Optical glossary
A clear guide to the abbreviations on a prescription. Find the term you need and check any unclear details before ordering your lenses.
Distance sphere
The spherical power prescribed for distance vision, measured in dioptres. A minus sign indicates correction for short-sightedness; a plus sign indicates correction for long-sightedness.
Copy the value and its sign exactly as prescribed. Do not guess an unclear sign.
Cylinder
The lens power used to correct astigmatism, in dioptres. It may be written with a positive or negative sign.
Read it together with the axis. Do not change the sign or convert these values yourself.
Axis
The orientation of the astigmatism correction. It is expressed in degrees and accompanies the cylinder value.
This is an orientation, not a number of dioptres. Keep the value shown on the prescription.
Near sphere
The spherical power prescribed for near vision. It may appear in a separate section of your prescription.
Do not confuse a complete near prescription with the addition value.
Addition
The additional positive power for near vision relative to the distance correction, used to correct presbyopia.
It is not the complete near prescription. If only ADD is given, check before entering a number in a sphere field.
Base curve
Describes a contact lens curvature using a radius measured in millimetres. It is one of the parameters used to fit the lens to the eye.
Base curve alone does not determine a suitable lens: design, diameter and professional fitting also matter. Do not replace a lens model just because its BC matches.
Ocular dominance
The preference for one eye in certain visual tasks. It may be assessed when fitting some contact lens solutions, such as monovision.
It does not indicate which eye has the better prescription. Assessment for lens fitting belongs with an eye care professional.
Pupillary distance
The distance between the centres of both pupils, measured in millimetres. A prescription may instead list monocular measurements, one for each eye.
Do not confuse the total with a monocular measurement or assume it should be halved. Confirm which measurement your lens fitting requires.
Sources and scope of this guide
General information, not a prescription or diagnosis. Glasses and contact lenses require specific instructions; their prescriptions are not interchangeable. If a sign or value is missing or illegible, ask the prescribing professional to confirm it. Reference sources: