ETDRS vs
Snellen.

Side By Side

The differences at a glance.

Why the redesign happened.

Snellen's 1862 chart standardized the optotype but not the chart: one giant letter at 20/200 and eight small ones at 20/20 means the task changes as the patient reads down — different crowding, different chances to guess, irregular jumps in difficulty. Bailey and Lovie's 1976 logMAR design fixed the geometry: the same five letters per row, sizes shrinking in constant 0.1 logMAR steps, spacing scaled in proportion so crowding stays equal at every level. The Early Treatment Diabetic Retinopathy Study adopted those principles with the ten Sloan letters in 1982, and the ETDRS chart became the instrument every serious trial measures with.

Scoring: lines vs letters.

Snellen scoring is coarse: the smallest line mostly read is the acuity, and a patient who reads half the next line gets a "+2" annotation at best. ETDRS scoring counts every letter — each one is worth 0.02 logMAR, five to a row — and trials record a letter score of roughly 85 − 50 × logMAR: 85 letters ≈ 20/20, 70 ≈ 20/40, 35 ≈ 20/200. When a retina study reports "a 15-letter gain," that's three full lines of real improvement, resolvable only because the chart measures in 0.02 steps. Convert any value between notations with our free acuity converter.

Which should your lane use?

Both — for different jobs. Snellen is fast, familiar, and perfectly adequate for routine refraction; ETDRS is what you reach for when change over time matters: low-vision work, co-managed retina patients, anything you might need to defend with numbers. The old constraint was owning two physical charts and the wall space for them. A calibrated digital lane erases it: Ocular Edge renders Snellen-progression charts and full ETDRS/LogMAR layouts from the same vector Sloan letterforms, scores per letter automatically in both notations, randomizes every presentation, and sizes everything from a physical ruler measurement — a 20/20 row at exactly 8.87 mm at 20 feet, on whatever screen is on your wall. The full letter-size math lives in our visual acuity chart reference.