Regarding ETDRS distance calibration, which statement is correct?

Regarding ETDRS distance calibration, which statement is correct?

• A. It is calibrated at 4 meters, with additional calibration at 2 meters and 1 meter
• B. It is calibrated only at 4 meters
• C. It is calibrated only at 2 meters
• D. It is calibrated only at 1 meter

Answer: (A)

The ETDRS distance calibration is built around a standard testing distance of 4 meters, so the letters are sized to provide a known acuity value at that distance. But clinics often can’t place a chart exactly 4 meters away, so the chart also offers calibrations for shorter distances—2 meters and 1 meter—so you can test at those distances without distorting the acuity measurement. Using the matching calibration for each distance keeps the results comparable on the same acuity scale. That blend of a 4-meter standard with additional 2-meter and 1-meter calibrations is why the statement describing calibration at 4 meters plus these shorter-distance calibrations is correct.

The ETDRS distance calibration is built around a standard testing distance of 4 meters, so the letters are sized to provide a known acuity value at that distance. But clinics often can’t place a chart exactly 4 meters away, so the chart also offers calibrations for shorter distances—2 meters and 1 meter—so you can test at those distances without distorting the acuity measurement. Using the matching calibration for each distance keeps the results comparable on the same acuity scale. That blend of a 4-meter standard with additional 2-meter and 1-meter calibrations is why the statement describing calibration at 4 meters plus these shorter-distance calibrations is correct.