Understanding Your Glasses Prescription

You've had your eye test and been handed a prescription, but what do all those numbers and abbreviations actually mean? Understanding your prescription helps you make sense of what's happening with your vision and have more informed conversations about your eyecare.

The basic measurements describe how light needs to be bent to focus correctly on your retina. A prescription for short-sightedness shows minus numbers, indicating you need lenses that diverge light to push the focus point back onto your retina. Long-sightedness produces plus numbers, with lenses that converge light to bring the focus point forward.

The sphere value, often abbreviated as SPH, indicates the main focusing power needed. The higher the number, whether plus or minus, the stronger your prescription. Someone with a sphere of minus one is mildly short-sighted, while minus six indicates significant short-sightedness.

Astigmatism appears as a cylinder value, abbreviated CYL, with an associated axis measured in degrees. This corrects for an eye that's more rugby ball shaped than football shaped, causing light to focus at different points. The cylinder power and axis direction work together to neutralise this distortion.

For those over about 45, there's usually an addition, written as ADD or Near Add. This is the extra focusing power needed for reading, on top of your distance prescription. It reflects the natural stiffening of the lens inside your eye that makes close focusing difficult.

The pupillary distance, or PD, measures the gap between your pupils in millimetres. This ensures the optical centres of your lenses align correctly with your eyes. It's essential for making glasses but isn't included on the prescription itself.

Your prescription belongs to you, and you're entitled to a copy. It's valid for two years unless your optometrist specifies otherwise, though this doesn't mean you shouldn't have your eyes tested more frequently if recommended.

At Atlantic Home Eyecare, we're always happy to explain your prescription and what it means for your vision. Understanding these numbers helps you appreciate why certain lens options are recommended and what changes between tests actually signify.