To determine if image magnification from ametropia or other factors required correction in the photogrammetric diagnosis of optic nerve hypoplasia, the following variables were studied using two standard 30 degrees field fundus cameras (Zeiss FF-3 and Kowa RC-2): (1) distance between the camera and the subject--variations in this distance do not change the image size; (2) decentration of the camera (i.e., decentration of the optic disc image within the field)--no effect was found using the Kowa camera, but in the Zeiss, a 2% (0.07 mm) increase in disc image size was found for each mm of optic disc decentration in the field; (3) ametropia, refractive experimental (induced in one subject with soft contact lenses, from +8.00 diopters (D) to -8.00D)--no significant change in image size was found (In addition, in 50 healthy adult eyes with various refractive errors, including ten with uncorrected aphakia, there was no significant difference found for the size of the optic nerve in different refractive ametropias. No correction factor was necessary for aphakia or for other refractive ametropias); and (4) ametropia, axial--axial myopia showed minor but significant magnification: a linear increase in the horizontal diameter of the optic disc image of 2% (0.07 mm) was found for each diopter of axial ametropia exceeding 3D. There was insufficient data for axial hyperopia. These magnification factors should be applied when measuring the optic disc on 35-mm transparencies to make a diagnosis of optic nerve hypoplasia.(ABSTRACT TRUNCATED AT 250 WORDS)