If you have been using prescription glasses for a long time and you have decided to start using contact lenses more often, or you are wondering if it is better glasses or contact lenses, you may realize that the graduation noted in the recipes of glasses and contact lenses is not the same. This is called dystometry, and quiet, because it happens a lot, especially when you have a very high graduation.
Why is this? Has your graduation changed? Is it an optometrist’s mistake?
None of these are the reason why the prescription of glasses and contact lenses do not match. If you are curious, read on and discover what dysmetry is and why your contact lenses and glasses have different prescriptions.
Why doesn’t the prescription of my glasses match those of my contact lenses?
We are going to propose a small experiment so you can check what happens with the lenses of glasses and contact lenses. You just have to take a magnifying glass and test the distance and position in which this magnifying glass should be placed so you can read more clearly. You will see that at a certain distance and in a position perpendicular to the direction of your gaze, the vision is much sharper.
The same goes for glasses and contact lenses. If you place the glasses farther or closer to the eye, your vision will be altered.
As the glasses are resting on the nose, a small distance is created between the lens and the cornea of your eye. This distance must be taken into account and must be covered by the lens to ensure a complete correction of your diopters. On the other hand, the contact lenses rest directly on your eye, so it is not necessary to have that extra distance when graduating the lens.
It is for this reason that the prescription of your contact lenses and glasses is different. Of course, this difference is much more noticeable when a higher diopter correction is required.
What is Dystometry?
Distometry is the calculation that is made to adapt the prescription of the lenses of glasses to contact lenses. Normally it is done in graduations greater than 4 diopters since in the lower graduations the difference is not so noticeable.
In order to calculate dystometry, it is also necessary to know the distance between the eye and the inner face of an ophthalmic lens.
This distance is called vertex distance and is expressed in millimeters. The standard size is between 12mm and 14mm. If you are curious to know the formula used to calculate dystometry, here it is:
Power (Contact Lenses) = Glasses Power / 1 – (Glasses Power * Distance to Vertex)
Now you know. When your optometrist marks in your contact lens recipe a different graduation than your glasses, he is not making a mistake nor does it mean that your diopters have changed. Simply the calculation of the dysmetry has been done so that you can enjoy perfect vision with your new contact lenses.
Go to your trusted optical center and let your optometrist check your vision periodically. If you have any questions or want to know the equivalence table of glasses and contact lenses, they can tell you what your specific diopters are at all times.