A refraction eye exam is a standard procedure that determines the precise lens power needed to correct a person’s vision. This test measures how light bends, or refracts, as it passes through the eye to focus on the retina. Its goal is to diagnose and quantify a refractive error, the optical irregularity that causes blurry vision. The result is the final prescription for corrective lenses, whether for glasses or contact lenses.
The Core Concept: How the Eye Focuses Light
The process of seeing begins when light rays enter the eye and are bent (refracted) by the cornea and the lens to land perfectly on the retina. The retina is the light-sensitive tissue that converts light into signals for the brain to interpret as images. If the shape of the eyeball or the curvature of the cornea or lens is imperfect, light focuses either in front of or behind the retina, causing a blurry image. These focusing imperfections are known as refractive errors.
Four main types of refractive errors are diagnosed during a refraction exam. Myopia (nearsightedness) occurs because the eyeball is too long or the cornea is too curved, causing light to focus in front of the retina. This results in clear near vision but blurry distance vision.
Conversely, hyperopia (farsightedness) happens when the eyeball is too short, causing light to focus behind the retina, which often makes close-up objects appear blurry. Astigmatism occurs when the cornea or lens is shaped like a football instead of a sphere, causing light to scatter and focus at multiple points. This irregular focusing leads to distorted or blurred vision at all distances.
Presbyopia is an age-related loss of focusing ability, typically starting around age 40, where the lens hardens and loses its flexibility to change shape for near tasks. The refraction exam measures the exact degree of these errors to determine the proper correction.
Steps in Determining Your Prescription
The refraction process is divided into two main phases: objective and subjective testing.
Objective Refraction
Objective refraction provides the eye care professional with an estimate of the refractive error without needing input from the patient. This is especially useful for young children or patients who have difficulty communicating their vision clearly. The most common tool for this is the autorefractor, a computerized instrument that measures the reflection of light off the retina to calculate a preliminary prescription.
Another objective method is retinoscopy, where the clinician shines a light into the eye and observes the movement of the light reflex from the retina. By placing lenses of different powers in front of the eye, the clinician neutralizes the movement of the light and determines the eye’s refractive state. This objective measurement establishes a reliable starting point for the final subjective phase.
Subjective Refraction
The second phase is subjective refraction, which relies on the patient’s feedback to finalize the prescription. This is performed using a phoropter, a large instrument that holds various lenses the patient looks through. The patient views a standardized eye chart, usually positioned 20 feet away. The clinician presents a series of lens choices, often asking, “Which looks better: one or two?”
This “one or two” technique presents minor, incremental changes in lens power. The patient judges which option provides clearer and sharper vision. The doctor systematically refines the sphere, cylinder, and axis powers for each eye until the patient reports the best visual acuity. This subjective refinement is considered the gold standard because it factors in the patient’s unique visual perception, ensuring the final prescription is optimally accepted.
Decoding Your Refraction Results
The final output of the refraction exam is a prescription containing three main values that quantify the correction needed for each eye.
Sphere (SPH)
Sphere (SPH) indicates the basic lens power required to correct nearsightedness or farsightedness. This value is measured in diopters. A minus sign signifies correction for myopia, while a plus sign indicates correction for hyperopia. A higher number, regardless of the sign, means a stronger correction is required.
Cylinder (CYL) and Axis
Cylinder (CYL) and Axis values are only present if astigmatism is detected. The Cylinder value specifies the lens power needed to correct the astigmatism, accounting for the eye’s irregular shape. This number is also measured in diopters and shows the magnitude of the error.
The Axis is a number from 1 to 180 degrees, indicating the precise orientation at which the cylinder correction must be placed in the lens. The Axis ensures the corrective lens is aligned perfectly to neutralize the specific irregularity caused by astigmatism. Pupillary Distance (PD), the distance between the centers of the pupils, may also be included to ensure the lenses are centered correctly in the frame.