Visual acuity in congenital nystagmus has proven to be primarily related to the duration of foveation periods, during which the image of a target falls onto the fovea and eye velocity slows down. It was found that the longer the foveation time the higher the visual acuity. However, the cycle-to-cycle variability of the eye position and velocity during foveation periods also contribute to visual acuity. A high variability of the eye position during the foveations hinders a stable placement of the target image on the centralmost fovea and consequently decreases visual acuity. To investigate the relationship between different nystagmus features and visual acuity, infrared- oculographic and electro-oculographic eye position recordings of 20 patients affected by congenital nystagmus were analysed in different gaze positions. In several patients' recordings, a high variability of the eye position during foveations (i.e. greater than 0.5 degrees) was detected. Correspondingly, low visual acuity was measured, in spite of sufficiently long foveation periods. The standard deviation of eye positions during foveation periods was used to measure this variability and it was found to be correlated to visual acuity, in conjunction with the mean duration of the foveation periods. On the basis of the data analysis, an exponential relationship is proposed to relate visual acuity and the standard deviation of the eye position during foveations.