This article explores the topic of whether or not it is possible to convert a contact lens prescription to a glasses prescription.
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:
- Converting Your Glasses Prescription to a Contact Lens Prescription
- Converting Eyeglass Prescription To Contact Lens
- 3 Ways to Convert Your Glasses Prescription to Contact Lenses
- Contact Lens Prescription VS Glasses Prescription
- Vertex Conversion Charts for Sphere and Cylinder Combinations
- Converting a Glasses Prescription To Contact Lenses TUTORIAL – Part 1
- Converting a Glasses Prescription To Contact Lenses TUTORIAL – Part 2
- Converting a Glasses Prescription To Contact Lenses TUTORIAL – Part 3
- Converting a Glasses Prescription To Contact Lenses TUTORIAL – Part 4
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).
Let’s examine both scenarios separately.
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. This formula looks like this:
Theoretically, this formula works both ways, meaning that you can use it to go from glasses to contacts, as well as from contacts to glasses.
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 of this:
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 a contact lens prescription. They are:
- 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, depending on the person, even a ‘Cylinder’ of -0.75 on glasses prescription can be completely removed from a contact lens prescription.
Let’s take this prescription for glasses:
The resulting contact lens prescription would most likely be:
In this example, both would be the same.
Now let’s look at this prescription for glasses:
Even though this prescription for glasses is different than the one above, it would result in the exact same prescription for contact lenses (most likely):
So if your starting point is this contact lens prescription…
… and you have no prior knowledge of your glasses prescription, there is absolutely no information that tells us whether or not a -0.25 ‘Cylinder’ was present in the original glasses prescription.
Now consider this prescription for glasses:
When a -0.50 ‘Cylinder’ is present in a glasses prescription, it is typically not carried over to the contact lens prescription. This is because the smallest ‘Cylinder’ that contact lenses are available in is -0.75.
But instead of just making it disappear completely from the prescription like we did with the -0.25 ‘Cylinder’, the -0.50 ‘Cylinder’ is removed by adding ‘-0.25’ to the ‘Sphere’.
Thus, the contact lens equivalent of the glasses prescription in Example 3. would likely be:
Are you starting to see how many different glasses prescriptions can lead to the exact same contact lens prescription?
Furthermore, if there were a -0.25 or -0.50 ‘Cylinder’ in the glasses prescription, we would have no idea what the ‘Axis’ should be.
Since the ‘Axis’ can be any number between 001 and 180, it means that this simple contact lens prescription…
|-1.00||-0.25||1,2, 3, 4 … 180|
|-0.75||-0.50||1,2, 3, 4 … 180|
That’s a total of 361 different possible glasses prescription combinations for 1 single contact lens prescription. And this is just the simplest case. It only gets more complex from here.
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.
Cylinder of contact lenses
There are only a very limited number of ‘Cylinder’ values available for contact lenses. Typically they are -0.75, -1.25, -1.75, and -2.25 (-2.75 is available for most monthly brands).
Cylinder of glasses
Glasses can be made with any ‘Cylinder’ number in -0.25 intervals i.e., -0.25, -0.50, -0.75, -1.00, -1.25, -1.50, -1.75, -2.00, -2.25, 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).
This contact lens prescription…
… could have come from any of these glasses prescriptions:
|-1.00||-1.25||085, 086, 087, 088, 089, 090, 091, 092, 093, 094, 095|
|-1.00||-1.50||085, 086, 087, 088, 089, 090, 091, 092, 093, 094, 095|
A single contact lens prescription that contains both ‘Sphere’ and ‘Cylinder’ power could have equally come from 22 different glasses prescriptions, and that is assuming that your eye doctor did not need to make any additional adjustments.
Problems With Converting The Axis
Converting the ‘Axis’ on a contact lens prescription to glasses is problematic for a few reasons.
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 i.e., 1,2,3,4,5,6,7,8,9,10,11, etc
The ‘Axis’ on a contact lens prescription can only be a multiple of 10 i.e., 10, 20, 30, 40, 50, 60, 70 etc.
That means that some information is lost when going from a glasses prescription to a contact lens prescription. This makes it impossible to convert back.
2. The ‘Axis’ is often adjusted by eye doctors to compensate for a rotated lens.
When you are being fitted with a toric contact lens by an eye doctor, he/she will typically start with the closest ‘Axis’ to the one on your glasses prescription. When the fit of these lenses is examined on your eyes, the eye doctor may decide to add or subtract to the ‘Axis’ if he/she finds that the contact lens is not sitting in the correct position on your eye.
This may be difficult to understand if you’ve never been taught how to fit contact lenses, so hopefully, this next example will help.
Let us consider this glasses prescription:
Firstly, the ‘Axis’ must be rounded to the nearest 10, in the case, 30.
However, when the eye doctor looks at this lens on a person’s eye, he/she sees that it is constantly rotated by 10 degrees counterclockwise. He/she will compensate for that by subtracting 10 degrees to the ‘Axis’, resulting in:
Unless you were specifically informed about these kinds of adjustments by your eye doctor, there’s no way for you know what was or was not done by your eye doctor to arrive at your final contact lens prescription.
Further confusing matters is the fact that certain axis adjustments may be needed for one brand which would not be needed for a different brand.
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!