Copyright © 2025 by refractionsteps.org. All rights reserved.
Refining the Sphere when there is no prior prescription
Briefly hold a +0.25 spherical lens in front of the trial frame and compare vision with and without it. Then repeat the test using a -0.25 lens. This comparison will indicate whether the current spherical lens should be replaced with a slightly stronger or weaker one.
In rare cases, a stronger trial lens (such as ±0.50) may be temporarily used to help guide for additional comparison.
If needed, replace the spherical lens in the rear slot of the frame. Repeat the test as often as necessary until the patient achieves optimal clarity.
Refining the Astigmatism when there is No Prior Prescription
Begin by checking whether the axis of astigmatism has shifted. The patient should rotate the cylinder lenses in the front of the frame, ensuring the etched lines remain at a 90° angle to each other. If the axis has changed, record the new axis values for both the concave and convex lenses. If the measurement is unclear, continue using the most recent reliable axis.
To evaluate the strength of astigmatism correction, use a ±0.25 Jackson Cross Cylinder (JCC). For higher levels of astigmatism, a ±0.50 JCC may be used instead.
If the patient has measurable astigmatism, there should be two cylinder lenses in the front of the frame. Their etched lines will be 90° apart, forming an incomplete crosshair pattern. The Jackson Cross Cylinder has four etched lines that also form partial crosshairs.
Hold the Jackson Cross Cylinder close to the eye being tested. Align its etched lines with those on the cylinder lenses in the frame. Ask the patient whether vision is clearer with or without the JCC. Then twirl the JCC (so the patient looks through it in the opposite direction), ensuring that its etched lines still align with those on the cylinder lenses. Ask the same question again.
If vision is worse with the JCC in both positions, the current cylinder strength is likely correct.
If vision improves with the JCC in one of the positions, twirl it again and identify which orientation gives the sharpest vision.
If the patient reports worse vision in one JCC position and no difference in the other, consider proceeding as if the position showing no change provided improved sharpness. Apply clinical judgment; as testing progresses, it will become evident if this was the appropriate decision.
If vision improves with the Jackson Cross Cylinder, make certain it is twirled to the position that gives the sharpest vision. 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 the red lines on the Jackson Cross Cylinder align with the red lines on the frame's cylinder lenses (and silver aligns with silver), use slightly stronger cylinder lenses.
If the red lines align with silver and vice versa, use slightly weaker cylinder lenses.
Never omit the cylinder correction entirely. Remove the original pair of cylinder lenses and insert a new pair (concave and convex) of the adjusted strength. The patient aligns the axis exactly as before - with a 90° separation between the two lenses.
Repeat the process as needed until maximum clarity is achieved.
If there were any changes in the cylinder strength or there were significant (non-trivial) changes to the axis since the last spherical adjustment, then go to Part 03 - Refining the Sphere and Astigmatism when there is No Prior Prescription to recheck both the sphere and astigmatism.
Finished Testing an Eye when there is No Prior Prescription
Before proceeding, make sure to record the following information:
The strength of both cylinder lenses in the front of the frame (equal strength, one concave and one convex).
The value of the sphere lens, along with whether it is concave (negative, red) or convex (positive, black).
The axis angles for the red and silver etched lines on the cylinder lenses (if not already recorded).
You've completed testing for one eye-but which one?
If the right eye was just tested, continue by testing the left eye: go to Part 02 - No Prior Prescription.
If both eyes are finished, proceed to convert the results into a usable prescription.
Converting Test Results into Something to Use as a Prior Prescription
For both eyes, two cylinder lenses were mounted in front. These lenses were equal in magnitude but opposite in polarity, with etched lines that form crosshairs.
The calculations can be done manually:
Step-by-Step Manual Conversion
Step 1: Start by treating all cylinder values as positive, for both eyes.
Step 2: Add the right and left sphere values using signed arithmetic:
If the total is ≥ 0, keep the cylinder values positive.
If the total is < 0, change both the right eye and left eye cylinder values to negative.
Step 3: For each eye:
Subtract the cylinder value from the sphere (signed arithmetic).
This result becomes the new sphere value.
Step 4: Multiply each cylinder value by 2.
Step 5: Assign the correct axis:
If using positive cylinders, use the axis for the silver etched lines.
If using negative cylinders, use the axis for the red etched lines.
**THIS IS NOT THE FINAL PRESCRIPTION**
This is an approximation only. It produces values that can be used as a prior prescription in subsequent testing.
To continue, skip to Part 05 - A Prior Prescription Exists, Test One Eye.