Image quality of the human eye for eccentric entrance pupils

The human eye has a considerable amount of chromatic aberration and a moderate amount of spherical aberration. If a small artificial pupil is placed in front of such optics and moved away from the centre towards the periphery the chromatic aberration will result in lateral colour shifting, and the spherical aberration will cause coma. Starting from available experimental data on the chromatic aberration and the spherical aberration, modulation transfer functions (MTF's) are calculated for central and eccentric positions of small pupils placed in front of the human eye. These calculations may help to separate optical and retinal contributions to the well-known reduction of visual acuity measured with eccentric pupils. It appears that measurements with pupils smaller than 1 mm in monochromatic light can be considered optically as diffraction limited. Changes of visual acuity found in this way must be assigned to the retina.