A computer model that incorporates the monochromatic aberrations of the eye is used to determine the optimal pupil size for axial and lateral resolution as it applies to retinal imaging instruments such as the confocal scanning laser ophthalmoscope. The optimal pupil size for axial resolution, based on the aberrations of 15 subjects, is 4.30 mm±1.19 mm standard deviation (sd), which is larger than that for lateral resolution [2.46 mm±0.66 mm (sd)]. When small confocal pinholes are used, the maximum detected light is obtained with a pupil size of 4.90 mm±1.04 mm sd. It is recommended to use larger pupil sizes in imaging applications where axial resolution is desired.