Visual Acuity

​by: Carmen Willings
teachingvisuallyimpaired.com
​Updated June 20, 2025

What Is Visual Acuity?

Visual acuity refers to the clarity or sharpness of vision and is a measurement of a person’s ability to distinguish detail. It is commonly expressed in Snellen notation (e.g., 20/20).
  • 20/20 is considered “normal” vision: the individual can see at 20 feet what a person with typical vision should see at 20 feet.
  • A person with 20/40 vision can see at 20 feet what a person with typical vision can see at 40 feet.
  • The larger the second number, the lower the acuity.
  • Individuals with 20/50 or worse are considered to have low vision.
  • Legally blind in the U.S. is typically defined as 20/200 or worse in the better eye with best correction, or a visual field of 20 degrees or less.
Note: Visual acuity alone does not determine functional vision. Two individuals with identical acuity may function very differently in real-world settings.

How to Read Visual Acuity on Eye Reports

Visual acuity is often abbreviated and coded on clinical eye reports. Key notations include:
OD Right eye (oculus dexter)
OS Left eye (oculus sinister)
OU Both eyes (oculi uterque)
cc With correction (e.g., glasses)
sc Without correction
V Distance vision
NV Near vision
Jaeger A system for near vision notation
Example:
  • OD 20/200 cc = Right eye has 20/200 vision with correction.
  • OS 20/100 sc = Left eye has 20/100 vision without correction.

Visual Acuity vs. Visual Functioning

Visual acuity describes what the eyes can see under standardized conditions. Visual functioning describes how a person uses their vision in everyday tasks. These are not always aligned. Environmental factors like lighting, contrast, movement, and glare can greatly affect visual functioning.
Vision is also a learned and developmental skill, especially for students with multiple disabilities or neurological conditions such as CVI (Cortical Visual Impairment).

Clinical and Educational Significance

A clinical eye exam should test both near and distance acuity, ideally with and without correction. This helps determine:
  • If the student tolerates corrective lenses
  • Whether the correction significantly improves visual function
  • How vision impacts educational access

Types of Visual Acuity Measurements

​1. Distance Visual Acuity
Measured at 20 feet, often using a Snellen chart or equivalent. This determines the student’s ability to:
  • View the board or smartboard in class
  • See information in assemblies or demonstrations
  • Read street signs, sports jerseys, or scoreboards
2. Near Visual Acuity
Measured at 12–16 inches, usually with Jaeger charts or symbol-based near vision cards. This determines the ability to:
  • Read textbooks and worksheets
  • Access menus, labels, forms, or worksheets
  • Engage in reading or writing activities
3. Functional Acuity Estimates
For students who are unable to participate in formal acuity testing, such as young children or those with multiple disabilities:
  • The ophthalmologist may provide an acuity estimate
  • The student may be described as Functioning at the Definition of Blindness (FDB)
  • Teachers may use functional vision assessments and print comparisons to estimate usable vision

Print Comparisons and Environmental Assessments

When students cannot complete formal vision tests:
  • Use real-world print samples (menus, worksheets, signs) to assess access
  • Note the distance at which the student can recognize objects or faces
  • Observe visual behavior in different lighting and contrast settings
These observational tools support educational planning and eligibility determination for vision services. The Print Comparison page provides environmental print comparisons to the Snellen chart.

Key Takeaways for TVIs

  • Always interpret acuity results in context—consider both medical findings and functional observations.
  • Document how vision affects classroom access, including reading, mobility, and participation.
  • Ensure a current eye report (within the last 1–3 years) is available for eligibility and program planning.
  • Coordinate findings with results from the Functional Vision Evaluation (FVE) and Learning Media Assessment (LMA).

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A pdf version is available on the Premium Printables page.
Understanding Visual Acuity Handout
Understanding Visual Acuity Handout