PURPOSE: To evaluate the effect of spherical aberration on visual acuity by correcting and inducing spherical aberration using an adaptive optics vision simulator. SETTING: Laboratory of Vision Science, Capital Medical University, Beijing, and Institute of Optics and Electronics, Chinese Academy of Sciences, Chengdu, China. METHODS: An adaptive optics vision simulator comprising a wavefront sensor and a 37-segmented deformable mirror was used to correct and induce aberrations of the eye. The effective ocular wavefront aberration was manipulated with the deformable mirror, as the resulting visual performance was simultaneously measured. Subjective visual acuity measurements were performed with a 6.0 mm pupil. Visual acuity at different contrasts was measured when spherical aberration was fully corrected and the other natural aberrations in the eye were present and when spherical aberration values were induced with the other aberrations corrected. RESULTS: The natural root-mean-square (RMS) value of spherical aberration in the 8 subjects examined was between -0.11 mu m and 0.14 mu m. There was no significant improvement in visual acuity with spherical aberration corrected and the subjects' natural aberrations present. When all aberrations were corrected, a decrease in visual acuity occurred when spherical aberration RMS was induced at 0.2 mu m and 0.3 mu m. CONCLUSIONS: When fluctuation of other natural aberrations in the eye were present, there was a slight effect on visual acuity when the spherical aberration RMS was approximately 0.1 mu m. Therefore, an RMS value of 0.1 mu m could be an acceptable amount of spherical aberration when correcting spherical aberrations. J Cataract Refract Surg 2009; 35:1389-1395 (C) 2009 ASCRS and ESCRS