By Linda Conlin, Pro to Pro Managing Editor
So many patients do it. They try on their new glasses or contact lenses, look around for a second or two, then cover one eye at a time and look around again. I’m always tempted to remind them that as soon as they leave, they’ll be using both eyes, not one at a time. I was reminded of this phenomenon when I read an entry in the Metropolitan Diary section of the New York Times. The section gives glimpses into scenarios that are uniquely ‘New York.’ In this scenario at a DMV, a man is asked to read an eye chart, and covers one eye. The clerk responded, “What are you going to do, drive with one eye? Read with both of them.”
Of course, eyes are tested one at a time during part of the exam, and with good reason. An Rx difference between the two eyes is the norm. In fact, 73% of the prescription population has a total power difference, sphere plus cylinder value, between the two eyes, of a quarter of a diopter or more. When the Rx has two diopters or more of difference, it is called anisometropia. As a result, image sizes between the two eyes are different, making it difficult for the brain to fuse them. This can result in the brain suppressing the image from the eye with the higher prescription, leading to amblyopia.
The differences in power between the spectacle lenses needed to correct anisometropia will also affect binocularity due to different prismatic effects in each eye as the eye moves from the lens’ optical center. Contact lenses eliminate this problem because each eye is always looking through the optical center of the lens, but what about spectacles? The difference in vertical prismatic effect is especially true in progressive lenses where the mid-range and reading portions are designed to occur at specified positions.
What’s the solution? The vertical prismatic difference can be controlled, to a certain extent, by adjusting the progressive power distribution for each eye individually, based on the known power value for each eye. Reducing this difference to the minimum possible ensures more comfort. This requires first, precise monocular measurements. Second, you may have noticed that labs may not make only one lens in certain designs. This allows for individual design compensation using prism to minimize vertical imbalance for better binocular vision.
As lens designs continue to improve, we will see continued innovation in binocular vision correction. Remember that we use two eyes, and learn more about equal accommodative support for anisometropia with our CE, Binocular Harmonization Technology, at 2020mag.com/ce.