Sphere, cylinder, add, PD: a glasses prescription is full of abbreviations that are hard to follow without an explanation. This second part of our glasses vocabulary explains the most important optical terms and words: dioptres, pupillary distance, lenses and coatings. Frames, frame shapes and the parts of glasses are covered in Glasses Terminology, Part 1.
Key points at a glance
- Your prescription lists values for the right (R) and left (L) eye separately: sphere, cylinder, axis, add, sometimes prism and pupillary distance.
- The dioptre (D) is the unit of lens power. Minus values stand for short-sightedness, plus values for long-sightedness.
- Pupillary distance (PD) and fitting height decide where the lenses sit in front of your eyes.
- Refractive index, anti-reflective coating and hard coating determine how thin, reflection-free and durable your lenses are.
Reading a glasses prescription: sphere, cylinder, axis, add and prism
Your values are recorded for each eye separately after an eye test with an optician or an eye doctor.
| Term | Abbreviation | Meaning |
|---|---|---|
| Sphere | SPH, S | Basic lens power in dioptres. Minus corrects short-sightedness, plus long-sightedness. |
| Cylinder | CYL, C | Corrects astigmatism (an unevenly curved cornea), also in dioptres. |
| Axis | AX, A | Direction of the cylinder in degrees (0 to 180). Always goes with the cylinder. |
| Add (addition) | ADD | Extra power for near vision in presbyopia, needed for reading, varifocal and office lenses. |
| Prism, base | PRISM, BASE | A prism bends the light, the base gives the direction. It is prescribed for certain misalignments of the eyes. |
When you order lenses, the values are entered separately for each eye. If your prescription includes a prism, ask customer service before you order.
Dioptres (D): the unit of lens power
The dioptre (D or dpt) shows how strongly a lens bends light. The further a value is from zero, the stronger the correction. Lenses are made in steps of 0.25 D, for example -1.25 or +3.75. Reading glasses usually have just a plus value, such as +1.5 or +3.5.
Don’t confuse it with visual acuity, which describes how sharp your vision is. Which value suits your eyes is measured by an optician or eye doctor, not by an online test.
Refractive errors and accommodation in brief
With a refractive error, the eye does not form a sharp image on the retina. These are the basic terms:
- Short-sightedness (myopia): Distance is blurred, near is sharp. Shown as a minus value.
- Long-sightedness (hyperopia): Near vision is harder, at higher values distance too. Shown as a plus value.
- Astigmatism: The cornea is unevenly curved. Shown as cylinder and axis.
- Presbyopia (age-related long-sightedness): The lens loses elasticity and accommodation, the eye’s adjustment to near distances, declines. Shown as the add.
Signs are not a diagnosis. Only an eye test shows whether you need glasses. More on presbyopia: Presbyopia: Causes, Symptoms and Solutions.
Pupillary distance (PD) and fitting height: where the lenses sit
Pupillary distance is the distance between the centres of your pupils in millimetres. More precise is the monocular PD, measured from the centre of your nose to each pupil, because pupils often do not sit symmetrically. Together the two values give the total PD.
Fitting height is the vertical distance from the pupil to the bottom edge of the lens in the frame. It decides where the vision zones lie in the lens and is measured with the glasses on. It matters most for varifocal lenses.
Measuring PD yourself: A ruler held in front of your eyes while you look in a mirror gives a rough value. For exact values, especially for varifocals, visit an optician near you.
Single vision, varifocal and office lenses
| Lens type | What it does | Typical for |
|---|---|---|
| Single vision | One power across the whole lens, corrects one distance. | distance in short-sightedness, simple reading glasses |
| Varifocal (progressive) | Distance at the top, intermediate in the middle, near at the bottom, with smooth transitions. | one pair for all distances in presbyopia |
| Office lens | For near and intermediate distance, such as desk and screen. Not meant for distance. | office work, in addition to everyday glasses |
Refractive index, anti-reflective and hard coating: material and coatings
Most lenses today are made of plastic. You can read more about the materials in Glasses Materials: Pros and Cons of Titanium, Acetate and More.
| Term | Meaning |
|---|---|
| Refractive index | A measure of how strongly a material bends light, for example 1.5, 1.6 or 1.67. The higher the index, the thinner and lighter the lens at the same power. |
| Anti-reflective coating | Reduces reflections on the lens. Others see your eyes better, and you get fewer distracting reflections in your field of view. |
| Hard coating | Makes plastic lenses more resistant to scratches. It cannot prevent scratches completely. |
| UV protection | UV400 lenses block UV radiation below 400 nm. On sunglasses, look for UV400 and the CE mark. |
| Tint, self-tinting | A tint darkens the lens, lightly or strongly. Self-tinting (photochromic) lenses darken in UV light and lighten again indoors. |
Many lenses already come with a hard coating and an anti-reflective coating. If light bothers you unusually strongly, ask an eye doctor.
Miu Miu MU 01XV VAU1O1£134.24Miu Miu MU 09XV VAU1O1£134.24Miu Miu MU 04UV 1AB1O1£134.24Miu Miu MU 01XV 19P1O1£133.92Ray-Ban RX3928V 2500£86.72Ray-Ban RX3928V 2501£86.72Ray-Ban ALAIN (RX7239 2001)£60.15Miu Miu MU 09XV 16K1O1£135.08Frequently asked questions about optician vocabulary
How many dioptres is a lot?
There is no fixed limit in everyday life. -1.0 D and -3.5 D are both minus values, but the second needs a much stronger lens. Whether you need glasses is decided by an eye test with an optician or eye doctor.
More terms: part 1 on frames and shapes
What is nylor, what are the parts of glasses called and what do the numbers inside the arm mean? You will find it in Glasses Terminology, Part 1. If you have questions about your own values, your optician or eye doctor can help.
This article does not replace medical advice.
