Now that you know who can or can’t wear contact lenses, it’s time to get the contact lens fitting started. The first step is to determine the power of contact lenses needed for your patient. To do this so it is crucial to know how to convert a glasses prescription to contact lenses.

Is A Contact Lens Prescription The Same As Glasses?

NO!

Well… sometimes yes.

Ok, it depends! let me explain.

What’s The Difference?

Glasses hold lenses up in front of the eyes, whereas contact lenses sit directly on the eyes. By understanding this key difference between contact lenses and glasses, you will understand why contact lens and glasses prescriptions are not always the same.

Why Does Distance Between Lens And Eye Matter?

It matters because of this:

In a nutshell, this simple equation states that the perceived power of lenses changes as you change the distance between the lens and the eye.

“The perceived power of lenses changes as you change the distance between the lens and the eye.”

Now, as we know, the lenses in glasses are at a different distance from the eyes than contact lenses are. The distance between a corrective lens and they eye is called the vertex distance.

• For glasses, the vertex distance is usually somewhere between 12-15mm
• For contact lenses, the vertex distance is always zero

This equation tells us that, for example, a -6.00D lens held 13mm from the eye will not feel like a -6.00D lens if placed directly on the eye. That’s why we need this equation in order to convert a glasses prescription to contact lenses.

Do We Really Need An Equation Though?

I can already sense that you’re trying to find a way around doing calculations using this or any other equation. So don’t worry! I only brought up the equation in order to help explain the concept and the math behind doing these conversions.

The much easier way is to refer to a chart which already has the results of the equation listed for every single possible lens power. These charts have several different names, but I tend to go with vertex conversion chart or vertex distance conversion chart.

Vertex distance conversion charts are easy to find online, but I’ve made my own just for you! If you find this chart useful you can download & print it for your office.

How To Use A Vertex Distance Conversion Chart

Using a vertex distance conversion chart is simple.

• The ‘Glasses Lens Power’ column refers to the power on the glasses prescription.
• If the power on the glasses prescription is negative, then the converted power is directly to the left.
• If the power on the glasses prescription is positive, then the converted power is directly to the right.

Let’s do a quick example to demonstrate this.

Say you have this glasses prescription and want to convert it to contact lenses:

You could use the formula:

Perceived lens power = -6.25 / {1 – [(0.014)(-6.25)]} = -5.75

Or, we can simply refer to the vertex distance conversion chart:

Find the number on the glasses prescription in the middle column. If that number is negative, the converted number is the one on the left as is the case in this example. If number on the glasses prescription was positive, the converted number would be on the right.

This is showing that a -6.25D lens placed at 14mm from the eye would only seem like a -5.75D if it was place directly on the eye’s surface. Recall that a prescription for glasses is measured with lenses at 14mm from the eye, not at the eye’s surface. So if -6.25D was given in contact lenses, it would actually feel like -6.75D at 14mm which would be too strong!

This may feel a little confusing but the minutia of the exact reason and mechanism by which this happens is not overly important to practitioners like yourselves.

What About Low Powers?

You probably noticed that the vertex distance conversion chart above only starts at glasses prescriptions of +/-4.00D. What about zero to +/-4.00D? Good observation!

The nature of the equation which populates vertex distance conversion charts is such that for small powers, the converted power is not significantly different than the original power. Remember that glasses and contact lenses are only available in steps of 0.25. So if the vertex distance conversion formula yields a difference of less than 0.25 than it is of no real significance.

So what happens to glasses prescriptions between zero and +/-4.00D? By and large, for these values the glasses and contact lens power will be the same.

For powers between 0 and +/- 4.00D, glasses and contact lens prescriptions are the same. 

The Exception: ASTIGMATISM!

If the sphere and the cylinder numbers combined add up to more than -4.00, a conversion is necessary. To learn how to convert a glasses prescription to contact lenses that has astigmatism, click right here!

If you don’t have time or learn the full the process, you can simply look up the conversion in my Astigmatism Vertex Distance Conversion Chart.

How Accurate Is This?

Using a vertex distance conversion chart to convert a glasses prescription to contact lenses is just the first of many steps involved in getting to the final contact lens prescription. Remember, it’s just a mathematical formula spitting out numbers. Every eye is different and every contact lens brand is different. Sometimes the vertex distance conversion chart will give the best result for your patient or client, and sometimes it will have to be modified. It’s the job of a good contact lens practitioner to verify and determine the best power for your patient or client.

How Can You Confirm The Results?

Once you’ve used a vertex distance conversion chart to convert a glasses prescription to contact lenses, the following things should be done to confirm the results.

1. Pull out some trial contact lenses in those powers and have your patient or client try them on.
2. Give the contact lenses a couple of minutes to settle in.
3. Measure the visual acuity of each eye individually.
4. Perform an over-refraction on eye each using loose trial lenses or a phoropter.
5. Use the results of the over-refraction to pull new trial contact lenses and repeat the process.

The process ends when the refraction over the contact lenses is zero for each eye.

Summary

So far we’ve covered how to convert a glasses prescription to contact lenses for a few different scenarios.

Briefy, they are:

• For glasses power between zero and +/- 4.00D –> No conversion needed.
• For glasses power over +/- 4.00D –> Use a vertex distance conversion chart.
• When using a vertex distance conversion chart it’s important to confirm the results with an over-refraction.

But major element are what are we still missing? That’s right. Astigmatism.

What you’ve learned here will work just fine so long as there’re no astigmatism present in the glasses prescription. To learn how to deal with astigmatism when converting a glasses lens prescription to contact lenses, we’ll need a whole separate article.

==>Click here to read ‘How To Convert A Glasses Prescription To Contact Lenses: Astigmatism'<==