Refraction in optometry refers to the process of measuring how light bends when it enters the eye. The eye’s optical system, consisting primarily of the cornea and the lens, must accurately focus light rays onto the retina, the light-sensitive tissue at the back of the eye. When this focusing power is imperfect, vision becomes blurred. This diagnostic procedure is central to comprehensive eye examinations and provides the necessary measurements to correct focusing errors with prescription lenses.
The Goal of Optometric Refraction
The purpose of the refraction test is to find the exact lens power required to correct any focusing deficiencies in the eye’s optical system. This process is designed to achieve the patient’s best corrected visual acuity (BCVA), which is the sharpest, clearest vision possible using corrective lenses. Achieving BCVA means light is focused precisely onto the retina. The final prescription is essentially a formula for lenses that will perfectly compensate for the eye’s natural focusing errors.
Common Refractive Errors Identified
Refraction identifies common focusing problems, known collectively as refractive errors, which prevent light from meeting precisely on the retina. Myopia, or nearsightedness, occurs when the eye focuses light in front of the retina, often because the eyeball is slightly too long or the cornea is too curved. This causes distant objects to appear blurry, while close-up vision typically remains clear. Hyperopia, or farsightedness, is the opposite condition, where light focuses theoretically behind the retina, often due to a short eyeball or a flat cornea. This generally makes near objects more difficult to see clearly.
Another prevalent condition is Astigmatism, which occurs when the cornea or sometimes the lens has an irregular, non-spherical shape. This uneven curvature causes light to be focused at multiple points, resulting in distorted or blurred vision at any distance. Presbyopia is an age-related focusing issue that occurs as the lens inside the eye naturally hardens and loses its flexibility, typically starting around age 40. This loss makes it difficult for the eye to change focus for close-up work, necessitating an additional power correction.
The Steps of the Clinical Refraction Process
The clinical refraction process is a methodical procedure that blends technology and patient feedback to finalize a precise prescription. It begins with Objective Refraction, where the eye doctor estimates the patient’s prescription without requiring subjective responses. Instruments like the autorefractor instantly measure how light is reflected from the back of the eye, providing a computerized starting point for the correction. Retinoscopy is another objective method where the doctor shines a light into the eye and observes the light reflex in the pupil to estimate the necessary lens power.
The process then moves to Subjective Refraction, which is the crucial refinement step relying on the patient’s direct input. This stage typically involves the phoropter, the large instrument containing many lenses that the patient looks through. The eye doctor presents a series of lens choices, asking the patient the fundamental question, “Which is better, number one or number two?” This is an iterative process of fine-tuning the spherical power and then using a specialized lens called a Jackson Cross Cylinder to accurately measure the power and orientation (axis) of any astigmatism.
Near Vision Assessment
Throughout the subjective testing, the doctor steadily adjusts the lenses in small, quarter-diopter increments, seeking the lowest power that still yields the clearest possible vision. Near vision is also assessed during this phase, especially for patients over 40, to determine the additional power needed for reading and close work.
Interpreting Your Eyeglass Prescription
The final eyeglass prescription is a written document detailing the lens powers required for correction, measured in diopters. SPH, or Sphere, indicates the main lens power needed to correct nearsightedness or farsightedness. A minus sign (-) next to the SPH number signifies correction for Myopia, while a plus sign (+) indicates correction for Hyperopia. CYL, or Cylinder, represents the extra lens power required to neutralize Astigmatism.
The CYL value is always accompanied by the Axis, a number between 1 and 180 degrees that specifies the precise orientation or angle at which the astigmatism correction must be placed in the lens. If your prescription includes an ADD value, or Addition, it indicates the supplementary magnifying power needed for reading or close-up tasks, primarily to correct Presbyopia. This ADD power is usually the same for both eyes and is combined with the distance prescription for the lower portion of bifocal or progressive lenses.