Because a single gonioprism is expected to fit every eye, gonioscopy often distorts the anatomy of the angle and prevents accurate evaluation. Keratometric gonioscopy, a method whereby the gonioprism contact surface corresponds to the corneal curvature, has solved this defect, permitting an undistorted view of the angle. Better visualization of the narrow angle is achieved by improved mirror position and height.
Becker SC. Unrecognized Errors Induced by Present-Day Gonioprisms and a Proposal for Their Elimination. Arch Ophthalmol. 1969;82(2):160-168. doi:10.1001/archopht.1969.00990020162003