Ever wondered what your contact lens prescription actually means? Us too. But fortunately, we’re here to help. Dr. Edwin Wallington, OD, breaks down what a contact lens prescription means and how to understand your own contact lens prescription.
First things first, it can help to understand the function of contacts in your eyes. Before light enters the eye, it passes through the tear film. This tear layer plays the role of keeping the cornea moist and healthy.
Next, light passes through the clear cornea, which is that clear dome over the iris (the color of the eye) where you place your contact lens. A contact lens has to fit nicely onto the cornea while still allowing the tear layer to still perform its job. This is all part of deciding your valid contact lens prescription.
The first part of a contact lense prescription is the base curve. Base curve is an indicator of the steepness of the contact lens curve. Imagine how a sphere bubble with a radius of 8.8 mm has a flatter outside curve than a sphere bubble with an 8.4 mm radius. Well, that is exactly what the base curve is. A contact lens base curve has an approximate value around 8.5 (give or take). It is a metric fitting curve based on the shape of your cornea. If your cornea is flatter, your doctor will reach for a higher numbered base curve, because it is flatter too.
Next up is diameter. An appropriately fit base curve has to be just a little loose so the cornea isn’t squeezed too tight. The contact lens diameter helps stabilize this soft fit to allow the contact lens to sit comfortably on the dome of the cornea and allow for tear movement.
Since a typical cornea is approximately 12 mm wide, a soft contact lens diameter will be around 14 mm so that there is about 1 mm of contact wiggle room around the entire edge of the cornea.
The style of the contact lens consists of individual materials, water content, and oxygen transmissibility along with other agents and curves to enhance the design. The style is what usually “makes or breaks” a contact lens fit.
Base curve is objectively determined, diameter is almost always predetermined, but style is subjective. Since different people have different natures to their eyes, different styles tend to be tolerated differently as well.
What Your Contact Lens Prescription Actually Means
OK, but what does all this actually mean? It might help to picture how your eyes work. As we mentioned earlier, light first goes through the tear film, then through the cornea. Beyond the cornea, light passes through the pupil and on through the lens of the eye. Your natural eye lens sits immediately behind the pupil and is made of elastic proteins that can change the lens shape when your internal eye muscles, called ciliary muscles, flex. This lens is important for focusing. If we are corrected for seeing far away, our ciliary muscles are relaxed and our lens is essentially flat.
If we want to see up close, our ciliary muscles flex and our lens turns rounder to add focus. The problem with this lens is that it loses its elastic properties between 40 years old and 53 years old. We do not lose the strength of our ciliary muscles; instead we lose the flexibility of this lens. Everybody in the world goes through this process. It’s called presbyopia.
Choose Your Power
If we have our correct prescription for far away, but we lose our lens ability to add focus, then we can get our focus in another form added to the lens power, which we call the ADD POWER. If we need a low amount of help, then we ask for a LOW ADD. If we need a lot of help, we may ask for a HIGH ADD. And, you got the idea, if we are somewhere in between, we take a MID ADD.
Add Some Direction
Once light passes through the lens, it needs to focus at a single point in the back of the eye called the fovea. The fovea is about as small as the tip of a pin and no thicker than saran wrap, but it contains over 5 million light-responsive cells that are responsible for your important detailed vision.
Your eye doctor’s job is to get that light that entered into your eye to focus directly on your fovea. If it already focuses there, then you do not need correction. If it doesn’t, then they have to move the focal point a certain distance. If light would focus beyond the fovea, say 5.25mm, then we would have to move that point forward 5.25 mm, right? Well, when we move a point forward, we refer to that as the “+” direction and would call that +5.25mm.
When we reference light, however, we measure light in diopters, and would call that distance +5.25 sphere diopters. And that is where the term “sphere” comes from. Sphere Power would be +5.25 sphere diopters. People who wear plus spheres are farsighted.
On the other hand, if we had to move the focal point backward, say 3.75 diopters, then we would reference that as sphere power of -3.75 sphere diopters. People who wear minus spheres are nearsighted. If a person had two focal points in the back of one eye, such as one point -1.00 diopters away from the fovea and the other point at -2.50 diopters from the fovea, then we would call that astigmatism.
In this example, the first point is at -1.00 (called the sphere power) and the second point is at -2.50. However, we would write the prescription like this: -1.00-1.50 X 180.
The -1.00 is obvious, but notice that the -1.50 represents how far -1.00 is from -2.50. The -1.50 (called the cylinder power) is the gap between the two points that we call astigmatism. The 180 is called the AXIS and it explains how the two powers are supposed to be oriented in the contact lens. This example says that the -1.00 spherical power will line up in the 180 meridian, while the -2.50 cylinder will line up on the 180 axis.
Take notice that the majority of your prescription for contact lens is simply based on math and measurements.
Your eye doctor has to find those specific measurements with their equipment and uses their knowledge, understanding, and their experience in their field to find your perfect contact lens prescription. And don’t forget, if you have any questions about your contact lens prescription, don’t hesitate to ask your eye doctor.