The question of the “worst eyesight number” is complicated because the term refers to two distinct measurements used by eye care professionals. One measurement describes the strength of the corrective lens needed to focus light, while the other describes the sharpness of vision achieved, even with correction. The highest numerical value for a prescription suggests the most misshapen eye, but the lowest functional vision score represents the most severe, often permanent, loss of sight. Determining the “worst” number requires looking at the extremes of both the corrective lens power and the remaining visual function.
Understanding the Measurement Scales
Vision is measured using two primary systems that assess different aspects of the eye’s function. The first is visual acuity, which measures how clearly a person sees at a distance and is typically expressed as a fraction. This measurement indicates the sharpness of vision achieved with the best possible correction.
The common measurement of 20/20 represents “normal” vision. This means a person sees at 20 feet what an average person also sees at 20 feet. If the measurement is 20/400, the individual must stand at 20 feet to see what a person with normal vision could see clearly from 400 feet away. This scale measures functional vision, regardless of the physical cause of the impairment.
The second primary measurement is the refractive error, which quantifies the power of the lens required to correct the eye’s focus. This is measured in units called diopters, or ‘D’.
A negative number, such as -5.00 D, indicates myopia (nearsightedness), where light focuses in front of the retina because the eyeball is too long or the cornea is too curved. A positive number, like +3.00 D, indicates hyperopia (farsightedness), meaning the eye is too short or the lens is too weak.
The numerical value of the diopter signifies the lens strength. A higher number, whether positive or negative, corresponds to a greater deviation from the eye’s ideal shape, requiring a stronger prescription for correction.
Extreme Refractive Errors and Diopter Limits
The “worst” number concerning corrective lens power relates to the highest diopter measurement recorded. Extreme refractive errors represent the greatest anatomical deviations in the eyeball’s structure. High myopia is generally categorized as a prescription of -6.00 D or stronger, while extreme myopia exceeds -10.00 D.
In these severe cases, the eye is significantly elongated, which stretches the delicate retinal tissue. This stretching can lead to pathological changes like retinal detachment or macular degeneration. While most prescriptions cap out in the -20s, a record-breaking case of myopia was documented at an astonishing -108 diopters.
High hyperopia, or severe farsightedness, is less common in the extreme ranges but is categorized as +6.00 D or greater. Most cases seen in practice are below +15.00 D, but unusual conditions, such as the removal of the eye’s natural lens, can result in prescriptions reaching +60.00 D.
For both severe myopia and hyperopia, standard glasses become very thick and heavy. Correction often requires specialized contact lenses or surgically implanted lenses to achieve functional vision.
The Threshold for Legal and Functional Blindness
The worst eyesight number in terms of functional vision is determined by the uncorrectable endpoint of the visual acuity scale. The legal definition of blindness in the United States requires a best-corrected visual acuity of 20/200 or worse in the better eye. It also includes a restriction of the visual field to 20 degrees or less, often described as tunnel vision.
Vision significantly worse than 20/200 falls into categories that describe minimal remaining function. For example, a person may only be able to “Count Fingers” (CF) at a very short distance, or merely perceive “Hand Motion” (HM) as a vague shadow. These measurements are used when the patient cannot read the largest letter on the eye chart.
The lowest level of functional sight is “Light Perception” (LP), where an individual can only distinguish between light and dark, but cannot discern shapes or objects. The absolute worst measurable eyesight is “No Light Perception” (NLP), which represents total blindness. This functional scale is the truest measure of the worst eyesight number, as it indicates a permanent inability to process light into a meaningful image.