Visual acuity is the most fundamental measurement in eye care. It quantifies the spatial resolving power of the visual system -- how well the eye can distinguish fine detail at a given distance. Every comprehensive eye exam begins with visual acuity, and as a paraoptometric, you will likely measure it dozens of times per day. Getting it right is not optional.
The Snellen chart, developed by Dutch ophthalmologist Herman Snellen in 1862, remains the most widely used acuity test in clinical practice. Its design is deceptively simple: letters (optotypes) of decreasing size arranged in rows, each calibrated so that the critical detail subtends a specific visual angle at a standard distance. Understanding not just how to administer this test but why each step matters will serve you on both the CPO and CPOA exams and in daily clinical work.
This guide covers the complete visual acuity testing protocol, from patient positioning through documentation, along with the underlying optical principles you need to understand for certification exams.
What Visual Acuity Actually Measures
Visual acuity measures the smallest detail the eye can resolve. Specifically, it tests the minimum angle of resolution (MAR) -- the smallest angular separation between two points that the visual system can distinguish as separate. A person with 20/20 vision can resolve details subtending 1 minute of arc (1/60 of a degree) at 20 feet.
This is not a complete measure of vision. Visual acuity does not assess peripheral vision, contrast sensitivity, color perception, depth perception, or how well the eyes work together. It is a specific measurement of central foveal resolution under high-contrast conditions. A patient can have 20/20 acuity and still have significant visual problems that other tests will reveal.
What VA Does and Does Not Test
VA measures:
- Central foveal resolution
- High-contrast letter recognition
- Spatial resolving power at distance
- Minimum angle of resolution
VA does not measure:
- Peripheral vision
- Contrast sensitivity
- Color perception
- Binocular function or depth perception
The Snellen Chart: Design and Principles
Snellen optotypes are designed on a 5x5 grid. The overall letter height at the 20/20 line subtends 5 minutes of arc at 20 feet, and each stroke of the letter subtends 1 minute of arc. This means the critical detail -- the gap in a C, the opening of an E, the space between strokes -- is 1 minute of arc at the designated distance.
Each line on the chart corresponds to a specific acuity level. The large E at the top is typically 20/200, meaning its critical detail subtends 1 minute of arc at 200 feet. At 20 feet, that detail subtends 10 minutes of arc, making it relatively easy to see. As you move down the chart, the letters get progressively smaller until the 20/20 line, where the critical detail subtends exactly 1 minute of arc at 20 feet. Some charts include lines below 20/20 (such as 20/15 and 20/10) for patients with better-than-average acuity.
Standard Snellen Lines
Typical Snellen charts include these acuity levels from top to bottom: 20/200, 20/100, 20/70, 20/50, 20/40, 20/30, 20/25, 20/20, 20/15, and 20/10. Not all charts include every line, and some use slightly different progressions. The 20/200 line is also the legal blindness threshold when best corrected acuity cannot exceed this level.
Understanding Snellen Notation
Snellen notation is expressed as a fraction. The numerator is always the testing distance -- in the United States, this is 20 feet. The denominator is the distance at which a person with normal acuity (20/20) could read that same line. This is the single most testable concept in visual acuity for certification exams, so make sure you understand it thoroughly.
Interpreting Common Acuity Values
In metric notation (used internationally), the testing distance is 6 meters. So 6/6 equals 20/20, 6/12 equals 20/40, and 6/60 equals 20/200. You may see either system on the exam, so be comfortable converting between them.
Step-by-Step Testing Protocol
Consistency in your technique ensures reliable, repeatable measurements. Follow this protocol every time without shortcuts, and your acuity measurements will be defensible and useful to the doctor.
Position the patient
Seat the patient at exactly 20 feet (6 meters) from the chart. The chart should be at eye level and uniformly illuminated. If using a projector chart, confirm calibration for the room length. Room lighting should be moderate -- not dark, not excessively bright.
Determine testing condition
The doctor's protocol will specify whether to test with or without correction. Typically you test uncorrected acuity (UCVA) first, then with current glasses or contacts (cc or BCVA). Always ask and document which condition you are measuring. If testing with correction, make sure the patient is wearing their current, habitual glasses -- not an old pair from the glove compartment.
Occlude the left eye (test OD first)
Always begin with the right eye (OD). Cover the left eye completely with a paddle occluder or opaque tissue. Never allow the patient to use their fingers -- they will inevitably peek between them. If testing with glasses, hold the occluder in front of the glasses lens, not behind it. Do not let the patient press on the covered eye, as this can temporarily blur vision when that eye is tested next.
Direct the patient to read the chart
Ask the patient to read the smallest line they can see. If they start too high, gently direct them lower. If they cannot see the top line, move to the sub-chart procedures below. Encourage guessing -- patients often stop when they feel uncertain, but partial recognition still provides useful information. A common instruction is: "Read the smallest line you can, and keep going even if you have to guess."
Record the result with plus/minus notation
Record the smallest line where the patient reads more than half the letters correctly. If they miss letters on that line, note with minus notation (e.g., 20/25-2). If they read extra letters from the next smaller line, note with plus notation (e.g., 20/30+1). This precision matters -- the doctor uses it to track subtle changes between visits.
Test the left eye (OS), then both eyes (OU)
Switch the occluder to the right eye and repeat. Then remove the occluder and test both eyes together. Binocular acuity (OU) is typically one line better than the better monocular eye. If it is not, or if it is significantly better, this may indicate a binocular vision issue worth noting.
When Acuity Is Below Chart Range
If the patient cannot read the largest letter on the chart (usually the 20/200 E), you must use a descending scale of tests to quantify their remaining vision. Each level represents a significant drop in visual function, and accurate recording is critical for the doctor's assessment.
Count Fingers (CF)
Hold up fingers at a measured distance and ask the patient to count them. Start at 3-4 feet and move closer if needed. Record as "CF at X feet" (e.g., CF at 3 ft). Always specify the distance -- "CF" alone is incomplete documentation.
Hand Motion (HM)
If the patient cannot count fingers at any distance, wave your hand in front of their eye and ask if they can see movement. Record as "HM" with the distance tested. This indicates very poor acuity but confirms some form perception.
Light Perception (LP)
Using a penlight or transilluminator, shine the light toward the eye and ask if the patient can see it. Also test light projection by shining from different directions (superior, inferior, nasal, temporal) to determine if the patient can identify where the light is coming from. Record as "LP with projection" or "LP without projection."
No Light Perception (NLP)
If the patient cannot detect a bright light shone directly into the eye in a darkened room, record "NLP." This indicates total blindness in that eye. NLP is a significant finding and should always be confirmed carefully before recording.
Types of Visual Acuity Measurements
Understanding the distinctions between different VA measurements is essential for both the exam and clinical practice. Each tells the doctor something different about the patient's visual status.
UCVA (Uncorrected)
Measured without any glasses or contacts. Tells the doctor the baseline refractive state of the eye. Abbreviated "sc" (sine correctione -- without correction).
BCVA (Best Corrected)
Measured with the optimal prescription in place. This is the gold standard -- if BCVA is reduced, something beyond refractive error is affecting vision. Abbreviated "cc" (cum correctione -- with correction).
Pinhole Acuity (PH)
Measured through a pinhole occluder when VA is reduced. Improvement with pinhole suggests the cause is refractive (correctable with lenses). No improvement suggests ocular pathology. A key diagnostic clue for the doctor.
Habitual VA
Measured with whatever glasses or contacts the patient is currently wearing in daily life. This tells the doctor how well the patient is actually seeing day-to-day, which may differ from BCVA if the prescription is outdated.
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Documentation Standards
Proper documentation of visual acuity is a frequent exam topic and a daily clinical requirement. Incomplete or ambiguous recordings can lead to misinterpretation, poor clinical decisions, and medicolegal problems.
Complete VA Documentation Example
Distance VA (sc):
OD: 20/40-1
OS: 20/30+2
OU: 20/25
Distance VA (cc, habitual Rx):
OD: 20/25-1 PH: 20/20
OS: 20/20
OU: 20/20
This documentation tells the doctor exactly what was measured, under what conditions, and with what precision. The pinhole result on OD suggests the remaining deficit is refractive, not pathological.
Documentation Checklist
Every VA recording should include: which eye (OD/OS/OU), the testing condition (sc, cc, or PH), the chart type and distance if non-standard, plus/minus notation for partial lines, and the date and your initials. If the patient was tested at a distance other than 20 feet, the numerator must reflect the actual testing distance.
Common Mistakes and Exam Pitfalls
Allowing finger occlusion
Patients naturally want to use their hand to cover an eye. Never allow this. Fingers separate, light leaks through, and the measurement becomes unreliable. Always use a proper occluder, tissue, or occluding paddle. This is a classic exam question.
Not encouraging guessing
Many patients stop reading when they feel uncertain. You should encourage them to try the next line and guess if necessary. A patient who "guesses" 3 out of 5 letters correctly on the 20/20 line has better acuity than one who stops at 20/30 because they felt unsure.
Confusing numerator and denominator
The numerator is always the testing distance (20 feet). The denominator is the distance at which a normal eye could read that line. A larger denominator means worse acuity. 20/200 is worse than 20/100, which is worse than 20/40. On the exam, you may be asked to rank acuity values from best to worst.
Testing OS before OD
The convention is always OD first, then OS, then OU. Testing in the wrong order introduces inconsistency in documentation and can cause confusion when comparing to previous records. This is a simple habit that should be automatic.