A refraction eye exam is a common part of a comprehensive eye evaluation, focusing on how light interacts with your eye. This assessment helps eye care professionals determine the precise optical power needed for corrective lenses, such as eyeglasses or contact lenses. It helps identify if your eyes are focusing light correctly, contributing to clear sight.
Understanding Refraction
Vision begins when light enters the eye, passing through structures that bend, or refract, these light rays. The cornea, the clear, dome-shaped front surface, performs the initial and most significant bending of light due to its curvature and the change in refractive index as light moves from air into the eye.
Following the cornea, light travels through the pupil and then encounters the crystalline lens, located just behind the iris. The lens adjusts its shape, a process called accommodation, to fine-tune the focus of light rays. This dynamic adjustment allows the eye to bring both distant and near objects into sharp focus on the retina.
The retina, a light-sensitive tissue lining the back of the eye, acts like the film in a camera. When light rays are accurately focused onto the retina, photoreceptor cells convert these light signals into electrical impulses. These impulses are then transmitted through the optic nerve to the brain, which interprets them as the images we perceive.
Purpose of the Refraction Exam
A refraction exam measures the eye’s refractive state to identify errors where light isn’t bent correctly, leading to blurred vision. The exam determines the optical prescription required for corrective lenses. This includes quantifying conditions such as myopia (nearsightedness), where distant objects appear blurry, and hyperopia (farsightedness), which affects near vision. Astigmatism, characterized by an irregularly curved cornea or lens causing distorted vision, is also identified and measured.
For individuals over age 40, the exam can also assess presbyopia, an age-related condition where the eye’s natural lens loses flexibility, making it difficult to focus on close-up objects. By accurately measuring these refractive errors, the eye care professional can prescribe lenses that help light focus correctly, aiming to achieve the clearest possible vision.
How a Refraction Exam is Performed
The refraction eye exam typically involves a combination of objective and subjective measurements to determine the most accurate lens prescription. The process often begins with an autorefractor, a computer-controlled machine that provides an initial, objective estimate of your refractive error. You sit with your chin on a rest and look into the device, which projects light into your eye and measures how it reflects off the retina.
After the objective measurement, the eye care professional refines the prescription using a phoropter, an instrument that contains various lenses of different strengths. During this subjective part of the exam, you look through the phoropter at an eye chart, typically positioned 20 feet away. The doctor will switch between different lenses, asking you to compare which option provides clearer vision, often phrasing it as “Which looks better, one or two?”.
This interactive process allows for fine-tuning the lens power based on your direct feedback. The eye care professional systematically adjusts spherical, cylindrical, and axis components until the sharpest possible vision is achieved for each eye individually.
Decoding Your Eye Prescription
After a refraction exam, you receive a prescription that contains several abbreviations and numbers. You will typically see “OD” and “OS,” which are Latin abbreviations for your right eye (oculus dexter) and left eye (oculus sinister), respectively. Sometimes, “OU” may be used for both eyes (oculus uterque).
The “SPH” or “Sphere” value indicates the main lens power, measured in diopters, needed to correct nearsightedness or farsightedness. A minus sign (-) before the number signifies nearsightedness, meaning distant objects appear blurry, while a plus sign (+) or no sign indicates farsightedness, where near objects are less clear. The further the number is from zero, the stronger the correction required.
If you have astigmatism, your prescription will also include “CYL” (Cylinder) and “Axis” values. The CYL number specifies the amount of lens power needed to correct the irregular curvature of the cornea or lens, which causes distorted vision. The Axis, a number between 1 and 180 degrees, indicates the orientation or angle at which the cylindrical correction should be placed on the lens.
For multifocal lenses, such as bifocals or progressive lenses, an “ADD” value may be present. This “Add” power represents the additional magnifying power, typically a positive number between +0.75 and +3.00 diopters, needed for reading and other close-up tasks. This addition is usually the same for both eyes and addresses presbyopia.