What is a Vertex Conversion Chart?
Vertex conversion charts are helpful tools that eye doctors, optician, para-optometric technicians, etc, use as part of the process of determining what power to use for your contact lenses.
Here is a conventional vertex conversion chart:
First, the prescription for the glasses is determined, then the numbers on the glasses prescription are ‘back vertexed’ using a vertex conversion chart to turn them into numbers are more appropriate for contact lenses.
If you’re not familiar with why numbers would be different for glasses and contact lenses, I suggest reading this article.
Limitation of Vertex Conversion Charts
The way that vertex conversion charts are made, they only allow you to convert one power at a time. And as we all know, to correct vision perfectly, sometimes glasses require 2 different powers called the Sphere and the Cylinder.
Using a conventional vertex conversion chart to convert both the Sphere and the Cylinder is complicated due to the way they are written out on prescriptions. Without going into the long explanation for it, converting a complicated prescription with Sphere and Cylinder usually requires using a vertex conversion chart in conjunction with drawing a power cross diagram, which can be a little time consuming.
I thought about what it would take to create vertex conversion charts for both Sphere and Cylinder powers that would prevent one from needing to do the power cross diagram. It turns out that all it took was a little bit of patience and a photo editor.
On this page, you will find a link to the charts I created which apply the back vertex formula to both Sphere and Cylinder powers at the same time. Lots of combinations are possible here, so it was necessary to create multiple charts and organize them by Sphere power.
Please note that these chart are for reference and informational purposes only. They are not meant to determine anything more than the result of the back vertex formula. A prescription for contact lenses requires many more steps, including a physical examination of the contact lenses on your eyes, and can only be issued by a licensed optometrist or optician.
Why Make These Charts?
When I was a para-optometric assistant doing back vertex calculations for the optometrists I worked for, I had to deal with a lot of calculations and diagrams that were pretty time-consuming. For simple prescriptions, I could do the math in my head, but when prescriptions got very high and complicated, I had to pull out my paper and pen and start drawing diagrams and adding and subtracting all sorts of numbers in order to arrive at the proper conversion. In retrospect, it would have been a lot easier to have these charts to refer to.
So I made them for everyone’s benefit.
The charts are organized by Sphere power. Every chart will show the conversion for the specified Sphere power and all of the Cylinders between 0 and -3.25 in increments of -0.25. Charts are available for Sphere powers between 0 and -7.75.
Here is a sample conversion chart for Sphere powers of -7.00.
I have arranged all the charts in a table below. If you’re a optometrist, optician, or a student and find these charts helpful, feel free to bookmark this page. Alternatively if you’d like a .pdf version for easy access in your clinic, leave a comment below!