How to understand it
The axis locates a meridian; it is not the amount of astigmatism. It matters only in context with the cylinder notation.
An example
−0.75 D at 90° and −0.75 D at 180° have the same cylinder magnitude and different directions.
A common mix-up
180° is not twice as severe as 90°, and neither is a dioptre value.
See the idea in a full guide →
Sources & verification
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