Vernier and letter acuities are both susceptible to degradation by image motion. In a previous study, we showed that the worsening of Vernier acuity for stimuli moving up to 4 degrees/s is accounted for primarily by a shift of visual sensitivity to mechanisms of lower spatial frequency. The purposes of this study were to extend the previous results for Vernier acuity to higher stimulus contrast and velocities, and to determine if a shift in spatial scale can similarly explain the degradation of letter acuity for moving stimuli. We measured Vernier discrimination for a pair of vertical abutting thin lines and letter resolution for a four-orientation letter 'T' as a function of stimulus velocity ranging from 0 to 12 degrees/s. Stimuli were presented at 20 times the detection threshold, determined for each velocity. To determine the spatial-frequency mechanism that mediates each task at each velocity, we measured Vernier and letter acuities with low-pass filtered stimuli (cut-off spatial-frequency: 17.1-1.67 c/deg) and analyzed the data using an equivalent blur analysis. Our results show that the empirically determined, equivalent intrinsic blur associated with both tasks increases as a function of stimulus velocity, suggesting corresponding increases in the size of optimally responding mechanisms. This progressive increase in mechanism size can account for the worsening of Vernier and letter acuities with velocity. Vernier discrimination is found to be more susceptible to degradation by various stimulus parameters than letter resolution, suggesting that different mechanisms are involved in the two tasks. We conclude that the elevations in Vernier and letter acuities for moving stimuli are the consequence of a shift of visual sensitivity toward mechanisms of lower spatial frequencies.