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James Dugas, OD and Associates

Optometrists in Austin Texas
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How to Get the Best Optical Prescription (Rx)
Abstract/Take Away Message
The goal of each test in a refraction is to find the end point of where you see no difference between the two lenses.  Don't worry about the idea of when you can't tell which is better  The end point of each sub-test is to find the point where they look about the same and you have no preference as to which better.  That's the end point of each sub-test of a refraction.  So, don't worry when you have no preference as to which is better; that's the end point we want to find. 
How this information helps you 
You will feel more comfortable with the results of the refractive procedure (refraction) after your eye exam, and you will have a more accurate prescription and less stress during the exam. If we have a more accurate distance prescription, we can more easily rule out eye disease.
The goal of this page is to explain an idea that will make you more comfortable with the lens comparing process of determining your prescription which is termed a "refraction", and increase your confidence in the results of the refraction, which is your eyeglass prescription.  An accurate refraction is a necessary starting point for your contact lens prescription and the refraction is a factor in your Lasik procedure. 
Years ago, a lady who was a Professor at UT Austin told me that she did not like eye exams because they were so stressful.
I asked her why she felt that way.  She told me that she did not know if she would get the correct prescription because sometimes she could not tell the difference when asked "which lens is better or clearer?".
I then asked her, "Did anyone ever tell you that the goal of each test is to find the point where they look equal and you can't tell the difference"?  She replied, "No. No one ever told me that".
After that encounter, I started giving more detailed instructions about each test and the goal of each test, which is to find the end point where they look about equal and you can't tell which is better.  During the exam, when I give instructions and mention the goal of the test, patients will usually say "No one ever told me that".
The Refraction Procedure
During the refraction, we do several tests and sub-tests which compare two different lenses, and we ask which is the better of the two.  We compare and "bracket" the lenses by comparing progressively smaller increments until we find the point where the patient can't distinguish any difference or they look about the same.  Sometimes they're both a little blurry, sometimes they're both fairly clear, and sometimes there's a big difference.
During the refraction, the question is always; which is better or clearer, or do they look about the same and you have no preference as to which is better.  Again, the goal is to find the point where you can't tell the difference and you have no preference as to which is better, which is the end point to that sub-test.  We have found our former patients remember these ideas and that makes subsequent exams easier for both of us, and yields more accurate prescriptions. 
The Sub-Tests and What We Look For 
The Spherical Lens Test
With the spherical lens test we will ask you if it gets clearer, or just smaller and darker, without an increase in clarity.  Becoming smaller and darker without a gain in clarity means it's over-powered.  We want to use as low a power as possible because it's more comfortable to read at near with a lower power, especially if your age is over 40 years. 
The Astigmatism and Axis Test 
This is a test where we ask which is better, but both are blurry all the time, and we ask which is the better of the two.  You will usually say, I was seeing better earlier.  We know that, but the goal of the astigmatism test is to find the point where the lenses are equally blurry and you have no preference as to which is better.  
At some point, on all sub-tests, you can't really see much difference and they look about the same and you have no preference.  That's the end point to each sub-test. Then we repeat the comparisons and tests, each time finding the point where they look about the same.  All data and end points are re-checked, to assure that each end point of each test was the same.  That's why you will see us doing the same test several times, to find the "repetitive and consistent end point".  When that occurs, we have the data needed to write an accurate prescription.
There are other instructions that we give for each test that make testing easier for you, however the end point of where you can't tell a difference is the main idea which you should understand regarding a refraction.
So, don't worry when you can't tell which lens is better or when they look about equal; that's the end point to the test that we want to find. 
Why We Do Refractions and Why We Want to Determine and Know Your BCVA
You want us to do a refraction so you can get a prescription for eyeglasses or contact lenses.
We want to do a refraction for the same reasons, but we also want to determine your Best Corrected Visual Acuity (BCVA) because it helps us diagnose if you have eye disease.  If your BCVA is 20/20 you probably (but not all the time) have no immediate problem.
If your BCVA is less than 20/20, we have questions as to why it's not 20/20.  We will do tests to determine if it's due to amblyopia or eye disease.  A gradual reduction in BCVA could be cataracts or macular degeneration.  A sudden reduction in BCVA could be retinal detachment, MS, tumor, or stroke.  Whenever your BCVA is less than 20/20, we want to know why. 
Scans, State Law, Auto-Refractors, Wave Scan Analysis, Distace Rx is Only a Starting Point
With the current state of technology, you may ask the question (which patients often ask), why can't you just do a scan and give me an Rx?  That's an excellent and very thoughtful question.  Here's the short answer for now.
Firstly and foremost, State law requires your Optometrist to do a interactive/manual refraction and for good reason.  An interactive and manual refraction is most the accurate and helps us diagnose eye disease by determining your BCVA.
Auto-Refactors give an approximation of your distance Rx only, and nothing else.  The results are not accurate enough to prescribe from, in my opinion, to receive 100% satisfaction from patients.  Furthermore, during the manual refraction the eye doctor discovers things about your accomodative facility, muscle imbalance, amblyopia, suppression and other things which, if not discovered, you would have questions about after you received your glasses, if the glasses were prescribed from only an accurate distance Rx.     
Wave scan analysis is done before your Lasik procedure and is compared to the results of the manual refraction before you have your Lasik procedure.  Wave Scan delivers a very accurate determination of your distance Rx and is why we have such a high success rate with Lasik procedures.  However, wave scan does not deliver information about your accomodative facility, etc, and cannot help us design proper multi-focal Rx's, reading Rx's and computer Rx's.  Wave scan does not measure your best corrected visual acuity (BCVA), which is important in diagnosing eye disease.  Wave scan does accurately quantify your distance Rx, but again, an accurate distance Rx is only a starting point for an accurate prescriptions for glasses, reading glasses, computer Rx's, or contact lenses of any type.   
Although Wave Scan is very accurate in determining your distance Rx, your doctor would still have to do other tests, discuss presbyopia, reading glasses and working distances, computer prescriptions, double vision at near due to a convergence insufficiency which is to be referred to a vision therapist, and so forth.
Summary of How to Get the Best Optical Prescription
  • Don't worry when you can't tell which lens is better.  That's the endpoint to the sub-test which we want to find.
  • The most accurate method of quantifying your distance prescription is to do an interactive and manual refraction.
  • An accurate distance prescription is a starting point for an accurate prescription for distance glasses, reading glasses, computer prescriptions, contact lens prescriptions and is a factor in your Lasik procedure.
  • An accurrate distance prescription helps us determine if your BCVA is 20/20 or less and helps us diagnose eye disease if it is not 20/20.
  • That's why we do do interactive and manual refractions and how you get the best optical prescription.