Visual acuity (VA) measurement is the single most common clinical task performed by ophthalmic assistants. Every patient encounter begins with it. On the COA exam, VA testing falls under the Assessments domain — the largest content area at 42% of the exam — and questions about it span technique, notation, documentation, and clinical interpretation.
This guide covers everything you need to know: how the Snellen chart works, how to perform distance and near testing correctly, when to use the pinhole occluder, how to record results across the full range from 20/20 to no light perception, and what the exam expects you to understand about each step. Solid knowledge here pays dividends across many COA questions.
Understanding the Snellen Chart
The Snellen chart, developed by Dutch ophthalmologist Herman Snellen in 1862, remains the global standard for distance visual acuity measurement. Each letter on the chart is called an optotype, sized so that the entire letter subtends 5 minutes of arc at a defined distance, with each stroke of the letter subtending 1 minute of arc.
The chart is read at 20 feet (or its optical equivalent). The large "E" at the top represents 20/200 — a person with normal vision can read it from 200 feet. The smallest line most people can read is 20/10 to 20/15. Standardized charts must be calibrated for the test distance used in your clinical setting.
Test Distance
20 ft
6 meters (standard)
Normal Vision
20/20
or better (20/15, 20/10)
Legal Blindness
20/200
best-corrected, better eye
What the Snellen Fraction Means
In the fraction 20/X, the numerator (20) is the test distance in feet. The denominator (X) is the distance at which a person with normal vision could read the same optotype. So:
- 20/20 — Normal acuity. The patient reads at 20 feet what a normal person reads at 20 feet.
- 20/40 — Reduced. The patient reads at 20 feet what a normal person sees at 40 feet.
- 20/200 — Severely reduced. Also the threshold for legal blindness in the better eye with correction.
- 20/10 — Better than normal. The patient reads at 20 feet what most people need to be within 10 feet to see.
Performing Distance Visual Acuity Testing
Proper technique is essential for reliable results. Deviations from protocol — incorrect distance, improper occlusion, allowing squinting — all introduce measurement error. The COA exam tests both the correct sequence and the rationale behind each step.
Position the patient at the correct distance
Position the patient exactly 20 feet from the chart (or the mirror-equivalent). The patient should be seated comfortably with the chart at eye level. Verify the illumination meets chart specifications — typically 85 to 160 candelas per square meter for projected charts.
Test right eye first, then left eye, then both eyes
The convention in ophthalmology is OD (oculus dexter, right eye) first, then OS (oculus sinister, left eye), then OU (oculus uterque, both eyes). Occlude the non-tested eye with an occluder paddle — not the patient's hand, which allows peeking and can induce accommodation. Never press the occluder against a closed eye or rub the eye.
Start at an appropriate line and work down
For most patients, start at the 20/40 or 20/30 line and work toward smaller letters. This is efficient and reduces fatigue. For patients with known poor vision, start higher. Ask the patient to read each line completely. The smallest line at which the patient reads more than half the optotypes correctly is recorded as their acuity for that line.
Record the result with plus or minus notation
Record the Snellen fraction for the smallest line the patient reads correctly. Add a plus sign for each additional letter read on the next smaller line (e.g., 20/40+2 means they read the 20/40 line and two letters on the 20/30 line). Use a minus sign for each letter missed on the recorded line (e.g., 20/40-1 means they read the 20/40 line but missed one letter). Also specify whether the measurement was uncorrected (sc, sine correctione) or best-corrected (cc, cum correctione).
The Pinhole Test
The pinhole occluder is a simple but diagnostically powerful tool. By limiting incoming light to the central rays that pass through the optical axis — bypassing the refracting surfaces of the eye — the pinhole eliminates or reduces the blur caused by refractive errors. This makes it an excellent screening tool for distinguishing refractive from non-refractive causes of reduced vision.
VA Improves with Pinhole
Suggests the reduced vision is primarily due to an uncorrected or under-corrected refractive error (myopia, hyperopia, or astigmatism). The patient may benefit from updated spectacle or contact lens correction.
VA Does Not Improve (or Worsens)
Suggests an organic cause: cataract, corneal scar, macular degeneration, glaucoma damage, or optic nerve disease. These patients need further evaluation by the ophthalmologist, not just a new glasses prescription.
Clinical Note
A dense cataract can sometimes cause vision to appear to worsen through the pinhole because the pinhole reduces total retinal illuminance. If the pinhole causes significant worsening, document this finding carefully — it may indicate advanced media opacity rather than retinal or optic nerve disease.
Near Visual Acuity Testing
Near visual acuity is tested at 16 inches (40 cm) using a near vision card. Unlike distance testing, near VA reflects both refractive status and the patient's ability to accommodate (or the adequacy of their reading addition for presbyopic patients). Near VA is an essential part of any comprehensive eye exam and is particularly important in presbyopic patients or when evaluating reading difficulty.
| Near VA Notation | Approximate Snellen Equivalent | Clinical Significance |
|---|---|---|
| Jaeger 1 (J1) | 20/20 near equivalent | Excellent near vision; reads newsprint easily |
| Jaeger 3 (J3) | ~20/40 | Adequate for most near tasks |
| Jaeger 5 (J5) | ~20/80 | Reading difficulty; large print needed |
| Jaeger 10 (J10) | ~20/200 | Severe near vision impairment |
| M notation (0.4M) | 20/20 | Metric system; used in low vision settings |
Near VA cards may use Jaeger notation (J1 through J20), M notation (metric print size), point notation (used by low vision specialists), or a near Snellen fraction. The Jaeger system is most common in ophthalmology offices in the United States, though it lacks the standardization of distance Snellen fractions because different manufacturers have produced slightly different Jaeger charts over the years.
Always document near VA at the specified working distance (16 inches or 40 cm). If a patient holds the card closer to see it, note the actual distance used. Deviations from the standard testing distance alter the accommodative demand and make the results less comparable to normative data.
Documentation Standards
Accurate documentation is as important as accurate measurement. The COA exam tests whether you know how to record VA findings properly. A complete VA entry includes the acuity for each eye, whether the measurement was with or without correction, the testing distance, and the notation system used.
Correction status
sc (sine correctione) = without glasses/contacts. cc (cum correctione) = with glasses/contacts. Always specify which was tested.
Eye order
Always OD first, then OS, then OU. Never reverse this order in documentation.
Plus/minus letters
Document additional letters read or missed using +/- notation: 20/40+2 means 20/40 line complete plus 2 letters on the 20/30 line.
Sub-20/400 findings
When below the chart, use: CF (count fingers) at X feet, HM (hand motion) at X feet, LP (light perception), or NLP (no light perception). Always include the distance for CF and HM.
Pinhole results
Document pinhole VA as a separate entry: PH (pinhole) 20/X. Include this whenever a pinhole test was performed.
Practice COA Visual Acuity Questions
Opterio includes VA testing questions across all COA content domains, with AI-powered explanations that teach the clinical reasoning behind each answer.
Special Testing Situations
Pre-verbal and Non-verbal Patients
For children or patients who cannot read letters, use HOTV cards (letter-matching), Lea symbols, or Allen figures. The child points to a matching card rather than naming the optotype. The same Snellen fraction notation system applies.
Low Vision Patients
When distance acuity is worse than 20/400, move the patient closer and record the actual distance. A patient who reads the 20/400 line from 10 feet has an acuity of 10/400. Also document the count-fingers, hand-motion, and light perception hierarchy if needed.
Patients with Nystagmus
Nystagmus creates special challenges. Test with both eyes open (binocular VA) in addition to monocular testing. Allow the patient to find their null point (gaze direction where nystagmus is minimized). Document head posture if the patient adopts a compensatory position.
Malingering and Non-organic Loss
When organic disease does not explain the level of vision loss, the pinhole test is a key tool. Inconsistent VA between test attempts, dramatic improvement with encouragement, or VA that varies widely may suggest functional (non-organic) vision loss. Document findings carefully and report to the physician.