How to Read an Eyeglass Prescription?

Wearing prescription glasses all day long is one thing and understanding prescription terminology is another!

When shopping for glasses online do we essentially know all the scientific abbreviations used on our prescription?

Well, we at Lensfit believe that a little know-how about prescription won’t do any harm. We might just end up educating someone or having an interesting optical conversation.

So here’s what you need to know about the terms popping up on an eyeglass prescription:

SPH (Sphere):

SPH is a representation of a curved lens that corrects vision by converging light that enters the pupil. The power is positive (+) for hypermetropia also known a long sightedness, and is negative (-) i.e. myopia for short sightedness. It is measured in dioptres.

CYL (Cylinder) & Axis:

Cylindrical axis is used to correct astigmatism and the geometry of the lens allows light to reflect at different focal meridians.

The transition of the two perpendicular powers is determined by the axis. A vertical axis is measured at 90 degrees and the horizontal axis at 1-180 degrees.

ADD (Addition):

ADD is the magnifying power denoted by (+) sign to correct presbyopia and instilled at the bottom of multifocal and bifocal lenses to facilitate reading.

PD (Pupillary Distance):

The distance between your left and right eye pupils measured in millimetres is the Pupillary Distance. An adult PD ranges between 55mm- 67mm. PD can be measured by pupil to pupil distance or measurement of pupil from centre of the face.


PRISM rectifies vision alignment concerns and is measured in P.D (prism diopters). It is given in a decimal form and denotes the thickness of the base rotation of prism. It is measured in dioptre strength and direction is classified as Base Up (BU), Base Down (BD), Base In (BI) and Base Out (BO) indicating the amount of binocular vision required.

Now that you know the unconventional terms appearing on a prescription, make sure you read well into it and make an informed decision when shopping for designer glasses online for yourself or for a loved one!

Happy learning and Happy shopping at!