• Comparison of visual acuity measurements under different conditions (eg, age, refractive error, disease progress, recovery from injury) depends crucially on the scale applied to values of the minimum angle of resolution. Four kinds of scales are examined: the minimum angle of resolution (MAR) in linear, logarithmic, exponential, and reciprocal measures. To help construct an equal-discriminability scale for visual acuity, the standard errors of the mean minimum angle of resolution in five different retinal locations were determined experimentally in two observers. Because the ratio of standard error/mean was found to be nearly a constant, the logarithmic scale of MAR is recommended for adoption as a standard for visual acuity: in it the just noticeable differences in acuity are everywhere equal on the scale.
Westheimer G. Scaling of Visual Acuity Measurements. Arch Ophthalmol. 1979;97(2):327-330. doi:10.1001/archopht.1979.01020010173020