BACKGROUND Practice-related improvements of performance are common in many areas of visual processing. There is preliminary evidence that this is also the case for standard optotype acuity tasks. The present study was designed to confirm and quantify the effect of practice under different feedback conditions and to track the dynamics of practice over several sessions. METHODS Subjects completed a total of 56 runs of a computer-based acuity test with randomly oriented Landolt C optotypes, split evenly over four sessions at intervals of 1 week. Half of the subjects received feedback indicating the correct response. RESULTS Over the course of the sessions, the test outcomes increased significantly by 0.11 logMAR with feedback and by 0.055 logMAR without feedback. In addition to an increase in acuity over the first few runs of the first session, a major part of the practice effect with feedback occurred not during a session, but in between the first and the second session. Without feedback, the increase in acuity occurred mainly within the first half of the first session. CONCLUSIONS Feedback has a drastic effect on the magnitude and dynamics of the practice effect, which is not explained by simple familiarization with the test procedure. If feedback is not given, practice effects can be neglected in most clinical routine applications even when many test repetitions are performed. However, they may become relevant on a group level in clinical studies without an appropriate control.