How To Convert A Glasses Prescription To Contact Lenses
In order to begin the contact lens fitting process, you must determine the power of contact lenses required for your patient. Once you have determined who can wear contact lenses and who cannot, you may proceed. In order to make this conversion, it is necessary to know how to convert glasses prescriptions into contact lenses.
How Do Contact Lens Prescriptions Differ From Glasses Prescriptions?
Okay, let me explain.
How Do They Differ?
A contact lens sits directly on the eye, whereas a pair of glasses holds lenses up in front of the eyes. By recognizing this key difference, you are able to understand why prescriptions for contact lenses and glasses may differ.
What is the importance of the distance between the lens and the eye?
There are several reasons why this is important:
Based on this equation, you can see that the perceived power of lenses changes as you alter the distance between the lens and your eye.
CHANGING THE DISTANCE BETWEEN THE LENS AND THE EYE CHANGES THE PERCEIVED POWER OF LENSES.”
The vertex distance between a corrective lens and the eye is the distance between the corrective lens and the eye. As we know, contact lenses are held at a different distance from the eye than glasses.
It is usually between 12 and 15 millimeters for glasses to have a vertex distance
The vertex distance for contact lenses is always zero
A -6.00D lens, for example, placed directly on the eye will not feel like a -6.00D lens if held 13mm away from it. Thus, we require this equation in order to convert a glasses prescription to a contact lens prescription.
But do we really need an equation?
I am able to tell that you are trying to circumvent doing calculations using this equation, but don’t worry! The purpose of bringing up the equation was to provide a straightforward explanation of how the calculations are done.
This is more convenient if you refer to a chart that provides results for every possible lens power. Many different names have been given to these charts, but I often refer to them as vertex conversion charts or vertex distance conversion charts.
An explanation of how to use a vertex distance conversion chart
Vertex distance conversion charts are easy to use.
There is a column titled ‘Glasses Lens Power’ which refers to the prescription power of the glasses.
An optical prescription with a negative power would indicate that the converted power would be directly to the left.
The converted power is directly to the right if the power on the glasses prescription is positive.
To demonstrate this, let’s look at a quick example.
Suppose you have a prescription for glasses that you would like to convert to a contact lens prescription:
The formula is as follows:
The perceived lens power is calculated as -6.25 / [1 – [(0.014)(-6.25)]} = -5.75
Alternatively, we can refer to the vertex distance conversion chart as follows:
This example demonstrates how to convert the prescription number to the converted number. If the prescription number is negative, you would convert to the left, while if it is positive, you would convert to the right.
Hence, a lens with a -6.25D rating placed directly on the eye surface would only appear -5.75D if it was placed directly on the eye. Recall that glasses prescriptions are measured with lenses placed at 14mm from the eye, not directly on the eye’s surface. In this case, -6.25D would actually feel as -6.75D at 14mm, which is too much force to handle.
Although this may seem somewhat confusing, practitioners like you are not overly concerned with the precise reason and mechanism as to why this occurs.
What About Low
I would like to draw your attention to the fact that the vertex distance conversion chart above only begins at a eyeglass prescription of +/-4.00D. What about a prescription of zero to +/-4.00D? Interesting point!
For small powers, the equation used to populate vertex distance conversion charts is such that the converted power does not differ significantly from the original power. The vertex distance conversion formula will yield no significant difference if the difference is less than 0.25 because glasses and contact lenses are only available in 0.25 steps.
It is generally true that the glasses and contact lens power will be the same for glasses prescriptions between 0 and +/-4.00D.
THE CONTACT LENS PRESCRIPTIONS FOR POWERS BETWEEN 0 AND +/- 4.00D ARE THE SAME AS FOR POWERS BELOW 0 AND +/- 4.00D.
There is one exception: astrogmatism.
A conversion is required if the sum of the sphere and cylinder numbers exceeds -4.00.
What is the accuracy of this information?
It is only one of many steps involved in converting a prescription for glasses to a contact lens. Using a vertex distance conversion chart to convert a prescription for glasses to contact lenses is only the first step. Remember, it is only a mathematical formula that produces numbers. Every eye is unique, and every contact lens brand is unique. The vertex distance conversion chart may give you or your client the best result, but it may also have to be modified from time to time. Contact lens practitioners are responsible for verifying and determining the optimal power for their patients or clients.
Is there a way to confirm the results?
After you have converted a glasses prescription to contact lenses using a vertex distance conversion chart, you should confirm your results by doing the following.
Provide your patient or client with a pair of trial contact lenses in those powers.
Allow the contact lenses to settle in for a few minutes.
Each eye should be measured individually for visual acuity.
Test the over-refraction of each eye with a phoropter or loose trial lenses.
The over-refraction results may be used to pull new trial contact lenses and repeat the procedure.
Each eye’s refraction over the contact lenses must be zero for the process to be complete.
We have covered a few different scenarios in which glasses prescriptions can be converted to contact lenses so far.