Zones of retinopathy of prematurity. Zone I corresponds to a circle centered on the optic disc, with a radius of twice the distance from the optic disc to the center of the fovea. A helpful clinical correlate for estimating the temporal extent of zone I is to place the nasal edge of the optic disc in the field of view of a 25- or 28‑diopter lens, with the limit of zone I being the temporal field of view. Zone II forms a circle peripheral to zone I with a radius from the optic nerve to the nasal ora serrata. Zone III is made up of the remaining temporal crescent of retina outside zone II.