On every prescription and every lens box you will see two rows or columns marked OD and OS:
The values for each eye often differ — different SPH, sometimes different BC or DIA on toric lenses, and almost always different axis on astigmatism prescriptions. Always insert the right-eye lens first and develop the habit of opening that box from the same side every time. Many wearers also colour-code their lens case lids — red for right, blue for left.
If you accidentally swap them, vision will feel "off" — blurry, with eye strain after 10–15 minutes. Take both lenses out, rinse, and reinsert in the correct eyes.
For a deep dive into all the prescription numbers see How to read your prescription.
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