One of the most common questions I have been asked is: What is astigmatism???
Historically, people have been told their eyes have been shaped like either soccer balls or footballs, but that really doesn’t describe how astigmatism works.
Astigmatism, when it comes to the optics of the eyes, simply refers to a difference in curvature. A difference in curvature can be found within the cornea, or the lens. Your prescription represents the amount of NET astigmatism; it is the sum of the corneal and lens astigmatism.
With optics, the curvier a surface is the more effect it has on bending light as it passes thru. For a structure that has the same curvature in all angles, light is bent the same regardless of the angle, and therefore all angles have the same focal point. When there is a difference in curvature, some of the light rays focus at a different point because of the difference in curvature.
Astigmatism can either be regular or irregular. Regular astigmatism is where the main difference of curvature is located 90 degrees away from each other. Irregular astigmatism would then be any other pattern.
Concerning regular astigmatism, it is not just the amount of astigmatism that effects vision, but also the angle. The more curvy angle can either be horizontal, vertical, or at some oblique angle. Concerning direction, horizontal is easier to focus thru, then vertical, and with greatest impact, oblique. Prescription lenses and contact lenses are designed to complement these angles, and to redirect all light rays to one point of focus for maximum clarity.
Astigmatism can change over time.
Total Astigmatism = Corneal + Lenticular
Over time, the lens within the eyes grows with more layers added on, and this growth can change its optics. Also, this growth increases the density of the lens which reduces its flexibility resulting in a reduction in the eyes’ ability to focus.
Some things that can change the shape and alter the astigmatism of the cornea include:
– Nearby growths (pinguecula or pterygium from UV exposure)
– Corneal conditions (keratoconus or other forms of corneal degeneration)
Astigmatism itself is not disease, but eye diseases can effect astigmatism. It is important for your eyes to be regularly examined to evaluate the optics and health.