In the application of Topcon SL-45 Scheimpflug slit and Neitz CTR retroillumination photography to in vivo documentation of cataractous change, several sources of variance affect the results of each technique. We have measured the between-person, between-photographer, between-focal plane (Neitz), between-photo session, replicate photograph (Topcon), between-image analyst and replicate image analysis variances encountered in in vivo documentation of human cataracts with the Topcon SL-45 and Neitz-CTR cameras and our system of computerized image analysis.