Realizing that the numbers in your glasses prescription are not always the same numbers that are needed for your contact lenses is an important first step in being successful with contacts. Ordering the wrong power in your contact lenses can result in blurry vision, eyestrain, headaches, double vision, etc.
When it comes to converting a glasses prescription to contact lenses, there are different ways of going about it for different people. This article will describe 3 methods of converting a glasses prescription to contact lenses and who should be using each one.
1. Plan a Visit to Your Eye Doctor’s Office
This is the method that most people are familiar with. If you need glasses or contact lenses, just head over to your eye doctor’s office and hope for the best when it comes to your wallet.
Your eye doctor is trained to assess the fit of contact lenses on your eyes and to convert the numbers on your glasses prescription to make them appropriate for contact lenses. At the end of a contact lens assessment, your eye doctor will provide you with a contact lens prescription that you can use to order contact lenses anywhere you please.
The Contact Lens Rule
Eye doctors are required by law to provide you with a glasses prescription after an eye exam and a contact lens prescription after a contact lens assessment. They cannot withhold your prescription in an attempt to force you to purchase your eye-wear from them. If your optometrist doesn’t give you a prescription, they are in violation of the law. If your eye doctor refuses to comply with the law, you can help by alerting the FTC of their unjust practices. You can do so here. To learn more about issuing a complaint, click here.
In the United States, online contact lens vendors are required to request your contact lens prescription information before they can sell contact lenses to you. The information you submit is sent to your eye doctor where he/she will have the option of reviewing it.
The law is written in such a way that upon receiving a prescription verification request from an online vendor, your eye doctor has 8 hours to respond to the request. This is where your eye doctor can intervene if they disapprove of what you’re attempting to order. However, failure of your eye doctor to give a response with 8 hours results in a defacto approval of your order.
Does every optometrist comply?
Unfortunately, just as with almost all jobs and professions, there are some optometrists who like to cut corners in order for them to save time, see more patients, and collect more fees. I have heard many accounts of optometrists charging for a “contact lens assessment”, while never doing more than a simple conversion of your glasses prescription on paper which can be performed in under 15 seconds (by anyone).
It sounds rather obvious when I type this out, but a contact lens assessment actually requires your eye doctor to assess your contact lenses. This means he/she must do a vision check for each eye wearing a contact lens to make sure it’s acceptable. They must also evaluate the centration and curvature of the contact lenses on your eyes. And if you’re lucky, they might even ask you how you feel in the contact lenses.
2. Using the Back Vertex Formula
In the United States, you cannot use methods 2 and 3 in this article to order your contact lenses. These methods are reserved for opticians, optometrists, and any eye-care professional in training for the purposes of practice.
The ‘Back Vertex Formula’ is a mathematical equation which is what eye doctors use to produce a theoretical power for contact lenses from a glasses prescription.
The Back Vertex Formula is:
As you can see, this is a fairly simple equation with only 2 variables.
Fc represents the theoretical power of contact lenses
F represents the power in the glasses
X represents the vertex distance
The vertex distance refers to the distance (in meters) between your eye and the back of your lenses on a correctly fitting pair of glasses. Generally, this distance is the standard testing distance (12-14mm) that optometrists use when measuring your glasses prescription. However, your optometrist may choose to use a different testing distance if he/she is so inclined.
So let’s do a sample calculation using the back vertex formula.
Let’s use the following parameters:
F = -5.00
x = 0.013 (remember that x is in meters)
Fc = -5.00 / (1 – [0.013 * -5.00] )
Fc = -5.00 / (1 – [ -0.065] )
Fc = -5.00 / 1.065
Fc = -4.70
Of course, a glasses prescription contains much more than 1 number, so which number do you have to use for ‘F’?. The answer is, it depends.
If there is a ‘Sphere’ number and no ‘Cylinder’ number, you use the ‘Sphere’ number as F, and you only perform the calculation once (per eye).
If there is a ‘Sphere’ number and a ‘Cylinder’ number, then you have to do the calculation twice. Once with the ‘Sphere’ number as ‘F’ and once with the ‘Sphere’ + ‘Cylinder’ number as ‘F’.
If you are working with a prescription that has both ‘Sphere’ and ‘Cylinder’, you may want to look into performing a power cross diagram. You can learn more about power cross diagrams in this tutorial.
If you think that doing all these calculations and diagrams is too annoying and troublesome, don’t worry you’re not alone. Even optometrists don’t perform these calculations. All of the answers have already been calculated and organized into easy to use charts called Back Vertex Charts. And this brings us to Method 3.
3. Using Back Vertex Charts
Again, these types of charts are designed to assist eye-care professionals in performing on-the-fly conversions to save them time in clinical practice. They are not meant to bypass getting a professional contact lens assessment.
In a busy optometry clinic or optical shop, optometrists and opticians don’t have time to do back vertex calculations using the formula I provided above. Instead, they use reference charts that list the solutions to all the possible prescriptions they might encounter on a day to day basis.
Here’s an example of how a Vertex Conversion Chart is used.
Right: Sphere -5.50 Cylinder none Axis none
Left: Sphere -6.25 Cylinder none Axis none
They would find the answer by looking up -5.50 and -6.25 in a chart such as this one:
In the example above, the converted numbers would be:
Right: Sphere -5.12 Cylinder none Axis none
Left: Sphere -6.75 Cylinder none Axis none
However, the issue with these charts is that they only convert one number at a time. If you have any astigmatism in your prescription (any numbers in the ‘Cylinder’ column), then you have 2 numbers that need to be converted.
But hold on…
The numbers in the ‘Cylinder’ column cannot be converted in the same way as the numbers in the ‘Sphere’ column.
It’s because the ‘Cylinder’ does not actually represent a power found in your glasses or contacts. It denotes the difference in power between your primary and secondary powers.
So what to do if a glasses prescription has both ‘Sphere’ and ‘Cylinder’ in it? Well, this is where things get a little complicated. Eye care professionals use a technique called a ‘power cross’ to deconstruct the numbers in the glasses prescription, they then use vertex conversion charts to convert each power, and then they reconstruct the traditional sphere/cylinder prescription format.
You can learn more about power cross diagrams here.
Confused? Don’t give up just yet!
Thanks to Contacts Advice, there is now an easier solution than using the power cross.
The Easy Method
If one performed all the calculations necessary to convert every single possible Sphere and Cylinder combinations that one might see on a glasses prescription, they could develop charts that bypass the need to do the calculations each time.
And it just so happens that I devoted the time to do this and created Contacts Advice’s Vertex Conversion Charts for Sphere and Cylinder Combinations.
If you’re dealing with ‘Sphere’ number that is between 0 and -8.00 and a ‘Cylinder’ number that is between 0 and -3.50, then no matter what the prescription is, you’ll find it on my charts as well as the calculated back vertex powers.
Here is an example prescription with both Sphere and Cylinder:
Right: Sphere -6.00 Cylinder -1.25 Axis 180
Left: Sphere -6.00 Cylinder -2.00 Axis 170
Simply find each of those combinations in the correct chart. In this example, you’d use the one below.
The results of the example above would be:
Right: Sphere -5.62 Cylinder -1.00 Axis 180
Left: Sphere -5.62 Cylinder -1.63 Axis 170
It’s as easy as that! No messy calculations or power cross diagrams required. You just look up the desired ‘Sphere’ and ‘Cylinder’ and you get the back vertex conversion for both instantly.
The chart above is only for ‘Sphere’ and ‘Cylinder’ combinations where the ‘Sphere’ is -6.00. If the ‘Sphere’ you are looking for isn’t -6.00, you will have to use a different chart.