No Two Pairs of Eyes are the Same
July 23, 2019
Do you ever wonder what makes your vision or prescription different from someone else's?
It may sound simple, but one of the most rewarding parts to me of being an optometrist, even after doing this for years, is the simple ability to correct someone’s vision. Having worn glasses myself since I was in elementary school, I know exactly how debilitating it can be to have to deal with blurry vision. Correcting someone’s poor vision can have a huge impact on their quality of life. I sometimes think that it is downplayed just how important this aspect is for an optometrist to play a role in because so much attention is placed on the health aspects of the exam. All aspects of an eye exam are obviously important, but most of the questions I get from patients every day are about how I can help correct their vision and how exactly the corrective lenses work. I thought I would take some time to then discuss exactly why some people’s eyes see poorly in the first place, and what a pair of glasses and contact lenses does to correct this.
Each eye is unique!
In order to understand how corrective lenses work, a basic understanding of eye anatomy is first required. Light first contacts the cornea which is the clear front surface of the eye. The light rays are then bent, or refracted, as they travel through the pupil. Behind the pupil, the crystalline lens further refracts the light rays and sends them to the surface of the back of the eye called the retina. The goal of an ideal eye set up with perfect uncorrected optics is for light to be evenly refracted from all surfaces and land directly on the retina forming a perfectly focused image that can be transferred to the brain. Practically this often does not happen because each eye is unique, and anatomy fluctuates from person to person. An ideal cornea is perfectly spherical, and all light rays are refracted equally, but in reality many corneas have irregularities causing light rays from different parts of the cornea to be refracted by different amounts. This can cause some of the image to be in focus and some to not be in focus. Another factor that is very important is the overall length of the eyeball called the axial length. Any deviation from the ideal axial length will cause the light rays to either focus in front of or behind the retina. The combination of corneal irregularity and deviations from the ideal axial length combine to cause a blurry distorted image to be transferred from the retina to the brain where it is then interpreted as a blurry image in the real world. These differences from normal can be very minimal, but even very minor changes can cause huge amounts of image distortion.
So I think I am nearsighted…..or maybe farsighted?
In order to correct a blurry image being placed on the retina, the light entering the eye must be altered in such a way to counteract what the eye is doing naturally. This is the entire point of the refraction part of the eye exam. When I am asking “which is better, 1 or 2” each of those lenses are positioning the light in different ways in order to land it right onto the retina. The best way to think about that portion of the eye exam is actually as a measure of distance. It is the distance the light must be moved in order to land precisely on the retina. The corresponding glasses or contact lenses for that prescription must then have a lens that bends the light the correct amount in order to focus onto the retina. The three types of refractive error that exist are myopia, hyperopia and astigmatism. Some people have only one of these, some have several and some people have two completely different eyes with some combination of all three! When a person has poor distance vision, they are myopic or nearsighted. In this case the light focuses naturally in front of the retina causing a blurry image to be produced. The cause of this is often an elongated axial length, and there is nothing the eye can naturally do to correct this. Corrective lenses in this case cause the light to move back onto the retina and be properly focused. These are usually the patients that are amazed with new glasses. It is an immediate improvement and the results can be pretty impressive especially for first time glasses wearers. The opposite of myopia is hyperopia or farsightedness. In this case light is naturally refracted too far behind the retina. The eye actually has a fix for this though as the crystalline lens can force the light to be further bent onto the retina. Long periods of this strain on the crystalline lens can be tiring and cause eye fatigue especially with near work as the closer an object is to the eye, the harder the crystalline lens must work to keep focus. Hyperopic eyes often see clearly, but these patients can suffer from headaches and eyestrain from constantly over focusing. Corrective lenses in this case are more used to relax the eyes back to their ideal position where the strain can be alleviated. These patients are often not as impressed at first, but once they realize they do not have a headache every day at work they are thrilled! The third category of refractive error is astigmatism. This is not related to the axial length, but to the irregularity of the cornea. With astigmatism, certain areas of the cornea will be more or less curved than ideal, and the light from each part of the cornea will refract the light differently. This causes multiple images to simultaneously be viewed, and the resulting image is not only blurry, but also can have other symptoms of shadowing or glare. The goal of the astigmatism correction is then to take these multiple images and independently align them correctly with the retina causing a clear crisp image. The combination of all of these measurement adjustments results in the patient’s final prescription.
Understanding the optics of vision correction, and the ways in which a patient’s prescription must be written to address all their visual needs is honestly my favorite part of optometry. I always love the immediate impact I can have on a patient’s quality of life with a new prescription. The refractive portion of an exam is so important to the overall experience, that I want to make sure it is always performed correctly the first time. Once I am confident in the new prescription, it is then the job of our lab technicians to translate that into a perfect pair of glasses. Next month I will discuss the entire process the lab goes through in order to create those lenses and get you seeing your absolute best!