Visual acuity describes the clarity or sharpness of vision and is commonly measured using the Snellen chart, which results in the familiar 20/X notation. This format compares a person’s vision to what is considered standard visual clarity, often referred to as 20/20 vision. When an eye is measured at 20/70, it signifies a significant reduction in the ability to see fine detail. The distinct challenge this presents is how the brain and the entire visual system adapt when this level of impairment affects only one eye.
Understanding the 20/70 Measurement
The Snellen fraction 20/70 indicates that a person must stand 20 feet away from a chart to see a letter clearly that a person with 20/20 vision can see clearly from 70 feet away. This represents a marked decrease in visual clarity at a distance.
The World Health Organization (WHO) classifies visual acuity between 20/70 and 20/160, even with the best possible correction, as moderate visual impairment, or moderate low vision. While this is significantly worse than near-normal vision, which is generally 20/30 to 20/60, it is not considered legal blindness. Legal blindness in the United States requires best-corrected visual acuity to be 20/200 or worse in the better-seeing eye.
Functional Impact of Monocular Impairment
Having 20/70 vision in one eye, while the other eye maintains good clarity, creates a condition known as monocular impairment. The most noticeable consequence is the loss of stereopsis, which is the brain’s ability to use two slightly different images to create accurate three-dimensional perception. This loss can impact tasks requiring precise hand-eye coordination, such as catching a ball, reaching for an object, or pouring liquids.
The visual field is also affected, as the impaired eye contributes less to the overall peripheral vision on that side of the head. To compensate for this reduced field and the lack of depth perception, a person often develops compensatory head movements, turning their head more frequently to scan the environment. This physical adaptation can lead to visual fatigue and chronic neck or shoulder strain, particularly during long periods of visual concentration.
The brain favors the eye with better vision, causing the stronger eye to perform most of the detailed work, which can increase overall visual strain and lead to headaches. This reliance on the stronger eye can make prolonged tasks like reading or driving at night more demanding. While many individuals with monocular vision can obtain a standard driver’s license if their better eye meets the minimum acuity standard, the reduced depth perception can complicate judging distances, especially at higher speeds.
State driving laws vary, but many require a minimum corrected acuity of 20/40 in the better eye for an unrestricted license. This impairment may lead to restrictions like mandatory outside rearview mirrors or specific license evaluations. For commercial driving licenses (CDL), the standards are typically stricter, though recent changes have allowed individuals with stable monocular vision to qualify after an evaluation by an eye specialist.
Common Underlying Causes and Correction Options
One common cause of 20/70 vision in one eye, particularly if the condition has been present since childhood, is amblyopia, sometimes called “lazy eye,” where the brain fails to fully develop the visual pathway from that eye. If diagnosed late in life, the vision loss from amblyopia may be permanent, but if detected early, it is often treatable with patching therapy or corrective lenses.
In adults, a unilateral 20/70 reading may point toward structural changes within the eye, such as an early-stage cataract, which is a clouding of the eye’s natural lens. Other possibilities include corneal irregularities, a localized retinal issue like a branch retinal vein occlusion, or damage to the optic nerve. A comprehensive examination by an optometrist or ophthalmologist is the first step to accurately diagnose the underlying cause of the reduced acuity.
Correction options depend entirely on the diagnosis, ranging from simple prescription eyeglasses or contact lenses to resolve uncorrected refractive errors. If a cataract is the cause, surgical removal and replacement of the lens can restore clarity, potentially bringing the vision back to 20/20. For conditions like severe amblyopia or permanent nerve damage, correction focuses on maximizing the remaining vision through specialized low-vision aids and rehabilitation training to help adapt to the monocular state.