Low vision is classified primarily by two measurements: how sharp your central eyesight is (visual acuity) and how wide your field of vision extends (visual field). The World Health Organization sets the main thresholds, but classification also depends on whether you’re being evaluated for legal benefits, rehabilitation services, or functional ability in daily life. Each system uses slightly different cutoffs and categories.
WHO Categories of Visual Impairment
The WHO defines low vision as visual acuity worse than 20/60 but equal to or better than light perception, even after treatment and the best possible glasses or contacts. A visual field narrower than 10 degrees from center also qualifies. Within that range, severity breaks into tiers. Mild low vision starts at worse than 20/60, moderate begins at worse than 20/200, and severe low vision covers worse than 20/400. Below that, you move into blindness categories.
These categories matter because they determine what kind of rehabilitation you may benefit from. Someone at the mild end might do well with magnifying lenses and better lighting, while someone at the severe end may need more intensive orientation and mobility training alongside assistive technology.
Legal Blindness in the United States
The U.S. Social Security Administration considers you legally blind if your vision cannot be corrected to better than 20/200 in your better eye, or if your visual field is 20 degrees or less in your better eye for a period lasting or expected to last at least 12 months. This is a benefits threshold, not a medical diagnosis. Many people classified as legally blind still have usable vision.
The 20/200 number means that what a person with normal sight can see from 200 feet away, you need to be within 20 feet to see. The visual field cutoff of 20 degrees is roughly like looking through a narrow tube. Normal peripheral vision extends about 180 degrees side to side, so 20 degrees represents a dramatic narrowing. You can meet the legal definition through either measurement alone.
Classification by Type of Vision Loss
Acuity and field measurements tell you how much vision is lost, but not where or how. A more practical classification system groups low vision into four types based on which part of your visual system is affected, because different types of loss create very different challenges in daily life.
- Type 1: Reduced acuity with a full visual field. You can see in all directions, but everything looks blurry or washed out. This is common in cataracts and some forms of macular degeneration. Reading and recognizing faces are the primary difficulties.
- Type 2: Any visual field loss with more than 10 degrees of remaining field. This includes both central blind spots and partial peripheral loss. You still have enough remaining field to navigate most environments, but you may miss objects on one side or struggle with tasks that require sharp central focus.
- Type 3: Peripheral field loss with less than 10 degrees of remaining field. This is sometimes called tunnel vision. It splits further into 3a (sharp central acuity preserved) and 3b (both peripheral and central vision affected). People with 3a can often still read but struggle enormously with mobility and orientation.
- Type 4: Visual field loss caused by a brain event. Stroke or brain injury can damage the visual processing centers rather than the eyes themselves. The eyes may be structurally healthy, but the brain cannot interpret what they receive. This type requires distinct rehabilitation approaches.
How Children Are Classified Differently
The WHO uses a separate definition for children. A child is considered to have low vision with acuity worse than 20/60, compared to the adult threshold. The visual field criterion stays the same at less than 10 degrees. The definition also includes an important qualifier: the child “uses, or is potentially able to use, vision for planning and/or execution of a task.” This forward-looking language reflects the fact that children’s visual systems are still developing, and early intervention can sometimes improve how effectively they use whatever vision they have.
Beyond the Eye Chart: Functional Vision
A standard eye chart measures only one thing: how well you can distinguish high-contrast black letters on a white background in a well-lit room. That test misses several dimensions of vision that directly affect how well you function day to day. Someone can score 20/20 on an eye chart and still have significant visual impairment if their contrast sensitivity is poor.
Contrast sensitivity measures your ability to distinguish objects from their background when the difference between them is subtle, like seeing a gray car against a gray sky, or reading a menu in dim lighting. It’s scored on a logarithmic scale where a score of 2 is normal, below 1.5 indicates visual impairment, and below 1 indicates visual disability. Many eye conditions erode contrast sensitivity while leaving acuity relatively intact, which is why some people feel their vision is worse than their eye chart results suggest.
A full functional vision assessment goes further still. It evaluates color vision, depth perception, how well your eyes track moving objects, how sensitive you are to glare, and how well you see at night. Each of these can be independently impaired. A person with good acuity but severe glare sensitivity may be functionally unable to drive, even though their eye chart score looks fine. These assessments guide rehabilitation by revealing exactly which visual tasks are compromised and which adaptations or devices will help most.
Why Classification Systems Overlap
If these systems seem like they don’t quite line up, that’s because they serve different purposes. The WHO categories create a universal language for tracking visual impairment across populations. The U.S. legal definition determines eligibility for disability benefits and services. The type-based classification system guides rehabilitation specialists toward the right interventions. And functional assessments capture what numbers alone cannot: how your specific combination of visual losses plays out when you’re trying to pour coffee, cross a street, or read your mail.
In practice, a person with low vision will encounter multiple classification systems. An ophthalmologist records acuity and field measurements. A government agency applies its legal thresholds. A rehabilitation specialist assesses functional vision across several dimensions. Each layer adds information, and together they build a more complete picture of what someone can and cannot see, and what support will make the biggest difference.