Understanding Your Prescription

Sphere values are measured in dioptres. Under about two dioptres is mild, two to five is moderate, and above six is generally considered high. Higher prescriptions benefit most from high index materials and from smaller frames.

A cylinder value means you have astigmatism, and the axis tells the lab which way round to orient the correction. Small amounts are extremely common and entirely normal.

A spectacle lens sits about twelve millimetres in front of your eye. A contact lens sits directly on it, which changes the effective power, particularly at higher prescriptions.

More importantly, a contact lens prescription includes fitting parameters that a glasses prescription has no reason to contain: base curve, diameter, material and brand. Those come from measuring your eye, which is why a separate contact lens examination is required.

Glasses prescriptions are typically valid for one to two years and contact lens prescriptions require annual renewal. That is not an administrative habit. Corneal health, tear film and lens fit all change over time, and an out of date prescription means an out of date eye health assessment.

Nothing about the health of your retina, your optic nerve or your eye pressure. A prescription describes how light needs bending. The eye health assessment is the part that finds glaucoma, diabetic changes and macular degeneration, and it is the part your own experience cannot substitute for.