Human visual acuity strongly depends on environmental conditions. One of the most important physical parameters affecting its value is the pupil diameter, which follows changes in the surrounding illumination by adaptation. Thus, the direct measurement of its influence on visual performance would require either medicaments or inconvenient apertures placed in front of the subjects? eyes to examine different pupil sizes, so it has not been studied in detail yet. In order to analyze this effect directly, without any external intervention, we accomplished simulations by our complex neuro-physiological vision model. It considers subjects as ideal observers limited by optical and neural filtering, as well as neural noise, and represents character recognition by templatematching. Using the model, we reconstructed the monocular visual acuity of real subjects with optical filtering calculated from the measured wavefront aberration of their eyes. According to our simulations, 1 mm alteration in the pupil diameter causes 0.05 logMAR change in the visual acuity value on average. Our result is in good agreement with former clinical experience derived indirectly from measurements that had independently analyzed the effect of background illumination on pupil size and on visual quality.