What is the purpose of using logMAR as a measurement on the ETDRS chart?

• A. The progression of letter sizes is logarithmic
• B. It is linearly scaled
• C. It uses a fixed number of letters per line
• D. It measures contrast sensitivity

Answer: (B)

Using logMAR on the ETDRS chart provides a measurement that is linear in acuity. In this system, each letter read reduces the logMAR value by a fixed amount, and each line represents the same, equal step in logMAR. That means small changes in vision produce consistent, evenly spaced changes on the scale, which makes scoring precise, comparable across observers, and suitable for statistical analysis. The design does involve equal geometric steps in print size, but the purpose of using logMAR is to create equal, linear increments in the acuity measurement. The chart’s fixed number of letters per line is a design choice, and it does not measure contrast sensitivity.