A refraction test is a common part of a comprehensive eye examination, serving to determine the precise lens prescription needed for clear vision. This assessment helps eye care professionals understand how light focuses within your eyes, identifying and measuring any refractive errors that might be blurring your sight.
What is a Refraction Test
A refraction test is a measurement of how light bends as it enters the eye and focuses on the retina. It involves both objective measurements, often using automated instruments, and a subjective assessment based on the patient’s responses. The test helps identify and quantify refractive errors, which are common vision problems where the eye does not correctly focus light, leading to blurry images. This process allows an eye doctor to determine the exact prescription required for eyeglasses or contact lenses. Ultimately, the test aims to achieve the clearest possible vision with corrective lenses.
How a Refraction Test is Performed
The refraction test begins with an objective measurement using an autorefractor. This computer-controlled machine provides an initial estimate of your prescription by measuring how light changes as it enters your eye. It uses infrared light to detect reflections from the retina, determining the eye’s focusing ability. This automated reading serves as a starting point for the eye care professional, especially for individuals who might have difficulty communicating, such as young children.
Following the objective measurement, the eye doctor performs a subjective refraction using a phoropter. This large device contains various lenses of different strengths, which are systematically placed in front of your eyes. You will look through the phoropter at an eye chart, with one eye occluded at a time. The eye doctor will then present different lens combinations, asking you to compare them and indicate “which is clearer, one or two?”. This interactive process allows for fine-tuning the prescription based on your feedback.
Common Refractive Errors Diagnosed
A refraction test helps identify several common vision conditions where the eye’s shape prevents light from focusing correctly on the retina, resulting in blurred vision. Myopia, commonly known as nearsightedness, causes distant objects to appear blurry while close objects remain clear. This occurs when the eyeball is too long, causing light to focus in front of the retina. Hyperopia, or farsightedness, results in nearby objects appearing blurry.
Astigmatism arises from an irregular curvature of the cornea or lens, leading to distorted or blurry vision at all distances. The uneven shape causes light to focus at multiple points on the retina rather than a single point. Presbyopia is an age-related condition that affects individuals over 40, making it difficult to focus on close-up objects. This occurs as the eye’s natural lens becomes less flexible with age, reducing its ability to change focus.
Understanding Your Prescription
An eyeglass prescription contains specific abbreviations and numbers that detail the necessary lens corrections for each eye. OD, short for “oculus dexter,” refers to the right eye, while OS, or “oculus sinister,” denotes the left eye. Sometimes, “OU” (oculus uterque) indicates that the prescription is the same for both eyes.
The “SPH” (sphere) value indicates the main lens power needed to correct nearsightedness or farsightedness, measured in diopters. A negative number signifies nearsightedness, while a positive number indicates farsightedness. The “CYL” (cylinder) value addresses astigmatism, indicating the amount of cylindrical power required to correct the irregular curvature. The “AXIS” specifies the orientation of the cylinder correction, given as a number between 0 and 180 degrees. Finally, “ADD” (addition) represents the extra magnifying power needed for reading or close-up tasks, a positive number.