Methods to Detect Mild Astigmatism. Three ways to detect if a small amount of astigmatism exists are:
1. Use a ±0.25 Jackson Cross Cylinder
2. Use concave and convex ±0.12 or ±0.125 cylinder lenses (see Part 02 - No Prior Prescription under the heading "Initial Astigmatism Testing when there is No Prior Prescription" for general ideas on how to proceed.)
3. Use a single ±0.25 cylinder lens (either convex or concave).
While methods 1 and 2 are listed here, only method 3 is described here.
By this point, you already know whether convex or concave cylinders are being used.
Testing Procedure:
There are theoretical advantages to using a slightly more negative sphere for this test (rather than slightly positive).
If using convex cylinders temporarily reduce the sphere by 0.25.
If using concave cylinders, no changes are made to the sphere at this time.
Mount the concave or convex 0.25 cylinder lens in the front of the frame.
Let the patient rotate the cylinder lens - they typically achieve better results than when the optometrist rotates it.
In some cases, abrupt changes in axis angle reveal better clarity than gradual shifts.
The goal is to locate the axis where the sharpest vision is achieved.
If no improvement in clarity is found regardless of the axis:
The patient likely has no detectable astigmatism. Follow these steps:
1. Remove the cylinder lens.
2. If the sphere was reduced for the test, restore it by adding 0.25.
