What distance is commonly used to measure ETDRS visual acuity?

What distance is commonly used to measure ETDRS visual acuity?

• A. 3 meters
• B. 4 meters
• C. 2 meters
• D. 6 meters

Answer: (B)

Standardizing the testing distance keeps each line of letters on the ETDRS chart at a precise visual angle, which is essential for the chart’s equal-steps (0.1 logMAR) progression. The chart is calibrated for four meters, so reading at that distance yields consistent, comparable acuity scores across visits and studies. If you tested from a shorter distance, letters subtend a larger angle and you’d likely record better-than-accurate acuity; from a longer distance, letters become too small and acuity would appear worse. That is why four meters is the standard distance for ETDRS visual acuity testing, while other charts like Snellen might use different distances such as six meters (or 20 feet).

Standardizing the testing distance keeps each line of letters on the ETDRS chart at a precise visual angle, which is essential for the chart’s equal-steps (0.1 logMAR) progression. The chart is calibrated for four meters, so reading at that distance yields consistent, comparable acuity scores across visits and studies. If you tested from a shorter distance, letters subtend a larger angle and you’d likely record better-than-accurate acuity; from a longer distance, letters become too small and acuity would appear worse. That is why four meters is the standard distance for ETDRS visual acuity testing, while other charts like Snellen might use different distances such as six meters (or 20 feet).