Understanding Refractive Conditions
Think of your eyes is like a camera. Throughout this article, we will be comparing your eyes to the function of the camera and highlighting its similarities and differences. If you’re a professional photographer, you’ll carry many lenses to capture your photo in different ways. Lenses have different powers and specifications that you can interchange with your camera. When you initially fit your camera lens to its body, the image you look through the viewfinder of your camera is often blurry. You would need to either let the camera autofocus by turning a few knobs on the side; you can focus the image manually.
The reason why the image is blurry initially is that the image is focus either in front of the film or behind/beyond the film plane.
If the natural lens of our eye focuses the image in front of the retina (or film), this refractive condition is called short-sightedness or myopia. If it focuses behind the retina (or film), this refractive condition is called long-sightedness or hypermetropia.
Your eyes have only one lens. You can’t interchange the lenses of your own eyes as quickly as your camera. Correcting these refractive conditions, either longsightedness or short-sightedness can be done by finding the prescription that is required in your glasses to bring the images back into focus.
This is a gross simplification of these two refractive conditions of the eye. You might notice that the film (or the sensor in digital cameras) lies on a flat plane. Our eyes (eyeballs), the “film”, the retina is curved. This poses a few challenges which are beyond the scope of this article. But this fundamental difference between a camera and the human eye introduces another form of refractive error called astigmatism.
Understanding The Structure of Your Eyes
Unlike a camera, your eyes are round. The retina or the film at the back of the eye is curved. Trying to keep all parts of the image focussed on a curved surface is difficult. This challenge is also experienced by cartographers, where they have to represent geological features of the Earth in two dimensions.
All the structure of the eye work cohesively to overcome this problem, so the images we interpret is in its correct representation. However, not all eyeballs are entirely spherical. It’s ever so slightly elongated, making it aspheric. If the eyeball is so stretched out making it look like an egg, the images we see would be stretched and distorted; like looking at the reflection of the back of a silver spoon. It is the aspheric nature of the eyeball that causes the refractive error in which optometrists call it astigmatism.
I hope you can appreciate that no eyeballs on Earth are entirely 100% spherical. It is more of an ellipsoid as opposed to a sphere. As a result, your optometrist measures the amount of astigmatism routinely. Everyone would have a small amount of astigmatism. Whether this is included in your final prescription for your glasses is entirely based on the consultation between you and your optometrist. By not having a prescription that includes astigmatism doesn’t mean you don’t have astigmatism, it means the amount of it is so small, it is negligible as it has very minimal impact to your vision. Or, the manufacturer doesn’t make the lenses for such a minimum astigmatic prescription.
Astigmatic refractive errors are routinely measured and incorporated into your prescription lenses. This is done by making one area of your lens thicker than other parts to correct for the aspherical nature of your eyes. This amount is dictated by the “CYL” or cylindrical correction that is required on your prescription. The “axis” part of your prescription dictates the part of the lens that is necessary to be made thicker or thinner to give the required power for correcting your astigmatism.
It is not unusual to have between 0.25DC – 0.75DC amount of astigmatism for a normal eye. From time to time, your optometrist may neglect up to 0.50DC correction every time you check your prescription glasses as it is very minimal. Most contact lens (toric contact lenses) companies only stock lenses that correct for astigmatism from 0.75DC. From our experience, patients would their astigmatism starting to affect their vision from 0.75DC and above.
Do You Have Astigmatism?
Everyone has some residual astigmatism. However, there are certain eye diseases which give rise to a very high degree of astigmatic correction. Several corneal conditions such as keratoconus and pellucid marginal degeneration, change the curvature of the front surface of the eye. The progressive thinning of this corneal disorder changes the astigmatic correction that is required, sometimes in the order of over 5.00DC. In those cases, other forms of optical correction such as rigid gas permeable (RGP) contact lenses are required to “neutralise” the asphericity.
The Bottom Line
As our eyeballs are never 100% spherical, everyone is subject to some astigmatic correction. During your consultation with your optometrist, you both would determine whether the astigmatism is either negligible or worth correcting. There are several eye conditions that can lead to a high degree of astigmatism, like keratoconus and pellucid marginal degeneration. At Capital Eye, your local optometrist, we use the latest technology like our corneal topographer to screen these eye diseases. It is normal to see your prescription to incorporate a small degree of astigmatism like from 0.25DC to 0.75DC.
Having a small degree of astigmatism is normal. It’s like having a moisture reading in your bathroom. However, if the moisture reading in your bathroom is exceedingly high or your astigmatism is significant, then we need to rule out certain eye diseases which may contribute to the astigmatic correction of your new prescription glasses.