As an Optometrist for many years, I have written LOTS of prescriptions for eyeglasses. I have noticed that the prescriptions I write are different from the prescriptions that my colleagues write for the same patient, even when the patient has not experienced significant vision changes.
Elements of a Prescription
Essentially, an eyeglass prescription contains three distinct numerical values: sphere, cylinder and axis. There is a fourth, called the ‘ADD’ power which is for bifocal or progressive lenses, but I want to focus on the other three.
The ‘SPHERE’ is the number that indicates whether you are nearsighted (myopia) or farsighted (hyperopia). Myopia and hyperopia are determined by the axial length of the eye — in plain terms, the measurement from the front of the eye to the back of the eye. Myopia is written with a (-) sign in front of the power and hyperopia is written with a (+) sign in front of the power.
The ‘CYLINDER’ is the number that describes the amount of astigmatism that the eye has. Astigmatism is a measure of curvature of the eye.
The ‘AXIS’ is related to the cylinder curvature and shows the direction of where the eye is curved.
These measurements are your eyeglass prescription numbers and are used to make glasses. When glasses are manufactured according to the prescription, there is a tolerance range of how far off the prescription can be from what is written. Generally accepted tolerance ranges are within 0.25D (diopters) of the prescription written for the sphere and the cylinder. For the axis, measured in degrees, it can vary depending upon how much cylinder power there is. Generally, the higher the CYL, the smaller the tolerance for axis.
Since the visual tolerances is small, that means that eyeglasses must be made very accurately to the prescription. I have found this accuracy to be very true, but why, then, do so many people still have difficulty with adapting and seeing with their prescription glasses? There is not a lot of information about prescription variance from one doctor to the next, but I often see that prescriptions for sphere and cylinder differ up to 0.75D of power (0.50D outside of the tolerance range). That is a large difference, and definitely can affect how we see.
Why are there differences in prescriptions for the same person?
So how could it be, that the same patient could get two different prescriptions from two different eye doctors, even with eye exams on the same day? The truth is, there is no ONE set of prescriptions that work. Human eyes can adjust and adapt to small differences in prescriptions because it is actually the brain that is interpreting the image that it is given by the eyes. The human brain wants to see as clearly as possible and will try to work through any prescription that is put in front of the eyes. If you’ve ever tried another person’s glasses on, you may have noticed that the images are very distorted or blurry; or worst, you may have even felt sick to your stomach. This feeling is your brain telling you that the prescription is not correct. If the prescription happens to be very close, you may think your friend’s glasses might just work for you. This feeling is your brain adapting to the power in the glasses.
Other factors, like how tired you are, the time of day, health conditions such as diabetes can all affect the prescription the doctor is giving you. Remember that in an eye exam, the doctor is asking you to respond to choices that are presented to you. If you are tired, can’t focus well or have fluctuations due to medical conditions, your responses may not be accurate. As eye doctors, we need the patient to help us determine the prescription; it is not just the doctor giving you the eyeglass numbers.
Your prescription measured at the eye doctor is a snapshot of your vision at that exact time. If you don’t go to the eye doctor often, it’s highly likely your prescription has changed. The more you test your vision, the more vision data you’ll have to compile the most accurate prescription possible. With the EyeQue Personal Vision Tracker, you can test your vision without using your co-pay from home. You an be a better patient by saving your vision history and sharing with your eye doctor to ensure your prescription is as accurate as possible!
How would I determine whether my glasses are right for me?
So, as a someone who wears glasses, what can you do if you are not seeing properly out of your new prescription? First, you should give it a few days to see if you adapt. For every change in sphere, cylinder and axis, it can take some time for the brain to feel comfortable and re-focus the eyes. If you are not adapting after a few days, I would recommend two steps:
- Testing your prescription with the EyeQue Insight, at home 20/20 vision screener
- If you don’t see 20/20 in your individual eyes and dual eyes, your prescription isn’t accurate, if you see 20/20, your prescription is correct.
- Making an appointment with your eye doctor for a glasses re-check
- Your doctor can check the tolerances and the prescription to make sure the lenses were made according to the prescription.
Prescribing glasses is a science, but it is also somewhat of an art. We have to take into consideration many factors besides just the prescription. Hopefully, this explanation helps you understand a little bit about the process.