In designing optical systems where the eye serves as the final detector, assumptions are typically made regarding the optical quality of the eye system. Often, the aberrations of the eye are ignored or minimal adjustments are built into the system under design to handle variations in defocus found within the human population. In general, the eye contains aberrations that vary randomly from person to person. In this investigation, a general technique for creating a random set of aberrations consistent with the statistics of the human eye is developed. These aberrations in turn can be applied to a schematic eye model and their effect on the combined visual instrument/eye system can be determined. Repeated application of different aberration patterns allows for tolerance analysis of performance metrics such of the modulation transfer function (MTF).