The optical performance of the eyes of nine vertebrate species was evaluated using optical system design techniques and schematic eye models. Essential features of eyes, including the modulation transfer function (MTF) and the MTF cutoff frequency are related to the numerical aperture of the eyes. Superior resolution for in vivo imaging of photoreceptors may be achieved by dilating the iris pupil of an eye, minimizing coherence, and using short wavelength illumination. The difference of lateral and axial resolution between a small and a large eye for imaging photoreceptors in vivo. © 2000 Society of Photo-Optical Instrumentation Engineers.