The human iris and pupil act as a diaphragm, not unlike that of a camera, which changes its opening size depending on how much light would be appropriate to enter the camera for a clear picture. This function is identical for the human eye and without a round pupil a patient will suffer visual deficiencies.
When light/images enter through openings other than the normal, relatively round pupil, or when the pupil is only partially filled by a lens optic, the potential exists for multiple images passing to the retina. This will cause double vision. Likewise, light striking the bare periphery of the lens will provide an opportunity for glare. The tears in the iris also will foster undesirable images to reach the retina.
Double vision medical expert witness specialties include ophthalmology retina vitreous ophthalmology neuroophthalmology