The Visual
Acuity Chart.

The Fundamentals

What a visual acuity chart actually measures.

A visual acuity chart presents optotypes — standardized letters, numbers, or symbols — in precisely decreasing sizes at a known distance, to find the smallest detail a patient's eye can resolve. The geometry is the whole point: a 20/20 optotype subtends 5 minutes of visual arc, with each identifying stroke subtending exactly 1 minute. Herman Snellen formalized this in 1862, and every chart since — Snellen rows, LogMAR/ETDRS panels, Tumbling E, Landolt C rings, pediatric picture charts — is a variation on that same angular math.

Because acuity is angular, letter size is a function of distance. Move the chart, and every optotype must resize. That single fact is why printed charts are locked to one lane length, why projector systems needed careful throw calibration, and why a properly calibrated screen — which can recompute sizes for any distance instantly — is the natural end point of the acuity chart's 160-year evolution.

Exact letter sizes, 20/200 to 20/10.

At the standard 20-foot (6.1 m) lane, the required physical letter heights follow directly from the MAR formula letterHeight = distance × tan(MAR × 5 × π/10800). These are the sizes a correctly calibrated chart must render — and the sizes you can verify on an Ocular Edge screen with a ruler:

Shorter lane? The whole table scales linearly — at 10 feet a 20/20 letter is 4.43 mm — and mirrored lanes double the optical path so a 10-foot room can test at 20-foot equivalence. Ocular Edge handles both automatically from the calibration wizard, and validates its sizing math with an automated suite of 41 clinical accuracy tests.

Snellen or LogMAR? Both — scored properly.

Snellen charts read fast and every patient knows them, but their irregular size steps and per-row letter counts (one 20/200 letter, eight 20/20 letters) make change hard to quantify. The LogMAR/ETDRS format fixes that: five Sloan letters per row, a constant 0.1 logMAR step between rows, per-letter scoring at −0.02 logMAR each — which is why it is the standard in clinical research. A complete chart system should offer both, plus Tumbling E and Landolt C for patients who don't read the Latin alphabet, and picture optotypes — HOTV, kids' symbols, Allen pictures, Pigassou — for pediatric exams. Ocular Edge ships all of them, in every plan, rendered as vector geometry following ETDRS letter conventions and the ISO 8596 Landolt ring.

The memorization problem.

Every optometrist has met the patient who "reads" E-F-P-T-O-Z from memory. A printed chart can't help it — it never changes. A digital visual acuity chart deals a fresh random line from the standard letter set on every press, isolates a single optotype for amblyopia work, adds crowding bars for pediatric testing, and flips the whole chart for mirrored lanes — all from the remote in your hand at the phoropter. That's the practical difference between displaying a chart and running one.

Paper, projector, panel — or software.

A wall chart costs a few dollars and does one immutable thing. Chart projectors improved size flexibility but demanded bulb, throw, and screen maintenance. Dedicated digital chart panels solved both problems and commonly list for $2,600–$4,200 per lane for the privilege. The software approach keeps the calibrated accuracy and drops the appliance: Ocular Edge turns any Smart TV, monitor, or tablet into the chart — paired to your practice with a 6-digit code, calibrated with a ruler, controlled from any remote, working offline once loaded, from $24.99 per lane per month or $449 once.

Keep Reading

From one chart to the whole lane.