. The ADD value carries a plus sign and is usually written the same for both eyes.

ADD is used in progressive lenses and bifocal lenses. In a progressive lens the power changes gradually from top to bottom; in a bifocal lens the distance and near zones are separated by a visible line. On some prescriptions ADD appears as a separate field, while others have two separate lines labelled distance and near. The near line shows the values written for reading glasses. If your prescription has only distance values, the ADD field stays empty.

What is PD (pupillary distance) for?

PD is the distance between the two pupils in millimetres and is used to centre the lens correctly in the frame. It is not a prescription power; it is a fitting measurement that places the optical centre of the lens in front of your pupil. Dioptre values are written with a plus or minus sign, while PD is a length without a sign.

PD can be written as a single number or as two separate numbers for the right and left eye. It may not appear on every prescription. Centring is especially decisive for progressive lenses, because the distance, intermediate and near zones have to sit in the right place.

What other fields appear on a prescription?

Besides SPH, CYL, AX, ADD and PD, some prescriptions also include a prism value and separate distance and near lines.

Not every prescription has all of these fields. A field left empty on your prescription shows that this correction has not been prescribed for you.

Is a glasses prescription the same as a contact lens prescription?

No, a glasses prescription and a contact lens prescription are not the same. A spectacle lens sits at a certain distance in front of the eye, while a contact lens sits directly on the surface of the eye. That is why a contact lens prescription includes extra values such as base curve (BC) and diameter (DIA) that a glasses prescription does not have.

In Türkiye, contact lenses are supplied against an ophthalmologist's prescription, and choosing and fitting the lens is the doctor's job. Do not adapt the values on your glasses prescription to contact lenses yourself. If you wear contact lenses, the basic rules in our article on contact lens hygiene will guide your daily care.

What do people most often mix up when reading a prescription?

The things people most often mix up when reading a prescription are the plus and minus signs, the axis value and the right and left eye lines. These points clear up the confusion:

1. The sign is part of the number. +1.50 and -1.50 are not the same value; one corrects long sight, the other short sight.
2. The axis is not a power. Numbers such as 90 or 180 in the AX column are degrees and do not show the power of the lens.
3. Right and left mean your right and left. They are written from the point of view of the person wearing the glasses, not the person looking at the prescription.
4. A figure you cannot read is not guessed. Because the values go up in steps of 0.25, a small reading mistake means a different lens.
5. Prescription and coatings are separate matters. The values on a prescription describe the power of the lens; for features such as anti-reflective coatings, see our article on lens coatings.
6. A prescription is tied to time. Your prescription can change over time; if the one you have is old, ask your doctor.

Who writes glasses prescriptions in Türkiye?

In Türkiye, glasses prescriptions are written by ophthalmologists. An optician does not examine the eyes or determine the prescription; they make up the prescription lenses according to the values the doctor has written. Prescription lenses are supplied against a prescription. If you do not understand a value on your prescription, or you notice a change in your vision, the person to see is an ophthalmologist.

If you are coming to Antalya, Belek or Kadriye on holiday, carrying a photo of your prescription on your phone makes things easier: if your glasses break or get lost, your values are at hand. More on this topic can be found in our visitor's guide for Belek and Kadriye.