Can You Convert a Contact Lens Prescription To Glasses?
JS
By Julie Sennett
·6 min read
This article explores the topic of whether or not it is possible to convert a contact lens prescription to a glasses prescription.
Medical disclaimer: I am not a doctor. The information on this page is for educational purposes only, and it is not to be taken as medical advice. Please read Contacts Advice Terms of Use before continuing.
A prescription for glasses is not the same as a prescription for contact lenses. The differences in how glasses correct vision compared to contact lenses causes the numbers on the prescription for each to vary slightly (in most cases).
When you visit your eye doctor, the first thing he/she does is measure your glasses prescription. The numbers on your glasses prescription serve as the basis for your contact lens prescription.
In essence, your contact lens prescription is a modified version of your glasses prescription. It is modified using mathematical formulas, industry conventions, as well as a certain amount of trial and error.
The question is, once you have obtained a finalized contact lens prescription, can you then use the information found on it to elucidate an accurate prescription for glasses?
Can You Convert a Contact Lens Prescription To Glasses?
The simple answer is: no. It cannot be done.
There are several reasons why this is not possible, and I will discuss them all below.
Understanding the Conversion Process is Key
The best way to understand why you cannot convert a contact lens prescription to glasses is to understand the process involved in converting a glasses prescription to contact lenses.
On this website, I have written extensively on this topic. From overall summaries to step-by-step tutorials, as well as shortcuts and tricks, I have explained the process in as much detail as you'll find anywhere. To read up on that process, I recommend the following articles:
If you don't feel like reading through all of that, I'll give a general overview below. I'll keep it as simple as possible, but if you feel like there's something you're missing, you can refer to the articles above.
Problems With Converting The Sphere
The first number found on your glasses and contact lens prescription is the 'Sphere Power'. Your prescription for glasses and contacts may contain only 'Sphere' power, or a combination of 'Sphere' and 'Cylinder' power with an 'Axis' (you cannot have 'Axis' without 'Cylinder' and vice versa).
A Contact Lens Prescription With 'Sphere' But No 'Cylinder'
In some instances where the Sphere is the only power in the prescription (i.e., there is no Cylinder or Axis) it would actually be possible to convert a contact lens prescription to glasses.
To go from a contact lens prescription to glasses, the sphere power is put through a mathematical equation called the Back Vertex Formula:
Fc = F / (1 − xF)
Theoretically, this formula works both ways. However, in reality, attempting to use it to go from contacts to glasses can easily lead you into error because it assumes that there is no cylinder in the original glasses prescription, which is not necessarily the case.
The problem arises because:
Just because there is no Cylinder on a contact lens prescription, it does not mean there is no Cylinder in a glasses prescription.
There are two possible 'Cylinder' power values that can be present in your glasses prescription which will never make it to a contact lens prescription:
Cyl = −0.25
Cyl = −0.50
If your prescription for glasses contains either −0.25 or −0.50, the 'Cylinder' will appear as blank (no Cylinder) on your contact lens prescription. Sometimes, even a 'Cylinder' of −0.75 on a glasses prescription can be completely removed from a contact lens prescription.
Example 1
Glasses prescription: Sphere −1.00 / Cylinder none / Axis none
Contact lens prescription: Sphere −1.00 / Cylinder none / Axis none In this example, both would be the same.
Example 2
Glasses prescription: Sphere −1.00 / Cylinder −0.25 / Axis 001
Contact lens prescription: Sphere −1.00 / Cylinder none / Axis none Even though this glasses prescription is different than the one above, it would result in the exact same contact lens prescription.
Example 3
Glasses prescription: Sphere −0.75 / Cylinder −0.50 / Axis 001
Contact lens prescription: Sphere −1.00 / Cylinder none / Axis none When a −0.50 Cylinder is present, it is removed by adding −0.25 to the Sphere.
Are you starting to see how many different glasses prescriptions can lead to the exact same contact lens prescription? A simple contact lens prescription of Sphere −1.00 / no Cylinder could have come from any one of 361 different possible glasses prescriptions. And this is just the simplest case.
A Contact Lens Prescription With 'Sphere' AND 'Cylinder'
If a contact lens prescription has 'Cylinder' power in it, the glasses prescription must also have 'Cylinder' power in it.
The problem is:
The 'Cylinder' power of a glasses prescription is the same as the 'Cylinder' power of a contact lens prescription in less than 50% of cases.
Contact lenses typically only offer Cylinder values of −0.75, −1.25, −1.75, −2.25, and −2.75. Glasses can be made with any Cylinder number in −0.25 intervals (−0.25, −0.50, −0.75, −1.00, −1.25, −1.50, etc.).
This means that for any Cylinder power found on a contact lens prescription, it could have come from at least 2 different Cylinder powers on the glasses prescription, plus all of the different Axis variations (1 to 180). A single contact lens prescription with both Sphere and Cylinder could have equally come from 22 different glasses prescriptions.
Problems With Converting The Axis
1. The 'Axis' is rounded to the nearest 10 for contacts.
The 'Axis' on a glasses prescription can be any number between 1 and 180. The 'Axis' on a contact lens prescription can only be a multiple of 10 (10, 20, 30, 40, 50, 60, 70, etc.). Some information is lost when going from a glasses prescription to a contact lens prescription, making it impossible to convert back.
2. The 'Axis' is often adjusted by eye doctors to compensate for a rotated lens.
When fitting a toric contact lens, an eye doctor may add or subtract to the Axis if he/she finds that the contact lens is not sitting in the correct position on the eye. For example, a glasses prescription with Axis 033 might be rounded to 030 for contacts, but if the lens rotates 10 degrees counterclockwise on the eye, the final contact lens Axis would be adjusted to 020 — entirely different from what the glasses prescription would suggest.
Unless you were specifically informed about these kinds of adjustments by your eye doctor, there's no way for you to know what was or was not done to arrive at your final contact lens prescription.
Summary
It would be much simpler if both glasses and contact lens prescriptions were exactly the same and could be used interchangeably. Unfortunately, that is not the case.
Although they both use 'Sphere', 'Cylinder', and 'Axis', there are many steps involved in converting and adjusting the numbers from one to the other.
Once the numbers are converted from glasses to contact lenses, there is no way of knowing for certain what the original numbers were unless you have a written record of it. That is why the only way to recover the numbers on your glasses prescription is to contact your eye doctor's office.
I hope this article was informative. I know I covered some pretty challenging topics so if you have any questions about it, feel free to ask!
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