Artificial pupils and Maxwellian view.

When control of the pupil size is required, the simplest method is to use a physical artificial pupil or aperture that is placed in the spectacle plane. In some clinical applications (e.g., the potential acuity meter) an optical artificial pupil is imaged in the plane of the natural pupil by a Maxwellian view optical system. We compared visual performance with physical and Maxwellian artificial pupils by measuring the effects of the pupil diameter (0.5-5 mm in range) and defocus (5-D myopia to 4-D hyperopia) on minimum angles of resolution (MAR's) and on angular blur disk diameters. For pupil diameters down to ~ 2.0 mm there were no meaningful differences between the visual resolution that is obtained with the physical and the Maxwellian pupils. At the smallest diameter (0.5 mm) the physical artificial pupils caused the MAR to increase because of the diffraction limitation on resolution, and defocus no longer affected MAR. With the small Maxwellian pupils vision did not become diffraction-limited so that maximum resolution could still be obtained. MAR was still affected by defocus. The angular blur disk diameters measured with the smaller Maxwellian pupils were slightly but significantly larger than those found with physical artificial pupils. For physical artificial pupils, field-of-view restrictions may result from vignetting with the eye pupil. Thus small physical artificial pupils can act as pinholes causing resolution to become impaired but insensitive to defocus. Also vignetting by the eye pupil can restrict the field of view. Small optical artificial pupils from Maxwellian viewing do not impair resolution, and the resolution may remain sensitive to defocus. The eye pupil does not cause any field restriction, although, if small, it may filter higher spatial frequencies out of the retinal image.