Visual acuity measurements

Modern visual acuity charts are designed so the letter sizes on each line follow a geometric progression (ie, change in a uniform step on a logarithmic scale).1–3 The accepted step size has been chosen to be 0.1 log unit, which is equivalent to letter sizes changing by a factor of 1.2589 between lines. This standard gave rise to the logMAR (logarithm of the minimum angle of resolution) notation as shown in Table 1, column 3. The bold values correspond to the accepted logMAR steps. Values that are not in logMAR steps such as 20/30, 20/60, 20/70, 20/150, and 20/300 are included because of their common appearance in older visual acuity charts. A geometric progression of lines on the visual acuity chart was chosen…