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‘OD’ and ‘OS’ are abbreviations of the Latin words ‘oculus dexter’ and ‘oculus sinister’, respectively. In the context of an eye prescription, ‘OD’ means ‘right eye’ and ‘OS’ means ‘left eye.

In the context of an eye prescription. ‘Sphere’ is basically a measure of how strong your prescription needs to be. The number marked ‘axis’ describes the orientation of astigmatism. It will be a number between 1 and 180. The number under ‘cylinder’ provides the lens power needed to correct the astigmatism.

Your prescription may have a ‘base’ column, or it may be included in the same column as ‘prism’ this gives further information about the required prism correction, telling us the orientation needed in the lens.

The number under the ‘add’ column tells us the 2nd lens prescription needed for multifocal lenses.

The higher the number on your prescription, the stronger your prescription is. A number with a minus symbol before it means you are nearsighted, and a number with a plus symbol, or no symbol, means you are farsighted. A number over 5 is generally considered to be on the stronger side.

If you have an astigmatism, indicated by an “axis” and “cylinder” number on your prescription, it means that the front of your eye is irregularly shaped. This stops light from focusing properly on your retina and makes your vision blurry. It usually occurs along with near or farsightedness.

Astigmatism is often present at birth, but it can develop, or become more severe, as you get older.

Diopter is the measurement used for the strength of eyeglass lenses. The number under the ‘OD’ and ‘OS’ measurement in your prescription is measured in diopters.