Most often presenting as a keyhole-shaped pupil, coloboma may affect one or both eyes. Persons with coloboma of the iris often have fairly good vision, but those with coloboma involving the retina may have vision loss in specific parts of the visual field, which can cause problems with reading, writing, and close-up work or play. Large retinal colobomas, or those affecting the optic nerve, can cause vision loss that cannot be completely corrected with glasses or contact lenses.