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Initial Sphere Testing when there is No Prior Prescription
The lens kit may include black lenses that block all light and frosted lenses that allow light through but blur the image. Use one of these lenses to block the eye that is not being tested. Place it in the front position of the frame, over the non-tested eye.
For bright outdoor testing, the frosted lens may work better. For indoor settings, the black lens typically gives more accurate results.
Begin by testing the right eye.
Identify the spherical lens that provides the clearest vision in that eye.
If the kit has limited options, you may combine two lenses to approximate the correct strength.
Once you find a close match, insert the lens into the back position of the frame.
Briefly hold a +0.25 spherical lens in front of the frame and compare the clarity.
Then do the same with a -0.25 lens.
In unusual cases, it may be necessary to use stronger lenses for the above test.
This comparison helps determine whether the current lens is slightly too strong or too weak.
When you've identified the best spherical lens, place it in the back position for that eye.
Repeat the test as needed.
Initial Astigmatism Testing when there is No Prior Prescription
Step 1: Estimate Astigmatism Strength
Estimate the patient's astigmatism strength by observing how blurry their vision remains after identifying the best spherical lens.
Maximum estimate: ±6.00
Minimum estimate: ±0.25
DIVIDE THIS ESTIMATE BY 2 to determine the cylinder test lens strength. This will yield a value between 0.125 and 3.00.
If your kit lacks a lens of the exact calculated strength, use the nearest available strength above or below that value. Do not use a cylinder strength of zero.
Step 2: Select and Mount Cylinder Lenses
Select one concave (minus) cylinder lens and one convex (plus) cylinder lens of the same calculated strength.
Mount both lenses in the front position of the trial frame.
Align the etched lines on each lens at 90° from each other, forming a crosshair pattern. Together, the two lenses function like a Jackson cross cylinder.
If both handles face the same direction, flip one of the lenses around.
Step 3: Rotate to Find the Best Axis
For optimal results, the patient-not the optometrist-should rotate the lenses.
The patient should rotate both lenses together as a unit, maintaining the 90° axis separation between them.
Identify the position that produces the sharpest vision.
If no position improves clarity, re-estimate the cylinder strength and try again with a different pair of cylinder lenses.
Even if vision appears sharper without the cylinder lenses, this step is still essential for identifying the correct axis and determining whether astigmatism is present.
If no configuration improves vision, the patient may not have astigmatism-or the astigmatism is too mild to detect using this method. In such cases, abrupt changes in the axis angle may yield better results.
Step 4: Record Axis Results
At this point, you should have identified two axis values, separated by 90°. Confirm that:
Both cylinder lenses are of equal strength.
Their etched lines remain 90° apart.
Note: Always record each axis result. Later testing may refine it further, but keeping earlier values is useful. Occasionally, placing a slightly more negative spherical lens in the frame can help reveal the correct axis more clearly.
Proceed to Part 03 - Refining the Sphere and Astigmatism when there is No Prior Prescription to recheck both the sphere and astigmatism.