Part 08 - A Prior Prescription Exists, Refining the Astigmatism, Nonzero Cylinder

Part 08 - A Prior Prescription Exists, Refining the Astigmatism, Nonzero Cylinder

It is important to keep track of the axis and the changes to the axis throughout the testing process for the eye. Sometimes when you get closer and closer to the right lenses for the eye, it becomes more difficult to accurately determine the axis. You want to keep track so you have some reference points for the axis. Sometimes if it is difficult to measure the axis, you can use a slightly more negative sphere to refine the axis, but once the axis has been refined, you can switch back to the accurate sphere. Sometimes, it is helpful to take the axis from the prior prescription into account. The patient can be encouraged to try to make abrupt changes in the axis. But sometimes smooth changes work better.

The patient, not the optometrist, should always be the one to adjust the axis to find the point where the vision is sharpest.

Refine the axis.
Use a ±0.25 Jackson Cross Cylinder (JCC) to evaluate whether the cylinder power is correct.
In rare cases of strong astigmatism, a ±0.50 JCC or stronger may be used if ±0.25 does not produce a clear result.

1. Hold the JCC in front of the eye. Align two of its etched lines exactly with the lines on the cylinder lens in the frame.

2. Ask the patient: "Do you see better with or without the JCC?"

3. Twirling test: Flip the JCC so the patient looks through it in the opposite direction, making certain the etched lines on the JCC align with the etched lines on the cylinder lens. Ask again: "Better or worse?"

    If vision is worse in both twirled positions:

        The current cylinder strength is likely correct. Go to Part 09 - A Prior Prescription Exists, Refining the Axis.

    If vision improves in either twirled position:

        Twirl the JCC to the position that gives the best clarity.
        Examine the etched lines:

            Hint: To make the red etched lines on the JCC easier to identify, use a toothpick dipped in red fingernail polish to place a tiny dot on either side of the red markings. This simple trick improves visibility without interfering with the lens.
            Note: The cylinder lenses are clearly labeled as concave or convex, so there's no need to rely on the etched line color to determine lens type.

            If red cylinder lines align with red JCC lines or silver cylinder lines align with silver JCC lines, increase cylinder power by 0.25, that is, if positive cylinders are used, make the cylinder more positive, but if negative cylinders are used, make the cylinder more negative.

            If red cylinder lines align with silver JCC lines or silver cylinder lines align with red JCC lines, decrease cylinder power by 0.25, that is, if positive cylinders are used, make the cylinder less positive, but if negative cylinders are used, make the cylinder less negative.

            Let the patient re-adjust the axis.

            The optometrist records the new results.

4. Final Check

    If either cylinder power or axis has changed since the last time the sphere was adjusted, return to Part 06 - Refining the sphere and Astigmatism.

    If the cylinder or axis was not changed since the last time the sphere was adjusted, then proceed to Part 09 - A Prior Prescription Exists, Refining the Axis