Fundus measurements with indirect ophthalmoscopy. An experimental approach

We examined the factors influencing the accuracy of indirect ophthalmoscopic estimates of choroidal tumor size by studying indirect ophthalmoscope photographs of solid black domes implanted in the suprachoroidal space of postmortem human eyes. Measurements of the indirect ophthalmoscope photographs showed clinically significant differences in the absolute diameter of the visualized fundus, which varied with differences in condensing lens power and width, anteroposterior location of viewed fundus, and axial length of the experimental eye. Linear regression analysis demonstrated that the field of view of each condensing lens varied similarly with axial length and that the majority of variation in field size for each condensing lens was attributable to axial length variation. Equatorial fields of view averaged 11% smaller than posterior fields of view. The accuracy of estimated absolute intraocular dimensions using indirect ophthalmoscopy potentially approaches +/- 5%.