Visual Acuity and Refraction

What is Visual Acuity and Refraction?

In simple terms, this is how your vision is measured. Your visual acuity is the line you read on the eye chart. This number is represented as a fraction. Standard visual acuity charts range from 20/20 to 20/400. Though 20/20 is considered normal, there are a small number of individuals who may even see 20/15 or 20/10, which is better than 20/20. However, there are other factors that contribute to your overall vision, which means you can have 20/20 vision and still have visual complaints. For example, you may have glare or halos from oncoming light sources, but be able to read the 20/20 line on the eye chart. So, functional vision is comprised of more than just the line you can read on the eye chart.

Best-corrected vision is a term used to describe vision that is corrected as much as it can be with glasses or contact lenses. In other words, your best-corrected vision is the best vision you can achieve with glasses or contact lenses. This can be either best-corrected distance vision or best-corrected near vision−they are not always the same. This is different from your uncorrected vision, which refers to what you see without your glasses or contact lenses.

Your refraction is the measurement taken when you are evaluated for glasses, and it is how your best-corrected vision is determined. This measurement tells your “prescription” for glasses and is written in “diopters.” This measurement can then be converted into a contact lens prescription. Your glasses and contact lens prescriptions are usually slightly different due to the different position in which each of these lenses sits relative to the surface of your eye.

In summary, there are four main types of refractive errors that can affect the eye and our ability to focus clearly, which can all be corrected with glasses or contact lenses. In the past, myopia and hyperopia were able to be corrected with intraocular lenses (IOLs) after cataract surgery, but presbyopia and astigmatism were not able to be corrected in this manner. Due to advances in intraocular lens technology, each of these refractive errors can now be corrected with intraocular lenses. They require slightly different types of intraocular lenses, and not every patient is a candidate to have a presbyopia- or astigmatism-correcting IOL. It is important for every patient considering cataract surgery to discuss these options with their operating surgeon. Every eye is different; the most important thing is to determine which lens will best meet your needs and is also appropriate for use in your eye.

© Vision Information Services, LLC, Mooresville, NC 2012