Have you ever been to the eye doctor? (Hopefully, yes). If so, have you ever wondered how optometrists determine new prescriptions? Then you’ve come to the right place! According to Doctor Duyen Tran, an optometrist at Warby Parker and several other offices, and graduate of the Massachusetts College of Pharmacy and Health Sciences, the first step to determining new prescriptions is by checking patients’ vision with their glasses (if they come in with glasses). “We use a device called a lensometer to scan the prescription of the glasses and see patients’ current prescription,” Dr. Tran states. 

After checking the current prescription, Dr. Tran typically uses an instrument called an autorefractor, which measures how light changes when it enters the eye. The autorefractor “gives an estimation of the patient’s prescription and front curvature of their eyes,” Dr. Tran says. Then, a test called a manifest refraction is performed. It’s a black and gray colored device that sits against the patient’s face while the optometrist switches through different lenses and asks whether lens 1 or lens 2 is better. Manifest refraction is subjective refraction because “the optometrist adjusts the patient’s prescription based on their responses until they can see clearly and comfortably.” Dr. Tran will then finalize the prescription based on what’s shown on the autorefractor. 

According to Dr. Tran, one common error recent graduate doctors make is that they tend to over-minus a nearsighted patient, which means making a prescription more negative than needed. The more negative the numbers on a prescription, the worse your eyesight is and the stronger your prescription needs to be to correct it. “When performing vision tests and giving patients a choice between two lenses, young kids especially will tell you the more negative option is clearer,” Dr. Tran explains. For example, a patient could see fine with -1.00 diopter–the unit of measurement used for an eye prescription–but often, they will say -1.25 diopters is clearer, and thus optometrists can potentially over-minus patients. “It’s not wrong but if you give them a stronger prescription, you’re going to over accommodate.” Like with anything you want to be good at, Dr. Tran says a lot of practice with a refractor is needed to determine the best prescriptions for patients. Hopefully, understanding the process of getting a new prescription will allow patients to be more comfortable in clinical settings and overcome any doctor-visit anxiety they may have.