Psychometric measurement of visual acuity not only determines an acuity threshold, but also, by the slope of the psychometric function, reveals how the percentage of correct responses changes with increasing target size. In this study we determined how the slope of the psychometric function changed for different threshold acuities. Psychometric visual acuity functions were obtained using a clinical contour-interaction chart for 35 naive, normal subjects with best refractive correction and with 1.5 and 3.0 diopters of spherical blur. The slope of the psychometric acuity function remained essentially constant for different threshold acuities when acuity was scaled logarithmically (log MAR), but varied with the threshold when inverse Snellen (MAR) and Snell-Sterling scales were used. Our results lend support to the use of the log MAR scale for acuity but, because normal subjects' slopes range tenfold, the use of this parameter in making clinical distinctions may be problematic.