If the axis is tilted right or CCW, what should you do to the spectacle axis to compensate for rotation?

If the axis is tilted right or CCW, what should you do to the spectacle axis to compensate for rotation?

• A. add to
• B. subtract from
• C. ignore
• D. invert axis

Answer: (B)

When a toric correction rotates on the eye, the line of correction (the axis) shifts by the same amount. To bring the axis back to the intended orientation, you rotate the spectacle axis in the opposite direction by that rotation amount. In practice this means subtracting the amount of rotation from the prescribed spectacle axis. For example, if the intended axis is 90 degrees but the lens sits rotated 5 degrees to the right, the axis on the lens is effectively 95 degrees; subtracting 5 brings it back to 90. This same subtracting rule is used to compensate for rotation in other directions, because the goal is to realign the axis with the original correction. Ignoring the rotation would leave the astigmatic correction misaligned, and inverting or simply adding would not restore the correct orientation.

When a toric correction rotates on the eye, the line of correction (the axis) shifts by the same amount. To bring the axis back to the intended orientation, you rotate the spectacle axis in the opposite direction by that rotation amount. In practice this means subtracting the amount of rotation from the prescribed spectacle axis. For example, if the intended axis is 90 degrees but the lens sits rotated 5 degrees to the right, the axis on the lens is effectively 95 degrees; subtracting 5 brings it back to 90. This same subtracting rule is used to compensate for rotation in other directions, because the goal is to realign the axis with the original correction. Ignoring the rotation would leave the astigmatic correction misaligned, and inverting or simply adding would not restore the correct orientation.