Infant and adult vernier acuity differed by a factor of only 4 to 6 when the stimuli were periodic and results were expressed in units of spatial phase. This ratio was much smaller than the factor of 50 to 100 obtained when we expressed our results and those of others in terms of the threshold spatial displacement in visual angle. We compared infant and adult vernier performance to performance on a “benchmark” contrast discrimination task, where the vernier and contrast discrimination stimuli contained identical Fourier contrast spectra. When we compared vernier performance directly to contrast discrimination performance, infant and adult data were remarkably similar, suggesting that similar parts of the visual system limit vernier and contrast performance of subjects of both ages. A control experiment on adults suggested that the superior performance of the contrast discrimination task is due to recruitment of visual pattern analyzers situated at a distance from the discontinuities in phase position and contrast.