How to read your eye prescription
SPH, CYL, AXIS, PD — what each number on your prescription actually means.
A prescription looks like a spreadsheet and reads like one too. It is really just four or five numbers per eye, and once you know what each column does, the whole thing becomes obvious.
OD and OS
OD is your right eye, OS is your left. Some prescriptions write RE and LE instead. If you see OU, that means both eyes together.
SPH — the main correction
Sphere is the strength of the lens, in dioptres. A minus number corrects short-sightedness, a plus number corrects long-sightedness. The further from zero, the stronger the lens.
CYL and AXIS — astigmatism
If your cornea is shaped more like a rugby ball than a football, you have astigmatism. CYL is how much correction that needs, and AXIS (between 1 and 180) is the angle it sits at. One is meaningless without the other.
ADD — reading power
Usually appears after 40. It is the extra magnification added to the lower half of a progressive or bifocal lens.
PD — pupillary distance
The distance between your pupils, in millimetres. This one is not about your eyes' focus at all — it tells the lab where to centre the optical middle of each lens. Get it wrong by more than 2mm and you will feel it as eye strain.
