Which scale is used on the ETDRS chart?

• A. Visual acuity scale
• B. logMAR
• C. MAR
• D. Snellen

Answer: (B)

The ETDRS chart uses the logMAR scale because it provides a precise, linear way to measure visual acuity. On this chart, each letter represents a fixed step of 0.02 logMAR, and there are five letters per line, making each line differ by 0.1 logMAR. This arrangement creates equal, evenly spaced steps in acuity, so small changes in vision are reflected as proportional changes on the scale. That linearity is why logMAR is favored in research and clinical trials—it allows accurate scoring (often by letter), easy summation of results, and meaningful statistical analysis.

Other scales don’t fit as well. The Snellen system uses varying step sizes between lines, which makes changes in acuity harder to quantify precisely. The MAR (minimum angle of resolution) is a measurement of angular size, not a scale for chart scoring, and using it as a chart scale wouldn’t provide the same uniform stepping. A generic “visual acuity scale” is not specific enough for the exact, linear progression required by ETDRS testing.

So, logMAR is the scale used on the ETDRS chart because it yields equal-interval, linear measurements of acuity that are ideal for precise assessment and analysis.