Visual Acuity Testing in Adults
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Introduction
Visual acuity (VA) is a fundamental measure of visual function that assesses the eye’s ability to distinguish fine details and shape of objects in the direct line of vision. It serves as a cornerstone of the ophthalmic exam informing diagnosis and treatment decisions. All adults, regardless of risk factors, must undergo a comprehensive medical eye evaluation with increasing frequency as patients age, aimed at preventing or minimizing vision through early detection of ocular diseases, visual dysfunction, and underlying systemic conditions. Visual acuity may also be performed periodically to monitor disease processes and progression in patients with a known eye condition1,2.
Contents
Conceptual Framework
Minimum Angle of Resolution
Types of Visual Acuity
Optotype Chart Types
Snellen Chart
ETDRS Chart
Bailey-Lovie Chart
Landolt C and Tumbling E Chart
Visual Acuity Testing Protocol
Distance Vision
Setup and Environmental Conditions
Correction Status
Monocular Testing
Low Vision Acuity Designation
Near Vision
Summary of Documentation and Reporting
Factors Affecting Visual Acuity
Functional Visual Loss
Legal Context
References
Conceptual Framework
Minimum Angle of Resolution
Visual acuity is defined as the reciprocal of the minimum angle of resolution (MAR) expressed in minutes of arc. The minimum angle of resolution is the smallest angle of separation at which two points can be distinguished as separate, best represented by the letter E on the Snellen chart. For an ideal human eye, under optimal conditions, the MAR is approximately 1 minute of arc.
The photoreceptor mosaic of the fovea centralis constrains the theoretical limit of resolution. The fovea centralis is the central 1–2 mm of the macula and contains the highest density of cone photoreceptors in the retina, with center-to-center cone spacing of approximately 0.5 arcmin3. According to the Nyquist–Shannon sampling theorem, accurate reconstruction of a spatial signal requires a sampling frequency at least twice the highest frequency present in that signal4. Because each cone acts as an independent sampling unit, the minimum resolvable spatial frequency is determined by the spacing between adjacent cones: with a cone separation of ~0.5 arcmin, the Nyquist limit of the foveal mosaic is ~1 arcmin, meaning the eye cannot reliably resolve details finer than 1 arcmin without aliasing artifacts. This 1 arcmin Nyquist limit corresponds precisely to the angular subtense of each stroke of the letter E at the 20-foot line on the Snellen chart, explaining why 20/20 represents the theoretical norm for best-corrected visual acuity in a healthy eye under optimal conditions5,6. In practice, factors such as optical aberrations, pupil size, and neural processing set the functional limit close to, but rarely better than this theoretical ceiling.
Types of Visual Acuity
| Type | Description | Clinical Use |
| Minimum (detectable) visible acuity | Detection of a single point of light against a background | Contrast sensitivity |
| Minimum (separable) resolvable acuity | Discrimination of two points as separate (MAR) | Standard chart testing |
| Minimum recognizable (optotype) acuity | Recognition of a letter or symbol | Snellen, ETDRS charts |
| Minimum discriminable acuity | Detection of a small positional change (Vernier) | Hyperacuity; amblyopia research |
Optotype Chart Types
Snellen Chart
The Snellen chart was created by Dutch ophthalmologist, Dr. Hermann Snellen, in 1862 and is the most widely used chart in clinical practice7. It portrays a row of letters with each element subtending 1 arcmin at the designated letter distance. The letters are sized on a 5x5 grid generating an overall height that subtends an angle of 5 arcmin. This can be used for evaluating both near and distance vision.
The Snellen chart provides quick and adequate evaluation of visual acuity however several limitations remain. The chart has an unequal number of letters per line, increasing from one letter at the 20/200 line to eight letters at the 20/20 line. As the lines include more letters, the spacing between letters decreases and can create a crowding effect. Lastly, some letters may be easier to read than others. This lack of standardization within a chart makes it unreliable for purposes of statistical analysis8.
ETDRS Chart
The ETDRS chart was developed in 1982 to overcome the limitations of the Snellen chart and standardize VA measurements for clinical research. It is the gold standard for clinical research. Tested from 4 meters (13 feet) with each row containing 5 letters each of equal spacing on a logarithmic scale. Scoring is represented as logMAR= 1.1- (0.02 x total letters correct)9.
Bailey-Lovie Chart
Developed in 1976 by Bailey and Lovie as an effort to redesign the Snellen chart, it became the precursor to the ETDRS chart. Containing a total of 14 rows with 5 letters each, featuring logarithmic scaling and equal letter counts per row. More commonly used in optometric and low vision settings10.
Landolt C and Tumbling E
Dr. Snellen developed the tumbling E chart for children as well as adults with language barrier or cognitive impairment. This displays the letter E facing different directions in which a patient can identify which direction the E is facing. Similarly, the Landolt C displays a capital C or open ring facing various directions. Patients may be able to point out the patterns as an alternative assessment of their visual acuity. The Landolt C, developed by Dr. Landolt, is regarded as the lab standard by the International Council of Ophthalmology11.
Several notation systems are used to express distance visual acuity and may allow conversions between systems. The Snellen fraction is expressed as distance tested/distance at which the letter subtends 5 arcmin (e.g., 20/20 in feet or 6/6 in meters). The logarithm of the minimum angle of resolution (logMAR) is the preferred notation in clinical research and represented by the following formula12:
logMAR = log10(MAR)
Snellen 20/20= log10(20/20)= logMAR 0
| Snellen (ft) | Snellen (m) | logMAR |
| 20/10 | 6/3 | −0.3 |
| 20/20 | 6/6 | 0.0 |
| 20/40 | 6/12 | +0.3 |
| 20/100 | 6/30 | +0.7 |
| 20/200 | 6/60 | +1.0 |
Visual Acuity Testing Protocol13
Distance Vision
Setup and Environmental Conditions
Testing Distance: 20 feet standard for Snellen (14 feet for ETDRS)
Illumination: controlled room lighting with backlit or front-lit chart with standardized luminance of 80-160 cd/m2
Positioning: chart should be positioned at eye level or slightly below
Correction Status
Test with current correction unless directed otherwise. Add notations as below:
SC (sine correctione): without correction
CC (cum correctione) with correction
PH - with pinholes
Pinhole vision may not improve for all patients, but if visual acuity improved with pinhole occluder, the deficit is likely an optical (refractive) etiology. If no improvement is noted, document as
“NI.”
Monocular Testing
Each eye is tested separately with right eye (OD) first (convention) then left (OS)
Occlude the fellow eye with an opaque occluder and ask patient to read left to right, line by line, until they miss more than half of a line.
Prefer showing multiple lines during test instead of isolating one line or one optotype.
Ensure patients are using the distance portion of their glasses. If fewer than half of the line's optotypes are missed: record as line minus missed (e.g., 20/20-2) and if more than half are missed: record the last full line read plus correct optotypes on the next line (e.g., 20/25+2).
Use Numbers, Tumbling E’s, or LEA Symbols for low literacy or non-English speakers.
After optimization of environment and addition of pinhole, if no letters are legible proceed to test at shorter distance explained below
Low Vision Acuity Designation
When a patient is unable to identify any letters on the chart, proceed with further testing in the order below. Allow patients to move their head or use eccentric fixation for best vision. If side/peripheral vision helps, note as Ecc or EF (eccentric fixation) or head tilt left, right, up or down, etc.
1. Count fingers (CF)- patient correctly counts fingers held at specified distance (e.g., CF at 3 feet)
2. Hand motion (HM)- patient detects direction of hand movement at specified distance
3. Light perception (LP)- patient detects presence/absence of light; if patient can identify direction of light (superior, inferior, nasal, temporal) record as LP w/ P (light perception with projection)
4. No light perception (NLP)- no perception of any light
Binocular testing
Test both eyes vision together in cases such as latent nystagmus or when filling out specific forms such as the Department of Motor Vehicles (DMV) form.
Near Vision
Near vision is tested at standard reading distance, typically 14-16 inches. Common systems include the Snellen-equivalent near cards. The same principles of optimized lighting and monocular testing apply. Ensure patients are using the bifocal portion of glasses, if applicable. Near acuity is important in evaluation of presbyopia, low vision rehabilitation, and macular disease monitoring.
Summary of Documentation and Reporting
1. Distance at which the chart was read (standard 20 ft / 6 m, or specify if modified)
2. Correction status (sc, cc, ph)
3. Occlusion (monocular, right or left, binocular)
4. Notation (Snellen, logMAR, etc.)
5. Number of letters read on the best line (important for ETDRS)
6. Testing conditions if non-standard (reduced distance, dim lighting)
Example: SC VA OD: 20/80+2, OS: 20/40−1. CC VA OD: 20/25 +2, OS: 20/20. Pinhole OD: 20/20. Testing at 20 ft, Snellen chart.
Factors Affecting Visual Acuity
Visual acuity is influenced by a multitude of optical neural, retinal, cognitive, and environmental factors. Optical factors are among the most common and include refractive error, media opacities, and pupil size. Neural etiologies span the entire visual pathways including macular disease, amblyopia, glaucomatous neuropathy, optic neuritis, and compressive lesions, each capable of reducing visual acuity through distinct mechanisms. The retinal aspect includes conditions such as diabetic macular edema, vascular occlusions, retinal detachment, and neovascular AMD that impair and disrupt retinal architecture. Beyond the structural diseases, cognitive and behavioral factors can affect measured VA. This includes patient cooperation, fatigue, motivation and non-organic visual loss. Standardized testing conditions (testing distance, ambient lighting, chart illumination) help ensure an accurate VA that reflects true visual function14.
Functional Visual Loss
Non-organic visual loss (NOVL), also known as functional visual loss, is defined as reported reduction in visual acuity without detectable organic cause. Prevalence reported at 5-12% in ophthalmology clinics and most commonly diagnosed in female patients15. NOVL is suspected when VA is inconsistent across testing with an otherwise normal comprehensive ophthalmology exam. Confirmatory tests have been developed to identify functional visual loss including the fogging test, vectograph charts, OKN drum test, and visual evoked potential16.
Legal Context
US Legal Blindness: A visual acuity of 20/200 or less in the better-seeing eye with best conventional correction (meaning with regular glasses or contact lenses). OR a visual field (the total area an individual can see without moving the eyes from side to side) of 20 degrees or less (also called tunnel vision) in the better-seeing eye17.
Driving Test: Most states require a minimum of 20/40 best corrected VA for unrestricted driving. Restrictions may be placed if corrective lenses are needed to reach 20/40. Driving with monocular vision is legal with no restriction. Vision worse than 20/100-20/200 fail to meet licensing standards18.
References
1. Dictionary of Eye Terminology. Accessed May 21, 2026. https://store.aao.org/dictionary-of-eye-terminology.html
2. Feder RS, Olsen TW, Prum BE, et al. Comprehensive Adult Medical Eye Evaluation Preferred Practice Pattern(®) Guidelines. Ophthalmology. 2016;123(1):P209-236. doi:10.1016/j.ophtha.2015.10.047
3. Rehman I, Mahabadi N, Motlagh M, Ali T. Anatomy, Head and Neck, Eye Fovea. In: StatPearls. StatPearls Publishing; 2026. Accessed May 21, 2026. http://www.ncbi.nlm.nih.gov/books/NBK482301/
4. Curcio CA, Sloan KR, Kalina RE, Hendrickson AE. Human photoreceptor topography. J Comp Neurol. 1990;292(4):497-523. doi:10.1002/cne.902920402
5. Riggs LA. Visual acuity. In: Graham CH ed. Vision and visual perception. New York: Wiley, 1965:321–349.
6. Westheimer G. Visual Acuity. Annu Rev Psychol. 1965;16(Volume 16, 1965):359-380. doi:10.1146/annurev.ps.16.020165.002043
7. Azzam D, Ronquillo Y. Snellen Chart. In: StatPearls. StatPearls Publishing; 2026. Accessed May 21, 2026. http://www.ncbi.nlm.nih.gov/books/NBK558961/
8. Kaiser PK. Prospective Evaluation of Visual Acuity Assessment: A Comparison of Snellen Versus ETDRS Charts in Clinical Practice (An AOS Thesis). Trans Am Ophthalmol Soc. 2009;107:311-324.
9. Ferris FL, Kassoff A, Bresnick GH, Bailey I. New visual acuity charts for clinical research. Am J Ophthalmol. 1982;94(1):91-96.
10. Bailey IL, Lovie JE. New design principles for visual acuity letter charts. Am J Optom Physiol Opt. 1976;53(11):740-745. doi:10.1097/00006324-197611000-00006
11. Colenbrander A. Visual Standards aspects and ranges of vision loss with emphasis on population surveys. Published online January 1, 2002.
12. Holladay JT. Proper method for calculating average visual acuity. J Refract Surg. 1997;13(4):388-391. doi:10.3928/1081-597X-19970701-16
13. Kniestedt C, Stamper RL. Visual acuity and its measurement. Ophthalmol Clin N Am. 2003;16(2):155-170, v. doi:10.1016/s0896-1549(03)00013-0
14. Levenson JH, Kozarsky A. Visual Acuity Change. In: Walker HK, Hall WD, Hurst JW, eds. Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd ed. Butterworths; 1990. Accessed May 21, 2026. http://www.ncbi.nlm.nih.gov/books/NBK215/
15. Kathol RG, Cox TA, Corbett JJ, Thompson HS, Clancy J. Functional visual loss: I. A true psychiatric disorder? Psychol Med. 1983;13(2):307-314. doi:10.1017/s0033291700050923
16. Manion GN, Stokkermans TJ. Nonorganic Vision Loss. In: StatPearls. StatPearls Publishing; 2026. Accessed May 21, 2026. http://www.ncbi.nlm.nih.gov/books/NBK599519/
17. Disability Evaluation Under Social Security | Disability | SSA. Accessed May 21, 2026. https://www.ssa.gov/disability/professionals/bluebook/
18. Vision and Driving. American Academy of Ophthalmology. May 14, 2024. Accessed May 21, 2026. https://www.aao.org/eye-health/tips-prevention/vision-driving
