Something we get asked about a lot at the Cataract & Refractive Institute of Florida is refraction. Refraction is the term used for the method by which we determine your glass prescription.
The instrument in the photo above is called a phoropter. It has lenses inside that allow us to offer a patient an almost unlimited amount of choices regarding glass prescription or what glass they think they see the best with.
The phoropter contains spherical lenses. They’re just round, normal lenses like a magnifying lens. These spherical lenses are used to correct the vision. So we’re gonna ask a patient, “Is this lens better than the first glass or the second one?” Then we change the glass, giving you an option of which glass you see better with.
In a round eye or an eye without astigmatism, the cornea is totally round, like a basketball. When light goes through it, all the light points are focused on one point. People with astigmatism have eyes shaped more like a football than a basketball.
If your eyes are shaped like a football, it’s steeper on one side and flatter on the other. When light goes through, it doesn’t get bent at the same amount; therefore, that light is focused sort of a line inside the eye. When people have astigmatism and they look at a light at night without glasses, they see a streak coming out of the light because that’s how the light is being focused in their eye.
Other lenses in the phoropter have astigmatism. We have a Jackson cross cylinder with astigmatism lenses at exactly 90 degrees opposite each other. Then we ask people which glass is better so we know the axis; that is, how many degrees on the axis do they like the astigmatism to be located. Then we flip the cross cylinder in the other direction and start asking again how strong of an astigmatism lens they want in that number of degrees; that is, 180 degrees or whatever the amount of degrees is. Then we come back from fine-tuning the cylinder, and that’s how you choose a glass prescription.
There is obviously some expertise on the part of the person doing the refraction to give you options, but still, as a patient, you’re the one who’s choosing that lens. It’s sort of a cooperative effort when trying to get the right glass prescription for a patient.
Refraction is very important as it does give medical information. Anytime we decide on someone’s eye health, a major factor is their best correctable vision. We need to know what that number is, whether 20/20 or another because all decisions depend on the best correctable vision.
If you come in and your vision’s 20/20 the year before or two years before, and now you come in, and it’s 20/25 or 20/30 or a little worse, Dr. Croley would want to know if that’s you developing astigmatism, becoming a little nearsighted or farsighted, or suffering from an eye disease.
It’s important for us to know what someone’s best correctable vision is because if you’re not correctable to 20/20 anymore, then Dr. Croley has to find out why. Sometimes it’s not always apparent in an eye exam, but if there is a vision problem and we can’t correct you to your 20/20 vision, we have to understand why you don’t see 20/20.
If you have any questions about refraction, please feel free to contact us online. If not, may God bless you with healthy eyes and great vision.
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At the Cataract & Refractive Institute of Florida, our board-certified ophthalmologist, Dr. James E. Croley III, uses state-of-the-art tools and techniques to help his patients manage their eye conditions. Dr. Croley is a member of the following professional organizations:
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