What it means and why
AXIS is an angle describing the orientation of cylinder correction. In the usual prescription convention it identifies the meridian with no added cylinder power; the full cylinder contribution is 90° away. The number is measured in degrees, not dioptres.
Directions repeat over a half-turn for this purpose. Values near 180° and near 0° are neighbouring orientations, so ordinary subtraction can exaggerate how far apart two axes are. Meaningful comparison also requires the same cylinder notation.
A worked example
Teaching example: −0.75 D at 90° and −0.75 D at 180° have the same cylinder magnitude but different orientations. They cannot simply be exchanged. Also, an axis near 180° is not “twice as serious” as one near 90°.
What it does not tell you
A changed axis is not interpreted in isolation from cylinder strength, repeatability and notation. A blank axis can be appropriate when no cylinder is prescribed; do not invent one for an order.
THINK IT THROUGHWhich describes amount: CYL or AXIS?
CYL gives the power difference; AXIS gives its orientation. Their units are different.
Sources & reading notes
Page updated:
- AOP · Your prescription explained ↗ Professional body guidance
Explains terminology and dispensing practice; it is not a head-to-head product trial.
- ABDO · Explaining spectacle prescriptions, part 1 ↗ Professional body guidance
Explains terminology and dispensing practice; it is not a head-to-head product trial.