Quora: How much noise is there in measuring astigmatism?

Posted on January 18, 2012 · Posted in Eye Exam, Instruments

The process of determining a patient’s prescription, or refraction, shouldn’t be thought of so strictly as to think that one option is correct and one is not. Prior to even asking for patient input, an eye doctor has a good idea of what your prescription will be. This is made possible by assimilating all the pre-test data (ie. visual acuity, ocular alignment, auto-refraction/keratometry, retinoscopy, etc.).

With this “idea” in mind, the doctor will then often use subjective input (“1 or 2”) to fine tune spherical and astigmatic correction. Option one might add a little power while option two might remove a little power. It’s important to note that the amounts in both directions are equal. The endpoint in generally considered to be a point of equality where the patient sees no difference between the two options presented.

So in your question, the guessing you do is likely due to the prescription being in the almost ideal spot/power resulting in you having trouble telling the difference between the two options…a near endpoint for the procedure. Similar idea for someone with no astigmatism correction. If there is zero astigmatism correction in the instrument and the doctor tries to add and remove equal amounts of correction (choice 1 or 2), both pictures should look fairly similar and difficult to see a difference.

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