Understanding Vision Tests: How Visual Acuity Is Measured
Visual acuity is the clarity or sharpness of vision, measured by the ability to discern letters or symbols at a standardized distance. The Snellen chart, developed by Dutch ophthalmologist Hermann Snellen in 1862, remains the most widely used visual acuity test in clinical practice. The familiar "20/20" notation means that at 20 feet distance, you can read what a person with normal vision reads at 20 feet. A score of 20/40 means you must be at 20 feet to read what a normal-sighted person reads at 40 feet, indicating reduced acuity.
The Snellen Chart and Visual Acuity Scale
20/200: Largest letter (legally blind if best corrected)
20/100: Very poor vision
20/70: Moderate impairment
20/50: Mild impairment
20/40: Driving standard (many states)
20/30: Slightly below normal
20/20: Normal visual acuity
20/15: Above average
20/10: Excellent (rare, fighter pilots)
Metric equivalent (at 6 meters):
20/20 = 6/6 | 20/40 = 6/12 | 20/200 = 6/60
LogMAR conversion:
LogMAR = log10(denominator / numerator)
20/20 = LogMAR 0.0 | 20/40 = LogMAR 0.3
Online test calibration:
At 10 ft (3m) from screen, letter sizes are
halved compared to standard 20 ft chart.
The 20/20 line at 10 ft = ~4.4mm tall.
Types of Vision Tests and What They Screen
The Snellen chart tests distance visual acuity — how clearly you see objects far away. It primarily detects refractive errors (myopia, hyperopia, astigmatism) and is the standard for driving license requirements. The Tumbling E test uses rotated E letters for children or people who cannot read the Latin alphabet. The patient indicates which direction the E's prongs face (up, down, left, right), making it universally applicable regardless of literacy.
The Amsler grid is a specialized test for macular degeneration — a condition affecting the central portion of the retina. The grid consists of evenly spaced horizontal and vertical lines with a central fixation dot. When viewed by someone with macular disease, the lines may appear wavy, distorted, or missing in certain areas. This test is particularly valuable for early detection of age-related macular degeneration (AMD), the leading cause of severe vision loss in adults over 60.
Color vision testing uses Ishihara-style pseudoisochromatic plates — circles filled with colored dots of varying sizes that form numbers visible to people with normal color vision but invisible or different to those with color deficiency. Approximately 8% of males and 0.5% of females have some form of color vision deficiency, most commonly red-green confusion (deuteranopia or protanopia). An astigmatism check uses a radial line pattern (fan chart); if certain orientations appear darker or sharper than others, it suggests the cornea has uneven curvature.