Optometry basics
What the values of a prescription mean. A pair only matches if both eyes fit, so every value is per eye.
The four values per eye
Each eye (OD = right, OS = left) uses steps of 0.25:
| Value | Meaning | Range |
|---|---|---|
| Sphere | base lens power (diopters) | -20 to +20, usually -6 to +6. Negative = near-sighted, positive = far-sighted |
| Cylinder | corrects astigmatism | 0 to -6, usually 0 to -3. Zero or negative only |
| Axis | orientation of the cylinder correction | 0 to 180, where 0 and 180 are the same. Only matters with a cylinder |
| Add | extra plus power for reading, multifocal only | +0.25 to +4, usually +0.75 to +3.50 |
Sphere and cylinder trade off
Less cylinder plus half that amount added to the sphere gives almost the same correction. That's the spherical equivalent. So a pair with less cylinder can still fit, given a sphere shift.
One more asymmetry
Giving a far-sighted patient (positive sphere) too little power is worse than giving too much, so the matching algorithm penalizes only that direction.